This last chapter breaks with the bird’s-eye view to engage in depth with three episodes that best highlight the intense interrelationship of longstanding scientific traditions in the Max Planck Society and global leadership in scientific fields. All three case studies have in common their roots in traditions that date from before Sputnik and that benefited from unique features of the Max Planck system, such as interdisciplinarity, embeddedness in international collaboration, and strong theoretical, experimental, and instrumental expertise. These all facilitated the rise of astro-particle physics and multi-messenger astronomy in Europe, in contrast to the difficulties experienced by their American counterparts, and fostered Max Planck scientists’ early participation in the entirely new field of gravitational wave astronomy. But the growing scale of scientific infrastructures and shifts in conditions at the end of the Cold War also heralded the constraints that Max Planck scientists would face in the 21st century, given that their scientific and technological achievements are meanwhile interwoven with vast multinational research organizations, where successes are not easily accredited.
1 Three Case Studies
This last chapter of the book will highlight three case studies—solar neutrinos, gravitational waves, and ground-based gamma astronomy—that directly exemplify how the longest-standing scientific traditions in the Max Planck Society, which have existed since the postwar era and whose roots go back even to the 1920s and ’30s, were especially effective in determining how the Society would successfully enter brand new research fields such as gravitational wave astronomy and astro-particle physics, a cross-disciplinary field at the intersection of particle physics, astronomy and cosmology that emerged in the 1970s and ’80s aiming to answer fundamental questions related to the history of the Universe.
Zooming in
Up to this point, our study has followed prominent threads in the history of the cosmic sciences in the Max Planck Society, constructing an explanatory framework centered around the interplay of various scientific traditions with the leading sociopolitical forces of the time, such as the atomic age and the space age. We have documented how, within this context, independent Max Planck Institutes learned to coordinate with one another their respective activities. In tracing the evolution of such scientific traditions, this survey has been organized as a loosely chronological and political narrative. We have focused on the ‘big picture,’ without attempting to provide an exhaustive account of all activities in the field; so far, we have also deliberately not examined any particular scientific development in close detail, only alluding to the most relevant cases.
This last chapter of the book, however, will engage in depth with the intense interrelationship of these longest-standing scientific traditions in the Max Planck Society, highlighting along the way three episodes that were key to its early presence—and its achievement of global leadership—in nascent fields of scientific interest in astrophysics and high-energy physics, and that even contributed to entirely new fields such as gravitational wave astronomy and astro-particle physics. This cross-disciplinary, multidimensional field, linking cosmology and particle physics, emerged in the 1970s and ’80s and was widely acclaimed by theoreticians studying the foundations of cosmology, nuclear/particle physics, and gravity, as well as by astrophysicists, astronomers, space physicists, and experimental particle and nuclear physicists.1
The choice of these case studies—solar neutrinos, gravitational waves, and ground-based gamma astronomy—is by no means based on a hierarchical evaluation of research in the Max Planck Society, but rather, most directly exemplifies how the longer-lasting traditions which have existed since the postwar era—and whose roots go back even to the 1920s and ’30s—were especially effective in determining how the Society would successfully enter new scientific fields.
The Deep Roots of Astroparticle Physics and Gravitational-Wave Research in the Max Planck Society
As mentioned above, the three selected case studies are rooted in research traditions dating from before Sputnik and that benefited from the unique features of the Max Planck system, such as interdisciplinarity, embeddedness in international collaboration, and strong theoretical, experimental, and instrumental expertise. In all of them, people and practices dating back to either Göttingen/Munich or Heidelberg served as springboards for, (in its most positive sense), opportunistic expansion into the most novel scientific questions of the day, with an impact lasting well into the 21st century, and even facilitating the rise of astroparticle physics and multi-messenger astronomy in Europe, in contrast to the difficulties experienced by the disciplines’ American counterparts. Within an increasingly globalized scientific landscape, it also happened that institutes belonging to different traditions entered the same field, alternately competing against and collaborating with one another, as in the case of neutrino physics and ground-based gamma ray astronomy. All the cases in this chapter follow a distinct pattern: instead of attempting to compete on an equal footing with their counterparts in the United States or even in France or the United Kingdom, Max Planck researchers increasingly identified the features that made their system unique, and considered how, in the increasingly global landscape, it could best take advantage of scientific competition and collaboration.2 These features were as follows:
- – An existing foothold in crucial international collaboration projects, which had first been gained through niche expertise in theory, small-scale experimentation, or the development of instruments.
- – The presence in several of the strongholds of the Max Planck Society, such as Munich and Heidelberg, of technical expertise and the capacity to develop radically new, medium-scale experimental systems, without the numerous hurdles that this type of research needs to overcome in other countries. These strongholds control flexible, highly competent, in-house technical workshops and maintain industrial contacts relevant to their specific requirements. These favorable conditions afforded further expansion as well as the retention, sometimes even attraction to Max Planck Institutes, of the global experts in the field.
- – An opportunistic trigger: the ability to identify, thanks to preexisting immersion in international collaborations, when leadership at the next scale of an experimental program could be achieved by Max Planck scientists, either in parallel with competitors or by taking over an international scientific research program. In all three cases mentioned in this chapter, the truly groundbreaking work had been done by others, sometimes over several decades, exceeding all the expectations and the opposition of their own scientific communities. Max Planck researchers accompanied such struggles, but from the wings, making a decisive move only once the pioneering work had been seen by those with deep expert knowledge of the field to show promising results, but had not yet been accepted by the mainstream scientific community which, in other countries, would determine whether the next scale of research should be funded. While their counterparts struggled to obtain the funding for scaling up their pioneering work, a good amount could be done in-house in Max Planck Institutes, and their early dominance in this scaling-up stage then allowed the rapid formation of collaborations with European partners in which Germans had the dominant instrumental role. In such pioneering enterprises, the relationship with the Americans was friendly but ambivalent: they all supported one another as dark horses against skeptics in their respective scientific communities; but at the same time, all sides played up the competition with other projects to highlight the urgency of their plans, and in Europe particularly, highlighted the opportunity for overtaking the Americans, a major justification in the eyes of their financial backers.
- – The cases treated in this chapter all feature decades-long searches for new, previously undetected signals, for which incremental improvements of several orders of magnitude in sensitivity and cost were expected. Measurements during the intermediate stages often only provided ‘upper limits,’ that is, null results, or non-detection at some statistical significance.3 Even more striking, false positives which could otherwise have derailed whole research fields, here rather provided the initial spark for revolutionary developments to come, by attracting a welcome flow of attention and resources. The long-term, independent support of the Max Planck Society and its ability to rely on in-house technical expertise and local networks of academic and industrial partners, combined with the theoretical expertise to confidently guide the search for something previously undetected allowed these projects to remain on the global leading edge for a vast range of experimental scales, up to financial limits of around 100 million USD.
Once the scale of research enterprises approached the billion dollar mark, at which global consolidation into just a few large-scale projects occurs, the outcome of Max Planck projects was quite heterogeneous, as the cases here illustrate: in some they obtained global dominance (ground-based gamma astronomy), in others they only narrowly averted defeat (gravitational-wave search), and on average, (also in neutrino research), they ended up on the front line of the enormous, flexible, horizontal collaborations that dominate much of 21st-century science.
Owing to how the Max Planck Society was formed in the postwar era, the scientific traditions involved in this discussion were led by physicists of both the theoretical (in Göttingen and, later, in Munich), and the experimental (in Heidelberg) varieties. This, in contrast to observational astronomy, which entered the Max Planck Society during the 1960s and followed an explicit logic of ‘catching up’ with the other major countries via well-trodden paths of incremental developments, such as telescope size, detector sensitivity, and optimized production techniques—what one might call ‘conservative innovation.’ As we have seen in previous chapters, such efforts were led by people and research traditions which were brought into the Max Planck Society only after Sputnik.4
This last chapter instead illustrates how conceptually ‘difficult’ fields, dealing with concepts and technologies that were far from the mainstay of most astronomers, astrophysicists, and particle physicists, entered the mainstream only after a long process, including decades of embryonic conceptualization and experimentation, eventually leading to experimental results which convinced a skeptical mainstream not only of the validity and scientific promise of the projects, but also of their financial and institutional feasibility. But this external skepticism also benefited these marginal new fields, lending them the particular dynamism of solidarity among competitors in different countries, aware that, while they were all racing to be the first and best in entirely new branches of research, they were all in it together, up against resistance from the more powerful, mainstream communities of accelerator physicists, observational astronomers, and space-based researchers.
The developments that follow—which initially, from the 1960s to the ’80s, were perceived as esoteric scientific quests—have during the time of writing this study advanced to become the core of the new ‘multi-messenger’ approach, which makes use of different cosmic messengers of the fundamental forces in nature—the electromagnetic, gravitational, weak and strong nuclear forces—to explore and understand the most violent phenomena in the Universe. The term multi-messenger astrophysics for this new field—a natural extension of traditional multi-wavelength astronomy—expresses the intimate connection between high-energy cosmic rays, astrophysical neutrinos, photons in multiple wavelength bands, and gravitational waves: the possibility of coincident observations of signals from diverse carriers can reveal inherently complementary information that is otherwise hidden, so finding the answers to some of the most important problems in astrophysics, and leading to the discovery of new phenomena by merging data from the world’s different detection sites.5 And more than half a century after their humble beginnings, and in the face of much skepticism from astronomers themselves, these traditions managed to establish that their instruments and sites are now also called ‘telescopes’ and ‘observatories.’ Multicomponent-based programs and experiments covering all four messengers in a broad energy range and using different techniques and detectors have currently become a main task for fundamental science, involving large international scientific collaborations working with big instruments in space and on the ground, that produce vast amounts of observational data to be analyzed and interpreted.
2 The Solar Neutrino Puzzle: Heidelberg between Cosmochemistry and Astroparticle Physics
The first newly emerging field benefited directly from the research tradition of cosmochemistry in southwestern Germany, introduced in Chapter 1. Through experimental techniques of mass spectroscopy and small sample radiochemistry, scientists from Freiburg and, later, the Max Planck Institute for Nuclear Physics in Heidelberg were able to collaborate with Brookhaven National Laboratory, where they met Ray Davis, father of solar neutrino detection and the ‘solar neutrino deficit’ paradox. Researchers from Heidelberg, led by Till Kirsten, improved the instrumentation and, in the 1970s, were even able to overtake the Americans by setting up the gallex collaboration, the next-generation experiment in which Germans, Italians, the French, and Israelis worked together with indirect support from the USA and the Soviet Union. Two decades after its conception, in the early 1990s, came the experimental results from gallex, which were part of the ‘Decade of the Neutrino’ that culminated in Nobel Prizes for the founders of the field. This leadership guaranteed a subsequent foothold in neutrino research even as it evolved away from cosmochemistry toward the electronic detection methods which have now become a central aspect of neutrino-based multi-messenger astronomy.
American Origins
The first newly emerging field benefited directly from the research tradition of cosmochemistry in southwestern Germany, introduced in Chapter 1. An intrinsically interdisciplinary research tradition with internationally recognized strengths in instrumental techniques, such as radiochemistry and mass spectrometry, was coupled with the ability to combine specific experimental approaches with the deepest theoretical questions of the time, something that had been the trademark of this Heidelberg tradition since the early collaboration initiated by Walther Bothe and Wolfgang Gentner in the 1930s.
Through experimental techniques of mass spectroscopy and small sample radiochemistry, scientists from Freiburg and, later, the Max Planck Institute for Nuclear Physics in Heidelberg were able to collaborate with Brookhaven National Laboratory, where Ray Davis, since the mid 1960s, had pioneered solar neutrinos detection using radiochemical techniques.6 Researchers from Heidelberg, led by Till Kirsten, improved the instrumentation and, in the 1970s, were even able to overtake the Americans by setting up the gallex collaboration, the next-generation experiment in which Germans, Italians, the French, and Israelis worked together with indirect support from the US and the Soviet Union. The cosmochemical path to neutrino research, which had initially been paved in collaboration with scientists in the United States, turned into one of the earliest examples of German researchers leading an international collaboration project, which at one point in the 1980s was the only spearhead in an otherwise neglected form of research falling right into the chasm between the interests of particle physics and astrophysics. Thanks to this groundbreaking work, in subsequent decades, the collaboration became part of a rapidly growing mosaic of interlocking research enterprises that led to a series of Nobel Prizes from the 1990s on. This cosmochemical tradition gradually gave way to the now prevalent forms of neutrino research based on electronic detection methods derived from experimental particle physics, which were adopted not only in Heidelberg, but also, in another manifestation of the most ancestral rivalry within the Max Planck Society, by researchers in Munich (see Chapter 1).
The neutrino plays a vital role in nuclear physics, particle physics, and astrophysics, and so intensive experimental efforts have always been conducted to elucidate its properties. For about 25 years, the neutrino was only a theoretical entity. Postulated by Wolfgang Pauli in 1930 to explain the apparent failure of conservation laws in nuclear beta decay,7 it was incorporated by Enrico Fermi in his successful theory of this process and taken to be convincing evidence of its existence.8 Neutrinos can travel several light years without interacting with matter because of the weakness of their interactions with other particles. For this reason, a neutrino was viewed as an ‘undetectable particle’ for many years and only in 1956 was the detection of a free (anti)neutrino finally announced by Frederick Reines and Clyde L. Cowan, who had used the newly developed technology of organic liquid scintillators, positioning a large tank of water near a nuclear reactor, a very intense source of antineutrinos.9 With this experiment—heralding the beginning of the era of neutrino detection—the status of the neutrino changed drastically: it was no longer a hypothesis, a mere theoretical construct, but a very solid fact.
By that time, thanks to many years of technological and scientific developments, it had become possible to think about detecting reactions triggered by neutrinos. The Sun, too, is a good source of neutrinos, which are produced by nuclear reactions taking place in its core. Detailed elaboration of the proton-proton chain (
In the early 1960s, Ray Davis Jr., a radiochemist from Brookhaven, took up the challenge and devised an experiment in the one-mile-deep Homestake Mine in South Dakota to detect the flux of these energetic neutrinos coming from the Sun. This was done using a huge tank of carbon tetrachloride (CCl4, used in the past as dry cleaning fluid) as target material and locating it deep underground—to minimize cosmic ray interactions producing reactions that could mimic neutrino capture—and then painstakingly extracting and counting with a small Geiger counter the tiny amounts of argon-37 atoms that were produced by the very rare interaction of neutrinos from the Sun with chlorine-37 atoms. The enormous volume—which would be characteristic of all future neutrino detectors—could overcome the problem of the very small probability for a neutrino to interact with chlorine nuclei of the radiochemical detector.14 The basic reason for doing solar-neutrino experiments was to test quantitatively the theories of nuclear energy generation in stars and of stellar evolution. The photons that are the subject of conventional astronomy come from the outermost layers of a star, whereas neutrinos, because of their large mean free path, can reach us directly from the deep interior of a star, where the nuclear reactions responsible for energy generation and stellar evolution occur. Davis himself originally set up his experiment in the context of solar physics, seeking to determine whether the measured neutrino flux coincided with the theoretical predictions made by solar and nuclear astrophysicists. However, the initial results, published in 1968, came as a surprise: the neutrino capture rate in the detector showed that the upper bound on the solar neutrino flux was two to three times smaller than expected on the basis of the Standard Solar Model.15 The deficit in the solar-neutrino flux resulting from the first large-scale experiment designed to detect neutrinos from the interior of the Sun was thus a blow to the then acknowledged theory of solar-type stellar physics and became known as the solar neutrino problem.
Over the following 20 years, Davis’s chlorine-argon experiment, which could be considered as the beginning of neutrino astrophysics, remained the only experiment providing data on solar neutrinos. For many years, these experiments faced resistance from the mainstream of accelerator-based particle physicists, who expected it would be too difficult to obtain reliable data due to the experimental design, which was also perceived as unglamorous chemistry carried out in abandoned mines, in contrast to their well-established work with nuclear reactors and particle beams in accelerators.16
An Unexpected Paradox between Astrophysics and Particle Physics
To address the difficult theoretical implications of his research results—which were steadily improved, as the unwanted background effects and uncertainties from nuclear decay-counting statistics of the few argon atoms produced in the tank were constantly lowered—Davis found his greatest ally in John Bahcall, a solar astrophysicist from Princeton, whose systematic work over the years proved that the low flux found in the solar neutrino experiments of Davis and others could not be explained by errors in the Standard Solar Model.17 Over the many years that Davis’s experiment was conducted, during which time results trickled down only slowly, the measured flux continued to be only around one-third of the theoretical prediction, launching almost three decades of scientific debates that covered a wide range of epistemological questions, from the accuracy of the experimental system to the validity of the nuclear fusion theories of the Sun, and ultimately, whether the problem may have originated in the nature of neutrinos themselves.
In 1967, before the first results were published by Davis,18 Bruno Pontecorvo had anticipated the solar neutrino problem, pointing out that neutrinos could oscillate between different states, and thus solar neutrinos (electron neutrinos) might transform into muon neutrinos during their journey from the Sun to the Earth. This phenomenon would lead to an observed deficit of neutrinos in chlorine-based experiments which could only detect neutrinos of a specific lepton flavor, electron neutrinos.19 The phenomenon of neutrino oscillation from a type (flavor) to another—a pure quantum mechanical phenomenon—could take place only if neutrinos had a mass, while neutrinos are assumed to be massless in the Standard Model, the Standard Theory of elementary particle physics describing not only the microscopic forces, but also the nature of the basic constituents of matter. In this case there turn out to be three ‘families’ of elementary particles called quarks and leptons, neutrinos belonging to the latter in three ‘flavors’ depending on their relationship to the three heavy leptons: electron, muon, and tau. If neutrinos had mass, they would alternate among these.20 The neutrinos that have jumped from one flavor to another would then escape detection by Davis’s radiochemical detector, as this was geared only toward detecting one kind of neutrino, electron neutrinos produced in the solar core.21 At the end of the 1960s, the first controversial—and disappointing—results of Davis’ solar neutrino experiment set the stage for the next decade, during which the necessity of clearing up the question about lepton charge conservation and the number of neutrino types (neutrino oscillations) became increasingly a pressing problem, in particular for the future of solar neutrino astronomy. Most people believed that a possible solution for the discrepancy could be an astrophysical problem, that is, the consequence of the Standard Solar Model providing an inadequate description of the internal workings of the Sun. But perhaps our understanding of the neutrino itself was at fault, as suggested by Pontecorvo: it may have been a problem in the Standard Theory of Particle Physics, according to which neutrinos are massless particles, as had been assumed by Pauli and Fermi. But massless neutrinos cannot oscillate.
The issue of neutrino mass and the solar neutrino deficit thus established itself as a question at the intersection of astrophysics and particle physics, becoming one of the most controversial proposals of fundamental physics at the time.22 The physical and cosmological implications of this possibility could be enormous, as massive neutrinos were initially proposed as the most natural particle candidate for dark matter,23 invisible unknown matter distinct from ordinary matter such as protons and neutrons, making up about 27 percent of the Universe, and whose presence can only be inferred from gravitational effects on visible matter. And so, not only could they play a role in the development of structure in the Universe (galaxies, clusters, etc.), but they were one of the first recognized illustration of the close relationship that exists between cosmology and elementary particle physics: cosmology could be used to put constraints on the properties of neutrinos and particle theory could have important consequences for cosmology.24
In the early hot and dense Universe, interactions between elementary particles were essential, determining the structure of the Universe we see today. A new class of theories proposed during the 1970s, called the Grand Unified Theories (guts), suggested that all interactions (strong, electromagnetic, and weak, but not gravity) are unified at a large energy scale and neutrino masses would be inversely proportional to this scale.25 As neutrino masses and mixing could represent a probe into the physics at Gut energy scales, the questions of neutrino mass and of solar neutrino flux grew in importance, both in high energy physics and cosmology.
Neutrinos had always provided “the major consummated connection between particle physics and astrophysics and cosmology.”26 Solar neutrino detection, as a challenge both for astrophysics (test of the Standard Model of the Sun and of the stars) and for particle physics (the observed deficiency of solar neutrinos might be due to the oscillation phenomenon, a test for physics beyond the Standard Theory of elementary particles), became one of the major contributors to the emergence of particle-astrophysics. The identity of the new field materialized in conferences held from the early 1980s on, where a widely diversified physics community had the chance to discuss and explore the conceptual links between theoretical and experimental particle physics, nuclear astrophysics, and fundamental topics, such as the early Universe, its large-scale structure, dark matter and dark energy, and cosmic background radiation.27 The Big Bang, and the very early Universe with its high temperatures and particle densities, became a “hot laboratory for the nuclear and particle physicist,” in the words of Zeldovich, one of the founding fathers of particle cosmology.28
Heidelberg’s Privileged Position in Experimental Cosmochemistry
But to solve the solar neutrino paradox, another series of solar neutrino-counting experiments on a different scale was needed in order to detect the most abundant but low-energy flux of neutrinos from the dominating pp chain of thermonuclear reactions occurring inside the Sun, converting hydrogen into helium starting from the fusion of two protons, which is responsible for about 99 percent of the energy production.29 Only the pp neutrino species could be predicted accurately. This is almost solar model independent and, consequently, more significant for testing the hypothesis that fusion of hydrogen powers the Sun. It could therefore serve as a ‘known source’ in the long-baseline neutrino oscillation experiment, the Sun–Earth distance of about
However, pp neutrinos were not accessible in Davis’s experiment, because their energy is below the threshold of the neutrino reaction converting nuclei of chlorine-37 into radioactive argon-37. But detection of the pp neutrinos could still be done using the radiochemical methods, with a tank full of fluid, deep underground, but containing the much rarer substance of gallium. This experiment would detect solar pp neutrinos by employing a reaction in which the impinging neutrino would transform a nucleus of gallium-71 into a nucleus of germanium-71 plus an antineutrino. The lower threshold of this neutrino capture reaction would allow detection of pp neutrinos.30
It was known that a realistic experiment would require tenths of tons of this aluminum-affine metal with the required radiochemical purity, but gallium was very expensive and thus the construction of specific gallium plants to extract it at an industrial level would require an investment to the order of 100 million dollars. Moreover, even before anyone could dare to ask for funding, a series of open questions would have to be answered, some of which related to the development of a suitable low-level counting procedure for germanium-71.31 The questions arising included whether it was possible to establish a committed international network of top scientists with the relevant expertise and support from their agencies, as well as whether there was a suitable underground laboratory available.32
It is at this stage of developments that Heidelberg’s scientists entered the business of solar neutrinos, but there were deep roots that made this possible in the first place.
Since the 1950s, Wolfgang Gentner had been collaborating most closely with American researchers at Brookhaven National Laboratory (bnl), where his disciple, the cosmochemist Joseph Zähringer, had been the first in a line of visitors to the Chemistry Department, among them, later, also Till Kirsten.33 German cosmochemists were valued in Brookhaven for their expertise in mass spectrometric detection of extremely small quantities of stable rare gas isotopes. In Heidelberg, geochemical investigations by mass spectrometry had been used in particular to determine the half-life for the double-beta decay of tellurium-130 in connection with studies on the xenon and krypton isotopic composition of meteorites.34 From 1966, during his postdoc stay at bnl, Kirsten collaborated with Oliver Schaeffer, working on the double-beta decay problem, a phenomenon which largely involves questions related to the nature of the neutrino.35 In Brookhaven, Kirsten was impressed by Davis’s work and followed with great interest his efforts to improve his low-level proportional counters to detect the very small activities from radioactive decay of nuclides that could also be applied to solar neutrino experiments. The same capabilities in low-level counting and determination of minute quantities of stable rare gas isotopes by mass spectrometry were applied over the following years in a collaboration between bnl and the Max Planck Institute for Nuclear Physics in Heidelberg, in nasa’s Apollo Lunar Sample Analysis Program (see Chapter 2), which meant the two institutes remained in contact. During this time, they discussed the possibility of using gallium, which would allow detection of the major neutrino flux coming from the Sun.
For a long time, this remained a dream because of the high cost of industrial production but, all the same, they kept an eye on the problem.
Gallium Experiment Proposals and American Failure
The gallium solar neutrino experiment became a practical method for observing the proton-proton (pp) reaction neutrinos when industry began extracting ton quantities of gallium as a by-product of the manufacture of aluminum. Producing gallium in quantity was motivated by the need for gallium to produce various electronic devices. In 1974, research on chemical procedures for extracting germanium from gallium began at Brookhaven National Laboratory in collaboration with the University of Pennsylvania. A similar effort was started in the Soviet Union, where a gallium experiment had been proposed by Vadim Kuzmin at the Lebedev Physics Institute in Moscow already in the mid-1960s, as a means of observing the low energy neutrinos from the basic proton-proton fusion reaction, whose flux is accurately calculated from solar models, and is essentially independent of many factors that influence the calculations of the boron-8 neutrino flux which represented the main source for Davis’ chlorine-argon experiment.36
Moreover, in the meantime, Davis had found a significant neutrino deficit in his Homestake experiment and, around the mid-1970s, Davis and Bahcall had begun to put forward the idea that another experiment was required “to settle the issue of whether our astronomy or our physics is at fault.”37 A measurement of the solar proton-proton neutrino flux was regarded as a critical test of our knowledge of neutrino physics and the fusion processes in the interior of the Sun. At that time, this possibility was being taken into consideration in Heidelberg, as Till Kirsten wrote to Davis:
we have some more or less speculative ideas about a gallium or bromine solar neutrino experiment and very much need your judgment and advice in order to facilitate a decision whether we should get serious […].38
The first official reference to these “speculative ideas” can be found in the Annual Report of the Max Planck Society for the year 1977, where investigations using cosmochemical methods were mentioned in connection with the possibility of identifying interactions of solar neutrinos with Earth nuclei.39 In early January 1978, a Solar Neutrino Workshop devoted to the status and future of solar neutrino research took place in Brookhaven, with an emphasis on possible new experiments.40 Till Kirsten participated, and a collaboration to develop a gallium radiochemical detector was set up between Heidelberg and Brookhaven, which included the University of Pennsylvania and the Institute of Advanced Study in Princeton.41 The discrepancy between the results of the chlorine-37 solar neutrino experiment and the predictions made using the standard model of the solar interior increasingly suggested that either some basic aspects of the standard theory of stellar evolution were wrong, or that neutrinos produced in the interior of the Sun did not reach the Earth, at least not in the form or quantity in which they are emitted. The idea was to demonstrate the feasibility of a gallium experiment that could distinguish between these two broad classes of explanation. The possibility that neutrinos could oscillate from one flavor to another, or even decay before they reached the Earth, was also considered.42 In 1979, the premises were laid for a joint mpi/ bnl gallium experiment in the Homestake mine, where Davis was conducting his own hunt for solar neutrinos.43 As an initial step, because of the high costs, a ‘pilot experiment’ with 1.5 tons of gallium was planned, which it was announced would be underway by the end of that year.44 It aimed to demonstrate that all steps in the planned experiment, from the extraction of the germanium-71 atoms produced by interaction of solar neutrinos with gallium to counting them, would be feasible at the level necessary for a full-scale experiment.45
Collaboration between Heidelberg and Brookhaven continued during the following two years, and the pilot experiment operating at Brookhaven National Laboratories was completed in 1983, having demonstrated the feasibility of a full-scale gallium detector.46 But there was also the problem of finding a suitable underground laboratory, because in fact there was not sufficient space at the Homestake site used by Davis. The search in North America, including Canada, had not worked out, and nor had options in Germany (Asse salt mine near Salzgitter).
In any case, the US Department of Energy (doe) decided not to fund the project, the evident potential of the pilot experiment notwithstanding. Funding for the full-scale project was denied in the United States overall, owing to what was later described by Bahcall as a typical problem of interdisciplinary research at the time: the astrophysicists recommended that it be funded as a particle physics experiment, and the particle physicists expected it to be funded from the astrophysics budget, and so “doe could not get the nuclear physics and the particle physics sections to agree on who had the financial responsibility for the experiment.”47 Kirsten later commented that, despite John Bahcall’s influential help, the funding effort failed
most probably because the whole conception of radiochemical neutrino experiments had the image of being exotic, at best. More often, it triggered late party amusement at conference banquets.48
In the meantime, in striking contrast to what was happening in the US, the collaborative effort in the Soviet–American Gallium Experiment (Sage) was going ahead under the leadership of Vladimir Gavrin, Georgii Zatsepin (from the Institute for Nuclear Research of the Russian Academy of Sciences) and Thomas J. Bowles (Los Alamos National Laboratory). Sage then went into operation in 1986 at the Baksan underground facility for neutrino physics in the Northern Caucasus.49
Europe Goes It Alone: Gallex Outcomes and the Beginning of the Neutrino Decade
The failure of the American–German attempt at a joint solar neutrino project led to a new collaboration formed in Europe and aiming for a full-scale experiment with 30 tons of gallium in the underground Gran Sasso National Laboratory in Italy (lngs, Laboratori Nazionali del Gran Sasso), whose creation had been strongly backed by Antonino Zichichi at the end of the 1970s, and which was then being built under the Gran Sasso massif, not far from Rome.50 This unique and timely infrastructure—built at the dawning of astroparticle physics—has since enabled Italy to take a leading role in this field. From 1984, the leading theoretical physicist Nicola Cabibbo, President of the National Institute for Nuclear Physics (INFN), strongly supported solar neutrino research as a major topic for the nascent laboratory, as recalled by Kirsten:
I explained my issue to Nicola Cabibbo. From there on I had an ally. I now had the valuable asset that in the difficult negotiations for funding and collaboration formation I could argue: Yes, we do have a place to go in Europe: Gran Sasso.51
The initial problem in forming the gallex collaboration was in fact “to activate the key factors needed to conduct the experiment,” solving what Kirsten called “a circular problem”:
If you need an annual world production of gallium, to get industry interested you must convince them that you are not crazy and that you can pay for it. For this, competent and influential players have to convince their home institutions to support the activity and to lobby for money acquisition at governments and foundations. But how can you ask for that unless you know where to go with your experiment—deep underground being an indispensable prerequisite?
Essential to solving the puzzle were the support of European funding institutions and foundations and, too, the
enthusiastic support of the pioneers that got the lngs underground laboratory going: Nicola Cabibbo, Luciano Maiani and Enrico Bellotti, the first director of lngs, member and—at the same time—great supporter of gallex in the critical initial phase, when Lngs still was in statu nascendi.52
Gallium funding was eventually secured through joint funding by the German Federal Ministry for Research and Technology, the Alfred Krupp von Bohlen und Halbach Foundation, and the Max Planck Society. The Italian National Institute for Nuclear Physics financed the underground Gran Sasso National Laboratory which would host the experiment, and the French made available their high-flux reactors, with which an artificial neutrino source was obtained in 1993 in order to conduct detection tests.53
Major goals of the gallium experiment, gallex, would be:
to provide the first experimental proof that the Sun produces its energy by nuclear fusion, to limit or identify neutrino mass differences through eventual electronic-neutrino disappearance between Sun and detector via neutrino oscillations, to identify the cause of the ‘solar neutrino puzzle’ posed by the chlorine solar neutrino experiment.54
The Sun was becoming a powerful laboratory for exploring physics beyond the Standard Theory of elementary particles, not only to investigate the nature of the neutrino, but also to search for other weakly interacting particles like the so-called ‘solar cosmions,’ which the Sun was supposed to have accreted from the dark matter of the Galaxy. In this case, such captured particles could alter the Sun’s thermal profile and thus change the predicted neutrino flux, so solving the solar neutrino problem and the missing mass problem of the Galaxy.55
Against this backdrop, while new fundamental questions were crying out for answers, favoring the blossoming of non-accelerator physics, the first gallex meetings were held in Milan, in February 1985, and in Heidelberg, in October of that year.56 The Heidelberg cosmochemists, led by Till Kirsten, had a unique constellation of factors in their favor. This ambitious project would shift the emphasis of the research program of the isotope group in Heidelberg toward a considerably broader perspective. Also involved in the project was their close ally Rudolf Mössbauer from the Technical University of Munich, who had studied under Bothe in the 1950s and now had his own Nobel Prize (1961).57 With this combined expertise, including Kirsten and Mössbauer, the Max Planck people in Heidelberg took over the leadership of the radiochemical gallium experiment that came to be known as gallex. It was the first case of an international collaboration of this type led by Germans, which was named ‘The European gallex collaboration,’ a designation to be used for all public presentations representing the work of the team.58 A key element was the support of the Italian National Institute for Nuclear Physics, namely provision of infrastructure in the fledging new Gran Sasso Laboratory shielded by about 1,400 meters of dolomite rock, the first large underground facility to be exclusively devoted to fundamental research.59
The laboratory was completed in early 1987, while construction and preparation of the gallex experiment went on from 1986 to May 1991, when the first solar neutrino recording started. It was among the first experiments to be conducted at Gran Sasso Laboratory.60 The project team also included scientists from other European countries and there was significant involvement on the part of Israel, further strengthening Heidelberg’s traditional link to the Weizmann Institute of Science mentioned earlier in this study. In 1986, agreement was reached also on the participation in the gallex project of a group from Brookhaven National Laboratory.61 As emphasized later by Kirsten during the inauguration ceremony,
gallex is a fine example for a smoothly functioning fruitful international collaboration, in particular for the potential of European nations if they combine their skills and resources without national egoism. Only by joining forces was it possible to get gallex going.62
The new radiochemical solar neutrino experiment, gallex, was specifically developed with the main objective of achieving a clear distinction between the astrophysical and the particle physics solution to the solar neutrino problem.63 Its scientific purposes were rooted in using refined radiochemical techniques for detection of neutrinos from the Sun, but its scope quickly widened and evolved within the emergent field of particle astrophysics, whose endeavor to detect cosmic neutrinos had long since been a major focus.64
Construction and test operations in the underground Gran Sasso National Laboratory lasted from 1986 to 1991, when gallex began recording the first solar neutrino flux, taking data over the course of six years.65
In 1992, the gallex experiment could claim to have observed for the first time the primary pp neutrinos, constituting nearly the entirety of the solar neutrino flux.66 As recalled by Kirsten, this result, announced at the Neutrino 92 conference held in Granada, Spain, in June 1992,
implied a definite contribution from pp-neutrinos and thus, their discovery. This converted ‘what nobody doubted to know about how stars produce their energy’ into an experimental fact.67
Indeed, such discovery represented a significant test of the hypothesis that hydrogen fusion powers the Sun. In the foreword to the conference proceedings, coeditor Angel Morales judged that “the first gallex results will mark a cornerstone in neutrino history.”68gallex continued to operate until January 1997. At the end of data-taking, the result was only 60 percent of what had been hoped, significantly (6 sigma) below the Standard Solar Model prediction and, hence, the disappearance of bulk (sub-MeV) neutrinos was established at > 99 percent confidence level.69
However, in early 1994, the solar neutrino situation, with results from the Homestake experiment, the Kamiokande experiment, gallex, and sage, was still ambiguous, as emphasized by David N. Schramm: the differences between observations and the Standard Solar Model might “still be due to either astrophysical inputs or new neutrino physics.”70 But from around 1994 onward, both gallex and sage, the gallium experiment carried out as of end of 1989 by the sage collaboration in the underground laboratory in the North Caucasus Mountains—which employed very different chemical procedures71 —began to give very similar solar neutrino results, in striking disagreement with Standard Solar Model predictions, so providing additional evidence for electron neutrino disappearance.72 Neutrino flavor changes remained as the only viable possible consistent explanation for this evidence.73 With these results, gallex and sage significantly contributed to making the 1990s the ‘decade of the neutrino,’74 inaugurated in 1987 by the first ever detection of a burst of neutrinos from the explosion of a supernova in the Large Magellanic Cloud, the first supernova since 1604 visible to the naked eye—an extraordinary event marking the birth of neutrino astronomy.75
Complementary Competitors: Real-Time Detectors Jump in
The radiochemical chlorine and gallium neutrino detectors were the first generation of large-scale solar neutrino experiments. Along with the Japanese Kamiokande, a water Cherenkov detector installed beneath one kilometer of earth and rock in the Kamioka Mine, they were in the early 1990s the only operational experiments, contributing to advances in our understanding of neutrino properties and in identifying the solar neutrino problem, as well as providing key elements along the path to its solution. Kamiokande had initially been devised by Masatoshi Koshiba for the search for the proton decay predicted by Grand Unified Theories, and it was modified in order to detect solar neutrinos. Kamiokande II went into operation in 1986, and on February 23, 1987, it happened to be sensitive enough to detect neutrinos produced by the Supernova 1987A, the most spectacular experimental outcome from this cataclysm.76 In fact, as we see later in this chapter, this supernova considerably boosted all the nascent fields treated in this chapter; but in all cases other than this neutrino detection—which marked the birth of extra-solar system neutrino astronomy—it was a ‘missed opportunity,’ as the other experiments were not yet ready to take advantage of the rare phenomenon.
After 1990, the emphasis in solar neutrino research shifted from solar physics to the particle physics realm, as the most likely cause of the missing electron neutrinos was new neutrino physics. In the meantime, after nine years of successful solar neutrino recording, Kamiokande was upscaled and replaced by the larger Super-Kamiokande imaging Cherenkov detector, under the guidance of Yoji Totsuka and Yoichiro Suzuki. In 1998, it showed evidence for the oscillations of atmospheric neutrinos, produced as decay products in hadronic showers resulting from collisions of cosmic rays with nuclei in the upper atmosphere.77 The deficit in the observed ratio of the flux of muon to electron flavor atmospheric neutrinos, which was inconsistent with expectations based on calculations of the atmospheric neutrino flux, could be explained by neutrino oscillations between muon neutrinos and tau neutrinos, providing indication for a small but non-zero mass for neutrinos.78
But the turning point was real-time neutrino detectors, for these see the neutrino interactions event by event by transmitting data to real-time monitoring and processing systems which analyze them, so yielding several neutrino parameters, in particular the oscillation pattern. In this way, without recourse to the Standard Solar Model, they are able to provide definitive proof of neutrino flavor oscillation.
At the turn of the millennium, the Sudbury Neutrino Observatory (sno) real-time Cherenkov detector for boron-8 neutrinos was being completed at the Creighton Nickel Mine in Sudbury, Canada. The advent of the sno experiment marked a breakthrough in solar neutrino physics. All the previous attempts had been electron-neutrino disappearance experiments. Oscillations produce neutrinos of different flavors and thus neutrino appearance experiments should be able to observe neutrinos of flavor different from the electron neutrinos produced by the Sun. The sno detector was able to provide a direct proof that the neutrinos from the Sun were not disappearing on their way to Earth. Instead, a part of the solar electron neutrinos had transformed into a different flavor, and they were captured with a different identity when arriving at the Sudbury Neutrino Observatory. These measurements revealed the existence of a large flux of muon and/or tau neutrinos in the flux coming from the Sun, and since all neutrinos generated deep inside the Sun are created with the electron flavor, the results clearly demonstrated that neutrinos can oscillate from one type to another, if oscillations have sufficient time to develop. The ‘missing solar neutrinos’ of previous experiments (that were sensitive only to the electron flavor) were not really missing at all, but only unobservable, being present as muon or tau neutrinos, which are not detected by the chlorine and gallium radiochemical experiments.79
The combined results of the Super-Kamiokande and sno measurements showed that new physics is required to describe the propagation of solar neutrinos, and that the Standard Solar Model prediction can be verified to high accuracy—provided that the electron neutrino mixes significantly with the muon neutrino and the tau neutrino. Building on the contribution made by all the previous experiments—now integrated with key results from sno—finally made it possible to determine the corresponding oscillation parameters.
Both the Super-Kamiokande and sno real-time experiments provided convincing smoking-gun evidence for the process of neutrino oscillation that many physicists had long regarded as an “intellectual luxury.”80 Such a scenario definitely implied a non-zero mass for the neutrino, clearly showing that the Standard Theory explaining the framework of elementary particles cannot be the complete theory of the fundamental constituents of the Universe.
The year 2002 became the annus mirabilis of solar neutrino physics. Convincing data from sno had validated the existence of neutrino oscillations and Ray Davis and Masatoshi Koshiba were awarded the Nobel Prize “for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos.”81 By that time, the first-generation experiments like Kamiokande, gallex, sage, (and since 2007, Borexino), had widely established the existence of the phenomenon of neutrino oscillations, providing a strong motivation for another Nobel Prize for neutrinos to be awarded in 2015 to Takaaki Kajita (Super-Kamiokande Collaboration, Japan) and Arthur B. McDonald (sno collaboration, Canada) “for the discovery of neutrino oscillations, which shows that neutrinos have mass.”82 This discovery, more than 40 years after the prediction of the phenomenon by Bruno Pontecorvo—who had unfortunately passed away in 1993—has profound implications for our understanding of the Universe.83 The study of solar neutrinos had made a fundamental contribution both to astroparticle and to elementary particle physics, providing both a test of solar models and relevant indications of the fundamental interactions among particles. In the process, this race, together with the detection of the burst of neutrinos from the Supernova 1987A, created the entirely new field of neutrino astrophysics and paved the way to completely new scenarios and to physics beyond the Standard Theory of particle physics.84 From then on, the neutrino field focused on precisely determining the fundamental properties of this unique particle.
The Beginning of Astroparticle Physics in Heidelberg
In the meantime, toward the end of the 1990s, the Institute for Nuclear Physics in Heidelberg was completely reorganizing its research activity. The field of nuclear physics was focused now on two main topics: many-body dynamics of atoms and molecules and the synergy between particle physics and astrophysics, including the area of non-accelerator physics, which was now one of the components of the Institute’s program on nuclear and particle physics. The Heidelberg–Moscow experiment on neutrino-less double-beta decay was fully operational at Gran Sasso Laboratory from 1996, dominating the scene in the 1990s and contributing to the changing scenario in Heidelberg at the turn of the century.85 This program definitely marked that shift from ‘cosmophysics’ to high-energy astrophysics underway at the Heidelberg Institute for Nuclear Physics, also owing to participation in the hegra experiment for high-energy gamma astronomy and the planning of its successor project, the new imaging Cherenkov telescope system h.e.s.s. (see later section on TeV gamma astronomy). Together with the solar neutrino measurements, these fields had become a main focus of astrophysical research at the institute. Moreover, a new phase of solar neutrino exploration was opening with the advent of Borexino (the Italian diminutive of the preliminary project borex (BORon solar neutrino EXperiment), a new challenging next-generation experiment conceived in the late 1980s, in which the Max Planck Institute for Nuclear Physics would play a major role.86 Unlike gallex, Borexino could provide a real-time view of the core of the Sun through direct detection of the neutrino interactions with the target nuclei of the liquid scintillator target, enabling sub-MeV solar neutrino spectroscopy for the first time.87 Built and operated by a large international collaboration again located at the Gran Sasso National Laboratory, Borexino laid the foundations for the analysis of the still unexplored sub-MeV energy region, isolating for the first time the monoenergetic beryllium-7 neutrinos.88 This would be essential to test the stability and consistency of the standard explanation of the oscillation mechanism, either confirming or disproving the presence of discrepancies between theory and experiments.89 At the turn of the new millennium, the focus of solar neutrino research was shifting from the relatively high-energy boron-8 neutrinos (maximum energy 14.1 MeV) to the low-energy beryllium-7 neutrinos, and to those produced by other nuclear reactions in the Sun, having energies less than or of the order of 1 MeV. Together with Super-Kamiokande and the Sudbury Neutrino Observatory, the planned Borexino experiment would be at the cutting edge of solar and particle physics.
One of the unique features of the Borexino detector was the very low radioactive background.90 Very low background levels were in fact required to detect beryllium-7 neutrinos, but what was even more challenging was detecting the signal of neutrinos from the rare proton-electron-proton (pep) reaction and of neutrinos from the even less common carbon, nitrogen, oxygen (CNO) fusion cycle, a major energy source of large and late stage stars.91 Running continuously since 2007, the Borexino experiment, using about 300 ton of ultra-pure organic liquid scintillator, has measured beryllium-7 neutrinos, pep neutrinos,92 boron-8 neutrinos, and pp neutrinos, confirming the theoretical prediction for all neutrinos formed in the multistage nuclear fusion process, providing information about neutrino oscillations and providing the most complete real-time insight into the core of our Sun so far.93 The study of solar neutrinos has been completed by the first detection ever, with a high statistical significance, of neutrinos from the CNO cycle, the Sun’s second fusion process. With this achievement, all the theoretical predictions on how energy is generated within the Sun have been experimentally verified, closing an era commenced in the 1930s, with the first theories on the nuclear fusion mechanisms that are active in stars. The CNO process, though sub-dominant in the Sun, plays a key role in understanding the fundamental properties of all stars larger than our Sun in the Universe, where the majority of energy is generated in the CNO cycle. Their size, temperature, brightness, and lifetime are determined by the concentration levels of carbon, nitrogen, and oxygen, acting as catalysts and intermediate products in the cycle in which a total of four hydrogen nuclei ultimately combine to form a helium nucleus.94
3 The Quest for Gravitational Waves
This second emerging field was the result of the research tradition in theoretical astrophysics in Göttingen and Munich. The 1960s saw an explosion of interest in the new field of relativistic astrophysics, boosted by the unveiling of the violent Universe by radio astronomy and by spectacular astrophysical discoveries. Then came the decisive push through the pioneering experiments of Joseph Weber, who claimed to have detected gravitational waves (1969). Munich scientists quickly entered the field with a three-branched approach: experimental detection, statistical analysis of the results, and a solid theoretical footing in general relativity, owing to the appointment of the renowned relativist Jürgen Ehlers. This initial strength then allowed them to shift toward the new method of laser interferometry, taking advantage of expertise at the nearby Max Planck Institute for Plasma Physics. In the 1970s and 1980s, this effort was led by an itinerant group of experts circulating through institutes in the Munich area, facilitating the transition from resonant bars towards laser interferometry and its innovation at increasingly large scales, eventually finding a dedicated site in Hanover in the early 1990s. Resistance from the worldwide astronomical community and financial constraints resulting from German reunification then compelled the project to ‘Europeanize’ and, ultimately, to downsize its proposed experiment to pilot scale. The German approach never developed into a fully-scaled detector, putting the emphasis instead on perfecting experimental systems and building excellence in technology and instrumental innovation. In parallel, Ehlers founded an institute for gravitational physics in Potsdam, and soon both branches were unified as the Albert Einstein Institute of the Max Planck Society, one of the central contributors to the detection of gravitational waves in 2015.
The ‘Renaissance’ of General Relativity, Quasars, and Relativistic Astrophysics
This second emerging field was the result of the research tradition in theoretical astrophysics in Göttingen and Munich. Since the early 1960s, Ludwig Biermann’s Institute for Astrophysics actively participated in the explosion of interest in the new field of relativistic astrophysics, which had been triggered by the discovery of quasars and boosted by the growing interplay of astronomy and astrophysics with general relativity, which was fast becoming “one of the most active and exciting branches of physics,” based on the premises laid down in the post-World War II period by the process dubbed “Renaissance of general relativity.”95
Technological progress during World War II had opened new horizons in the study of astronomy and the advent of radio astronomy had revealed that much in the Universe is of an explosive nature and that violent events occur within galactic nuclei. Astrophysicists had tried to understand the source of the tremendous energy stored in cosmic rays and the magnetic fields of some powerful radio galaxies.96 The realization that the energy released within strong radio sources can exceed an energy equivalent of millions of solar masses led William Fowler and Fred Hoyle to explore the possibility that
at the centers of the galaxies there are star-like objects with masses ranging from about
up to about solar masses for abnormal galaxies [our emphasis].
Fowler and Hoyle’s opinion was that
only through the contraction of a mass of
– solar masses to the relativity limit can the energies of the strongest sources be obtained.97
This article appeared in August 1962, but in the meantime, Hoyle and Fowler took a further step. In February 1963, they argued that nuclear energy could not be the key to the problem, being unable to maintain sufficient internal pressure to provide support against gravity for such massive astrophysical objects, and observed that gravitational energy, instead, could be of decisive importance for bodies in that range of masses. The energies demanded by the strong sources were “so enormous as to make it clear that the relativity limit must be involved.” As this limit was approached “general relativity must be used” [our emphasis].98
The conclusion was now clear; that at a certain stage of its contraction (at about the size of the whole solar system) a very massive object would implode catastrophically, in about 100 seconds.99
Soon after, in the following March, Fowler and Hoyle’s suggestions appeared to materialize in the “star-like” objects—celestial bodies with a very large redshift and corresponding unprecedented large radio and optical luminosities—whose identification was announced in four consecutive articles in Nature.100 The most pressing problem in astrophysics at the time became how to explain the mechanism whereby such ‘superstars,’ the most bizarre and puzzling objects ever observed through a telescope to date, and which proved to be the most powerful energy sources in the sky, managed to radiate away the energy equivalent of five hundred thousand suns in short order. In recognition of their small size, they were called quasi-stellar radio sources, soon renamed quasars.101
The connection with Fowler and Hoyle’s proposed mechanism involving gravitational collapse—a purely relativistic phenomenon at the time not yet fully understood—turned a spotlight on the bonds between general relativity, astronomy, and astrophysics. In December 1963, the international Symposium on Gravitational Collapse and other Topics in Relativistic Astrophysics took place in Dallas, organized by three relativists: Ivor Robinson, Alfred Schild, and Engelbert Schücking. Bringing together optical and radio astronomers, theoretical astrophysicists, and general relativists, it marked the start of a new era bridging the gap between the still exotic world of general relativity and the realm of astrophysics. Moreover, it officially opened the discussion on topics ranging from neutron stars to the possibility of gravitational collapse or a singularity in space-time, setting the stage for a dialogue between different scientific communities. This conference, the first of a long series of Texas Symposia, officially launched the brand-new field of relativistic astrophysics, merging two seemingly distant fields, so far removed that the organizers had to invent a new label for it.102 This event took place at a time when the complex process developing since the aftermath of World War II, which had set in motion the ‘renaissance’ of Einstein’s theory after a long period of stagnation, was being completed. After remaining cut off from mainstream physics for a generation, this formerly dispersed field was attracting an increasing number of practitioners, becoming the basis for the standard theory of gravitation and cosmology. New connections were now on the verge of being established with astrophysics and physical cosmology, through which general relativity would enter its ‘astrophysical turn,’ becoming an established branch of physics.103
During the 1960s, the detection of the cosmic microwave background radiation by Arno Penzias and Robert Woodrow Wilson104 —together with the interpretation, by Robert Dicke and his associates, of such radiation as a relic of the Big Bang—and, too, the discovery of pulsars that were immediately identified as neutron stars, became part of a wider scenario connecting the rise of the ‘golden age’ of general relativity105 to the birth of relativistic astrophysics. In providing the first definite proof of the existence of these highly compact stars—previously only theoretical entities—this discovery radically widened the perspective, firmly establishing the belief that strong gravitational fields may be of key importance for quasars, for violent events in the nuclei of galaxies, for supernova explosions and remnants, for the death by collapse of very massive stars and, in general, for the very compact astrophysical objects that were beginning to populate the Universe in the 1960s. Toward the end of the decade, black holes—exotic objects hitherto having only a purely theoretical status—became serious, albeit much debated, astrophysical hypotheses.106 The discovery of pulsars did settle the existence of neutron stars as endpoints of the stellar evolution of massive stars, and had the effect that
rather less was heard about the inherent absurdity of the more radical end-state, especially after Wheeler had dignified it with a name: ‘black hole.’107
Setting the Stage for a Gravitational Wave Experiment at Biermann’s Institute for Astrophysics
In Germany, Ludwig Biermann’s sub-institute at the Max Planck Institute for Physics and Astrophysics was uniquely well situated to make contributions to this revolution in relativistic astrophysics. The longstanding commitment at the Max Planck Institute for Astrophysics to study of the structure and evolution of stars, also conducted with computer simulations, developed into research on very dense stars such as white dwarfs or neutron stars.108 In addition to expanding from already dominant fields, entirely new perspectives and research pathways opening up in the astrophysical sciences were mirrored by research activities conducted at Biermann’s Institute for Astrophysics.109 From 1964, the young researcher Peter Kafka began to work on topics related to general relativity and cosmological questions in Munich, also related to the existence of quasars, the most exciting topic of the time.110 He investigated the problem of gravitational collapse in general relativity, but in particular he explored the space-time distribution of the quasars and radio galaxies as deduced from observational evidence.111 From radio astronomical observations it appeared that there were relatively more quasars at larger distances, and so that must mean they were more common in the early life of the Universe. This could be explained as an effect of its evolution: if their redshifts were of cosmological origin, quasars—whose very nature was still a subject of debate—must have existed only very far away in time and space, contradicting the perfect cosmological principle, which was at the core of steady-state cosmology.112 The counting of radio quasars as recognized sources at cosmological distances might thus help to confirm the existence of the Big Bang model, ruling out the steady state model of the universe, according to which the expanding universe would maintain a constant average density, with matter being continuously created to form new stars and galaxies, implying the notion of a Universe on average homogeneous and isotropic in space, and constant in time. However, there was disagreement about the meaning of relations, between the observed numbers, redshifts, and brightness of quasars, and in the abstract of his article in Nature, Kafka remarked that “no decision can be made, from a statistical count of quasars, between steady state and other cosmological models.”113
In the same 1960s scenario, in which the Max Planck Institute for Astronomy in Heidelberg and Max Planck Institute for Radio Astronomy in Bonn were both finally founded in the 1960s, and while gamma ray and X-ray astronomy activities were in progress at the Institute for Extraterrestrial Physics in the Munich suburb of Garching, new conditions for the interaction between nuclear physics, astrophysics, cosmology, and optical and new astronomies were being created, widening the scope and context of what was being relabeled as the field of ‘cosmic physics.’
Pulsars, Black Holes, and Other Possible Sources of Gravitational Waves
According to Einstein’s theory of general relativity, accelerated masses produce gravitational waves, which propagate at the speed of light through the Universe. The existence and physical properties of gravitational radiation became central to various research agendas as one of the important open questions addressed by the general relativity and gravitation community emerging from the mid-1950s onward, when “the availability of appropriate notions of what a gravitational wave is allowed physicists to put forward heuristic arguments for their existence and detectability.”114 Towards the end of the 1950s, gravitational radiation became a key focus of theoretical studies in general relativity, reinforcing Joseph Weber’s interest at the University of Maryland. Encouraged by John Wheeler, one of the leading figures in the renaissance of general relativity in the US, Weber accepted the challenge and pioneered the quest for the experimental detection of gravitational waves from astronomical sources. For several years, however, his experiments remained an isolated example. Weber had also mentioned as possible sources “events which might be associated with supernovae, neutron stars or closely spaced binaries.”115 His work in turn inspired interest in such astrophysical objects as possible sources of gravitational waves. At that time, Freeman Dyson had pointed out that the usual formula giving the gravitational-wave energy flux from a binary star, leads—in the extreme relativistic case of a close binary collapsing system formed from a pair of neutron stars—to the prediction of a huge output of radiation. The powerful burst of gravitational waves should be detectable by Weber’s existing equipment.116 This remark gave an extra stimulus to the pioneering experimental work of Weber, also prompting the physics and astrophysics communities to consider gravitational radiation—whose physical reality was becoming evident—as a phenomenon of great potential importance in the physical world. More generally, gravitational radiation was being considered during the 1960s also as a possible mechanism for both the dissipation and transfer of energy in the domain of relativistic astrophysics,117 and spinning compact objects, too, were candidate sources of gravitational waves. Pulsars were thus quickly recognized as promising sources of detectable gravitational waves, attracting wider attention to Weber’s ongoing efforts.118
In mid-June 1969, Weber published his famous article claiming to have observed coincidences on gravitational radiation detectors based on resonating metal bars separated by a distance of about 1,000 km at Argonne National Laboratory near Chicago and at the University of Maryland: “There is good evidence that gravitational radiation has been discovered.”119 The announcement immediately caused a sensation in the physics community. Soon, gravitational waves—as well as hard X-rays and gamma rays—would be envisaged by John Wheeler and Remo Ruffini as one of the most promising ways to detect black holes.120 In 1970, Franck Zerilli analyzed the problem of the pulse of gravitational radiation given off when a star falls into a black hole and Stephen Hawking’s prescient article of 1971 even discussed gravitational radiation resulting from the collision of two black holes.121
Between Theory and Experiment in Munich: The Appointment of Jürgen Ehlers and Billing’s Resonant Bar
The Max Planck Institute for Astrophysics quickly reacted to the new exciting perspective opened by Weber’s claims. His article was published in the June 16 issue of Physical Review Letters and by July there was a telephone conversation between Weber and Biermann, who was at the time in the United States, where he was a regular visitor every year. During the call, Biermann expressed his strong interest in Weber’s experiments, which he had most probably discussed with his collaborators immediately before leaving Munich.122 The characteristics of Weber’s gravitational wave antennae were immediately studied by Hermann Ulrich Schmidt, while Kafka explored in detail the possible consequences of the gravitational waves “supposedly discovered by Weber.”123
By early summer of 1969, both Biermann and Heisenberg were working toward intensifying research on gravitation theory and relativistic astrophysics.124 They shared the common aim to invite the renowned relativist Jürgen Ehlers to spend a long period of time at their Max Planck Institute. Ehlers had studied general relativity with Pascual Jordan, one of the pioneers of quantum physics, who had formed a research group in this field in Hamburg back in the early 1950s, which was one of the seeds fertilizing the renaissance of general relativity. After obtaining his PhD and habilitation (German post-doctoral lecturing qualification) with Jordan, Ehlers had moved to Syracuse University in 1961, where he had worked several years with Alfred Schild’s group.125 He had now a professorship at the University of Texas, Austin, and held visiting professorships in Germany.126 At the time, Ehlers had just published a broad survey of the state of cosmology in relation to the impact of the recent discoveries of quasars, pulsars, the cosmic microwave background, and the beginning of experimental gravity physics.127 It became Biermann and Heisenberg’s ambition to have him back in Germany.
During this effervescent wave of new astrophysical phenomena in the late 1960s, things moved quickly. Biermann proposed that Ehlers should move to the Max Planck Institute for Astrophysics128 and, at the end of October, Heisenberg and Biermann sent a joint letter to Adolf Butenandt, then President of the Max Planck Society, in which they emphasized how during the last year general relativity and the gravitation question had become relevant at the Institutes for Astrophysics and for Physics, especially in relation to gravitational waves and neutron stars. For this reason, the Munich institutes would strongly benefit from the presence of a renowned relativist like Jürgen Ehlers.129 Ehlers became a Scientific Member of the Institute for Astrophysics on June 1, 1971.
In the meantime, in late November 1970, the possibility of starting a gravitational wave experiment was being seriously considered by Biermann’s group.130 With this incursion, the Institute for Astrophysics would also move into experimental astrophysics based on a strong theoretical standpoint, a process that we have shown is characteristic of the Munich family of institutes. In parallel with intense theoretical work on general relativity and relativistic astrophysics, planning continued for the gravitational wave experimental activity at the Max Planck Institute for Astrophysics, also involving Heinz Billing, who was still leading the computing group but now successfully returned to physics. Wheels were put in motion and work began in earnest in 1971, when the gravitational wave experiment had its own specific section in the Annual Report.131 The aim was “to confirm or disprove the existence of gravitational pulses suggested by Weber as an explanation of his results.”132 With the arrival of Ehlers in June 1971, the new Department for Gravitation Theory and Relativistic Astrophysics was established.
International collaboration was an inherent aspect of this experimental enterprise: for a coincidence experiment, similar to the one conducted by Weber, they needed a second antenna, far from Munich. They were lucky, because, independently from them, a German colleague, the electronics engineer Karl Maischberger, and the physicist Donato Bramanti had also begun to work on a Weber-type gravitational wave antenna at the European Space Research Institute (esrin) in Frascati, near Rome, with which the institute had already interacted in recent years.133 While intending to be as close as possible to the original experiment, they still made several improvements, which made their detector—together with the similar one built in Frascati—“the most sensitive room-temperature bar experiment at that time.”134 The Munich resonant bar—a long aluminum cylinder reproducing Weber’s setup, and that would ring at a certain frequency in response to a gravitational wave—began operating in October 1972.135 The aim was to test whether the pulses of gravitational radiation reported by Weber were detectable in coincidence between Munich and Frascati. The first negative results, in conflict with Weber’s, were presented in June 1973, in Paris, at the International Colloquium on Gravitational Waves and Radiation.136 By that point, the Munich–Frascati pair was a respected participant among the growing number of locations around the world where gravitational waves were being hunted experimentally.
Triggered by Weber’s announcement that he had observed coincident pulses between resonance gravitational wave detectors located at the University of Maryland and at Argonne National Laboratory, other groups had also started experiments to analyze and test Weber’s results: in the United Kingdom,137 in the United States at ibm Research Center in Yorktown Heights (New York), and Bell Laboratories in Holmdel (New Jersey),138 in Japan,139 and in the Soviet Union, where discussions on gravitational wave detection began already around 1960 and led to experimental efforts repeating the search for coincident signals on separated Weber-type antennae.140 Results obtained were negative.141
The Munich–Frascati experiment reported results of the first 150 days of coincident data in 1975,142 and in March 1976, after 580 days of total useful observation time, the detectors were dismantled and the experiment stopped.143
Jumping on the Laser Interferometer Bandwagon
In the early 1970s, the Max Planck Institute for Astrophysics considerably expanded its research activities and the growing number of international visitors corresponded to a similar flux of internal members visiting scientific centers abroad, as evident from the Annual Reports at the time. This happened in parallel with the explosive developments in astrophysics and cosmology, strongly supported by the rapidly evolving field of the new astronomies, whose birth had been fueled by the advent of the space age. These new technological windows also promised to facilitate studies on astrophysical processes that only seemed possible within the framework of general relativity. For instance, black holes and the search for their observational evidence, theories of quasars, neutron stars, compact X-ray sources, and high-energy phenomena in galactic nuclei, the physics of high-density and nuclear matter, and the distribution of quasars in the universe were becoming popular subjects addressed at conferences. The possibility of emission of gravitational wave pulses, as well, was proving the crucial role which relativistic gravity could play in these frontier astrophysical phenomena. Just a decade before, it was difficult to find an application of relativistic gravity to astrophysics outside of cosmology. On theoretical grounds, the role which general relativity must play in resolving such issues as the end point of stellar evolution and the importance of gravitational collapse as a source of energy had been anticipated since the late 1930s by pioneers such as Robert Oppenheimer in the US and Lev Landau in USSR,144 and in the 1950s by Wheeler,145 Hoyle, and others.146 Their ideas were now being proven relevant to observation. The objects whose properties these theories predicted were white dwarf and neutron stars, but there was every reason to believe that the other objects predicted by relativistic gravity should also exist, notably black holes.
The first wave of gravitational wave experiments—as well as the discovery of pulsars and the longstanding aim to detect gravitational radiation pulses produced in catastrophic collapse of stars resulting in supernovas or black holes—had prompted other researchers to propose alternative detectors claiming sensitivities rather better than those of Weber’s original experiments, but within about an order of magnitude of them.147
Impinging gravitational waves cause free test bodies to exhibit displacements which are proportional to their distance. It is this extremely small change in separation which has to be experimentally detected against a background of perturbing influences such as thermal and seismic vibrations. While these groups were concentrating on the problem of reduction of the background effects, an alternative approach to improving sensitivity could be obtained by increasing the displacement caused by the wave. As the variation of the distance between the test masses induced by the passage of a gravitational wave is proportional to the distance between the masses, one must increase the separation between the test masses as much as possible. It is this extremely small change in separation which has to be experimentally detected against a background of perturbing influences such as thermal and seismic vibrations. The basic idea behind this new approach was to continually compare the lengths of the arms of optical interferometers by bouncing laser beams between pairs of mirrors at the ends of each arm, and then making the two beams converge on a point and overlap. In the absence of gravitational waves, the beams’ electromagnetic oscillations cancel one another out. If there is a space-time disturbance caused by gravitational waves, the arms change length, and the laser beams no longer cancel one another out: light is detected.148 Any relative distance changes in two optical paths at right angles to one another would detect displacements, due to a gravitational wave propagating in a direction normal to the plane of the system. The advantage of this method is that the mirrors, acting as test masses, can be placed kilometers apart, so that a gravitational wave induces larger relative motions. The changes in optical path might be further increased by reflecting each beam back and forward many times between each pair of masses to enhance displacement sensitivity. But a most important feature of interferometer antennae—which are potentially more sensitive than resonant-bar detectors—is that they are inherently broadband, being also sensitive over a much wider range of frequencies than had been practicable with bar detectors, and can detect and measure the wave forms of all classes of sources. However, laser systems of course had also the disadvantage of being technologically more complex and, in particular, more expensive than bars. The pioneer of this technique was Robert L. Forward, a disciple of Weber who was the first to build a small size interferometer in the late 1960s at Hughes Aircraft Company Research Laboratories in Malibu, and to put into operation the first prototype detector in 1971,149 improving it until 1978. He demonstrated that this idea could work in practice but did not obtain funds to move to a more sophisticated instrument.
Simultaneously, Rainer Weiss from mit, especially intrigued by pulsars,150 had since the end of the 1960s been actively exploring the idea of laser interferometry as a better chance of detecting gravitational waves, starting a very detailed theoretical analysis of the ultimate sensitivity and of the noise sources of an interferometer. After the failure of a first attempt in 1972, Weiss sent another funding application to the National Science Foundation (nsf) in August 1974, proposing the construction of a prototype interferometer with arms nine meters in length.
This is the point at which the concentration of expertise in gravitational topics, theoretical and experimental, became decisive for Munich: because of Kafka’s deep involvement in the analysis and evaluation of Weber’s experiment, he was asked to be one of the reviewers of the project. As a theoretician, he felt like an outsider in the experimental side of the field (—“I didn’t understand much about the experimental possibilities […] I had to talk to the experimentalists anyhow”), and decided to circulate the proposal among the experimental groups in Munich.151 It was unavoidable that they discussed all these things in detail as they were actually planning to upgrade their experiment, investigating the possibility of designing an antenna that was to be kept at very low temperatures—near absolute zero—to reduce thermal noise, in parallel with other technical improvements to achieve better sensitivity. However, the Munich/Garching group was so enthusiastic about Weiss’s plans that they immediately determined it would be possible to replicate the interferometric experiment using in-house resources, even if this meant starting from scratch again and exploring a whole new technology. In the meantime, Weiss did not get the money from the National Science Foundation, and so the original American project was delayed, while the Munich group quickly moved forward with the new project. At the same time, the Americans themselves would use the Germans’ success (and the fact that the project proposal had been inspired by Weiss’s leaked proposal) to receive funding in the end, and over the next decades, to an ever-increasing extent, they eventually took back control over the largest effort in gravitational wave detection experiments.152 Walter Winkler, who worked at the Munich project from the very beginning, recalled: “Rai Weiss stated in this respect: ligo would not have been funded without the results from the Munich/Garching group.”153
In the meantime, the discovery of the first pulsar in a close binary system, in 1974, had opened up new possibilities for the study of relativistic gravity.154 The decrease of the orbital period (obtainable from the observed time variation of the pulsar period), while the two stars gradually spiral closer to one another as gravitational waves carry energy away—a consequence predicted by Einstein’s theory—would thus constitute a test for the existence of gravitational radiation.
An Itinerant Gravitational-Wave Group
In March 1976, while observations with the Weber-type resonant antennae ended, a 3 m interferometer was being built by the Munich group.155 In December 1978, the Ninth Texas Symposium on Relativistic Astrophysics, which had become the principal international meeting where relativists and astrophysicists met and discussed recent research, was held in Munich.156 For the first time in the history of these series of meetings, a Texas Symposium was held not just outside Texas but also outside the continental United States. The Texas Symposium held in Munich was also the occasion of the first public announcement of the experimental evidence for the reality of gravitational radiation damping in the binary pulsar discovered by Hulse and Taylor, which was published shortly afterwards.157
Discussions on continuing research on the gravitational wave experiment with laser interferometry were ongoing, also in view of Heinz Billing’s retirement in 1982.158 Ludwig Biermann officially retired in March 1975, but continued to be active at the institute. In promoting the gravitational wave experiment, he had added a last fruitful item to his rich and enduring legacy.159 The heroic era of gravitational wave experiments at the Institute for Astrophysics was coming to an end and at the same time the development of laser interferometers was changing globally the scale of gravitational wave experiments. A prototype 3 m gravitational wave antenna was in the preliminary phase of testing, in view of the more ambitious project for a 30 m antenna.160 By October 1980, a decision had been taken to transfer the gravitational wave experiment group to the Max Planck Institute of Quantum Optics, which was founded on January 1, 1981.161
In May 1982, when the gravitational wave group became officially part of the Max Planck Institute of Quantum Optics in Garching, construction of a new prototype interferometer, which would have a 30 m path, had already started; construction was completed in mid-1983, but improvements continued to be made over the years.162 Weiss himself very well expressed the valuable efforts made by the group:
So, the Max Planck group actually did most of the very early interesting development. They came up with a lot of what I would call the practical ideas to make this thing better and better.163
In the meantime, an Italian–French collaboration was being established in view of a project for an interferometric antenna.164 It was led by Adalberto Giazotto, working at the University of Pisa from 1982, and by Alain Brillet working on laser interferometry at csnsm (Centre de Sciences Nucléaires et de Sciences de la Matière) in Orsay.165
The group at Glasgow University, too, had moved towards the development of techniques for the detection of gravitational radiation using optical interferometry since 1975. As in Garching, the strategy had been based on developing the monitoring instrumentation on prototype detectors of small arm length, in the hope that the sensitivity to gravity waves could be improved fairly rapidly by scaling up the length of the arms, without making major changes to the instrumentation by which the length difference was monitored. In Glasgow they had built and were further developing a system with an arm 10 m in length,166 and were considering the possibility of building a larger detector with an arm approximately 1 km in length.167
Meanwhile, in Garching, after encouraging progress with the 30 m prototype, the group was stepping up efforts in order to prepare for a big leap in size: a full-sized laser interferometer with arms 3 km in length.168 Both the British and German groups had gained considerable experience in the design and operation of prototype versions of interferometric detectors since the early 1970s, but in 1988 it became clear that the British proposal for a 1 km antenna would not be financed by the Science and Engineering Research Council (serc).169 In Germany, preliminary investigations for this ambitious project, led by Gerd Leuchs at MPI of Quantum Optics, were financed by the German Federal Ministry for Research and Technology (bmft) for the period 1987–89.170 In a context of funding difficulties and increased emphasis on international collaborations, the Garching project for a 3 km interferometric gravitational wave detector resurfaced in 1989 as a joint German–British proposal,171 strongly encouraged by the two funding agencies bmft and serc.172 As stated in the preface to the proposal, it was expected that
all the long baseline detectors to be built [the ligo project and the Italian–French Virgo project] will operate as part of a coordinated worldwide network [our emphasis].
At that time, the prospects for the realization of a big interferometer looked excellent.173 From 1990, the gravitational wave project at the Max Planck Institute of Quantum Optics in Garching was led by Karsten Danzmann, who had come back from Stanford University, where he had moved in 1982 after gaining his PhD at the Technical University in Hanover.174
In 1991, the German–British project, now named GEO, was presented as an interferometer with arms each 3 km in length, to be built near Hanover, in the German state of Lower Saxony.175 In summer 1992, a 3 km GEO interferometer was still part of a list of the detectors at that scale being planned in the world: the French–Italian 3 km Virgo (comprising nine groups from both countries) to be built near Pisa; the American 4 km ligo (Laser Interferometer Gravitational-Wave Observatory) project (approved in fall 1991) with scientists at mit and Caltech; and a more recent Australian collaboration proposing aigo, a 3 km detector near Perth (not yet approved at the time). They were meant not to be in competition with one another, on the contrary, “a world-wide network of four detectors,” each “crucially dependent on the others,” would be required “to fully unravel the information contained in the signals with respect to the source direction, time structure, and polarization.”176
However, in spite of ongoing contacts between the European groups during the second half of the 1980s, the Italian–French collaboration and the British–German venture had not merged into a real pan-European joint effort, a European network of gravitational-wave telescopes that might have followed and matched the successful example of effective cooperation in the cern enterprise.177
Retreat from Full-Scale Experimentation: Rescuing Excellence through Technology and Instrumental Innovation
By 1992, the existence of gravitational waves had been demonstrated by the motion of the double neutron star system PSR 1913+16, in which one of the stars is a pulsar emitting electromagnetic pulses at radio frequencies at precise, regular intervals, as it rotates. Arrival-time measurements of the radio signals running since 1974 showed an orbital-motion decay consistent with the gravity-wave emission according to general relativity, with an accuracy better than 0.5 percent.178 This timely result would lend further momentum to ongoing plans to build large-scale, ground-based laser interferometers.
However, following the fall of the Berlin Wall in November 1989, after nearly three decades in existence, the Reunification Treaty was signed by the two German states in August 1990, and the German Federal Ministry for Education and Research (bmft) took a stance that was justified both by the critical situation ensuing from German reunification and the challenge of assuming responsibility for the process of restructuring East German science:179 the ambitious dream of a 3 km interferometer was definitely not to be considered a priority with respect to other planned physics projects in which bmft had programmed huge investments since the mid-1980s.180
German reunification had changed the circumstances in a truly dramatic fashion. With plans underway for similar large-scale antennas both in Europe and the US, the German and British teams who had pioneered research in the field since the early 1970s, and had long since collaborated owing to their respective commitment to building prototype interferometers, were deeply disappointed and struggled to find an alternative strategy, such as that pursued by Karsten Danzmann, who led the gravitational wave project at the Max Planck Institute of Quantum Optics in Garching. A reduction in arm length would cut down the detector cost considerably, making the plan to build a much smaller facility a realistic aim for the British–German teams. Max Planck scientists thus joined forces with British researchers to build the smaller GEO600 experiment, a gravitational-wave antenna with arms 600 m in length.181GEO600 itself, construction of which began in September 1995, was not explicitly funded as a detector, but obtained money from different sources, even from the bmft, for each innovative technology development.182
In 1993, Danzmann became professor at the University of Hanover as well as Director of the Institute for Atomic and Molecular physics, and from 1994 he continued, in parallel, to lead the project as leader of the Hanover branch of the Max Planck Institute of Quantum Optics. In March 1991, the Max Planck Institute for Physics and Astrophysics had been split up into three independent institutes: the MPI for Physics in Munich (Werner-Heisenberg-Institut), the MPI for Astrophysics, and the MPI for Extraterrestrial Physics, the last two in Garching.183 In 1995, a Max Planck Institute for Gravitational Physics—named after Albert Einstein, the physicist who developed the theory of general relativity (Albert-Einstein-Institut, AEI)—was founded, with Directors Jürgen Ehlers and Bernard F. Schutz, the latter also remaining part-time in Cardiff.184 Immediately after its foundation, Hermann Nicolai was appointed as third Director of AEI.185 Initially located in a temporary seat in Potsdam, where it began operations in April 1995, the institute moved to its new building in Potsdam-Golm in 1999. The creation of a new astrophysics-oriented Max Planck Institute in the new Bundesländer, following German reunification, resulted from a further ‘cell division’ in the Munich area,186 and increased the dominance of the Max Planck Society in the astronomical-astrophysical research fields. At the time, the strong pressure to move research institutes eastwards was particularly felt in Munich, which in addition to the AEI, was forced to continue the expansion of its flagship Institute for Plasma Physics to Greifswald on the East German Baltic Sea coast. The epicenter of gravitational-wave research also moved away from the Munich area, albeit for altogether different reasons already in place before reunification: while Bavaria at the time concentrated the specific expertise in gravitational waves, the laser technologies involved in their detection were pioneered in Lower Saxony, among a collection of institutes in Hanover and nearby Braunschweig. At the moment of generational change, regional actors led by Danzmann’s mentor Herbert Welling and backed by funders such as the Volkswagen Foundation exerted their influence to win the location of the interferometer (then with arms 3 km in length).187 Even though this full-scale project did not come about, the expertise continued to be focused in the area, and during the period of transition, when the founding generation of researchers in Bavaria reached retirement age, the new positions to replace them were created in Lower Saxony.
In 1995, in parallel with the founding of the Albert Einstein Institute, the construction of the 600 m-long gravitational wave detector GEO600 started in Ruthe, a site 20 km south of Hanover. Soon afterwards, this activity became one of the main research focuses of the new Institute, following the decision to transform the preexisting research center at the Max Planck Institute of Quantum Optics, based in Hanover and led by Karsten Danzmann, into a branch of the Albert Einstein Institute. The founding of a ‘center of excellence’ for gravitational wave research thus brought both experimental and theoretical activities under the same roof.188
In 2001, Danzmann was promoted to Director of the Laser Interferometry and Gravitational Wave Astronomy Division, the first of the two divisions planned when the Quantum Optics branch in Hanover became officially part of the Albert Einstein Institute, which has since maintained sites in both Potsdam and Hanover.189
While the German 3 km interferometer project had to be put aside in favor of the smaller GEO600, the American proposal for the Laser Interferometer Gravitational-Wave Observatory (ligo), consisting in two widely separated longbased installations (4 km arms) within the United States, was funded, as was the Italian Virgo.190 The Virgo project for a 3 km interferometer was approved between 1992 and 1994 by the Centre National de la Recherche Scientifique (cnrs) and the Istituto Nazionale di Fisica Nucleare (infn), eventually leading to the construction of the Virgo interferometer at Cascina, near Pisa, beginning in the late 1990s.
In 1997, the British–German collaboration finally entered into partnership with ligo, becoming part of the worldwide network of gravitational wave detectors and contributing to the next generation of US detectors with new advanced technologies.191 A collaboration linking the three ligo detectors in the US with its partner GEO600 in Germany and the Virgo detector in Italy was established in early 2007. Many of the technologies developed at GEO600 thus became instrumental in enabling the unprecedented sensitivity of ligo and Virgo.
On September 14, 2015, at 09:50:45 UTC, 100 years after Einstein formulated the field equations of general relativity, the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational wave signal matching the waveform predicted by general relativity for the inspiral and merger of a pair of black holes of about 30 solar masses each. The signal caused the mirrors at the ends of each interferometer’s 4 km arms to oscillate with an amplitude of about
Many of the instrumental innovations that eventually led to the first 2015 detection of gravitational waves using the ligo detectors had been pioneered by the Max Planck Institute researchers.194 They also played a key role in the computational tasks related to the detection efforts.195
Events such as the first one detected by the ligo collaboration, which was given the name GW150914, are invisible for traditional astronomical instruments, as any signal other than gravitational waves is emitted near the merging black holes. But then, on August 17, 2017, four decades after Hulse and Taylor discovered the first neutron star binary, the Advanced ligo and Advanced Virgo observatories made their first direct detection of a swell of gravitational waves from the coalescence of a neutron star binary system, which was followed after 1.7 seconds by a burst of gamma rays detected by the orbiting Fermi Gamma-Ray Space Telescope and integral observatory.196 The detection of this new gravitational-wave signal (GW170817) offered a novel opportunity to directly probe the properties of matter in the extreme conditions found in the interior of these stars, while the unprecedented joint gravitational and electromagnetic observation of this astronomical cataclysm marked the beginning of a new era in multi-messenger astrophysics.197
4 From Cosmic Rays to Ground-Based Gamma-Ray Astronomy
This final emerging field is the complex result of the evolution, throughout the entire 20th century, of the question about the origin and nature of cosmic rays. Until the 1960s, cosmic ray particles were one of the key research areas in experimental nuclear and particle physics, part of all the research traditions mentioned in Chapters 1–3. From the late 1950s onward, however, ground-based cosmic-ray research declined, as most of its stellar researchers moved toward accelerators or jumped on the Sputnik bandwagon to become space scientists. In the following three decades, cosmic rays were studied at less prestigious institutions, such as in Kiel, which nonetheless obtained results in the early 1980s that attracted worldwide attention. A new generation of accelerator-based particle physicists from both the Max Planck Institute for Physics and the Max Planck Institute for Nuclear Physics then began collaborating with Kiel, which was crucially also joined by a community of Armenians from the Yerevan Physics Institute, who had pioneered the innovative technique of stereoscopic Cherenkov Atmospheric Imaging, to detect the light and image the cascades of subatomic particles generated by cosmic gamma rays. This technique turned out to be its most promising feature, finalizing this tradition’s leap towards ground-based gamma-ray astronomy. Armenian success with Cherenkov telescopes, increasingly supported by Max Planck scientists, sparked competition between two Max Planck Institutes, in Munich and Heidelberg, to become world leaders in what promised to become an entirely new form of ground-based astronomy, thereby absorbing the Armenian scientists. The Max Planck Institutes then built the most successful telescopes of the subsequent generation, magic and h.e.s.s., while competing both with each other and with other global players. Thanks to their complementary double presence in the field, the two Max Planck Institutes won the race towards ground-based, gamma-ray telescopes, leading to the global Cherenkov Telescope Array (cta) collaboration with over 100 telescopes, which the Americans then entered as junior partners.
Cosmic Rays as an Entity between Particle Physics and Astrophysics
Finally, the third example of an emerging field in this volume is the entirely new field of Very-High-Energy (VHE) gamma-ray astronomy with ground-based Imaging Atmospheric Cherenkov Telescopes (iact), a most sensitive technique for the observation of the most energetic form of gamma rays. After the detection of solar and supernova neutrinos, the very-high-energy gamma ray photons recorded by ground-based Cherenkov telescopes were the second new window opened by astroparticle physics.
Like the programs that led to neutrino research and the search for gravitational wave signals, treated earlier in this chapter, ground-based gamma-ray astronomy was a development rooted not in traditional astronomy, but rather in experimental physics, namely in both the Munich and Heidelberg research traditions. In fact, to this day, the field’s technological and cultural practices remain deeply tied to the particle physics community. Furthermore, we will see how this new field benefited from several other non-astronomical traditions such as classical cosmic ray research and plasma physics. Like in the two previous cases, ground-based gamma-ray astronomy had a long latency period of several decades of slow improvements, which only started to bear fruit in the 1980s in the United States and the Soviet Union. But this case is more complex than the other two studied, and much more articulated, involving cosmic-ray physics, high-energy physics at accelerators, high-energy astrophysics and astronomy. In particular, it also includes a gradual shift in the very object of research, initially focused on cosmic-ray particles, only later transitioning towards gamma rays, having the smallest wavelengths and the most energy of any wave in the electromagnetic spectrum. These photons, which are thought to be correlated with the acceleration sites of cosmic rays, are born in the most extreme environments of the known Universe: supernova explosions, active galactic nuclei, and gamma-ray bursts.
Technologically, this shift in the object of research also implied a shift from the exclusive use of arrays of detectors for ground-level recording of extensive cascades of the ionized particles and electromagnetic radiation produced by cosmic rays interacting with nuclei in the upper atmosphere (Extensive Air Showers), towards the development and deployment of techniques that use the atmosphere itself as a detector. In this case, the by-products of the interaction of cosmic rays and gamma rays with atmospheric particles can be traced via the Cherenkov light that they emit in the air and which is bright enough to be picked up by large focusing mirrors on the ground. With this information, the nature, energy, and direction of the primaries can be calculated, including their time of arrival, in order to determine the variability in the source emission.
The field of gamma-ray astronomy is closely linked to the physics of cosmic rays. The communities that contributed to the developments outlined in this last part of the chapter originated in cosmic ray research, a tradition that used particles coming from outer space to inquire into fundamental physical processes in their interactions with matter. After a slow start in the 1930s, these experimental studies came to be of central importance for the branch of fundamental physics which deals with the ultimate structure of matter. New elementary particles, such as the positron, the muon, the pion, as well as the kaons and also certain hyperons, were discovered in cosmic rays. From the early 1930s to the mid-1950s, the study of elementary particles and the interactions of these particles at high energies was the basic part of cosmic ray physics. Experiments were often conducted at high altitudes—in airplanes and balloons, or on mountaintops—so as to capture the extensive showers of particles generated by ‘primary’ cosmic rays in the atmosphere. As the energy of the incoming primary increases, secondary particles can also reach sea level. Until the mid-1950s, cosmic ray particles were one of the key research areas in experimental physics, part of all the research traditions mentioned in Chapter 1. However, these cosmic-ray-based communities were dealt a blow by the advent of accelerators of higher energy, which were a more dependable source, providing controlled, intense, high-energy particle beams, and hence repeatable, statistically solid experiments.198 From the late 1950s onward, ground-based cosmic ray research declined, as most of its stellar researchers moved toward particle accelerators or jumped on the Sputnik bandwagon to became space scientists, or even began pioneering gamma and X-ray astronomy. In parallel, the center of gravity of this traditional aspect of cosmic ray physics moved into the high-energy range (energies above
At the same time, the astrophysical perspective on cosmic rays became reinforced. While the ‘little science’ program conducted by the cosmic ray physicists began to be overshadowed by the advent of accelerator-based research, investigations of the primary cosmic rays during the 1940s had increasingly raised questions connected with the rays’ origin and potential importance for astrophysics, which had been traditionally part of the field since its very beginning. In this period, it was definitely established that the bulk of the high-energy primary cosmic rays are mainly protons and, to a lesser extent, helium and heavier atomic nuclei, solving a problem which had preoccupied scientists since Hess’s balloon flight of 1912 showed that some type of ‘high-energy radiation’ from outer space is constantly bombarding the Earth.199 Starting from the 1950s, the transition to cosmic ray astrophysics diverted scientists’ attention from the interest in cosmic rays shown by high-energy physics, and such astrophysics studies became a great source of information about the energy spectrum and detailed chemical composition of the primary radiation. Investigations of the nuclear aspects of cosmic rays continued to evolve also because of the growing postwar role of nuclear astrophysics, whose realm spans explanations of the huge energy output from stars and other cosmic objects, while also providing a coherent picture of the abundance of the nuclides in the Universe and their evolution through time and space. Big-Bang nucleosynthesis—the formation of nuclei of helium, lithium, and deuterium from neutrons and protons in the very early Universe—would become a way to understand both particle physics and cosmology. At the same time, the discovery of the existence of primary particles characterized by energies much higher than could be previously imagined revived interest in the sources of cosmic rays and the mechanisms able to accelerate them at such super-high energies, on the whole one of the key questions of 20th century physics. Thus, the very existence of the primary cosmic radiation presented itself as an astrophysical and cosmological problem of great interest and importance, while the field underwent an evolution which gradually changed its old character, and its practitioners began to look at cosmic rays more and more as astrophysicists.200 Since their discovery, these particles had been also considered an important probe with which to explore the space beyond the Earth’s atmosphere, that is, the interplanetary space; this latter perspective being the complex result of the evolution, throughout the entire 20th century, of questions about the origin and nature of cosmic rays.
New discoveries coming from astronomy at the beginning of the 1950s also considerably assisted the birth of cosmic-ray astrophysics. After the development of radio astronomical methods, it became possible to obtain information about cosmic ray activity far away from the Earth, through observation of radio synchrotron emission produced by ultra-relativistic electrons accelerated within magnetic fields at a distance from the Earth, which also occurs in the optical and X-ray part of the spectrum. Up to that time, all hypotheses concerning the origin of cosmic rays were almost purely speculative and there was no real hope of investigating cosmic rays beyond the limits of the solar system.201 With the development of radio astronomy, and also of cosmic electrodynamics, which focuses on astrophysical and space plasma phenomena in which electromagnetic interactions play an essential role, it became possible to determine the character of the energy spectrum of relativistic electrons in different regions of our Galaxy and far beyond it. The question of cosmic ray origin became a truly astrophysical problem and transcended the realm of mainly hypothetical constructions. Moreover, radio astronomy demonstrated that cosmic rays are a universal phenomenon, as they are present in the interstellar space of the galaxy and of other galaxies, in quasars, in supernova remnants, and collectively they constitute an important energetic and dynamic factor.
Then, with the launch of Sputnik, outer space appeared to be a better platform than balloons, airplanes, and short-duration rocket flights for the study of the ‘primary’ cosmic rays before they interact with the atmosphere. Solar cosmic rays, their generation, Earth-bound motion, and effect on processes taking place in the near-Earth space, could now be studied, too, along with the giant, doughnut-shaped swaths of magnetically trapped, highly energetic charged particles that surround the Earth—the Van Allen radiation belts—the first of which was discovered in January 1958 by Explorer 1, the first satellite launched by the United States.202 Artificial satellites and space probes, together with the general progress in solar geophysics and physics, and the rapid development of radio astronomy and astrophysics, led to the appearance of a large number of investigations of all these interconnected questions, including the composition, energy spectrum, and spatial distribution of the primary cosmic rays on the Earth, the effect of the interplanetary medium and interplanetary magnetic fields, and other phenomena affecting variations in cosmic rays on the Earth and within the solar system.
Such studies of the chemical and isotopic composition and, in particular, of the ‘energy spectrum’ of the primary cosmic rays—strongly linked to their origin and mechanisms of acceleration—became intertwined with the appearance of observational gamma-ray astronomy, the branch of high energy astrophysics that studies the cosmos in gamma-ray photons, the most energetic form of electromagnetic radiation. Charged particles are continuously bent by magnetic fields embedded in interstellar and intergalactic plasmas, so that their direction is almost completely uncorrelated with that of their sources. This is why the quest for such cosmic accelerators should mostly rely on photons that, being electrically neutral, do not undergo deflections in galactic and intergalactic magnetic fields, and might provide valuable and otherwise unattainable information on the hidden sources. As seen in Extensive Air Showers in the atmosphere, decay processes of charged cosmic rays—that occur as protons, electrons, heavier nuclei, and their antiparticles—can ultimately produce high-energy photons, and thus possible sources of cosmic rays (like supernovae and their remnants, interactions of energetic electrons with cosmic magnetic fields, or places where cosmic rays can be confined, such as the Galaxy) were expected to be visible in gamma-ray astronomy.203 And so, while high-energy electrons and positrons can be indirectly detected by their synchrotron radiation, the really energetic photons produced by cosmic-ray interactions with matter and radiations fields now appeared to be the clue to gaining more direct insight into the acceleration sites and the targets where such high-energy interactions take place, thus broadening the potential to study cosmic rays in the Universe, and opening a window onto investigations of phenomena in extreme astrophysical environments and processes. Cosmic-ray astrophysics could therefore be based on the study of primary cosmic rays near the Earth, as well as on the new fields of radio astronomy and gamma-astronomy.
However, the Earth’s atmosphere is entirely opaque not only to many bands of the electromagnetic spectrum, but also to all cosmic ultraviolet, gamma, and X-rays; it is transparent only to electromagnetic radiation in the radio and optical regimes. A direct detection of such emissions is impossible without going beyond the atmosphere. Beginning in the early 1960s, gamma radiation generated in outer space in interactions between cosmic rays and interstellar matter was observed first by sending telescopes aloft with balloons that took them to within a few grams of residual atmosphere, and then with satellites experiments, which make accessible gamma-rays below a certain energy (about 20 GeV), that are quickly dying out at the top of the atmosphere. On the other hand, at significantly higher energies, the photon flux falls off very rapidly and satellite instruments lose sensitivity due to their limited collection area. The most energetic gamma rays (at GeV–TeV energies), messengers of the relativistic Universe which provide us with much information—on the conditions prevailing in remote regions, such as magnetic and electric fields or matter and radiation densities; on the acceleration mechanisms of charged particles and their distribution; and in particular, on the sources and mechanisms of their production—cannot be observed with balloon—or satellite-borne detectors. Fortunately, observations of GeV–TeV gamma rays can be carried out by ground-based experiments using the atmosphere itself as part of a giant gas detector: incoming gamma rays interact with the atmosphere, producing an electromagnetic shower, a broad distribution of secondaries that can be recorded by ground-based detectors spread over tens of thousands of square meters.204 The problem is that only a tiny fraction of extensive air showers are initiated by gamma rays. Charged cosmic rays outnumber ultrahigh-energy gamma rays by many orders of magnitudes and so it is instrumentally and epistemically challenging to discriminate the effects of the interaction of gamma rays with the atmosphere from those of energetic particles of matter which constitute a significant background and limit the sensitivity of such measurements. In any case, the techniques developed to detect high-energy cosmic rays and gamma rays from the ground were developed in line with a single tradition, with the epistemic focus shifting between them throughout the century. As a consequence of this shift, ground-based researchers using detector arrays specialized in aspects that it would be impossible to pursue anywhere else, namely those higher-energy particles that could not be produced using the existing accelerators, as well as collection areas wider than what could ever be set up in outer space. And this new focus increasingly directed attention toward very-high-energy gamma-ray astronomy, and consequently, to its full integration into high-energy cosmic ray astrophysics.
Cosmic Ray Research Traditions
Research on cosmic rays had arrived at the prewar Kaiser Wilhelm Society in the ‘golden age,’ the 1930s, courtesy of three of the world’s most respected physicists: Erich Regener, Walter Bothe, and Werner Heisenberg, the founders of the ‘fundamental research’ traditions research traditions described in detail in Chapter 1. Regener and Bothe had a common trajectory insofar as each was admitted to the KWG under the auspices of its then President, Max Planck, who thus provided an alternative arrangement to their difficult circumstances in universities during the early Nazi era. Within the KWG, Regener investigated cosmic radiation in ionization chambers, as well as Geiger-Müller counters in the atmosphere with balloons and under water; and at Friedrichshafen, on the shores of Lake Constance, he developed a research station for studies of the stratosphere, from where he could also continue his altitude-based cosmic-ray research and, during the war, also contribute to pioneering military rocket instrumentation for measuring the ultraviolet radiation from the Sun. Walther Bothe was appointed Director of the Institute for Nuclear Physics at the Kaiser Wilhelm Society for Medical Research, where he continued his detector-based tradition of nuclear research, which innovated many of the instruments he himself used for the detection of cosmic rays; and he was among the first to become a member of the Uranium Club at the start of the war, thanks to his expertise in nuclear physics and accelerators.
Werner Heisenberg entered the KWG directly, in the context of the wartime Uranium Club, but he had been making cosmic rays one of his central areas of research since the early 1930s in Leipzig.205 It was in Leipzig that he launched the career of Erich Bagge, a key player in the chain of developments outlined here. Bagge’s relationship with cosmic rays dated back to his doctoral studies with Heisenberg in the late 1930s.206 A theoretician, Heisenberg continued to closely follow developments even during the war, when the German nuclear project was absorbing all his energies, and cultivated cosmic ray studies, which provided the clue to understanding nuclear processes taking place at energies greater than those provided by radioactive sources or early accelerators. Throughout the war, in parallel to the nuclear efforts, Heisenberg and his team, now at the Kaiser Wilhelm Institute for Physics in Berlin-Dahlem, dedicated much of their open scientific activities to cosmic rays.207 After the war, cosmic ray research continued in Heisenberg’s new Institute in Göttingen,208 where an active experimental group worked at mountain laboratories or with balloons, before moving into accelerator physics in the second half of the 1950s. After returning from internment at Farm Hall in January 1946, together with the other members of the original Göttingen staff (seven of the ten detained at Farm Hall), Bagge became Heisenberg’s assistant, and in 1948, was appointed to the Chair of Physics at Hamburg University; but, as we will see later, he continued to cultivate cosmic ray studies, which in postwar Germany were also the sole means to study nuclear and subnuclear processes.209 In the foundational years of the Max Planck Society, cosmic rays actually played an even greater role as one of the few remaining experimental activities permissible in the context of ‘nuclear’ research. Regener relaunched the high-altitude balloon research program at his research facility, which had recently become associated with the new Max Planck Society and in 1952 became known as the Institute for the Physics of the Stratosphere. As administrator of the Kaiser Wilhelm Institute site in Hechingen, he had also provided a base for his disciple Erwin Schopper, whose balloon-based cosmic ray research especially aimed to observe high-energy nuclear disintegrations.210 Heisenberg’s institute, while still in Göttingen, maintained mountain stations for cosmic ray research on the Zugspitze (Germany’s highest peak), and the Wendelstein, both in Bavaria. And Walther Bothe’s disciple Wolfgang Gentner, based in Freiburg in the first postwar decade, created a cosmic ray observatory atop the Schauinsland, in the Black Forest, which was later inherited by his new Max Planck Institute for Nuclear Physics in Heidelberg.
During the 1950s, both Heisenberg and Gentners’ teams maintained experimental research groups on cosmic rays, but they were also aware that the future of physics lay in particle accelerators and also, after Sputnik, in outer space. They were both involved in starting cern research activities in Geneva, and as Director of the newly formed Max Planck Institute for Nuclear Physics in Heidelberg (successor to Bothe’s Institute), Gentner promoted the construction of a Tandem accelerator dedicated to advanced nuclear research. So, at these two Max Planck Institutes, as in many other physics institutes around the world, the 1950s was a decade of ‘retraining,’ away from cosmic rays and toward particle accelerators, while still maintaining a provisional foothold in their old cosmic research stations.211 Meanwhile, in 1954, Erich Regener died and his successor Georg Pfotzer continued the high-altitude cosmic ray tradition. The younger Schopper, however, left for the University of Frankfurt, where he became Director of the Institute for Nuclear Physics. There he pursued nuclear and accelerator physics and later contributed to space-based cosmic-ray research.
As was described in Chapter 2, Sputnik provided an enormous new path of expansion for these three research traditions, so that, in both Heidelberg and Munich, the two most powerful Max Planck Institutes increasingly focused their research on either experimental particle physics with accelerators on the ground, or space-based research, initially in fields such as space plasmas and cosmochemistry, while in Lindau, the airborne cosmic ray tradition progressed over time more in the direction of planetary science and solar system research. All these space traditions had close epistemic, instrumental, and methodological links to earlier cosmic ray research, but from the 1960s onwards, they deliberately left behind their ‘cosmic’ legacy as part of the general reorientation to the more topical questions relating to the scientific use of outer space for astronomy, astrophysics, and fundamental particle physics.
In Heidelberg, the experimental line of cosmic ray research proper closed down in 1964,212 while in Munich, the experimental cosmic ray group at the main institute in Freimann was moved to Garching to become part of the new Max Planck Institute for Extraterrestrial Physics, the aim being to continue research from outer space, making this thus an early example of recycling internal expertise to launch a new research field. While the founder Reimar Lüst and his collaborators focused on space plasmas and cosmic ray particles in near-Earth space, the cosmic ray people from Munich instead joined forces with the second director at the institute, Klaus Pinkau, who had moved in late 1965 from Bagge’s institute in Kiel to the Max Planck Institute for Extraterrestrial Physics. Pinkau, who had been educated at Bagge’s institute in Hamburg first, and then in Kiel, had developed into a skilled cosmic ray physicist during his stay in Britain with Powell’s group in Bristol in the second half of the 1950s, gaining his PhD in 1958 on electromagnetic cascades that originated in high-energy gamma rays.213 He had later started balloon experiments in Kiel,214 before being involved by Lüst in starting gamma-ray astronomy and in launching space research at the newly founded Institute for Extraterrestrial Physics.215 Through Bagge, Pinkau had indirectly benefited from the research tradition established by Heisenberg and he was now becoming part of that same tradition, contributing to its evolution toward the new fields being opened up by novel observational and technological windows. Throughout the 1960s, Pinkau transitioned from high-altitude cosmic ray physics to space-based gamma-ray astronomy.
Satellites
Cosmic gamma rays are mostly absorbed in the upper atmosphere and their signal has to be separated from the background produced by the interaction of cosmic rays with local matter. The turning point in interest in cosmic gamma rays had thus come with the opportunities afforded by the launch of Sputnik. In a seminal paper published just one month after the launch of the Soviet satellite, Philip Morrison, inspired and encouraged by the cosmic ray physicist Giuseppe Cocconi, his colleague at Cornell University,216 listed the possible physical mechanisms for the production of gamma rays in astrophysical sources, and calculated that outside the atmosphere it should be possible to detect a very high flux of gamma rays leading to a potential new ‘window’ of astronomy, similar to what was occurring with radio astronomy.217 His publication single-handedly triggered the start of gamma-ray astronomy as a means of obtaining fresh information about high-energy astrophysical processes and on cosmic structures in far regions of the Universe. Beyond Sputnik, the link between the rising interest in gamma rays, and the spectacular successes of radio astronomy since the postwar era is quite direct. Until the 1940s, almost all the information on the cosmos was obtained via optical channels, and the appearance and increasing use of two new channels of astrophysical information, the ‘radio channel’ and the ‘cosmic ray channel,’ introduced a fundamental new feature into the development of astronomy during the 1950s.
Establishing a connection between cosmic radio emission and cosmic ray particles—mainly relativistic electrons, gyrating in galactic and intergalactic magnetic fields, and radiating electromagnetic waves in such circular, accelerated motion—led to a renewed interest in the old problem of the origin of cosmic rays, which, broadly speaking, is the problem of their acceleration and propagation under various conditions. It was recognized that in all regions of the Universe there are sources of acceleration mechanisms leading to the emission of synchrotron radiation, which is a non-thermal radiation determined by processes different from thermal random motion of particles in matter with temperature above absolute zero, which are instead at the origin of the electromagnetic radiation generated in hot objects such as stars, hot gases, and galaxies.218
These brilliant successes combining radio astronomy with optical astronomy emphasized that much could be gained from exploring new wavelengths. But radio emissions appeared rather complex in origin, while gamma ray is a form of radiation which is more directly related to high-energy and nuclear processes in astronomical objects of various classes than is optical or radio emission.
With the prediction by Morrison in 1958, and the first observation of solar gamma-ray lines in 1959,219 particle physicists around the world sought to deploy their instruments for the new space-based challenge. As we have just mentioned, this was notably the case with Klaus Pinkau, who had started his scientific career in Hamburg and Kiel under the direction of Erich Bagge, who was one of the world’s pioneers in a new kind of particle detector called the spark chamber, a descendant of particle counters based on wartime advances in electronic timing circuits, spawned from work on cosmic rays.220 This device was the first attempt to use electronic equipment to reconstruct the trajectories of particles inside a chamber; it could operate up to a thousand times faster than the traditional methods of cloud chambers, bubble chambers, and photographic emulsion stacks.221
The spark chamber, which emerged during the 1960s as one of the primary instruments of particle physics research (notably used for the discovery of muon neutrino in 1962),222 was also a ready-made gamma-ray telescope. It needed only to be made robust and compact enough to be carried by high-altitude balloons and satellites. Through the early 1960s, Pinkau, like many others around the world, initiated a program of high-altitude balloons, first in Kiel and later at MPE, which sought to detect the gamma rays at an altitude around 10 km. These early observations were due in large part to the development of high-altitude ballooning and experimental techniques pioneered by the cosmic-ray physics community, many of which could be quickly adopted by the early gamma ray researchers.223 It would turn out that the prediction was too optimistic, as the flux was too low to trigger the small detectors carried by balloons. Moreover, balloon experiments are complicated by the fact that the flux of primary gamma rays of extraterrestrial origin has to be separated from the background produced by secondary gamma rays generated by cosmic ray interactions in the atmosphere of the Earth. The mid-1960s, when this disappointment had sunk in—and when X-ray astronomy was beginning to be a consolidated field with a number of successful important detections made employing balloon or rocket launches—was exactly the moment when space-based platforms became possible, so allowing the true flux of primary gamma rays to be reliably measured by means of satellite instrumentation, which afforded the new reality of observations from space in several wavelengths, including the low-energy gamma-ray regions of the electromagnetic spectrum. Since then, the development of relativistic astrophysics has increasingly showed how high-energy astrophysical processes can produce relativistic particles and associated gamma radiation over an enormous range of energies. Gamma rays can traverse great distances in space without being absorbed by intergalactic dust and gas; however, as most gamma rays coming from space are absorbed by the earth’s atmosphere, gamma-ray astronomy could not develop until it was possible to get detectors above the Earth’s atmosphere, using balloon, spacecraft and, in particular, satellites. In 1961, the American satellite Explorer XI, launched by mit physicists, provided the first view of the Universe at the shortest wavelength of the electromagnetic spectrum, by identifying gamma rays originated outside the Earth, with a detector consisting in a scintillation-Cherenkov device.224 However, in the five months during which it remained operational, Explorer XI detected only 22 cosmic gamma rays, too few to establish where they came from. Firm detection of gamma-ray sources was finally achieved with the short-lived SAS-2 satellite launched by nasa in 1972, which was able to show that the galactic center and the Vela and the Crab pulsars are strong gamma-ray emitters.225 The next major high-energy gamma-ray mission was Pinkau’s COS-B, the first esa scientific satellite (after the esro-eldo merger), with a high-energy gamma telescope as single payload, launched in 1975 and successfully operational until April 1982.226 This satellite, by establishing the field of low-energy gamma-ray astronomy, was able to provide the first complete gamma-ray map of the disc of the Milky Way, recording galactic continuum emission (mainly from interactions of cosmic rays with gas and radiation in the interstellar medium), as well as a catalogue of point sources, and detailed studies of several of them, also thanks to its long-lived spark chamber, the result of metal-ceramic technology.227
During the 1970s and ’80s, this first successful generation of gamma-ray satellites revealed the large-scale features of the gamma-ray sky and the diffuse emission from the galactic plane resulting from the interaction of cosmic rays with interstellar matter. The Crab Nebula and the Vela pulsars were identified as emitters of high-energy gamma rays and the periodicity of their light curves was mapped.228 The observation of intense point sources of gamma-rays was somewhat surprising, because of the incredible amount of energy required to produce them. Surprising, too, was the fact that the two most intense sources, namely the Vela pulsar and Geminga, were relatively weak emitters in other wavebands. In parallel to all these developments, the completely unexpected phenomenon of gamma-ray bursts, the ultra-high energy transient emissions of gamma rays of cosmic origin, was likewise discovered by the Vela defense satellites.229 In the 1970s, the interaction between X-ray, optical, and radio astronomy had already begun to unravel the physics and astrophysics of neutron stars and black holes. These results from gamma-ray spacecraft were now also showing that gamma-ray astronomy—as in the case of radio and X-ray astronomy—might well find new types of sources with the potential to disclose further aspects of the relativistic universe.
The Revival of Extensive Air Shower Arrays
However, at energies above 100 GeV, the fluxes of nearly all possible sources (see more details later) are in the order of one or less photons per year for square meter instruments, the maximum practical size of balloon- or satellite-borne gamma-detectors.230 This problem of detector size is unsurmountable, as the collection areas would need to be in the order of square kilometers. Finally, at even higher energies, one reaches the range above around 100 TeV. With gamma-ray primaries at this energy range—providing evidence of the existence of energetic accelerating mechanisms—the flux is extremely low, as in the case of ultra-high-energy-charged cosmic rays, whose estimated flux is so low that even large arrays of detectors spread across areas tens of square kilometers would catch only a few events in several years of operation.
What became pivotal, here, was the air shower detection method developed since the 1930s by investigating the cascades of ionized particles and electromagnetic radiation produced in the atmosphere, to a width of several kilometers, by primary cosmic rays.231 The extensive showers of secondary particles generated by high-energy primaries and interacting with nuclei in the high atmosphere manage to reach the ground and can be detected, thus providing adequate information about the energy, direction, and type of primary particle or cosmic gamma ray originating the shower, based on the study of spatial and temporal properties of secondary cascade products. The deployment of shower arrays (appropriate particle detectors spaced at different distances from each other, depending on the energy range) over large surfaces (order of 10000–100000 m2 and above) allows direct detection of the shower particles (electrons, muons, hadrons), so increasing the effective area of detection by several orders of magnitude greater than the compact experimental set-ups that can be flown to balloon altitudes or carried in satellites. This technique had long been crucial for the historical development of ground-based gamma-ray astronomy, during the 1980s, and it underwent a revival also at the Max-Planck Institutes for Physics in Munich and for Nuclear Physics in Heidelberg.
In an extensive air shower, the energy of the primary cosmic ray interacting with a nucleus in the upper atmosphere is shared among the shower particles. As the energy of the primary increases, the number of particles produced in the first few collisions increases, and also the number of generations that contribute to the nuclear cascade.232 But it took many decades to understand the intricacies and mechanisms of the showering process, so progress in disentangling the fine structure of the shower was slow. When the first powerful accelerators like the Cosmotron and the Bevatron went into operation in the early 1950s in the United States, elementary-particle and cosmic-ray physics began to drift apart and only nuclear interactions at exceptionally high energies remained within the province of cosmic rays. Particle detectors and large air shower arrays were developed because it was recognized that the phenomenon of Extensive Air Showers could offer a major tool for the study of the very-high-energy interactions provided by cosmic rays, still well beyond the realm of particle beams that could be obtained in terrestrial laboratories. Studying the showering process still offered the possibility of answering main questions regarding nuclear and particle physics, the arrival direction of the high-energy particles, the energy spectrum, and the mass composition of the primary cosmic rays.233 In the late 1950s and early ’60s, it was recognized that the very high energies can extend, in the rarest, highest-energy events, up to millions of TeV (
By the early 1960s, the search for ‘point sources’ of charged particles had been virtually abandoned and thoughts had turned to looking for extra-terrestrial gamma-rays, with considerations revolving around the mechanisms by which such gamma rays might be produced, and in which celestial objects—at the least, those gamma rays which could be identified as the products of cosmic ray interaction with gas nuclei and photons in the interstellar medium and elsewhere. Thus, if the target gas could be identified and the gamma rays could be measured, then the cosmic ray intensity could be inferred at places remote from the Earth. One of the problems with gamma-ray astronomy is that the rays are scarce relative to other entities that resemble them experimentally, and more so, as one looks for the more energetic ones. Gamma rays actually constitute only one or two out of every 100000 cosmic rays, which means a big signal-to-background challenge. The flux of cosmic rays at ‘low’ energies is great enough that they can be observed directly with detectors above the atmosphere, which can be shielded from practically all incoming radiations other than gamma rays, by using the so-called anticoincidence technique to reject the majority of charged cosmic-ray particles; but already above a few GeV, one would need a very large detector area in orbit for years to increase the detection probability for such far rarer, very-high-energy and ultrahigh-energy events; moreover, these detectors are also very massive. Thus, it was beyond this natural limit for space gamma-astronomy that the appeal of ground-based gamma-ray astronomy grew.
As we have seen, gamma-ray astronomy in the 100 MeV region was boosted by the 1958 seminal paper of Philip Morrison,235 while detectability of the higher-energy TeV gamma rays from the Crab Nebula—through the detection of gamma-induced air showers by ground-based arrays of detectors—was suggested the following year by Giuseppe Cocconi during the 6th International Cosmic Ray Conference held in Moscow.236 Although, like Morrison, he overestimated the eventual detected flux, his article is considered to have “sowed the seeds for the first serious atmospheric Cherenkov experiments to detect very high energy gamma rays from cosmic sources.”237 It certainly stimulated further attempts to use angular anisotropy as indirect evidence for the presence of gamma-ray sources.238 At the end of the 1950s, the idea that the science of gamma ray astronomy could be a tool for answering questions in high-energy astrophysics and cosmology gained currency. Ground-based gamma-ray astronomy, however, would need to rely on discriminating primary gamma rays of extraterrestrial origin from the overwhelming background of cosmic ray particles and, especially, of secondary gamma rays generated by cosmic rays in the atmosphere on the basis of air shower characteristics.
Above 10 TeV, in the ultrahigh-energy (UHE) region, where the number of cascade particles reaching the observation level is large enough to be recorded directly by air shower arrays, particle detectors are dispersed over a large area, generally at mountain altitudes, even if detection beyond a few hundred TeV is possible at sea level. Other detectors record the associated muon content, an important feature depending on the nature of the primary. The muon content in gamma-ray-induced showers is much reduced, compared to a proton shower; consequently, the careful analysis and selection of such extensive air showers appeared to be practically the only possibility to obtain experimental information about the existence of primary gamma rays with energies larger than
Early efforts to identify point-source anomalies in the arrival direction were not successful, beyond establishing meaningful upper limits for the flux of diffuse gamma rays, and interest waned for several years. Moreover, these conventional ground-based scintillator arrays are only practical for the highest energies, leaving a gap in the energy range between 10 GeV to around 10 TeV (the Very-High-Energy region, VHE), where the majority of interesting, high-energy cosmic processes take place. This is actually the energy range dominated by the Imaging Atmospheric Cherenkov Technique (iact), which will be the main focus in the following pages. We will also see, as with the previous two examples (solar neutrinos and gravitational waves), how the Cherenkov technique had originated in several decades of developments, in this case primarily in the United States, Britain, and the Soviet Union.
New Horizons for Ground-Based Gamma-Ray Astronomy: The Imaging Atmospheric Cherenkov Technique
In the energy range of 10 GeV to 10 TeV, the cascades die out in the upper atmosphere and the number of cascade particles reaching the observation level is too small. In this case, showers can be detected by means of a technique based on a phenomenon discovered already in the 1930s. The ultra-relativistic electrons and positrons generated by gamma rays in the upper atmosphere have velocities exceeding the speed of light in air and consequently emit Cherenkov radiation, the electromagnetic equivalent of a sonic boom generated by an aircraft flying at supersonic speed.240 Thousands of relativistic charged particles in the shower emit light almost simultaneously, as a fast light flash that can be detected on the ground during clear, dark nights. The required collection areas to work in the VHE region, which are in the range of
During the 1950s and ’60s, the technique was pioneered by John V. Jelley and William Galbraith of the Atomic Energy Research Establishment (aere) at Harwell, UK, and by Alexander Chudakov from the Lebedev Physical Institute of the Soviet Academy of Sciences in Moscow.241 In the late 1950s, while Chudakov was deeply involved in cosmic-ray experiments in space with rockets and satellites, Georgii T. Zatsepin, his colleague at the Lebedev Physical Institute and a real expert in cosmic-ray physics, proposed that he discuss the possibility of using Cherenkov light as a tool to search for local gamma-ray sources.242 Inspired by Cocconi’s talk at the Moscow conference about the detectability of TeV gamma rays from the Crab Nebula and other sources with an “air shower telescope,” they formulated by 1960 the plan and principles of the VHE Cherenkov technique and the world’s first gamma-ray telescopes to focus the Cherenkov light onto photon detectors—typically, photomultiplier tubes—were mounted in Katsiveli, Crimea, on the shores of the Black Sea. The following year, the number of mirrors was increased up to 12.243 They conducted the first systematic searches for gamma ray sources, observing ten target sources, among others, the Crab Nebula, supernova remnants, and radio galaxies recently identified by radio telescopes as sources emitting non-thermal, synchrotron radiation; but no statistically significant positive effect from any of these was found. And so, contrary to Cocconi’s overly optimistic predictions, no point sources of TeV photons could be detected.
However, Chudakov and collaborators’ pioneering experiments in Crimea were followed with interest by other groups. In the early 1960s, a collaboration between Jelley, at Harwell, and Neil A. Porter, formerly at Harwell but meanwhile at the University College in Dublin, led to a further experiment with two 90 cm mirrors at Glencullen, a dark site in the Wicklow Mountains, Ireland, to where the installations built at Harwell were transferred in 1963.244 It was with this experiment that the Dublin-born Trevor C. Weekes, a young member of the Irish–UK collaboration, began his career and the long quest to refine the Atmospheric Cherenkov Technique. Weekes then joined Giovanni Fazio’s group at the Smithsonian Astrophysical Observatory and the Harvard College Observatory, Cambridge, Massachusetts, which had been interested in detecting primary gamma rays with balloons and satellites since the early 1960s.
It turned out that neither the shower arrays nor the atmospheric Cherenkov experiments conducted for several years in the early 1960s could detect gamma-ray sources. Upper limits could be set only on the flux of high energy photons from supernova remnants, even if they also provided evidence that electrons are directly accelerated in the Crab Nebula.245 Technically, it was a painstaking problem with 1960s electronics to detect the faint and very brief flashes of light, separate them from other light sources, and analyze them to obtain physically meaningful information about the originating gamma rays. The search for point sources thus proved frustrating for a long time.246
In 1968, one of the first pulsars was discovered at the center of the Crab Nebula, as a pulsating radio source,247 and was identified as the remnant star of the supernova explosion.248 The spinning neutron star hypothesis for the origin of the signal provided the high-energy scenario in which particles could be accelerated to high energies in several possible ways.249 The possibility that pulsars could emit X-rays and even gravitational waves—and the observation of the star’s pulsation also in the MeV gamma-ray energy region by satellites—led to a parallel revival of interest in further observations of the Crab Nebula in the TeV and PeV gamma-ray energy domains, inaccessible from small outer space satellites. In that same year, 1968, Giovanni Fazio and his group, now including Trevor C. Weekes from the University College of Dublin, began to build a 10 m optical reflector at the Whipple Observatory on Mount Hopkins in Arizona, aiming in particular to detect gamma rays from the Crab Nebula.250 Construction of this reflector had been boosted by the prediction that if the radiation from radio to X-rays from the Crab Nebula was due to synchrotron radiation by relativistic electrons, then these same electrons colliding with the low-energy synchrotron-radiated photons would boost them to gamma ray energies through the Compton interaction process. The resultant gamma-ray spectrum would be most easily detectable at 100–1000 GeV energies, according to the Compton-synchrotron model of the Crab Nebula developed by Robert J. Gould in 1965, following Morrison’s early suggestions about the possibility of such an effect.251
Around the early 1970s, another newcomer to the field, Arnold Stepanian’s group at the Crimean Astrophysical Observatory in USSR, also searched for point sources of high-energy gamma rays using extensive air shower Cherenkov flashes detection, and reported what was considered a controversial observation, namely a very-high-energy gamma ray outburst from Cygnus X-3, a well-known compact X-ray binary source discovered by a rocket flight as early as 1966.252 During the 1970s, the search for gamma-rays from the direction of the X-ray source Cygnus X-3 was carried out by several groups applying different observation modes and experimental techniques and covering a wide range of energies. But the experimental results were somewhat contradictory, ranging from claims of a “clear excess” of gamma-rays from Cygnus X-3 to “no effect,” as for example, in the data of the COS-B satellite mission.
By the mid-1970s, while observations from space were becoming a reality, ground-based gamma-ray detectors proved unsuccessful in the search for sources in the TeV range. More sensitive and sophisticated methods and detectors were needed to deal with an overwhelming background of charged cosmic rays, in order to study the cosmic gamma radiation by means of ground-based instruments.
The Cosmic-Ray Group in Kiel Moves Toward High-Energy Gamma-Ray Astronomy
Pinkau’s trajectory from cosmic-ray showers, first in Hamburg then, from 1957, at the Pure and Applied Nuclear Physics Institute in Kiel, to space-based gamma-ray astronomy at the Max Planck Institute for Extraterrestrial Physics in Munich, brings us back to Bagge’s cosmic-ray group, of which also Otto Claus Allkofer and Joachim Trümper were members.253 This incubator for talented physicists, while also having minority participation in German and international satellite projects, could maintain its national leadership solely with cosmic-ray studies from the ground, by further specializing in its expertise in detectors and cosmic ray showers, and by following a more articulate path, eventually leading to high-energy ground-based gamma ray astronomy.
With the lifting of restrictions on nuclear research in 1955, Bagge benefited from the promises of the nuclear age and had a main role in the production of Germany’s first nuclear-powered vessel.254 In parallel to his activity as Technical Director of the nuclear facility, he had moved from Hamburg to Kiel in 1957, as Director of the brand-new Institute for Pure and Applied Nuclear Physics at the Christian-Albrechts-University. While he largely dedicated himself to the new applied nuclear enterprise, Bagge brought along the researchers he had trained in Hamburg, and continued to favor and promote research in cosmic ray physics at his new institute. As Trümper recalled:
At the Kiel Institute we were quite free in our choice of research topics. After the PhD (1959), I started a big experiment in cosmic rays, a so-called air shower experiment to study cosmic radiation at very high energies from
to eV […] One question was: What are the sources of cosmic radiation and how are the particles accelerated?255
And so, in the early 1960s, they redirected some of the activities carried out with traditional techniques and detectors—as well as with balloon flights—to a more ambitious project: the construction of a multipurpose shower array, including different detection systems and electronic equipment.256 The extensive air shower experiment went into operation in Kiel in June 1965 and, together with Allkofer, until 1970–71, Joachim Trümper was a leading member of the Kiel cosmic-ray group, before moving to Tübingen University, where he was appointed the Chair of Astronomy, as successor to Heinrich Siedentopf. There, he began to develop his program in the promising field of X-ray astronomy. Sometime in 1969 or ’70, Trümper visited the Max Planck Institute for Extraterrestrial Physics in Garching,257 to where his colleague and friend Klaus Pinkau had moved from Kiel. The foundations for more ambitious X-ray astronomy projects were laid on this occasion, and led to Trümper’s later appointment as Director of the Max Planck Institute for Extraterrestrial Physics (in 1975)—as well as to the planning and construction of rosat, the successful German X-ray satellite—although he still continued his strong collaboration with the Tübingen Institute, which became the MPE’s partner in all X-ray astronomy activities. Trümper had long since felt that such a long-term enterprise could be tackled only within the Max Planck Society.258
The fate of these two researchers exemplifies quite well what happened in the field of cosmic ray research after the 1950s. On the one hand, particle accelerators replaced cosmic rays as the source of collisions for experimental particle physics. On the other, the advent of the space age heralded the availability of satellites for research formerly done with balloons. Many prominent researchers with astrophysical interests, and their disciples, too, left cosmic rays behind from the late 1950s onwards, and turned instead to the development of satellite-based gamma-ray astronomy, as described in Chapter 3.
On the other hand, their colleague Otto Claus Allkofer, who remained in Kiel, continued to work on cosmic-ray research,259 and from 1975 to ’76, considerably extended and modified their air shower experiment. After ten years of operation, further scintillation counters and larger detector areas were added; this improved the reliability of detector response and allowed more detailed and more accurate data to be compiled, also thanks to a now completely automatic scan of each neon hodoscope photograph by a computer-controlled device that stored data on a magnetic tape for further analysis.260
In the early 1980s, interest in continuing the search with the shower array technique arose in the wake of various successful studies of gamma ray astronomy: by satellites, up to the GeV energy region; cosmic gamma ray bursts observed by satellites; and interesting results obtained from Extensive Air Shower (eas) measurements at ground level (threshold energies above
Before 1980 […] despite considerable effort, there were few results. The general feeling amongst astrophysicists was disinterest, if not disbelief. ‘Gamma-Ray Astronomy’ was interpreted as a branch of space science and presumed to terminate where satellites ceased to be useful, i.e., at 1 GeV.261
The field was languishing and then, in 1983, Manfred Samorsky and Wilhelm Stamm from Kiel surprised the community by reporting that, after four years of operation (from March 18, 1976 to January 7, 1980), measurements from their air shower array showed “a significant excess of extensive air showers”—presumably from gamma rays—emanating from the direction of Cygnus X-3. The signal was claimed to have a significance of 4.4 standard deviations.262 At the energies involved, it could be estimated that there must be charged particles with energies up to about
Part of the excitement aroused by Samorsky and Stamm’s puzzling claim came from events observed in parallel at massive underground proton decay detectors, built after the advent of the Grand Unified Theory of the electroweak and strong interactions, and predicting a likely instability of the proton, with a lifetime of less than
No subject has aroused such interest (and controversy) as the apparent detection of Cygnus X-3 in underground nucleon-decay experiments […] The existence of a new particle that would fit within the rather narrow constraints imposed by the underground experiments has come as a challenge to theoretical particle physicists at a time when there is not too much excitement in the field [our emphasis].268
Conventional physics could not explain those puzzling signals from Cygnus X-3, which was becoming a very topical object in conferences, and “in the flood of exotic theoretical predictions for an energy range inaccessible to HEP accelerator experiments,”269 an obvious name—cygnet—was coined to denote such hypothetical exotic primaries, unobserved particles with unique characteristics, which were especially intriguing for theoretical particle physicists.270 In this climate of unexpected results in the ultra-high-energy regime, in particular the one related to the striking observation of muon-rich air showers from the direction of Cygnus X-3, the name CYGNUS was given even to a new air-shower array located at Los Alamos National Laboratory in the US.271
In that same year, 1983, the experimental confirmation at CERN of the existence of the heavy vector bosons W± and Z, one of the main consequences of the Glashow-Weinberg-Salam unification of the weak and electromagnetic interactions, was achieved due to a major advance in high-energy physics: the creation of a proton-antiproton collider providing the necessary collision energy, which had been far beyond the reach of existing accelerators and detectors. Such detection showed that the unified electroweak theory had made a very good start, and supported the theoretical expectation that unification of strong, weak, and electromagnetic forces would reveal itself at the extremely high energies and particle densities available in the first instants of our Universe, so reinforcing the establishment of the new deep connection between particle physics and cosmology. Within the framework of the hot Big Bang model (based on Einstein’s theory of general relativity and the hypothesis that the Universe is isotropic and homogeneous when viewed over sufficiently large distances), the laws of particle physics could be applied in an attempt to trace the evolution of the cosmos in very early times.272
The impact originating from Kiel’s tantalizing observations was actually part of the growing symbiotic relationship between particle physics, astrophysics, and cosmology, which in those days of its very earliest appearances was being given a label of its own: the title of a talk given at the fourth Marcel Grossmann Meeting on General Relativity in 1985 by Abdus Salam, one of the protagonists of the unified electroweak theory, was Astro-Particle-Physics.273
In the wake of the excitement aroused by their 1983 announcement, the Kiel group was stimulated to continue the work in this new field of research with a far more ambitious experiment dedicated to the detection of gamma-ray sources in the energy region
The Max Planck Society’s Return to Air Shower Arrays: Munich and Heidelberg at hegra
The Kiel group’s claims that very energetic gamma-rays from Cygnus X-3 produced copious hadronic showers attracted the attention of several elementary particle physicists. Eckart Lorenz, then at the Max Planck Institute for Physics, after a successful career at cern and other laboratories,276 was fascinated by the Kiel announcements. Lorenz later recalled his scientific ‘leap,’ from the underground tunnels hosting cern accelerators to the highest mountaintops:
I remember it perfectly: I was standing in the dark, in the cern grounds, and a colleague approached me and said, “Somewhere up there is this Cygnus X-3. And there you have it: new physics.” Then I said: “I’m interested in what is going on up there…” [our translation].277
Particle physicists, accustomed since many years to working on immense experiments in huge collaborations, were bringing their skills and strategies—and of course novel technologies—to astrophysical projects rapidly growing in size and complexity, hopefully rejuvenating the field of particle physics itself and expanding its boundaries. The Max Planck Institute for Physics thus joined the hegra project with the initial intention of increasing the scintillator array by a significant factor.278
The 37 detectors of the initial hegra array planned by the Kiel group were twice read out for control purposes, from July 1989 to November 1990, by two independent electronic systems. The collected data were analyzed, including potential objects for which claims for very-high-energy or ultra-high-energy gamma-ray emission existed; but there was not
the slightest indication for an excess from any of the 9 sources. Especially for Cygnus X-3 (and some other sources), even less showers were detected than due to the expected average background.279
The search for ultrahigh-energy gamma-ray emission from the direction of sources observed from the satellite COS-B were likewise unsuccessful. Moreover, by April 1991, the data gathered by Jim Cronin’s array casa-mia (the Chicago Air Shower Array—Michigan muon Array), at the time the eas experiment with the highest sensitivity, put a stringent upper limit to the signal from Cygnus X-3, which ruled out this source excluding earlier observations.280 Cronin, a former accelerator physicist, had switched to the study of cosmic rays shortly after being awarded, together with Val L. Fitch, the Nobel Prize in Physics 1980, for the discovery of a slight asymmetry between matter and antimatter known as CP violation. Cronin, too, had become intrigued by the Kiel report and, leading a team from the Universities of Chicago and Michigan, had proposed that the large air shower array casa-mia search for high-energy gamma-ray sources, in particular for signals from Cygnus X-3.281 His array, which went into operation in early 1990, had pushed to the limit the possibility of a point source of cosmic rays, like Cygnus X-3, almost a factor of 100 lower than the original reports. So, it appeared that small experiments like the hegra/Kiel array, employing electron detectors only, would have no chance of finding sources, even if run for many years. The only alternative to the situation was to make hegra much larger and more sophisticated, increasing the detection area and, at the same time, the angular resolution, both by using many more electron detectors with fast-timing facilities, and adding large-area muon and Cherenkov light detectors to suppress the background showers. And this is the way the hegra experiment eventually evolved, becoming the most comprehensive instrument for ground-based gamma-ray astronomy, with a unique combination of detector capabilities. By early 1990, a proposal for extension of the installation at La Palma was in progress under the name of ‘hegra Collaboration,’ now including seven groups of scientists from German and Spanish institutions, and supported by funds from the Deutsche Forschungsgemeinschaft (DFG, German Research Association) and the Land Schleswig-Holstein.282
Meanwhile, the particle physicist Werner Hofmann, recently appointed Director at the Max Planck Institute for Nuclear Physics in Heidelberg,283 had witnessed the enormous interest triggered by the observation of the Kiel group of an excess of high-energy extensive air showers from the direction of Cygnus X-3—in particular its puzzling muon content—and subsequent revival of this field of cosmic ray physics.284 Starting from October 1988, in parallel with his intense high-energy physics research at accelerators, Hofmann began to set up his own cosmic ray project (Cosmic Ray Tracking), in collaboration with the Physics Institute of Heidelberg University.285 And so, besides improving the sensitivity for point sources, the aim of the proposal was to fill the gap left by the Air Cherenkov Telescopes and the Extensive Air Showers detectors with a new type of eas array based on the measurement of the direction of individual shower particles.286 As we will see, Hofmann’s step into the realm of particle astrophysics would be a premise for the later involvement of Heidelberg’s Institute for Nuclear Physics in the hegra project.
By the beginning of the 1990s, it was definitely clear that cosmological and astrophysical observations were a valuable complement to accelerator experiments. High-energy gamma-ray astrophysics could explore energy regions beyond the reach of accelerators. In October 1993, due to budget problems, the US Congress officially canceled the Superconducting Super Collider (ssc) project, after about ten years of planning and some 2 billion dollars already spent.287 It is interesting to recall here Jim Cronin’s comment about the different scales of estimated costs for building a big accelerator, in comparison with those for a large cosmic ray project like casa-mia, which he had successfully constructed and operated: 50–60 million dollars would be “only 10 percent of an ssc detector and 1 percent of the cost of the ssc itself.” Taken as “dollars per electron volt,” it might “sound like a bargain.”288
Not long after the cancellation of ssc, a “small and simple” space-borne cosmic-ray detector was proposed by the high-energy physicist Samuel Ting, who had been awarded the Nobel Prize in Physics 1976, jointly with Burton Richter, for the discovery of a new heavy particle known as J/Ψ, which confirmed the existence of the charmed quark. The new particle physics experiment in space, coming out of high-energy physics, was meant as an alternative project to the large coalition for a detector at ssc, put together by Ting, which had been rejected in 1991. Similarly, cern had rejected Ting’s proposal for an experiment at the future Large Hadron Collider, an upgraded version of the successful L3 experiment he had led at the Large Electron-Positron Collider. The Alpha Magnetic Spectrometer (ams) experiment, to be hosted at the International Space Station (iss), was considered controversial,289 but the context in which it was born, and its main scientific goals, both in the domain of astrophysics (cosmic-ray origin, age, and propagation, as well as the exploration of the most energetic gamma-ray sources) and in the domain of particle astrophysics (the search for cosmic antimatter and dark matter), are a further relevant example of the migration of high-energy physicists—Ting, like Jim Cronin, was actually a high-profile refugee from the world of particle accelerators—now taking advantage of the opportunities offered by the Universe as a ‘great cosmic accelerator.’
Similarly, particle physicists awaiting the planned yet still remote future Large Hadron Collider were hoping to get answers from Imaging Atmospheric Cherenkov telescopes (iact), the new type of Cherenkov detectors working on high-energy physics at the TeV scale, which could not be addressed by accelerators. In the months following the first ever observation, in February 1987, of a burst of neutrinos from the explosion of the Supernova 1987A, the possibility of detecting very-high-energy gamma rays from this source was likewise examined. However, within a year, it was the Crab Nebula, the result of a supernova explosion first recorded by Chinese astronomers in 1054, which came once again to the fore. The group working at the Whipple Observatory at Mount Hopkins, Arizona, submitted to the Astrophysical Journal a paper announcing the observation of a steady flux of gamma rays above 0.7 TeV from this source, at a very high level of statistical significance.290 Such a 9-sigma signal had been recorded using a refined version of the Imaging Atmospheric Cherenkov Technique, the Imaging Air Cherenkov Telescope (iact), in which a camera containing an array of photo multipliers is placed in the focal plane of a large mirror.291 After so many statistically suspicious and controversial ‘discoveries’ of gamma ray sources, the Whipple Telescope result represented the first uncontroversial detection of a source of TeV gamma rays, completely changing the future research perspective of high-energy gamma-ray astronomy.292
In order to fully grasp the significance of this breakthrough, one has to go back to the 1960s, when, on the ground, resources for detecting gamma rays with the Cherenkov Technique were practically nonexistent, and it took many years with the detection techniques then available to solve several fundamental problems, step by step. This was “largely due to inadequate instruments, slowly developing theories about particle interaction, slowly oncoming additional information from accelerator experiments and the lack of powerful computers.”293 Astrophysics depends on theory and modeling to a greater degree than most other physical sciences, because observations can be done only remotely. The ability of astrophysicists to extract physical insight from observational data necessarily relies on more powerful computers and computer programs that incorporate realistic physics. The rapid advance of computers, and the concurrent development of analytical techniques mentioned earlier, revitalized the community. But the 1970s, too, saw little progress and very few gamma-ray observatories were in operation. Only in the 1980s, when also computing power increased enormously and several major programs were developed for the interpretation of data and modeling, did the field of ground-based gamma-ray astronomy start to look promising to outsiders.
After decades of slow improvements, the American team led by Trevor Weekes, who had initiated gamma-ray studies with a reflector in 1968, started developing the ‘imaging’ technique in 1981.294 By 1985, Michael Hillas’s Monte Carlo simulations indicated that it should be possible to discriminate between the gamma-ray and proton-induced showers, rejecting up to 97 percent of the background events.295 This was indeed the really effective strategy for tackling the overwhelming and unwanted background of charged cosmic rays: by comparing the real-data parameter distributions with those of the gamma-ray and hadronic shower simulations, it became possible to enhance the gamma-ray content of any set of observations. Promising results were finally reached in 1989, when the first robust detection of TeV gamma rays from an astrophysical object was announced, a steady 9-sigma gamma-ray signal from the Crab Nebula, obtained with the 10 m optical reflector of the Whipple Observatory, equipped with a fast camera for imaging Cherenkov light from Extensive Air Showers.296
As we already emphasized, the opening of this new observational window onto the cosmos occurred in concomitance with the gradual appearance, over the 1980s, of astroparticle physics, as well as with a crisis in accelerator-based physics related to the end of the Cold War
Stereoscopic Cherenkov Imaging: An Armenian Tradition with International Reach
But the end of the Cold War also gave a key boost to cosmic ray research in Europe, crucially aided by the involvement of Soviet researchers. In the mid-1980s, physicists at the prestigious Yerevan Physics Institute in Armenia also decided to move into the field, which was already being pursued in the Soviet Union by Arnold Stepanian in Crimea. Stepanian had been using a four-mirror system since the end of the 1960s,297 but the more recent arrivals in the field felt that he, as an astronomer, had little particle physics insight and lacked credibility among the global community.
As mentioned earlier, the Yerevan Physics Institute in Armenia had begun developing the concept of stereoscopic approach in the mid-1980s, using the novel technique of multiple Imaging Atmospheric Telescopes.298 These were also part of larger efforts in the field of high-energy cosmic rays, in the course of which a complex shower array site similar to hegra was to be installed on Mount Aragats.299
However, these plans were laid in the final years of the Soviet Union, and economic collapse paralyzed such efforts. Previous contact with Claus Allkofer at the Nuclear Physics Institute in Kiel led to Razmik Mirzoyan and electronic engineers from the Yerevan Institute being invited in 1990 to work with Samorsky and Stamm.300 Allkofer unfortunately passed away in January 1990. Their initial bargaining chip was not the Cherenkov telescopes, but rather their access to military-grade scintillation detectors with which they offered to extend the collection area of the shower array. Before 1992, German interest in the Armenian group was largely due to such detectors, to be used to enlarge the hegra array. The Cherenkov array, alien to the tradition of the other participating groups, was a rather independent addition to the site, whose worth remained unproven during its first years of existence.
The final goal of the hegra Collaboration was now to build a large-area, multi-detector experiment that would enable the simultaneous measurement of extensive air showers and many shower parameters.301 Plans were made by scientists from the Yerevan Physics Institute, the Crimean Astrophysical Observatory, and the Munich Max Planck Institute for Physics and Astrophysics to complement the hegra array by a system of five Imaging Air Cherenkov Telescopes, each consisting of a multi-mirror reflector of 5 m2 collection area and a fast, 37-pixel camera in its focus. For each event, the light level in each pixel would be digitized and recorded. The darkness of La Palma and the large number of clear nights made the area one of the best sites for the atmospheric Cherenkov technique. The simultaneous operation of the ultra-high-energy air shower detector and the system of atmospheric Cherenkov light receivers would cover the energy spectrum from
While Kiel had first made contact with the Armenians, and scientists were invited to Kiel for short periods in 1990, the center of gravity was starting to shift to Munich. In June 1990, Eckart Lorenz, the most senior researcher from the Max Planck Institute for Physics in the hegra Collaboration, invited Razmik Mirzoyan to Munich, and it was decided that the mechanical mountings of the detector systems would be redesigned and built at the Institute for Physics, while the imaging camera and electronics would be made in Kiel.304 After many years of work in particle physics, Lorenz was now changing field, entering into cosmic rays and astroparticle physics: Lorenz “was someone with a vision,” had an “immense energy,” and “was the driving force in Munich,” Mirzoyan recalls.305 Eckart Lorenz was then setting up his shower-array-based contribution called airobicc,306 the first version of which was completed in fall 1992, but he was already seeing the potential in the Cherenkov telescopes. The five telescopes for atmospheric Cherenkov light—of 5 m diameter, with 19 mirrors to be operated in the brand-new Imaging Atmospheric Cherenkov Technique—would actually extend the energy range of the experiment down to about
hegra’s first telescope was designed as a somewhat modified version of the first prototype of the five-telescope array the Armenians had planned to build at Nor Amberd in 1989.308 It was installed in spring 1992. Until that moment, hegra had been an overlap of several types of detectors occupying the same space. The Cherenkov telescopes were just one more component of the mix, with the promise of extending the lower end of the detection energy range; and they comprised the only system based on direct observation of the interaction of gamma rays and the showers they cause with the atmosphere above, rather than on detection of the ‘tail’ end of the showers on the ground.309
The first Imaging Atmospheric Cherenkov Telescope at hegra, commissioned in the summer of 1992, confirmed the Crab Nebula as a source of very high-energy gamma rays, only two months after installation of the electronics and the imaging camera.310 This detection put hegra on at least an equal footing with the Americans of Whipple Observatory, and boosted worldwide confidence in the new technique. But more significantly, the plans underway for an array of five telescopes promised to quickly make hegra the most advanced system in the world.
A Second Entry Point for Heidelberg
The detection by the Whipple Observatory of the Crab as a steady source of TeV gamma rays—which, like the replication of this detection at hegra, had put the field of Very-High-Energy gamma-ray astronomy on a firm observational basis—had also triggered the interest of Heinrich Völk at the Institute for Nuclear Physics in Heidelberg (see Chapters 3 and 4 for more on his trajectory in the Max Planck Society). Throughout his career in the United States, Munich, and Heidelberg, Völk had investigated the question of acceleration mechanisms of cosmic rays from a theoretical perspective,311 which led him in turn to consider this problem in the light both of the connection to gamma-ray sources312 and the possible role, in the production of gamma rays, of cosmic rays penetrating a dense interstellar medium (the so-called molecular clouds), a research question raised by COS-B observations.313 As Völk himself has emphasized,
One knows that essentially all the known universe is filled with this non-thermal component… That’s what I called “the non-thermal Universe.” And so, our idea was to study the non-thermal Universe, which in other terms you could call cosmic rays… but which is much, much more than just what people call cosmic rays…
In this perspective, “the combination of plasma astrophysics and particle physics is basically gamma-ray astronomy” [our emphasis].314 Considering the high energies that the gamma rays achieve, thermal mechanism cannot be responsible for their production and one needs to evoke non-thermal processes to explain their origin. Such non-thermal radiation, typical of all gamma-ray sources, may originate from different processes, in particular from the interaction of non-thermal particle populations with photons and matter. In this sense, as Völk emphasized, gamma rays provide a window onto the non-thermal physics in our Universe, a view on a variety of objects—such as neutron stars, black holes, stellar explosions and the remnants thereof—which emit a significant fraction of their energy through non-thermal processes. This non-thermal astrophysics-oriented research became a basic hallmark of the Heidelberg Institute.
During a trip to Chicago, Völk met Felix Aharonian, the theoretician and director of the Armenian team, who shared with him very similar theoretical interests.315 Völk was very impressed and thus invited Aharonian to Heidelberg in 1993, where they started collaborating.316 Aharonian in turn brought with him the team’s leading engineer, Ruben Kankanian. In July 1993, at the 21st International Cosmic Ray Conference, the official announcement of the observation of a very-high-energy gamma emission from the Crab with the first of the hegra Air Cherenkov Telescope Array was cosigned by physicists from Heidelberg.317
Since the late 1980s, as we have already seen, Werner Hofmann’s group was proposing at MPIK—in collaboration with the Institute for Physics of the University of Heidelberg—the Cosmic Ray Tracking (crt) project, a new type of extensive air shower array, the basic concept and construction and performance details of which were presented at various conferences.318 Their Cosmic Ray Tracking system, based on the high-energy physics detector technology of a time projection chamber, promised to improve the sensitivity for the detection of point sources by about a factor about 100 greater than existing conventional eas arrays or Atmospheric Cherenkov Telescopes, which extended the energy range to a few TeV, while there were indications for gamma point sources at higher energies. The proposed project was expected to bridge the observational gap for cosmic rays in the energy range from TeV to PeV, opening a new energy window in astronomical observation and the potential to discover new phenomena in high-energy physics. Initially, the proposed location was the astronomical observatory at Llano del Hato, close to Merida in the Venezuelan Andes, at 3600 m above sea level. But in May 1992, when presenting the design and construction of the first full-size detector module, which was already running in coincidence with a small scintillator array in Heidelberg, it was announced that a series of ten full-size prototypes was under construction to form a “realistic small array,” and would be later moved to La Palma, to be tested in the hegra eas array.319 In January–February 1993, the first two detectors of the crt project were delivered and installed in La Palma. They could be operated standalone, as well as together with the hegra array.320 At the 23rd International Cosmic Ray Conference (icrc), held in July 1993 in Canada, the aforementioned observation of the Crab with the first operative hegra Imaging Atmospheric Cherenkov Telescope was announced by a collaboration now also including the Heidelberg scientists.321 By 1995, both the Max Planck Institutes for Physics and Nuclear Physics were fully participating in the multi-detector experiment hegra, and in August that year, Heinrich Völk and Werner Hofmann were among the signatories of the hegra Collaboration report on the results of observation of gamma rays from the Crab Nebula by the second hegra Imaging Atmospheric Cherenkov Telescope installed and taking data since February 1994.322 The analysis of the observations during the period October 1994–March 1995 revealed a positive signal at 10-sigma confidence level, a remarkable result that was submitted to Astroparticle Physics, the journal founded in 1992 as a dedicated publication channel for this nascent field. In this way, two Max Planck directors, from Heidelberg, strongly supported the scientific operations of the hegra project, marking the beginning of a new era in which major experiments in the field were to be designed and run from the start by larger groups. This was a definite change of scale in Max Planck involvement, compared to the situation in Munich. Once again, as at several times in the history of the Cherenkov technique, newcomers with a significantly higher scale of resources and prestige seemed to be gradually taking over from a more modest, previous generation. The Heidelberg Institute contributed to building up the hegra–iact system, which from 1998 consisted of five telescopes and thus could convincingly demonstrate the power of stereoscopic observations and the great potential of the technique; and it also served as the prototype for the third-generation instruments, guiding the evolution of TeV astronomy from a branch of cosmic ray studies into a full-fledged astronomical discipline.323
By the mid-1990s, the two originally ‘nuclear’ Max Planck Institutes in Munich and Heidelberg, which had participated in cosmic ray research since the middle of the century, before abandoning it during the space age, had again clearly attained global leadership in the field, thanks to their involvement in hegra and the absorption of experts such as Mirzoyan, Aharonian, and Kankanian from the Armenian group, and, in particular, their combined capacity to mobilize their in-house workshops for the construction of Cherenkov telescopes, a task beyond the ability of the smaller partners in hegra.
At the same time, there were already significant differences in the approach of each institute. Munich had been first on the scene, but was limited in scope by a ‘Mittelbau’ (non-director) researcher in a struggling experimental department. Heidelberg had arrived later, and with the full power of not one, but two Max Planck directors. At the experimental level, there was also a divergence in scientific style: Munich’s approach was based on the collaboration between Eckart Lorenz and Razmik Mirzoyan, both experimental physicists with an interest in understanding and innovating their instruments. Heidelberg was a more heterogeneous mix, with Völk and Aharonian on the theoretical end, Hofmann, a newcomer to Cherenkov astronomy, as the most important experimental physicist, with a wide expertise in instrumentation for high-energy physics, and technicians like Ruben Kankanian, who had a firm preference for stable, reliable instruments.
Parallel Continuation of hegra as h.e.s.s. and magic
By the mid-1990s, this divergence in approach was expressing itself in tensions between the factions gravitating towards the different Max Planck Institutes. As a consequence of such divergences and personal differences, Heidelberg and Munich ended up taking separate paths when proposing the next generation and scale of ground-based Cherenkov telescopes: h.e.s.s. (High Energy Stereoscopic System) in the south,324 and magic (Major Atmospheric Gamma Imaging Cherenkov Telescopes) in the north.325
In the case of Heidelberg, the preference was for a conservative approach that would be a gradual continuation of the systems proven successful at the Whipple Observatory and hegra. This was to be an array of multiple, middle-sized telescopes consisting of easily manufactured steel structures, for the study of sources in the energy range between 100 GeV and 100 TeV. Such an array favored a site away from La Palma, in which Heidelberg had less of a stake anyway. The end of apartheid in South Africa and the independence of Namibia spelled a new opportunity to build on the Gamsberg, where the Max Planck Institute for Astronomy had planned to set up its observatory in the early 1970s (see Chapters 3 and 4). This would be the first gamma-ray observatory in the southern hemisphere.326 In choosing the southern location, Heidelberg also gained strong backing from the French groups, which were to become the main collaborators. Finally, the choice between northern or southern hemisphere was related also to which kind of sources the new astronomy was expecting to focus on: the southern hemisphere makes available most of our own galaxy, the Milky Way—and in particular, the galactic center—so was the obvious choice for the detection of potentially smaller, closer sources. Conversely, a northern location would be more limited to extragalactic sources, which would need to be more powerful to be detected; plus, in terms of accessible skies, a northern site would be in direct competition, but also constructive overlap, with the Americans.
In proposing two separate projects, the institutes entered into competition with each other. While the Munich researchers favored a larger detection dish and finer detector technology to obtain higher sensitivity,327 the Heidelberg team, following on the already proven tradition with stereoscopic systems, went instead for an array of multiple, medium-sized dishes.328 But the key to the worldwide leadership of the Max Planck Institutes at this scale was that both Heidelberg and Munich had enough in-house technical competence in their respective workshops to produce the experimental systems internally, with relatively minor financial help from the Max Planck Society.329 In the mid-1990s, both Heidelberg and Munich had the independent capacity to create the world’s most important projects in the field, in competition with each other. This ‘competition’ was, therefore, more a case of massive potential rooted in complementarity, brought about by the different experimental choices which had driven their divergence in the first place. Munich was betting on a few large telescopes built with novel materials, and an emphasis on detector systems, while being conservative on their siting. Heidelberg more closely followed the Soviet tradition and American third-generation proposals (the future veritas, the Very Energetic Radiation Imaging Telescope Array System), opting for a larger number of cheaper, smaller, but also sturdier telescopes. As previously mentioned, Heidelberg was also making the best of the necessity of finding a new site, following the break-up with the project in La Palma, siting projects in Namibia instead, thanks to contacts within the Max Planck Institute for Astronomy (see Chapter 3),330 and the encouragement of their French partners.331
The hegra telescopes system ceased operations after six years of observations, in September 2002, in coincidence with the start-up of h.e.s.s., whose first telescope was inaugurated in late summer 2002.332 The first-stage array of h.e.s.s. was quickly built between 2002 and 2004 with, thanks to Völk’s requests, the help of external funding from the German Ministry of Research, which complemented the internal backing by the two participating departments in Heidelberg. The decision to opt for reliable, proven technologies paid off, for the system quickly yielded results, such as the more than one hundred new sources identified by the first survey of the galactic center, at a time when no other system was yet in stable operation.333
The situation in Munich was more haphazard, with success slower to mature. The funding of magic had been more difficult in the first place, with only internal funds and resources from the institute, supplemented somewhat by an insurance payment following severe fire damage to the hegra site in 1997.334 Still, Italian and Spanish partners helped maintain the project. Furthermore, when it started, the search for a new director was still on in Lorenz and Mirzoyan’s department. In 1993, with the arrival in Munich of Masahiro Teshima,335magic was finally on a par with Heidelberg, as far as directorial support went. Technically, magic was more innovative and daring than h.e.s.s., but this created many delays. Also, since the funding for magic had been restricted to a single telescope, in its first iteration it could not benefit from the advantages of the stereoscopic approach. The operation of magic started to stabilize around 2005, by which stage Munich was ready to apply for the funds to expand the system to the intended scale.336 While the earliest plans had suggested up to four dishes, the mid-2000s expansion stage foresaw only a second one; and it was autumn 2009 when the second telescope (magic ii) went into operation, so enabling stereoscopic observations. Mostly owing to input by Eckart Lorenz and the Munich MPI group, magic was specifically designed to search for VHE emission from gamma-ray bursts (grbs) and fast transient phenomena in general. This required a light mechanical structure, fast and precise movement, and the capability to quickly focus the mirrors after pointing to a source when alerted by satellites to do so.337 In any case, the lower energy threshold of magic meant an area of overlap with the Fermi satellite that turned out to benefit the system. In 2008, the magic collaboration detected for the first time a pulsed gamma emission from the pulsar in the Crab Nebula at energies above 25 GeV, creating again a bridge between satellite detectors and Imaging Atmospheric Cherenkov Telescopes.338magic became the first facility to report unambiguous VHE emission, with energies up to 1 TeV.339magic repointing procedure and new VHE data opened a novel pathway for understanding grbs that will be further extended by current instruments, a new generation of ground-based gamma-ray telescopes.
Finally, as mentioned earlier, the situation in the northern hemisphere necessitated specialization in extragalactic sources, which, even though fewer in number than the abundant ‘harvest’ of the galactic plane, were very different in kind: extreme, faraway objects with more potential for answering fundamental questions, including the ‘holy grail’ of the field: establishing the origin of high-energy cosmic rays.340 Today, the study of cosmic rays is in fact increasingly based on complementary approaches: on the one hand, the measurement of the energy spectrum, chemical composition, and anisotropy of charged ultra-high-energy cosmic; and on the other, the search for their sources through the observation of neutral radiation like photons and neutrinos, which point back to the emitting sources and are tracers of acceleration sites of charged cosmic rays. The physical connection between high-energy cosmic ray interactions and the resulting very-high-energy neutrinos and gamma-rays can in fact provide clues about their unknown astrophysical sources. Hopes in this direction were, for example, confirmed in the period 2017–18 by the first detection of a high-energy cosmic ray source, identified via the sequential, multi-messenger detection, firstly, of energetic neutrinos with the IceCube telescope in the South Pole; then by the Fermi gamma-ray observatory in space determining the approximate location of the associated gamma rays; and, thirdly, by magic identifying the particular source of the highest energy photons, a so-called Blazar—a particular class of Active Galactic Nuclei (agn), whose relativistic jet of ionized matter points toward the observer—that had already been registered in previous surveys.341 Previous detections of individual astrophysical sources of neutrinos had been limited to the Sun and the supernova 1987A, and thus this event was a giant leap in the growing field of multi-messenger astrophysics, whose fundamental goal is to understand the properties of the high-energy astrophysical sources by means of new observational strategies, integrating the study of high-energy charged cosmic rays, neutrinos, and electromagnetic radiation across a broad range of wavelengths.342
Over the past few years, detecting gravitational waves and neutrinos has become almost routine. Using information carried by photons, cosmic rays, neutrinos, and gravitational waves to investigate violent astrophysical phenomena, multi-messenger astrophysics has definitely been established as a “new kind of big science” (involving big collaborations, big instruments, and big data), thus, not only deeply affecting and expanding our scientific understanding of astrophysical processes, but also reshaping “the very way science is carried out.”343
Evolution towards a Single Global Collaboration
In the year 2005, when both magic and h.e.s.s. were applying to the Max Planck Society and external sources for funding, competition between the two projects peaked. Based on their initial successes, each system was planning to expand into its final configuration as a stereoscopic array. By this point, there were deep concerns within the Max Planck Society regarding the projects’ competition, potential duplication of efforts, and, especially, the perceptibly growing animosities.344 In a rare case of direct intervention, the MPG President and Vice-President brought together the main players in Heidelberg and Munich and indicated that it expected them to eventually reconcile and join forces; and, furthermore, that the current stage of expansion, h.e.s.s. ii and magic ii, was contingent on their promise to do so. This ultimatum would be the origin of the next-generation project, later named the Cherenkov Telescope Array (cta). The immediate effect of this high-level meeting, which did indeed result in an agreement on future cooperation, was the release of funds allowing both extant systems to expand and fulfill their intended potential; and hence, over the next decade, while still separate and in competition, they cemented their global dominance. And this dominance assured them enormous leverage when negotiating the next-generation project with their international partners. Moreover, this approved second stage permitted the competing projects, while still separate, to converge in terms of their technical capabilities, their respective upgrades moving each in the direction of its rival: magic ii, by adding a second large telescope, finally obtained stereoscopic capabilities for the Palma site, while h.e.s.s. ii brought to Namibia the largest Cherenkov telescope ever built, with a 28 m-diameter mirror.345
By the first decade of the 21st century, it was firmly established that the Max Planck Society should not own or administer large scientific infrastructures (Chapter 4). So, since its inception, cta included one additional German partner in the form of the Helmholtz Institute of desy-Zeuthen, successor to the Institute for High-Energy Physics, the main East German particle physics institute, which after reunification had been merged with the much larger and famous Hamburg-based research center for particle physics, founded in 1959 around a powerful electron-synchrotron project.346 The desy branch location at Zeuthen had been traditionally involved since the 1980s in neutrino astrophysics and at the time was a main collaborator in the large-scale IceCube experiment, the neutrino telescope at the South Pole, now also extending its astroparticle physics program to gamma rays, through participation in cta.
The two Max Planck Institutes, their international partners in h.e.s.s. and magic, and the Helmholtz, represented by desy-Zeuthen, constituted the core of cta, which was being bolstered at the time of its constitution by the good performance of the existing competing projects in Namibia and La Palma. The plans reflected what has become the standard logic of expansion for each new generation of large scientific projects: a change in scale and sensitivity of an order of magnitude. That is, the total amount of telescopes in cta would reach beyond one hundred, and the expected costs were in the order of half a billion dollars, thus, were starting to resemble the scale of upcoming astronomical projects such as the alma in radio astronomy and the European Extremely Large Telescope (elt) in optical astronomy. Moreover, the resemblance with alma is striking, in that the cta project was proposing a practical monopoly in the field of ground-based gamma astronomy. The crucial step in this direction was the absorption of the American competitors into cta. This was the first ever such case in a scientific field, of Americans joining an existing collaboration as ‘minority partners.’347 This was, however, a very special kind of monopoly, and could more aptly be named an agglomeration of heterogeneous partners within a single organization.348 The geographical choices reflect this heterogeneity, maintaining a foot in each hemisphere, with each site showcasing a path-dependent continuity with either magic or h.e.s.s, while adding features that are the specialty of the other global partners. Once the US, Brazilian, and Indian groups had joined, along with the strong Japanese participation, cta represented a worldwide effort, extending well beyond its European roots.349
On the northern site of La Palma, a traditional stronghold of the Max Planck Institute for Physics in Munich, now the Heisenberg Institute, the new cta telescopes are already under construction, on the site that previously hosted hegra and still hosts magic. The southern site already selected will be near the eso Paranal Observatory in Chile (see Chapter 4), and is to maintain a stronger connection to h.e.s.s. and the Max Planck Institute for Nuclear Physics in Heidelberg. The deployment of the project’s German telescopes, a responsibility of desy, still reflects the convergence of two separate paths (large telescopes and middle-sized arrays), and each of these ultimately derives heavily from Armenian expertise and a vast tradition in high-energy astrophysics extending back over half a century into the Soviet era.
cta, in building on the technology of current ground-based detectors and utilizing three classes of telescopes to cover the full cta’s energy range, and in improving the performance of the current iacts, is expected to facilitate the expansion of our knowledge of several scientific subjects, including the study of the origin of cosmic rays and the exploration of extreme particle acceleration (investigating in detail processes happening close to black holes, and within relativistic jets or winds); and, last but not least, also to shed light on the nature of the still mysterious dark matter and its distribution in the Universe.350
From January to February 2020, the prototype Large-Sized Telescope (lst), lst-1, while still in the commissioning phase, detected very high-energy emission from the Crab Pulsar: “This milestone shows us that the lst-1 is already performing at an extraordinary level, detecting a challenging source in record time,” said Masahiro Teshima, Director of the Max Planck Institute for Physics in Munich and Principal Investigator of lst.351
In joining “the field of telescopes capable of detecting gamma-ray pulsars”—particularly challenging sources, because of their weak signals and the dominance of the foreground gamma-ray signal from the surrounding nebulae—this last-generation telescope, covering the low-energy sensitivity range, has inaugurated the cta Observatory era for the worldwide astronomical and particle physics communities.
From here on, we use the more common terms ‘astroparticle physics’ or ‘particle astrophysics.’ For a preliminary survey of the field, see Vanessa Cirkel-Bartelt: History of Astroparticle Physics and Its Components. Living Reviews in Relativity 11 (2008), 2–58. doi:
The issue of international scientific collaborations was the main focus of the Senate meeting of November 1987 (Minutes of the 117th Senate meeting of 11.19.1987, in Munich, AMPG, II. Abt., Rep. 60, No. 117).
On the problem of negative results, see P. Astone, and G. Pizzella: Upper Limits in the Case That Zero Events Are Observed: An Intuitive Solution to the Background Dependence Puzzle. hep-ex/0002028. 1st Workshop on Confidence Limits,
CERN, Geneva, Switzerland, 17–18 Jan 2000. Geneva: CERN 2000, 199–205. doi:
The early generation of directors in ground-based observational astronomy came from different work cultures, namely optical astronomy (Heidelberg) and engineering (Bonn); these were soon supplemented by directors in space-based astronomy, many of whom started their careers in nuclear/particle physics and plasma research. These were closer to the earlier Max Planck traditions. Both ground-based and space-based astronomy were, however, much more directly determined by geopolitical considerations of the space race than the examples treated in this chapter. In ground-based astronomy, the choice of location, often abroad, combined with national competition to build the world’s largest instruments. In space-based astronomy, West Germany’s peculiar position in international politics, which precluded independent access to outer space, overdetermined the maneuvering options of its scientists. See Chapters 3 and 4.
Péter Mészáros et al.: Multi-Messenger Astrophysics. Nature Reviews Physics 1/10 (2019), 585–599. doi:
Kirsten, Till: interview by Luisa Bonolis and Juan-Andres Leon, Heidelberg, October 24–25, 2017. DA GMPG, BC 601051.
Wolfgang Pauli: On the Earlier and More Recent History of the Neutrino. In: K. Winter (ed.): Neutrino Physics. Cambridge: Cambridge University Press 1991, 1–25.
Enrico Fermi: Versuch einer Theorie der Beta-Strahlen. Zeitschrift für Physik 88 (1934), 161–171. doi:
The reactor was expected to produce fluxes to the order of
Alastair G.W. Cameron: Nuclear Astrophysics. Annual Review of Nuclear Science 8/1 (1958), 299–326. doi:
Luisa Bonolis: Bruno Pontecorvo. From Slow Neutrons to Oscillating Neutrinos. American Journal of Physics 73/6 (2005), 487–499. doi:
Luis W. Alvarez, A Proposed Experimental Test of the Neutrino Theory, Report UCRL-328, University of California Radiation Laboratory, April 18, 1949.
John N. Bahcall: Solar Neutrinos. Theoretical. Physical Review Letters 12/11 (1964), 300–302. doi:
By 1954, Davis had already used a large tank of carbon tetrachloride in the basement of one of the Savannah River reactors, at that time the most intense antineutrino source in the world, to try to detect reactor antineutrinos; and he established that they did not interact with chlorine nuclei. Raymond Jr. Davis: Attempt to Detect the Antineutrinos from a Nuclear Reactor by the Cl37(Anti ν, e−)A37 Reaction. Physical Review 97/3 (1955), 766–769. doi:
See Davis and Bahcall’s back-to-back articles: Raymond Jr. Davis, Don S. Harmer, and Kenneth C Hoffman: Search for Neutrinos from the Sun. Physical Review Letters 20/21 (1968), 1205–1209. doi:
Bahcall, John N., and Raymond Jr. Davis: The Evolution of Neutrino Astronomy. Publications of the Astronomical Society of the Pacific 112/770 (2000), 429–433. doi:
John N. Bahcall: Two Solar Neutrino Problems. Nuclear Physics B—Proceedings Supplements 43/1–3 (1995), 41–46. doi:
Davis, Harmer, and Hoffman, Neutrinos from the Sun, 1968, 1205–1209. Bahcall, Bahcall, and Shaviv, Present Status, 1968, 1209–1212.
Bruno Pontecorvo: Neutrino Experiments and the Problem of Conservation of Leptonic Charge. Pis’ma v Zhurnal Èksperimental’noi i Teoreticheskoi Fiziki 53 (1967), 984–988. http://inspirehep.net/record/51319?ln=en. Last accessed 11/10/2017. In 1962, it was experimentally established that a second type of neutrino (the muon neutrino) exists, which is paired with the muon in the same way the already known (electron) neutrino is paired with the electron: the neutrino involved in nuclear beta-decay and the one in muon-decay are thus two different particles. G. Danby et al.: Observation of High-Energy Neutrino Reactions and the Existence of Two Kinds of Neutrinos. Physical Review Letters 9/1 (1962), 36–44. doi:
Pontecorvo, who had devised the chlorine-argon radiochemical method for detecting neutrinos in 1945–46, put forward the idea of neutrino flavor oscillations in different forms from the 1950s onward. Neutrinos, like the charged particles electron, muon, and tau (and their antiparticles) to which each neutrino type is associated (three types of neutrinos: electron-type/muon-type/tau-type), are leptons, that is, elementary particles that do not undergo strong interactions. During the transition from one type to another, lepton numbers (i.e., the number of leptons minus the number of corresponding antileptons in each reaction) would not be conserved, contrarily to what is implied by the Standard Theory of elementary particle physics. The fact that the lepton number is not a conserved quantum number in the phenomenon of neutrino transition from one flavor to another leads to neutrino mixing, and consequently, oscillations of neutrinos, and thus to physics beyond the Standard Theory, as was suggested by Gribov and Pontecorvo one year after the first chlorine results were published by Davis. Vladimir N. Gribov, and Bruno Pontecorvo: Neutrino Astronomy and Lepton Charge. Physics Letters B 8/7 (1969), 493–496. doi:
In 1957, Pontecorvo put forward for the first time the idea of neutrino oscillation suggesting that antineutrinos from the reactor might transform into neutrinos and be able to trigger Davis’s detector. But this fruitful idea was only due to rumors reaching Pontecorvo that Davis had actually observed such events, which eventually were not confirmed.
For an overview of the problem in the late 1960s and early 1970s, see John N. Bahcall, and R. L. Sears: Solar Neutrinos. Annual Review of Astronomy and Astrophysics 10 (1972), 25–44. doi:
Semen Solomonovich Gershtein, and Yakov Borisovich Zeldovich: Rest Mass of Muonic Neutrino and Cosmology. Soviet Journal of Experimental and Theoretical Physics Letters 4 (1966), 120–122. http://adsabs.harvard.edu/abs/1966JETPL...4..120G. Last accessed 8/8/2020. For a discussion about neutrinos as candidates for dark matter on a cosmological scale, see also Virginia Trimble: Dark Matter in the Universe. Where, What, and Why? Contemporary Physics 29/4 (1988), 373–392. doi:
Michael S. Turner: Neutrinos and Cosmology. AIP
Conference Proceedings 72/1 (1981), 335–355. doi:
Andrzej J. Buras et al.: Aspects of the Grand Unification of Strong, Weak and Electromagnetic Interactions. Nuclear Physics B 135/1 (1978), 66–92. doi:
David N. Schramm: Neutrinos and Cosmology. Nuclear Physics B—Proceedings Supplements 38/1–3 (1995), 349–362. doi:
In bringing together experts in the fields of nuclear and particle physics, astrophysics, and cosmology, the international conference Weak and Electromagnetic Interactions in Nuclei, organized in October 1986 by the Max Planck Institute for Nuclear Physics, in conjunction with the 600th anniversary of the University of Heidelberg, testifies with its wide program to the early and deep involvement of the Institute in the novel trend connecting the laws of microphysics, astrophysics, and cosmology. Hans Volker Klapdor (ed.): Weak and Electromagnetic Interactions in Nuclei. Proceedings of the International Symposium, Heidelberg, July 1–5, 1986. Berlin Heidelberg: Springer 1986. In this regard, see also a slightly later volume exploring the close connections between micro (nuclear and particle physics) and macro physics (astrophysics and cosmology) induced by the weak interactions, and paying special attention to neutrinos. Klaus Grotz, and Hans Volker Klapdor: The Weak Interaction in Nuclear, Particle and Astrophysics. Bristol: Adam Hilger 1990.
Yakov Borisovich Zeldovich: The Universe as a Hot Laboratory for the Nuclear and Particle Physicist. Comments on Astrophysics and Space Physics 2 (1970), 12–17. http://adsabs.harvard.edu/abs/1970CoASP...2...12Z. Last accessed 9/19/2020.
In a proton-proton cycle, four hydrogen nuclei (protons) are fused, combining to form one helium nucleus. A small percentage of the original mass is lost in the process, mainly by conversion into heat energy, but some energy escapes in the form of neutrinos.
The capture of neutrinos by gallium-71 to produce germanium-71, an isotope with an 11-day half-life (the time needed for half the neutrons to decay) had a threshold of 233 keV, which was ideal for observing neutrinos from the pp reaction. This gallium method meant it was possible to detect the neutrinos from all solar thermonuclear reactions, including the initial proton fusion chain, which represents more than 98 percent of the neutrinos produced in the Sun, as preliminarily discussed in John N. Bahcall, and Raymond Jr. Davis: Solar Neutrinos. A Scientific Puzzle. Science 191/4224 (1976), 264–267. doi:
When very small activities of radionuclides are to be measured by direct observation of the radioactive decay, a certain amount of effort is required to choose and adapt detector systems, in order to attain high counting sensitivity and keep instrument background as low as possible.
Till A. Kirsten: Radiochemical Solar Neutrino Experiments and Implications. Physica Scripta 2000/T85 (2000), 71–81, 52. doi:
See, for example, articles published by Zähringer and Kirsten with Oliver Schaeffer while they were at Brookhaven: Oliver Schaeffer, and Josef Zähringer: Helium- und Argon-Erzeugung in Eisentargets durch energiereiche Protonen. Zeitschrift für Naturforschung A 13/4 (1958), 346–347. doi:
Till A. Kirsten, W. Gentner, and O. Müller: Isotopenanalyse der Edelgase in einem Tellurerz von Boliden (Schweden). Zeitschrift für Naturforschung A 22/11 (1967), 1783–1792. doi:
The double-beta decay transition from selenium-82 into krypton-82 was studied for the first time by Gentner, Kirsten, and Schaeffer (the authors thanked Davis for “valuable discussions”). Till A. Kirsten, W. Gentner, and O. A. Schaeffer: Massenspektrometrischer Nachweis von ββ-Zerfallsprodukten. Zeitschrift für Physik 202/1 (1967), 273–292. doi:
Vadim A. Kuzmin, and Georgii T. Zatsepin: On the Neutrino Spectroscopy of the Sun. Proceedings of the 8th International Cosmic Ray Conference, December 2–14, 1963, Jaipur, India. Bombay, India: Commercial Printing Press 1965, 1023–1024. https://ui.adsabs.harvard.edu/#abs/1965ICRC....2.1023K. Last accessed 1/6/2019. Kuzmin was the first to suggest, in 1965, the reaction scheme related to the transformation of gallium-71 into germanium-71, whose low threshold of 233 keV would allow the detection of pp neutrinos, by far the most abundant solar neutrinos, but with a very low energy <420 keV. Vadim A. Kuzmin: Detection of Solar Neutrinos by Means of the Ga71(ν,e−)Ge71 Reaction. Journal of Experimental and Theoretical Physics 22/5 (1966), 1051–1052. http://www.jetp.ac.ru/cgi-bin/e/index/e/22/5/p1051?a=list. Last accessed 4/30/2019.
John N. Bahcall et al.: Proposed Solar-Neutrino Experiment Using 71Ga. Physical Review Letters 40/20 (1978), 1351–1354. doi:
See timeline of “Early events” related to the preliminary phase in Till Kirsten’s papers (Till Kirsten, private collection, DA GMPG, BC 600004). We are very grateful to Till Kirsten for giving us an opportunity to consult such relevant documents related to the early phase of the Heidelberg solar neutrino project. From conversations with Kirsten we derived great insight into the development of the gallex experiment (Till Kirsten: interview by Luisa Bonolis and Juan-Andres Leon, Heidelberg, October 24–25, 2017. DA GMPG, BC 601051).
Max-Planck-Gesellschaft zur Förderung der Wissenschaften (ed.): Max-Planck-Gesellschaft Jahrbuch 1977. Göttingen: Vandenhoeck & Ruprecht 1977, 494.
G. Friedlander: Report on an Informal Conference on the Status and Future of Solar-Neutrino Research. Comments on Astrophysics 8 (1978), 47–54. http://adsabs.harvard.edu/abs/1978ComAp...8...47F. Last accessed 1/5/2019.
A draft of the proposal was signed by Till Kirsten on February 21, 1978: “Zum Forschungsprogramm SOLARE NEUTRINOS” am MPI für Kernphysik, Abteilung Kosmochemie (Isotopengruppe)” (AMPG, III. Abt., Rep. 68 A, No. 166/1.1). The proposal stressed how the expertise of the Cosmochemistry Department in low-level counting techniques, mass-spectrometry, and neutron reactions analysis would be an excellent basis for participating in such a project, even if this meant that the lunar sample investigations would be restricted. The end of the document mentions Oliver Schaeffer’s sabbatical year, beginning in late 1978 and to be spent in Heidelberg, in order to organize the collaboration. The participation of Wolfgang Hampel, as head of the Low-Level Laboratory, would be fundamental for such an experiment looking for rare events, where the background identification plays a key role, requiring the development of detectors for extremely low count rates. Hampel eventually became a leading figure in the solar neutrino experiment. Wolfgang Hampel: Particle Physics. Can the Gallium Detector Solve the Solar Neutrino Problem? Nature 308/5957 (1984), 312. doi:
Bahcall et al., Solar-Neutrino Experiment, 1978, 1351–1354.
See minutes of the first meeting of the solar neutrino collaboration, bnl, February 1–2, 1979 (Till Kirsten, private collection, DA GMPG, BC 600004). Pilot counting experiments, foreseen in both Brookhaven and Heidelberg, were discussed.
Within the pilot experiment, counting techniques were investigated in the course of 1980. The group was formed by Kirsten, W. Hampel, G. Heusser, M. Hübner, J. Kiko, O. A. Schaeffer, and R. Schlotz.
Neutrino capture in gallium-71 leads to germanium-71, which then decays back to gallium-71 by electron capture with a half-life of 11.4 days. Fifty tons of gallium as gallium trichloride solution would be needed for one neutrino capture per day (only neutrinos deriving from pp or pep reactions). A photo of the proportional counter specially developed to observe decay of the few germanium-71 atoms produced by reactions triggered in the gallium tank by neutrinos was reproduced in the 1981 Report. Max-Planck-Gesellschaft zur Förderung der Wissenschaften (ed.): Max-Planck-Gesellschaft Jahrbuch 1981. Göttingen: Vandenhoeck & Ruprecht 1981, 536. The international team collaborating on the pilot experiment had the following members: R. Davis, B. T. Cleveland, G. Friedlander, S. Katcoff, J. K. Rowley, and J. Weneser (Brookhaven National Laboratory); T. Kirsten, W. Hampel, G. Heusser, M. Hübner, J. Kiko, O. A. Schaeffer, and R. Schlotz (Max Planck Institute for Nuclear Physics, Heidelberg); I. Dostrovsky and Y. Eyal (Weizmann Institute of Science, Rehovot); J. N. Bahcall (Institute for Advanced Study, Princeton); K. Lande, W. Frati, R. I. Steinberg (University of Pennsylvania, Philadelphia). Assuming there was no problem with funding, they were supposed to obtain the necessary amount of 50 tons of gallium in stages of 10 or 15 tons per year over the next three or four years. The 1.5 ton gallium (2 million USD) were eventually financed by the Max Planck Society, indicating that the Society had full confidence in the relevance of such an enterprise. In the final section of his contribution to the conference proceedings, Hampel discussed the prospects for the detection of neutrinos emitted from collapsing stars: Wolfgang Hampel: Low-Energy Neutrinos in Astrophysics. In: Ettore Fiorini (ed.): Neutrino Physics and Astrophysics. Boston MA: Springer 1982, 61–79. The pilot experiment was concluded in 1983 and results were presented at the conference on Solar and Neutrino Astronomy held in August 1984 in Lead, US. Wolfgang Hampel: The Gallium Solar Neutrino Detector. In: M.L. Cherry, K. Lande, and W.A. Fowler (eds.). Solar Neutrinos and Neutrino Astronomy. 23–25 August 1984, Lead, SD, USA. American Institute of Physics 1985, 162–174. doi:
In this phase, Israel Dostrovsky and his colleagues from the Weizman Institute of Science in Rehovot, representing Israel at gallex, made a major contribution (we thank Till Kirsten for this remark). A report on the status of the gallium solar neutrino experiment conducted by the collaboration team, updated to early 1983, can be found in the paper presented at the conference Science Underground, held in Los Alamos in 1982. W. Hampel: The Gallium Solar Neutrino Detector. AIP
Conference Proceedings. American Institute of Physics 1983, 88–96. doi:
Bahcall, John N., and Davis, Evolution of Neutrino Astronomy, 2000, 429–433, 431.
Kirsten, Radiochemical, 2000, 71–81, 53.
Vladimir N. Gavrin: The Russian-American Gallium Experiment SAGE. Physics-Uspekhi 54/9 (2011), 941–948. doi:
Antonino Zichichi was President of the National Institute for Nuclear Physics from 1977 to 1982, and in the late 1970s, when a tunnel under the Gran Sasso mountain was under construction, as part of the highway connecting Rome to the Adriatic Sea, he saw this as a unique opportunity for the excavation of the large halls of an underground laboratory, which would also have an excellent connection to the road network. Antonino Zichichi: The Gran Sasso Project. AIP
Conference Proceedings 96/1 (1983), 52–64. doi:
Till Kirsten, personal communication with the authors (August 29, 2019). From Ettore Fiorini, Kirsten learned about Zichichi’s project to build the Gran Sasso underground laboratory, for which excavations had started in 1982. Kirsten had known Fiorini a long time, both being pioneers of double-beta decay experiments (even on the same isotope, tellurium-130, with different techniques). In 1984, Fiorini mediated the first connection between Kirsten and the National Institute for Nuclear Physics (infn), in particular with Nicola Cabibbo, universally known for his seminal theoretical work on the weak interaction. Cabibbo was President of infn until 1993, his successor being the theoretical physicist Luciano Maiani, who was President until the end of 1998, when he became Cern Director-General.
Till A. Kirsten: GALLEX/GNO. Context and Recollections. In: Mikko Meyer, and Kai Zuber (eds.): Solar Neutrinos. Proceedings of the 5th International Solar Neutrino Conference, Dresden, Germany, 11–14 June 2018. Singapore: World Scientific 2019, 47–68. doi:
The agreement with Brookhaven National Laboratory implied that they would take care of providing the enriched isotope chromium-50 to be used at Grenoble and Saclay to produce chrome-51 and test the neutrino capture process, but their funding request to the US Department of the Interior—supported by Rudolf Mössbauer—was not accepted. Unfortunately, the lack of enriched chromium source would be “a real tragedy,” which might have caused the French to withdraw from the agreement, as Ettore Fiorini wrote in a letter to Nicola Cabibbo, proposing that the whole question of funding and retrieving the source (from Oak Ridge Laboratories) could be handled by the Italian collaboration, with financial support from infn (Fiorini to Cabibbo, President of infn, September 25, 1989, Till Kirsten, private collection, DA GMPG, BC 600004).
Wolfgang Hampel et al.: Results of Ultra-Low Level 71Ge Counting for Application in the Neutrino Experiment at the Gran Sasso Underground Physics Laboratory. In: F.C. Jones, J. Adams, and G.M. Mason (eds.): Proceedings from the 19th International Cosmic Ray Conference, La Jolla, USA, August 11–23, 1985. NASA. Goddard Space Flight Center 1985, 422–425. http://adsabs.harvard.edu/abs/1985ICRC....5..422H. Last accessed 10/31/2018.
D. N. Spergel, and W. H. Press: Effect of Hypothetical, Weakly Interacting, Massive Particles on Energy Transport in the Solar Interior. The Astrophysical Journal 294 (1985), 663–673. doi:
A detailed schedule of the topics discussed at the first and second meetings can be found in the document “AGENDA—FIRST GALLEX-MEETING, February 19–21, 1985” and in “2nd gallex-meeting, October 10–11, 1985, in Heidelberg, MPI Kernphysik.” See list of all the meetings from 1985 to 1997. A proposal to the Bundesministerium für Forschung und Technologie/German Federal Ministry for Research and Technology (see a copy of the proposal “Messung der Sonnenneutrinos mit einem Gallium-Detektor”) had been presented in December 1984 (Till Kirsten, private collection, DA GMPG, BC 600004).
Mössbauer’s discovery of the effect later named after him, which inaugurated a new tool for precision spectroscopy, had been the last success of Bothe’s Institute for Physics at the MPI for Medical Research, which he led until his death in 1957. Rudolf L. Mössbauer: Kernresonanzabsorption von Gammastrahlung in Ir191. Die Naturwissenschaften 45/22 (1958), 538–539. doi:
For a snapshot of the gallex Collaboration at the beginning of the 1990s, see P. Anselmann et al.: Solar Neutrinos Observed by gallex at Gran Sasso. Physics Letters B 285/4 (1992), 376–389. doi:
The then President of Infn, Nicola Cabibbo, was instrumental in the approval process, supporting solar neutrinos as a major research area in the Gran Sasso National Laboratory. An official letter from Cabibbo to Kirsten was sent on July 30, 1985, confirming that the gallex experiment had been approved by the Gran Sasso Scientific Committee, given the great importance that infn attached to the success of gallex (Till Kirsten, private collection, DA GMPG, BC 600004).
Another experiment being installed there, and operational since 1989, was the Macro (Monopole, Astrophysics, and Cosmic Ray Observatory) large-area detector, designed to search for super heavy magnetic monopoles (cosmic relics from the early Universe predicted by Grand Unified Theories), high-energy gamma and neutrino cosmic sources, and, more in general “rare exotic phenomena in the cosmic radiation.” The Macro collaboration included researchers from 10 Italian universities and laboratories, 6 US universities and one Moroccan university. The MACRO Collaboration: MACRO, a Large-Area Detector at the Gran Sasso Laboratory. Il Nuovo Cimento C 9/2 (1986), 281–292, 282. doi:
See “Memorandum of Understanding,” signed by Gerhart Friedlander (bnl) and Till Kirsten on May 24 1986 (Till Kirsten, private collection, Da Gmpg, Bc 600004).
Document “infn-lngs. gallex Solar Neutrino Experiment. Inauguration Ceremony. November 30, 1990” (Till Kirsten, private collection, Da Gmpg, Bc 600004).
Till A. Kirsten: Das GALLEX-Sonnenneutrino-Experiment. Mitteilungen der Astronomischen Gesellschaft 68 (1987), 59–70. https://ui.adsabs.harvard.edu/#abs/1987MitAG..68...59K. Last accessed 4/30/2018. Theoretical related issues also connected to the nature of neutrinos were also investigated at the Institute for Nuclear Physics: K. Grotz, H. V. Klapdor, and J. Metzinger: Microscopic Calculation of Neutrino Capture Rates in 69,71Ga and the Detection of Solar and Galactic Neutrinos. Physical Review C 33/4 (1986), 1263–1269. doi:
Christian Spiering: Towards High-Energy Neutrino Astronomy. A Historical Review. The European Physical Journal H 37/3 (2012), 515–565. doi:
The inauguration ceremony took place on November 30, 1990. In his presentation, in the name of the whole collaboration, Kirsten emphasized the many meanings that gallex—which was expected to be a really important scientific adventure—had for him. He mentioned three of them: “[…] it is not a formal body but a vivid association of scientists, engineers, and technicians, individuals which are driven by their curiosity to get a deeper insight into the fundamental laws of nature […] To achieve this goal, they are forced to give up some of their individualism for the common goal, and they do so voluntarily in respect for each other.” As second goal, Kirsten mentioned the importance for Europe of such a collaboration and then outlined that “gallex is a challenge and by no means a ‘simple’ experiment. It is not a technocratic exercise where you plan, construct, push the button, and get a programmed output. Instead, it extends into new frontiers of experimental techniques, like separating single atoms of a reactive chemical element out of a reservoir of
Anselmann et al., Solar Neutrinos, 1992, 376–389. Anselmann et al., GALLEX Determination, 1992, 390–397. The most important results from gallex were outlined in the 1993 Annual Report: Generalverwaltung der Max-Planck Gesellschaft (ed.): Max-Planck-Gesellschaft Jahrbuch 1993. Göttingen: Vandenhoeck & Ruprecht 1993, 437–444. See also material related to the gallex project in AMPG, II. Abt. Rep. 66, No. 1990, 1991, 1992, 1993, 1994.
Kirsten, Radiochemical, 2000, 71–81, 55.
Kirsten, Radiochemical, 2000, 71–81, 55.
Till A. Kirsten: Solar Neutrino Experiments: Results and Implications. Reviews of Modern Physics 71/4 (1999), 1213–1232. doi:
David N. Schramm, and Xiangdong Shi: Solar Neutrinos: Solar Physics and Neutrino Physics. Nuclear Physics B—Proceedings Supplements 35 (1994), 321–333. doi:
V. V. Kuzminov: The Baksan Neutrino Observatory. The European Physical Journal Plus 127/9 (2012), 113. doi:
Early outcome from sage showed a strong discrepancy with the average value found by gallex. A. I. Abazov et al.: First Results from the Soviet-American Gallium Experiment. Nuclear Physics B Proceedings Supplements 19 (1991), 84–93. doi:
There was actually another explanation: neutrino decay. But nobody really believed in it, especially since observation of neutrinos from Supernova 1987A spoke against it. We are grateful to Christian Spiering for this comment. At that time, discussions on what appeared to be “the last hope for an astrophysical solution to the solar neutrino problem” led to the conclusion that the standard neutrino solution to the solar neutrino problem was strongly disfavored (or excluded) in favor of a non-standard neutrino, i.e., one with properties beyond the Standard Theory; and thus, “the last hope turned out to be a no-hope case.” Vadim L. Berezinsky, G. Fiorentini, and M. Lissia: LAST HOPE for an Astrophysical Solution to the Solar Neutrino Problem. Physics Letters B 365/1 (1996), 185–192, 191. doi:
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Electron neutrinos and muon neutrinos are produced mainly by the decay chains of charged pions produced in interactions between cosmic rays and atmospheric nuclei. The observed flux ratio of muon neutrinos (+ muon antineutrinos) and electron neutrinos (+ electron antineutrinos) showed a deficit of muon-neutrino events. In 2001, very precise information was provided by results based on more than 18000 solar neutrino events, increasing the number of previously reported events by an order of magnitude. Super-Kamiokande Collaboration et al.: Solar 8B and Hep Neutrino Measurements from 1258 Days of Super-Kamiokande Data. Physical Review Letters 86/25 (2001), 5651–5655. doi:
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The Nobel Prize in Physics 2002. Nobelprize.org. Nobel Media AB 2014.: The Nobel Prize in Physics 2002. https://www.nobelprize.org/nobel_prizes/physics/laureates/2002/. Last accessed 3/31/2018. Davis and Koshiba both shared the prize with Riccardo Giacconi, who had pioneered X-ray astronomy in the early 1960s together with Bruno Rossi. The latter, who had been his mentor, had unfortunately passed away in 1993.
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As clearly explained by Frank Close, Pontecorvo, who devoted much of his later career to studying the neutrino, for which he was given the sobriquet “Mr. Neutrino,” would have deserved at least three Nobel Prizes related to neutrinos. Close, Half-Life, 2015, 7.
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The nuclear double-beta decay research was expanding its broad potential—already explored with the Heidelberg–Moscow experiment—into the search for new physics beyond the Standard Theory of particle physics. Two new experimental proposals could increase the sensitivity for double-beta decay and dark matter search: the underground setup genius (GErmanium in liquid NItrogen Underground Setup) and the hdms (Heidelberg Dark Matter Search) experiment. In addition, the technology of producing and using enriched high purity germanium detectors produced for the Heidelberg–Moscow experiment found application in high-resolution gamma-ray astrophysics using balloons or satellites. See, for example, S. I. Svertilov et al.: Hard X-Ray and Gamma-Ray Spectrometer of High Resolution and Sensitivity on Board the International Space Station (ISS). Advances in Space Research 25/3–4 (2000), 901–904. doi:
This new challenging experiment, a large-volume liquid scintillator detector, viewed by about 2000 photomultipliers, characterized by a very low background level due to an unprecedented radio-purity of the detector material (liquid scintillator), would make it possible to study the entire spectrum of solar neutrinos from very low energies.
The experiment’s goal was the direct measurement of the flux of beryllium-7 solar neutrinos of all flavors via neutrino-electron scattering in an ultra-pure scintillation liquid. See also a description of the experiment in Max-Planck-Gesellschaft zur Förderung der Wissenschaften (ed.): Max-Planck-Gesellschaft Jahrbuch 2001. Göttingen: Vandenhoeck & Ruprecht 2001, 515–520.
Member countries (including several different institutions) were Germany, Russia, France, Italy, the US, the UK, and Poland.
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Its expected interaction rates were a few counts per day in a 100-ton target, while the main background of cosmogenic and radiogenic origin is one order of magnitude more intense. Detection of neutrinos from the CNO cycle has important implications in astrophysics, as it provides direct evidence for the nuclear process that is believed to fuel massive stars, with more than 1.5 solar masses.
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See chapter with this name in Kip S. Thorne: Black Holes & Time Warps. Einstein’s Outrageous Legacy. New York, NY: Norton 1994, 258–299. Thorne actually identified the ‘golden age’ of general relativity with the explosion of interest in black hole research.
The term ‘black hole’ began to circulate and was officially launched by John Wheeler in 1968: John Archibald Wheeler: Our Universe. The Known and the Unknown. American Scientist 56/1 (1968), 1–20. However, it is not clear who used it first, although it appears that it circulated as early as September 1963, during the first Texas Conference, as reported in the January 24, 1964, issue of Life magazine by Al Rosenfeld, Life’s science editor, who had heard the term mentioned at the symposium. The story is told in Tom Siegfried: 50 Years Later, It’s Hard to Say Who Named Black Holes. Science News (2013). https://www.sciencenews.org/blog/context/50-years-later-it’s-hard-say-who-named-black-holes. Last accessed 7/19/2018.
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See, for example, a letter by Kippenhahn to Biermann, referring to white dwarfs, collapsing stars, binary systems, and mentioning the problem that for the study of such non-linear dynamical effects one needed a powerful computing machine and that they had further perfected their own program on the evolution of stars, being at the forefront compared with other groups (Kippenhahn to Biermann, July 10, 1968, AMPG, III. Abt., ZA 1, No. 18).
Ludwig Biermann, and Reimar Lüst: Jahresberichte astronomischer Institute für 1964, München Max-Planck-Institut für Physik und Astrophysik, Institute für Astrophysik und extraterrestrische Physik. Mitteilungen der Astronomischen Gesellschaft 18 (1965), 57–66, 61. https://ui.adsabs.harvard.edu/abs/1965MitAG..18...57. Last accessed 10/30/2018.
Peter Kafka later recalled that at the time he did his ‘Diplom’ in Physics, Arnulf Schlüter, who had become Director of the Institute for Plasma Physics, had developed an interest in general relativity and asked him to work on this topic for his dissertation. Quasars were discovered at that time and so it became quite clear that general relativity would play a growing role in astrophysics and in cosmology; and as an expert on such topics, Kafka got a position at the Max Planck Institute for Astrophysics. Peter Kafka: interview by Michael Langer, March 27, 1999. Live-Gespräch-Sendung “Zwischentöne,” Deutschlandfunk, http://www.gegen-den-untergang.de/zwischentoene1999.html. Last accessed 11/4/2018.
Ludwig Biermann, and Reimar Lüst: Jahresberichte astronomischer Institute für 1965, München Max-Planck-Institut für Physik und Astrophysik, Institute für Astrophysik und extraterrestrische Physik. Mitteilungen der Astronomischen Gesellschaft 20 (1966), 67–79, 71. http://adsabs.harvard.edu/abs/1966MitAG..20...66. Last accessed 10/30/2018.
The static Universe proposed by Fred Hoyle and, independently, by Hermann Bondi and Thomas Gold, rejected the idea of an initial singularity, maintaining that a steady-state Universe could be compatible with the drifting apart of galaxies if new matter (approximately one hydrogen atom per cubic kilometer per year) were continuously generated in the intergalactic space. Since the mid-1950s, complete, new catalogues of radio sources had shown that the number of intense sources increased with distance, while, according to the steady-state theory, they were expected to be uniformly distributed throughout the Universe. Apparently the most distant objects of the Universe, quasars, had an impact in cosmology. If the high redshift of observed quasars was of cosmological origin, it meant that they were at distances such that the Universe was much younger than it is now, when the radio waves were emitted. This implied that galaxies produced more radio waves in the past, and thus began to call attention to the conflict between the Big Bang as a theory of cosmic expansion from a hot early Universe and the steady-state cosmology, according to which the observable Universe is basically the same on the large scale at any given time, a view called the “Perfect Cosmological Principle.” An intense controversy developed between proponents of different theories of the Universe, as discussed in Helge Kragh: Cosmology and Controversy. The Historical Development of Two Theories of the Universe. Princeton, New Jersey: Princeton University Press 1996.
Peter Kafka: How to Count Quasars. Nature 213/5074 (1967), 346–350. doi:
Blum, Lalli, and Renn, Gravitational Waves, 2018, 534–543, 534.
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Weber himself estimated the expected fluxes of gravitational radiation from such objects. Joseph Weber: Gravitational Radiation from the Pulsars. Physical Review Letters 21/6 (1968), 395–396. doi:
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Biermann to Weber, March 19, 1970, AMPG, III. Abt., ZA 1, No. 48. A telephone call was made between Aspen, Colorado, where Weber spent part of his time (as acknowledged in his articles), and Boulder, Colorado, where Biermann had spent the months of July and August in 1969, giving lectures. Ludwig Biermann, and Reimar Lüst: Max-Planck-Institut für Physik und Astrophysik. Institut für Astrophysik und Institut für extraterrestrische Physik. Mitteilungen der Astronomischen Gesellschaft 28 (1970), 79–105, 79. http://adsabs.harvard.edu/abs/1970MitAG..28...79B. Last accessed 10/30/2018.
These research activities, together with Biermann’s studies on some characteristics of the density of pulsars were announced in the new section of the Annual Report entitled “Relativistische Astrophysik, Quasare und Pulsare.” Biermann, and Lüst, Report 1969, 1970, 79–105, 86–87. See also, Peter Kafka: Discussion of Possible Sources of Gravitational Radiation. Mitteilungen Der Astronomischen Gesellschaft 27 (1969), 134–138, 138. https://ui.adsabs.harvard.edu/#abs/1969MitAG..27..134K. Last accessed 11/3/2017.
See minutes related to the meeting of June 9, 1969, of the search commission for Heisenberg’s successor in AMPG, II. Abt., Rep. 62, No. 437, Fol. 273.
Jordan had even favored Ehlers as his own successor in Hamburg (see related correspondence between Jordan and Heisenberg during winter 1967–68 in Heisenberg’s papers, AMPG, III. Abt. Rep. 93, No. 1745). Ehlers had emerged as a candidate successor to Heisenberg in spring 1969, when the search committee had not yet decided whether a theoretical or an experimental physicist should lead the Institute for Physics after Heisenberg’s retirement. During discussions about the possibility of appointing a theoretician, in particular an expert in general relativity, it was also mentioned that Einstein’s theory had somewhat receded into the background at universities in Germany, something that Jordan had pointed out on several occasions. In May–June 1969, Gentner (who presided over the commission tasked to find Heisenberg’s successor) and Jordan exchanged correspondence on this question, and Ehlers’s name was definitely the most favored, according to the opinion of several relativists (Gentner to Jordan, May 13, 1969, and Jordan to Gentner, May 19 and June 2, 1969, AMPG, II. Abt., Rep. 62, No. 437, Fol. 42–59).
Bruce Allen et al.: Jürgen Ehlers. 29.12.1929-20.05.2008. Jahresbericht der Max-Planck-Gesellschaft. Annual Report 2008. Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. 2009, 24–26.
Jürgen Ehlers: Probleme und Ergebnisse der modernen Kosmologie. Mitteilungen der Astronomischen Gesellschaft 27 (1969), 73–86. http://adsabs.harvard.edu/abs/1969MitAG..27...73E. Last accessed 4/25/2018.
Minutes of the 15th meeting of the Board of Trustees (Kuratorium) of the Max Planck Institute for Physics and Astrophysics, 17.03.1970, AMPG, II. Abt., Rep. 66, No. 3069.
L. Biermann and W. Heisenberg to Adolf Butenandt, October 31, 1969, AMPG, III Abt., Rep. 93, No. 1667. On November 7, a commission to appoint Ehlers as a Scientific Member of the Institute for Astrophysics was formed. The same commission was also involved in Heisenberg’s succession (CPTS meeting minutes of 07.11.1969, AMPG, II. Abt., Rep. 62, No. 1757). Documents clearly show how both Heisenberg and Biermann’s scientific interests would benefit from having Ehlers at the institute in order to build a bridge between unified field theory and gravitation theory, also in connection with new related interests in astrophysics and the idea of creating a working group on gravitational wave experiments. This would thus also create a better relationship between theory and experiment (minutes of the 15th meeting of the Board of Trustees (Kuratorium) of the Max Planck Institute for Physics and Astrophysics, 17.03.1970, AMPG, II. Abt., Rep. 66, No. 3069). On February 9, 1971, during the meeting of the CPT Section of the Scientific Council, it was communicated that Ehlers had accepted, and that he would take up his position on June 1, 1971 (AMPG, II. Abt., Rep. 62, No. 1761). On March 3, the Senate confirmed the appointment, remarking that Ehlers’ visit to Munich had shown that his presence would be of the greatest importance for both the Institute for Physics (Hans-Peter Dürr’s theoretical group) and the Institute for Astrophysics, as well as for the Institute for Extraterrestrial Physics.
See Biermann to Gerhard Börner, November 26, 1970, AMPG, III. Abt., ZA 1, No. 20. Hermann Ulrich Schmidt, who was spending some time at the National Solar Observatory at Sacramento Peak in New Mexico, wrote to Biermann about a discussion he was having with Ehlers, Weber, and Remo Ruffini about beginning a gravitational wave experiment in Munich. See also Biermann’s answer (Schmidt to Biermann, November 26 1970, and Biermann to Schmidt, December 8, 1970, AMPG, III. Abt., ZA 1, No. 21).
They specified that the decision to repeat Weber’s gravitational-wave experiment had been taken because of both its great astrophysical significance and the still pending difficulties in evaluating Weber’s findings. The prerequisites for this were particularly favorable at the Institute, as the necessary engineering and electronic experiences were available at the Numerical Calculators Division, while the local astrophysicists would be able to handle the theory and the statistical problems, and the addition of Ehlers would guarantee the close connection with the general theory of relativity. It was further emphasized how Weber’s detector could be improved. Ludwig Biermann, and Reimar Lüst: Max-Planck-Institut für Physik und Astrophysik. Institut für Astrophysik und Institut für extraterrestrische Physik. Mitteilungen der Astronomischen Gesellschaft 31 (1972), 323–350, 326. https://ui.adsabs.harvard.edu/#abs/1972MitAG..31..323B. Last accessed 4/29/2018.
Heinz Billing et al.: Results of the Munich-Frascati Gravitational-Wave Experiment. Nuovo Cimento, Lettere 12/4 (1975), 111–116, 111. doi:
Donato Bramanti, and Karl Maischberger: Construction and Operation of a Weber-Type Gravitational-Wave Detector and of a Divided-Bar Prototype. Nuovo Cimento, Lettere 4/17 (1972), 1007–1013. doi:
Walter Winkler: History of Physics (19). Fundamental Research for the Development of Gravitational Wave Detectors in Germany. SPG Mitteilungen 54 (2018), 14–18, 15. https://www.sps.ch/fileadmin/doc/Mitteilungen/Mitteilungen.54.pdf. Last accessed 5/30/2018.
Both Munich and Frascati built detectors as close to Weber’s as possible, including a close match with his resonant frequency of 1660 Hz. Donato Bramanti, Karl Maischberger, and Donald Parkinson: Optimization and Data Analysis of the Frascati Gravitational-Wave Detector. Lettere al Nuovo Cimento 7/14 (1973), 665–670. doi:
Peter Kafka: On the Evaluation of the Munich-Frascati Weber-Type Experiment. In: Y. Choquet-Bruhat (ed.): Ondes et Radiations Gravitationelles. International Conference, Paris, France, June 18–22, 1973. Paris: Centre National de la Recherche Scientifique 1974, 181–200.
The group in Glasgow (James Hough, Jon R. Pugh, Roger Bland), was at that time led by Ronald W. P. Drever, who later became a member of the team initially running the ligo project, after working for some time in parallel on Glasgow and US projects. On early work in Glasgow and interaction with German scientists, see Ronald W. P. Drever: interview by Shirley K. Cohen, June 1997. Transcript, Caltech Archives, http://resolver.caltech.edu/CaltechOH:OH_Drever_R. Last accessed 4/19/2021. Drever mentioned their friendly competition: “And we felt a bit envious, because they seemed to have more people, more money, more of everything. And everything they built was so beautifully built, and ours was kind of thrown together […] they were very friendly. We got on very well with them […] I wouldn’t say we were jealous, but we envied them.”
James L. Levine, and Richard L. Garwin: Absence of Gravity-Wave Signals in a Bar at 1695 Hz. Physical Review Letters 31/3 (1973), 173–176. doi:
Hiromasa Hirakawa, and Kazumichi Narihara: Search for Gravitational Radiation at 145 Hz. Physical Review Letters 35/6 (1975), 330–334. doi:
Vladimir B. Braginskii, Ya B. Zel’Dovich, and V. N. Rudenko: Reception of Gravitational Radiation of Extraterrestrial Origin. Soviet Journal of Experimental and Theoretical Physics Letters 10 (1969), 280–283. https://ui.adsabs.harvard.edu/#abs/1969JETPL..10..280B. Last accessed 10/25/2018.
Vladimir B. Braginskii et al.: Search for Gravitational Radiation of Extraterrestrial Origin. Journal of Experimental and Theoretical Physics Letters 16/3 (1972), 108–112. http://www.jetpletters.ac.ru/ps/1759/article_26757.shtml. Last accessed 6/12/2018. In December 1974, also the Glasgow group reported a negative result. James Hough et al.: Search for Continuous Gravitational Radiation. Nature 254/5500 (1975), 498–501, 501. doi:
Billing et al., Results, 1975, 111–116. The Frascati-Munich group claimed to have “set the lowest limits so far obtained for the rate of incoming short gravitational pulses stronger than a few times
Peter Kafka, and Lise Schnupp: Final Result of the Munich-Frascati Gravitational Radiation Experiment. Astronomy and Astrophysics 70 (1978), 97–103, 97. http://adsabs.harvard.edu/abs/1978A&A....70...97K. Last accessed 11/18/2017.
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A first obvious approach involved using larger bars of aluminum—or of new types of material—and cooling them down to very low temperature (2 K or less, near the absolute zero) to reduce thermal noise, measuring their oscillations by totally new types of mechanical/electrical transducers. Developments in this direction had been proceeding for several years at Stanford University, at Louisiana State University, and at Sapienza University in Rome. Another very challenging proposal from a technical point of view came from Braginsky’s group at Moscow University. Instead of bars, they were experimenting the possibility of building relatively small gravitational wave detectors using single sapphire crystals weighing only a few kilograms, which were supposed to be very efficient in discriminating between thermal noise and gravitational wave pulses.
In a Michelson laser interferometer, a laser beam will be split into two identical beams by a partially reflecting mirror, with one beam reflected at 90 degrees from the first, but preserving the original frequency. Each beam travels down an arm of the interferometer and both are reflected back and merged into a single beam before arriving at the photodetector. As long as the arms do not change length while the beams are traveling, light waves will stay perfectly aligned, canceling out in the recombined beam (totally destructive interference). Gravitational waves cause space to stretch in one direction and squeeze in a perpendicular direction simultaneously. For this reason, one arm of an interferometer will lengthen, while the other one shrinks, and constructive interference pattern will be observed in the photodetector. If one arm gets longer than the other, one laser beam will take longer to return, creating a phase difference between the two beams which will affect the interference pattern, showing that something happened to change the distance traveled by one or both laser beams. The interference pattern can be used to measure precisely how much change in length occurred and to extract information. The longer the arms of an interferometer, the smaller the measurements they can make. But this is an incredibly tiny effect, as gravitational waves, for example, can just change the length of a 4 km arm interferometer by 1/1000th the width of a proton, that is,
G. E. Moss, L. R. Miller, and R. L. Forward: Photon-Noise-Limited Laser Transducer for Gravitational Antenna. Applied Optics 10/11 (1971), 2495–2498. doi:
Harry Collins: Gravity’s Shadow. The Search for Gravitational Waves. Chicago, IL: University of Chicago Press 2004, 274.
Collins, Gravity’s Shadow, 2004, 276–277. See also, Peter Kafka: interview by Harry Collins, available at http://sites.cardiff.ac.uk/harrycollins/webquote/. Last accessed 6/10/2018. According to Collins’s interview with Robert L. Forward (also available at the same URL), Maischberger at ESRIN, in Italy, was involved as a reviewer, too, and he immediately thought of carrying out the interferometric experiment himself.
Peter Kafka: interview by Harry Collins, available at http://sites.cardiff.ac.uk/harrycollins/webquote/. Last accessed 5/5/2019.
Walter Winkler, personal communication with the authors, March 23, 2019.
Russel A. Hulse, and Joseph H. Taylor: Discovery of a Pulsar in a Binary System. Astrophysical Journal 195 (1975), L51–L53. doi:
The group comprised Billing, Kafka, Maischberger, Schnupp, and Winkler. Their Weber-type coincidence experiment had been run between July 1973 and February 1976. Kafka, and Schnupp, Final Result of the Munich-Frascati Gravitational Radiation Experiment, 1978, 97–103, 103.
Jürgen Ehlers, J. J. Perry, and M. Walker (eds.): 9th Texas Symposium on Relativistic Astrophysics. 14–19 December 1978, Munich. New York, NY: New York Academy of Sciences 1980.
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The fate of Billing’s group, still named “Numerische Rechenmaschinen,” came under discussion from March 1977 (CPTS meeting minutes of 08.03.1977, AMPG, II. Abt., Rep. 62, No. 1780). The committee specified that the preliminary gravitational wave experiment was of fundamental importance and should be continued (CPTS meeting minutes of 22.06.1977, 01.02.1978, AMPG, II. Abt., Rep. 62, No. 1781, 1783). During the meeting of the CPT section on October 29, 1980, it was reported that “Mr. Walther agreed to take the group into the Institute of Quantum Optics” (which would be founded soon after), CPTS meeting minutes of 29.10.1980, AMPG, II. Abt., Rep. 62, No. 1791. Reimar Lüst, then President of the Max-Planck Society, “was obviously ready to support the research after Billing’s retirement in 1982.” Walter Winkler, personal communication with the authors, April 4, 2019.
Biermann became Emeritus on March 31, 1975. In the Annual Report, signed by his successor Kippenhahn, his instrumental role during almost 30 years at the Institute for Astrophysics in opening and promoting new research fields—ultimately leading to the foundation of new institutes—was emphasized, and the dynamic effect of his establishment of the Department for Gravitation Theory and Relativistic Astrophysics and promotion of the Munich gravitational experiment was recalled. Rudolf Kippenhahn, and Klaus Pinkau: Max-Planck-Institut für Physik und Astrophysik. Institut für Astrophysik und Institut für extraterrestrische Physik. I. Institut für Astrophysik; II. Institut für extraterrestrische Physik. Reports 1975. Mitteilungen der Astronomischen Gesellschaft 39 (1976), 112–134, 112. http://adsabs.harvard.edu/abs/1976MitAG..39..112K. Last accessed 4/29/2018.
Albrecht Rüdiger et al.: The Garching 30-Meter Prototype and Plans for a Large Gravitational Wave Detector. In: Melville P. Ulmer (ed.): 13th Texas Symposium on Relativistic Astrophysics. Chicago, December 14–19, 1986. Singapore: World Scientific 1987, 20–22. https://ui.adsabs.harvard.edu/1987txra.symp...20R/abstract. Last accessed 10/29/2019.
In fact, the roots of the Max Planck Institute of Quantum Optics dated back to the establishment on January 1, 1976, of a Laser Research Group at the Max Planck Institute for Plasma Physics (IPP), a result of an agreement between the German Federal Ministry for Research and Technology, as it was called at the time, and the Max Planck Society. The aim of the group was to work on the development of high-power lasers and their application to plasma physics, chemistry, spectroscopy, and other fields. This issue was discussed at the meetings of the Max Planck Society’s ‘Senatsausschuss für Forschungspolitik und Forschungsplanung’ (Senate committee on research policy and research planning) in 1975 (see copies of the minutes in AMPG, III. Abt., Rep. 68 A, No. 151). The committee discussing the future of this group and its transformation into the Institute of Quantum Optics (with Herbert Walther, Karl-Ludwig Kompa, and Siegbert Witkowski as Directors of the three departments: Laser Physics, Laser Chemistry, and Laser Plasmas), was formed on June 14, 1978, and during the CPT Section meeting of May 5, 1979, the final formal decision was unanimously taken (CPTS meeting minutes of 14.06.1978, 30.01.1979, 09.05.1979, AMPG, II. Abt., Rep. 62, No. 1784, 1786, 1787). In 1981, the research group was given separate status as the Institute of Quantum Optics and the Research Group on Gravitational Waves became involved with the development of laser interferometers. The group at IPP initially had 46 members and quickly grew to 105, so that the space made available by IPP, including additional barracks, soon became too small. In 1986, when the institute moved to a dedicated new building, there were 184 staff members. See preface and Section 3.2.10, entitled “Messung von Gravitationswellen—eine Revolution in der Astronomie?” in Max-Planck-Gesellschaft: Max-Planck-Institut für Quantenoptik Garching b. München. Max-Planck-Gesellschaft. Berichte und Mitteilungen 6/86 (1986), 1–129.
A description of the laser interferometric project related to that stage of activities can be found in Heinz Billing et al.: The Munich Gravitational Wave Detector Using Laser Interferometry. In: Pierre Meystre, and M.O. Scully (eds.): Quantum Optics, Experimental Gravity, and Measurement Theory. Conference “Quantum Optics and Experimental General Relativity”, held August 16–29, 1981 in Bad Windsheim, Federal Republic of Germany. Boston, MA: Springer 1983, 525–566. doi:
Reiner Weiss: interview by Shirley K. Cohen, May 10, 2000. Transcript, Caltech Archives, https://resolver.caltech.edu/CaltechOH:OH_Weiss_R. Last accessed 1/19/2019.
Carlo Bradaschia et al.: The VIRGO Project: A Wide Band Antenna for Gravitational Wave Detection. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 289/3 (1990), 518–525. doi:
Adele La Rana, and Leopoldo Milano: The Early History of Gravitational Wave Detection in Italy. From the First Resonant Bars to the Beginning of the Virgo Collaboration. In: Salvatore Esposito (ed.): Società Italiana Degli Storici Della Fisica e Dell’Astronomia. Atti Del
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Convegno Annuale / Proceedings of the 36th Annual Conference. Napoli 2016. Pavia: Pavia University Press 2017, 185–196. doi:
N. A. Robertson et al.: Passive and Active Seismic Isolation for Gravitational Radiation Detectors and Other Instruments. Journal of Physics E: Scientific Instruments 15/10 (1982), 1101–1105. doi:
James Hough et al.: The Development of Long Baseline Gravitational Radiation Detectors at Glasgow University. Gravitation, Geometry and Relativistic Physics. Proceedings of the “Journées Relativistes” Held at Aussois, France, May 2–5, 1984. Berlin: Springer 1984, 204–212. doi:
The concept of the large antenna was described in Karl Maischberger et al.: Vorschlag Zum Bau Eines Grossen Laser-Interferometers Zur Messung von Gravitationswellen. MPQ 96. Max-Planck-Institut für Quantenoptik 1985. Plans for the large detector were described also in Rüdiger et al., The Garching 30-Meter Prototype and Plans for a Large Gravitational Wave Detector, 1987, 20–22. Karl Maischberger et al.: Status of the Garching 30-Meter Prototype for a Large Gravitational Wave Detector. In: Peter F. Michelson, Hu En-ke, and Guido Pizzella (eds.): International Symposium on Experimental Gravitational Physics. 3–8 August 1987. Guangzhou, China. Singapore: World Scientific 1988, 316–321. https://ui.adsabs.harvard.edu/1988egp..conf..316M/abstract. Last accessed 10/29/2019.
Serious financial problems in the UK led to fierce competition with projects put forward by the astronomy/astrophysics community, also because at the end of the 1980s, many conventional astronomers were still very suspicious and did not consider gravitational waves as something worth funding. This also happened in the US, where astronomers and astrophysicists felt that the LIGO project was competing for their resources. On this question see Collins, Gravity’s Shadow, 2004, 500–504.
See AMPG, II. Dpt., Rep. 66, No. 3122, 3853, 3868.
James Hough et al.: Proposal for a Joint German-British Interferometric Gravitational Wave Detector. MPQ 147. Garching: Max-Planck-Institut für Quantenoptik 1989. http://eprints.gla.ac.uk/114852/7/114852.pdf. Last accessed 4/18/2018.
In Appendix A of the proposal, the two groups presented the results of a 100-hour period of coincident observation using the two prototypes at Garching and Glasgow (the 30 m and the 10 m arm length), which had been solicited by bmft and serc to show that such detectors could be operated in the production fashion by the two teams working together. It was the first time that two detectors had been run continuously in a data-taking mode, demonstrating the potential for long-term operation of laser interferometric detectors.
Funds for the preliminary investigations of the German project were provided by bmft and searches for a suitable site went on during the second half of the 1980s. See documents on the financing of a grant for “Voruntersuchungen für den Bau eines großen Laserinterferometers zur Messung von Gravitationswellen” starting in November 1987 and ending in December 1990, and for a second tranche covering the period from January 1, 1990, to December 31, 1992 (AMPG, II Abt., Rep. 66, No. 3853, 3868 and Rep. 68, No. 65).
Gerd Leuchs, who had led the Garching group from 1985 to 1989, moved to work in industry and later became Director of the Max Planck Institute for the Science of Light.
Karsten Danzmann et al.: The GEO—Project a Long-Baseline Laser Interferometer for the Detection of Gravitational Waves. In: J. Ehlers, and G. Schäfer (eds.): Relativistic Gravity Research With Emphasis on Experiments and Observations. Proceedings of the 81
WE-Heraeus-Seminar Held at the Physikzentrum, Bad Honnef, Germany 2–6 September 1991. Berlin: Springer 1992, 184–209. doi:
Karsten Danzmann: Laser Interferometric Gravitational Wave Detectors. In: R.J. Gleiser, C.N. Kozameh, and O.M. Moreschi (eds.): Proceedings. 13th International Conference on General Relativity and Gravitation. Cordoba, Argentina, June 28-July 4, 1992. Bristol: Institute of Physics 1993, 3–19, 19. https://inspirehep.net/record/348400. Last accessed 1/24/2019.
According to the Italians, “In spite of a few European meetings, and a good collaboration with German and British colleagues through 2 European grants, we were actually pushed in the direction of a bi-national project, rather than a joint two-detector European project, by the fact that the German team at Garching and the British team (mainly at Glasgow) were pushing their own national projects, and feared that the settlement of a European collaboration would delay their acceptation.” Carlo Bradaschia: VIRGO 20th Anniversary. H—The Gravitational Voice. Special Anniversary Edition (2009), 2–12, 6. http://www.ego-gw.it/public/hletter/doc/h_Special_Edition.pdf. Last accessed 1/29/2019. Both the tension existing between national ambitions and efforts towards international collaboration, and the problem of the lack of a really coordinated gravitational-wave community at a European level, played a negative role in this phase. On the question of why European leading groups in the field of gravitational-wave interferometry did not join forces to build a European observatory with at least two detectors at kilometer scale see Adele La Rana: The Origins of Virgo and the Emergence of the International Gravitational Wave Community. In: Alexander S. Blum, Roberto Lalli, and Jürgen Renn (eds.): The Renaissance of General Relativity in Context. Cham: Springer International Publishing 2020, 363–406. doi:
Joseph H. Taylor et al.: Experimental Constraints on Strong-Field Relativistic Gravity. Nature 355/6356 (1992), 132–136. doi:
Bernhard A. Sabel: Science Reunification in Germany: A Crash Program. Science 260/5115 (1993), 1753–1758. www.jstor.org/stable/2881355. Last accessed 11/17/2019.
In mid-1990, bmft formed a multidisciplinary commission led by the theoretical physicist Siegfried Großmann, which was supposed to make recommendations on fundamental research in Germany. A 124-page report was officially released in April 1992. Notwithstanding the special sympathy with which the commission regarded the large-scale experiment of a gravitational-wave detector “because of its novel scientific objectives,” also acknowledging its “special charm” due to its innovative approach to gravitation, “the smallness, possibly the still-undetectability, of the effect,” was also highlighted in the report. A copy of the pages in the commission’s report related to the project of building a detector for gravitational wave astronomy (pp. 76–78) can be found in the Archives of the Max Planck Society in Munich (Akten der Registratur und des Archivs der Max-Planck-Gesellschaft, MPI für Quantenoptik, Gravitationswellenexperiment III, 1991–1997, Aktenzeichen 18140907, Barcode 233163, Fol. 373–380). bessy ii, the upgraded new electron storage ring producing synchrotron radiation for materials research purposes, to be built in Berlin, was instead considered a “high priority” initiative, and already in July of that year the project got the “green light.” By contrast, fundamental particle physics did not receive favorable treatment, but a financial horizon which “should not enlarge, nor back off in the next few years.” As for gravitational waves, the commission had not recommended “immediate implementation, but swift prosecution with intensive scientific discussion” E. Dreisigacker: Grundlagenforschung—quo vadis? Großmann-Kommission legt Empfehlungen vor. Physikalische Blätter 48/5 (1992), 372–374, 374. doi:
Karsten Danzmann et al.: GEO
600. Proposal for a 600 m Laser-Interferometric Gravitational Wave Antenna. MPQ-190. Garching: Max-Planck-Institut für Quantenoptik 1994. Karsten Danzmann et al.: GEO 600—A 600m Laser Interferometric Gravitational Wave Antenna. In: Eugenio Coccia, Guido Pizzella, and Francesco Ronga (eds.). First Edoardo Amaldi Conference on Gravitational Wave Experiments. World Scientific 1995, 100–111. doi:
In the period 1993–2000, this strategy was supported by Hermann Schunck, then Director of the bmft and responsible for fundamental research, especially for Physics, who was able to channel “leftover” money from other projects that had not been able to spend it, in order to finance specific GEO600 needs justifiable as “standalone projects” such as vibration isolation, data acquisition, novel optics, laser stabilization, and novel vacuum system design (Hermann Schunck: Written interview by Adele La Rana, May 14, 2019, and personal communication with the authors, November 20, 2019).
Minutes of the 127th Senate meeting in Frankfurt am Main, 08.03.1991, AMPG, II. Abt., Rep 60, No. 127.SP, pp. 23–24.
The foundation of the Max Planck Institute for Gravitational Physics has its roots in the period of German unification and in the role and commitments of the Society in the new Federal States, which were discussed in a long report by the MPG President Hans F. Zacher during the CPT Section meeting of October 2, 1990 (CPTS meeting minutes of 10.02.1990 AMPG, II. Abt., Rep. 62, No. 1821). Following the recommendation of the German Council of Science and Humanities (Wissenschaftsrat) to close the Einstein-Laboratorium of the Academy of Sciences in Potsdam, the Max Planck Society was involved to advice about the establishment of a thematically new Albert Einstein Institute for Gravitational Physics. A memorandum for the reorganization of the Einstein-Laboratorium into an International Einstein Center had already been formulated by Hubert Goenner and Friedrich Hehl in February 1991 and announced in Hubert Goenner, and F.W. Hehl: Zur Gründung des Albert-Einstein-Instituts für Gravitationsphysik. Physikalische Blätter 47/10 (1991), 936–936. doi:
CPTS meeting minutes of 09.02.1995, 21.06.1995, 19/20.10.1995, 8/9.02.1996, AMPG, II. Abt., Rep. 62, No. 1834, 1835, 1836, 1837. Ehlers, Schutz, and Nicolai led research activities respectively in general relativity, relativistic astrophysics, and quantum gravity/unified theories. Numerical relativity and computer simulations, also related to collapsing relativistic binaries and their associated gravitational waves, were an active part of the research activity since the very beginning of AEI, see, for example, Bernard S. Schutz: Max-Planck-Institut Für Gravitationsphysik (Albert-Einstein-Institut). Mitteilungen Der Astronomischen Gesellschaft 82 (1999), 649–656. https://ui.adsabs.harvard.edu/abs/1999MitAG..82..649S/abstract. Last accessed 12/1/2019.
Joachim Trümper: Astronomy, Astrophysics and Cosmology in the Max Planck Society. In: André Heck (ed.): Organizations and Strategies in Astronomy. Dordrecht: Springer Netherlands 2004, 169–187.
Interview with Karsten Danzmann, March 29, 2018, Deutsche Physikalische Gesellschaft e. V., Stern-Gerlach-Medaille 2018, available at https://www.youtube.com/watch?v=tNTB74bFGuc. Last accessed 2/23/2020. Danzmann is considered part of the so-called ‘Welling Laser Family’ (Laserfamilie Welling), and just a few years before, Welling had consolidated the region’s footprint in this field with the establishment of the Laser Zentrum Hannover e.V. Gerd Liftin, and Jürgen Mlynek: Zum 80. Geburtstag von Herbert Welling. Physik Journal 8/10 (2009), 45. https://www.pro-physik.de/restricted-files/98106. Last accessed 4/14/2020. Liftin, and Mlynek, Herbert Willing, 2009, 45. For the latest account of his career, see “Grosses Verdienstkreuz für Professor Herbert Welling,” Presseinformation des Niedersächsischen Ministeriums für Wissenschaft, 31.8.2019, available at https://www.mwk.niedersachsen.de/startseite/aktuelles/presseinformationen/grosses-verdienstkreuz-fur-professor-dr-herbert-welling-180202.html. Last accessed 2/23/2020.
In June 2000, the founding of a center for gravitational-wave research was discussed during a meeting of the CPT Section. As stressed by Bernard Schutz, the whole operation would assure participation of the Max Planck Society, with a cutting-edge role, in the outstanding projects EURO and the laser-interferometric detectors ligo and lisa (the latter, the Laser Interferometer Space Antenna mission, a giant interferometer to be placed in space). A committee was formed to examine the whole plan (CPTS meeting minutes of 07.06.2000, 19/20.10.2000, 15/16.02.2001, AMPG, II. Abt., Rep. 62, No. 1851, 1852, 1853).
CPTS meeting minutes of 15/16.02.2001, 20.06.2001, 18/19.10.2001, AMPG, II. Abt., Rep. 62, No. 1853, 1854, 1855.
The ligo construction proposal was approved by the National Science Board in 1990, and in 1992 the ligo cooperative agreement for the management of ligo was signed by nsf and Caltech, while construction at the chosen sites Hanford and Livingston began between 1994 and 1995.
B. P. Abbott et al.: Detector Description and Performance for the First Coincidence Observations between LIGO and GEO. Nuclear Instruments and Methods in Physics Research Section A. Accelerators, Spectrometers, Detectors and Associated Equipment 517/1 (2004), 154–179. doi:
LIGO Scientific Collaboration & Virgo Collaboration et al.: Observation of Gravitational Waves from a Binary Black Hole Merger. Physical Review Letters 116/6 (2016), 061102. doi:
Blum, Lalli, and Renn, Gravitational Waves, 2018, 534–543.
As an example, we cite here a series of articles related to the first realization by the German group of innovative detector technologies which made Advanced ligo and Virgo so sensitive. Jun Mizuno et al.: Resonant Sideband Extraction: A New Configuration for Interferometric Gravitational Wave Detectors. Physics Letters A 175/5 (1993), 273–276. doi:
The development of highly accurate analytical and numerical models of gravitational-wave sources—in particular of gravitational waves that neutron stars or black holes generate in the final process of orbiting and colliding with each other—have allowed extraction of astrophysical and cosmological information from the observed waveforms. These waveform models are then implemented and employed in the continuing search for binary coalescences. To significantly increase the probability of identifying gravitational waves in ligo and Virgo data, the search for burst-like events in turn requires detailed knowledge of the expected signals from different sources and such search tools are sensitive because of systematic development in the algorithm and methods which can be used as templates to filter the data. Numerical relativity simulations with supercomputers not only play an important role in predicting gravitational waveforms that are used for gravitational wave detection, but allow, in general, exploration of general relativistic phenomena and other high-energy phenomena, such as gamma-ray bursts and stellar core collapse, or mass ejection with related nucleosynthesis processes.
B. P. Abbott et al.: Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger. GW170817 and GRB 170817A. The Astrophysical Journal Letters 848/2 (2017), L13. doi:
B. P. Abbott et al.: Multi-Messenger Observations of a Binary Neutron Star Merger. The Astrophysical Journal Letters 848/2 (2017), L12. doi:
In 1953, the seminal international cosmic-ray conference was held at Bagnères-de-Bigorre, at the foot of the Pyrenees dividing France from Spain, where the famous French high-altitude Pic du Midi Observatory was located. During this meeting, the word ‘hyperon’ was announced for the first time, and the ‘invasion’ of accelerators was explicitly mentioned. James W. Cronin: The 1953 Cosmic Ray Conference at Bagnères de Bigorre. The Birth of Sub Atomic Physics. The European Physical Journal H 36/2 (2011), 183–201, 197. doi:
Marcel Schein, William P. Jesse, and E. O. Wollan: The Nature of the Primary Cosmic Radiation and the Origin of the Mesotron. Physical Review 59/7 (1941), 615–615. doi:
The evolution of speculations and theories in cosmic ray origin up to the 1960s can be followed in a volume containing 76 of the most outstanding original papers in the field and including translations of some of the important Russian papers. Stephen Rosen (ed.): Selected Papers on Cosmic Ray Origin Theories. New York: Dover 1969. On the transition to cosmic-ray astrophysics in the 1950s, see Vitaly L. Ginzburg: On the Birth and Development of Cosmic Ray Astrophysics. In: Yataro Sekido, and Harry Elliot (eds.): Early History of Cosmic Ray Studies. Personal Reminiscences with Old Photographs. Dordrecht: Springer Netherlands 1985, 411–426. doi:
At a time when no electrons had yet been detected in the primary cosmic radiation, suggestions about radio waves being emitted by ultra-relativistic electrons moving in interstellar magnetic fields (see Chapters 1 and 3) was taken up and developed by Vitaly Ginzburg and Iosif Shklovskii, who showed that radio-astronomy could give quantitative information about cosmic rays in distant regions of the Universe, vastly improving the chance of understanding their origin. These deductions are outlined in a paper summarizing the results of Ginzburg’s earlier work. Vitaly L. Ginzburg: The Nature of Cosmic Radio Emission and the Origin of Cosmic Rays. Il Nuovo Cimento 3/1 (1956), 38–48. doi:
During the 1950s, Van Allen had launched rockets and ‘rockoons’—rockets carried aloft by balloons—to carry on cosmic ray experiments above the atmosphere, and from 1956, had proposed the use of satellites for cosmic-ray investigations. The launch of Explorer 1 and its scientific payload was the culmination of his work for the 1957–58 International Geophysical Year. Van Allen’s simple cosmic-ray experiment consisted of a Geiger-Müller counter and a tape recorder. Follow-up experiments on further Explorer missions established that there were two belts of radiation circling the Earth. See the first article summarizing results already presented on official occasions or in scientific reports: James A. Van Van Allen, and Louis A. Frank: Radiation Around the Earth to a Radial Distance of 107,400 Km. 4659. Nature 183/4659 (1959), 430–434. doi:
High-energy electrons and positrons interacting with magnetic fields or low-energy photons produce gamma rays by so-called leptonic processes, while hadronic processes can produce gamma rays through the decay of neutral mesons originated by high-energy protons and nuclei interacting with matter through nuclear interactions.
Interactions between GeV–TeV photons result in secondary relativistic electron-positron pairs which lose energy, emitting bremsstrahlung radiation producing new high-energy photons. Further pairs are produced in the cascading process, which in turn create new gamma rays and so on, until the energy of the impinging photon is redistributed as the shower develops in the atmosphere, until only low-energy electrons and positrons are produced, which lose energy only through ionization processes.
Helmut Rechenberg: Werner Heisenberg—Die Sprache der Atome. Leben und Wirken—Eine wissenschaftliche Biographie. Die “Fröhliche Wissenschaft” (Jugend bis Nobelpreis). Berlin: Springer-Verlag 2009, 12.4. Since the very early 1930s, Heisenberg had an exchange of correspondence on cosmic ray physics with Bruno Rossi, as discussed in Luisa Bonolis: International Scientific Cooperation During the 1930s. Bruno Rossi and the Development of the Status of Cosmic Rays into a Branch of Physics. Annals of Science 71/3 (2014), 355–409. doi:
See Bagge’s articles between 1939 and 1941 on nuclear processes in cosmic rays, closely related to his Habilitation dissertation on nuclear processes and heavy particles in cosmic rays (Chapter 1).
For research activities on cosmic rays during the war period, see Werner Heisenberg (ed.): Vorträge über Kosmische Strahlung. Berlin: Springer-Verlag 1943.
Werner Heisenberg: Kosmische Strahlungen und Atomphysik. Jahrbuch 1951 der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. Göttingen: Max-Planck-Gesellschaft zur Förderung der Wissenschaften 1951, 229–263. Werner Heisenberg (ed.): Kosmische Strahlung. Vorträge gehalten im Max-Planck-Institut für Physik Göttingen. 2nd ed. Berlin: Springer 1953. See Chapter 1 for Heisenberg’s publications in the field between the 1940s and early 1950s.
Bagge was also asked to prepare a chapter on the origin and nature of cosmic rays for the well-known series Ergebnisse der Exakten Naturwissenschaften. Erich Bagge: Ursprung und Eigenschaften der kosmischen Strahlung. In: S. Flügge, and F. Trendelenburg (eds.): Ergebnisse der Exakten Naturwissenschaften. Berlin: Springer-Verlag 1949, 202–262. doi:
See, for example, Erwin Schopper: Early History of Shock Waves in Heavy Ion Collisions (The Frankfurt Group). In: Walter Greiner, and Horst Stöcker (eds.): The Nuclear Equation of State: Part A: Discovery of Nuclear Shock Waves and the
EOS. Boston, MA: Springer US 1989, 427–446. doi:
As mentioned in Chapter 1, Klaus Gottstein was one of the protagonists of this evolution at Heisenberg’s Institute. Klaus Gottstein: interview by Luisa Bonolis and Juan-Andres Leon, Munich, September 7, 2017. DA GMPG, BC 601006.
See institute timeline with cosmic ray division: MPI für Kernphysik: 50 Jahre Max-Planck-Institut für Kernphysik. Von Kernphysik und Kosmochemie zu Quantendynamik und Astroteilchenphysik. Heidelberg: MPIK 2008, 8–9.
On Pinkau’s cosmic-ray activities in Kiel and his appointment at MPE, see Chapter 3. Work for his PhD dissertation was done in Bristol, based on research on electromagnetic air showers: Klaus Pinkau: Energy Determination of Electromagnetic Cascades in Nuclear Emulsions. The Philosophical Magazine: A Journal of Theoretical Experimental and Applied Physics 2/23 (1957), 1389–1392. doi:
Diederich Köhn, Klaus Pinkau, and Gerd Wibberrens: Gamma-Ray Spectrometer for Balloon Flights. Composition, Origin, and Prehistory, Proceedings from the 8th International Cosmic Ray Conference. Jaipur 1963, 203–205. http://adsabs.harvard.edu/abs/1963ICRC....3..203K. Last accessed 2/7/2020. Klaus Pinkau et al.: Balloon Experiment Using Spark Chambers and an Ionization Spectrometer. Proceedings of the 9th International Cosmic Ray Conference. London, UK. 1965, 821–823. http://adsabs.harvard.edu/abs/1965ICRC....2..821P. Last accessed 10/30/2018. Instead of using simple nuclear emulsions, the study of high-energy cascades both from charged particles and gamma rays was improved, combining spark chambers with a ionization spectrometer. Diederich Köhn, Klaus Pinkau, and Gerd Wibberenz: Messung des sekundären Gammaspektrums von 0,2 bis 2 GeV in der oberen Atmosphäre. Zeitschrift für Physik 193/3 (1966), 443–458. doi:
Reimar Lüst, and Klaus Pinkau: Theoretical Aspects of Celestial Gamma-Rays. In: J.C. Emming (ed.): Electromagnetic Radiation in Space. Proceedings of the Third ESRO Summer School in Space Physics, Held in Alpbach, Austria, from 19 July to 13 August, 1965. Dordrecht: Springer 1967, 231–248.
Philip Morrison: interview by Owen Gingerich, Session I, February 22, 2003, AIP, https://www.aip.org/history-programs/niels-bohr-library/oral-histories/30591-1. Last accessed 10/3/2019. Morrison acknowledged discussions with Cocconi—who in the meantime had moved to high-energy physics with accelerators, and would soon after become Director of cern’s Proton Synchrotron—and with the well-known cosmic-ray physicist Kenneth Greisen, an expert in cascade theory, who had earned his PhD at Cornell University in 1942 under Bruno Rossi.
Philip Morrison: On Gamma-Ray Astronomy. Il Nuovo Cimento 7/6 (1958), 858–865. doi:
Cosmic rays are the best-known example of non-thermal particle population, with a spectral energy distribution showing no characteristic temperature scale and with energies (up to
Laurence E. Peterson, and J. R. Winckler: Short Gamma-Ray Burst from a Solar Flare. Physical Review Letters 1/6 (1958), 205–206. doi:
Arthur Roberts: Development of the Spark Chamber. A Review. Review of Scientific Instruments 32/5 (1961), 482–485. doi:
A spark chamber is an instrument created for detecting electrically charged particles. When a charged particle travels through a stack of metal plates inside a sealed box filled with a noble gas it ionizes the gas and sparks form along the trajectory, if a high enough voltage is applied between each adjacent pair of plates. The effect becomes visible as a line of sparks. In the case of gamma rays, their presence is made visible through the electron-positron pair production. In this interaction, the incident gamma ray is completely annihilated, with its energy transferred to the pair, which is strongly beamed forward; and thus, the trajectory of the gamma ray can be inferred from the pair’s trajectory.
Danby et al., Observation of High-Energy Neutrino Reactions, 1962, 36–44.
For a historical review of gamma-ray astrophysics at MPE see Volker Schönfelder, and Jochen Greiner: Half-a-Century of Gamma-Ray Astrophysics at the Max-Planck Institute for Extraterrestrial Physics. 1. The European Physical Journal H 46/1 (2021), 27. doi:
William L. Kraushaar, and George W. Clark: Search for Primary Cosmic Gamma Rays with the Satellite Explorer XI. Physical Review Letters 8/3 (1962), 106–109. doi:
On this phase of research in gamma-ray astronomy, see Klaus Pinkau: History of Gamma-Ray Telescopes and Astronomy. Experimental Astronomy 25/1–3 (2009), 157–171. doi:
Klaus Pinkau at the Institute for Extraterrestrial Physics in Garching had been the Principal Investigator of this European mission, using as detector a spark chamber descendant from the ones pioneered while he was still in Kiel, which was able to determine the direction and energy of incoming gamma rays. The COS-B gamma-ray instrument was designed and built by the Caravane collaboration, a consortium of five institutes from Germany, the Netherlands, France, and Italy. The premises for such collaboration dates back to the years the young Pinkau spent in Great Britain with Powell’s group, which paved the way to his participation in European space activities.
COS-B long period of observation (1975–1982) created the premises for the preparation of new ESA missions like exosat, xmm-Newton, and the integral mission. Pinkau, The Early Days, 1996, 43–47. Pinkau, History of Gamma-Ray Telescopes and Astronomy, 2009, 157–171. About the history of COS-B satellite see John Krige, Arturo Russo, and Lorenza Sebesta: A History of the European Space Agency 1958–1987. The Story of
ESA, 1973 to 1987. Vol. 2. European Space Agency 2000. Chapter 7. Volker Schönfelder: The History of Gamma-Ray Astronomy. Astronomische Nachrichten 323/6 (2002), 524–529. doi:
Several point gamma-ray sources were observed, including four radio pulsars, a result considered particularly striking, since only one radio pulsar had been seen at either optical or X-ray frequencies. C. E. Fichtel: Gamma-Ray Astrophysics. Space Science Reviews 20/2 (1977), 191–234. doi:
The first of these gamma-ray bursts, lasting typically less than one minute and whose nature remained a mystery, was detected in 1967, but they were not reported in the scientific literature until 1973. Ray W. Klebesadel, Ian B. Strong, and Roy A. Olson: Observations of Gamma-Ray Bursts of Cosmic Origin. Astrophysical Journal 182 (1973), L85–L88. doi:
From observations, we know that the flux of cosmic rays, especially gamma rays, drops as a function of energy following approximately a smooth power law
For a detailed review of the history of studies on Extensive Air Showers, see Karl-Heinz Kampert, and Alan A. Watson: Extensive Air Showers and Ultra High-Energy Cosmic Rays. A Historical Review. The European Physical Journal H 37/3 (2012), 359–412. doi:
The shower is a mixture of nucleonic and electronic cascades, but the nuclear active particles constitute the backbone of the shower, and though relatively few in number these particles are the most energetic and keep supplying secondaries around the core of the shower. The most obvious differentiating characteristic of hadronic versus purely electromagnetic cascades is the presence of pions, and subsequent production of muons in pion decays as well as electromagnetic sub-showers. The muons are very penetrating: they interact so weakly and have such a long lifetime that they can practically cross the rest of the atmosphere undisturbed. A part of the muon component decays as electrons and neutrinos. Low-energy muons can be detected with scintillation or tracking detectors. High-energy muons are captured with deep underground detectors.
Kampert, and Watson, Extensive Air Showers and Ultra High-Energy Cosmic Rays, 2012, 359–412, 360.
John Linsley: Evidence for a Primary Cosmic-Ray Particle with Energy
Morrison, On Gamma-Ray Astronomy, 1958, 858–865.
His proposal was to search for gamma-ray sources as a narrow-angle anisotropy in the distribution of Extensive Air Showers at mountain altitudes, near 1/2 of atmospheric depth with characteristic energy of 1 TeV and angular resolution about 1°. In particular, he suggested that high-energy protons in the Crab Nebula could produce neutral pions and thus generate a considerable flux of gamma rays from their decay. Giuseppe Cocconi: An Air Shower Telescope and the Detection of
Trevor C. Weekes: The Atmospheric Cherenkov Technique in Very High Energy Gamma-Ray Astronomy. Space Science Reviews 75/1 (1996), 1–15, 1. doi:
Aleksandr E. Chudakov: VHE and UHE Gamma Ray Astronomy. History and Problems. In: Maurice M. Shapiro, and John P. Wefel (eds.): Cosmic Gamma Rays, Neutrinos, and Related Astrophysics. Proceedings of the
NATO
Advanced Study Institute on Cosmic Gamma Rays and Cosmic Neutrinos Erice, Italy 20–30 April, 1988. Dordrecht: Springer Netherlands 1989, 163–182, 164. doi:
The ratio of the total number of muons to the total number of electrons of a shower was a parameter widely used and applied since the end of the 1960s to distinguish gamma-ray initiated showers from hadronic background. The muon component arises almost exclusively from the decay of charged pions and kaons within hadron showers. A small fraction, however, originates from photonuclear reactions occurring when primary high-energy cosmic gamma rays are absorbed by nuclei or by individual protons or neutrons, resulting in the production of charged and neutral pions or other elementary particles, whose decay products also contain muons. But in this case, the muon content is much reduced compared to a proton shower, as a consequence of the extremely low probability of photonuclear interaction with air nuclei (small cross-section). The low production or muons could thus be a criterion for identifying gamma-ray induced showers, helping in reducing the nearly overwhelming background from the dominant hadron-induced cosmic-ray showers. K. Kamata et al.: Predominantly Electromagnetic Air Showers of Energy
Alan A. Watson: The Discovery of Cherenkov Radiation and Its Use in the Detection of Extensive Air Showers. Nuclear Physics B—Proceedings Supplements 212–213 (2011), 13–19. doi:
Independently of Jelley and Galbraith, the Air Cherenkov Technique applied to cosmic ray showers was also experimented in 1953–57 by Chudakov, at the same Lebedev Physical Institute where Pavel A. Cherenkov, experimental discoverer of the phenomenon, worked. On the Pamir mountains, in a high altitude site traditionally used for cosmic ray research, Chudakov pioneered studies of Cherenkov radiation from Extensive Air Showers with an array of eight Cherenkov detectors. N. M. Nesterova, and A. E. Chudakov: On the Observation of Cerenkov Radiation Accompanying Broad Atmospheric Showers of Cosmic Rays. Journal of Experimental and Theoretical Physics 1/2 (1955), 388–389. http://www.jetp.ac.ru/cgi-bin/e/index/e/1/2/p388?a=list. Last accessed 11/9/2018. An account of early efforts in the USSR can be found in Aleksandr S. Lidvansky: Air Cherenkov Methods in Cosmic Rays. Review and Some History. Radiation Physics and Chemistry 75/8 (2006), 891–898. doi:
Aleksandr S. Lidvansky: G T Zatsepin and the Birth of Gamma-Ray Astronomy. Physics-Uspekhi 61/9 (2018), 921–925, 922. doi:
For a historical excursion on these early experiments at the Lebedev Institute, see Chudakov, VHE and UHE Gamma Ray Astronomy, 1989, 163–182.
John V. Jelley, and N. A. Porter: Čerenkov Radiation from the Night Sky, and Its Application to γ-Ray Astronomy. Quarterly Journal of the Royal Astronomical Society 4 (1963), 275–293. http://adsabs.harvard.edu/abs/1963QJRAS...4..275J. Last accessed 12/23/2018. J. H. Fruin et al.: Flux Limits for High—Energy γ—Rays from Quasi—Stellar and Other Radio Sources. Physics Letters 10/2 (1964), 176–177. doi:
Chudakov’s group set an upper limit in the Katsiveli experiment, showing the gamma-ray flux above 5 TeV to be two orders of magnitude less than had been anticipated by Cocconi. Aleksandr E. Chudakov et al.: On the High Energy Photons from Local Sources. Extensive Air Showers, Proceedings from the 8th International Cosmic Ray Conference. 1963, 199–204. http://adsabs.harvard.edu/abs/1963ICRC....4..199C. Last accessed 11/8/2018.
For a survey of early atmospheric Cherenkov radiation studies see Chapter 2 in David Fegan: Cherenkov Reflections. Gamma-Ray Imaging and the Evolution of TeV Astronomy. Singapore: World Scientific 2019.
Antony Hewish et al.: Observation of a Rapidly Pulsating Radio Source. Nature 217/5130 (1968), 709–713. doi:
Thomas Gold: Rotating Neutron Stars as the Origin of the Pulsating Radio Sources. Nature 218/5143 (1968), 731–732. doi:
Franco Pacini: Energy Emission from a Neutron Star. Nature 216/5115 (1967), 567–568. doi:
G. G. Fazio et al.: A Search for Discrete Sources of Cosmic Gamma Rays of Energies Near
Robert J. Gould: High-Energy Photons from the Compton-Synchrotron Process in the Crab Nebula. Physical Review Letters 15/14 (1965), 577–579. doi:
B. M. Vladimirsky, A. A. Stepanian, and V. P. Fomin: High-Energy Gamma-Ray Outburst in the Direction of the X-Ray Source CYG X-3. Proceedings of the 13th International Cosmic Ray Conference, University of Denver, Denver, Colorado, August 17–30, 1973 1 (1973), 456–460. https://ui.adsabs.harvard.edu/#abs/1973ICRC....1..456V. Last accessed 12/6/2018. The installation consisted of four 1.5 m parabolic mirrors with fast photomultipliers in the focal planes. The mirrors were combined to form a system of two completely independent pairs of detectors. During the 1950s, Arnold Stepanian and his group at the Crimean Astrophysical Observatory had studied cosmic rays also with balloon probes and from the mid-1960s had started to develop simple gamma-ray telescopes with the aim of detecting ultrahigh energy gamma rays.
In the early 1960s, Allkofer and Trümper collaborated in an experiment using spark chambers built in Kiel to measure the energy distribution of cosmic-ray muons at 3,000 m (an unprecedented altitude for this kind of experiment) in the cosmic-ray laboratory belonging to Heisenberg’s Institute in Munich, on top of the Zugspitze, Germany’s highest peak. Otto Claus Allkofer, and Joachim Trümper: Das Muonen-Spektrum in 3000 m Höhe. Zeitschrift für Naturforschung A 19/11 (1964), 1304–1309. doi:
See, for example, Hans-Willy Hohn, and Uwe Schimank: Konflikte und Gleichgewichte im Forschungssystem. Akteurkonstellationen und Entwicklungspfade in der staatlich finanzierten außeruniversitären Forschung. Frankfurt am Main: Campus Verlag 1990.
Joachim Trümper: interview by Helmuth Trischler and Matthias Knopp, March 18, 2010. Transcript, HAEU, https://archives.eui.eu/en/oral_history/INT076. Last accessed 1/4/2019.
The main aim was to study the electromagnetic core structure of showers, which could provide information about the chemical composition and mass of the primary particles at the highest energies and the nuclear interactions involved in the cascade process. The central laboratory building consisted of a former air raid shelter from a marine base in Kiel, hosting the muon spectrometers and a liquid Cherenkov counter, on top of which a light laboratory building was constructed. It contained most of the detectors: an array of 16 scintillation counters, each of 1 m2 area, and a neon hodoscope of 32 m2 area, consisting of 36 compact units comprising about 180000 neon tubes to track the charged particles. Erich Bagge et al.: The Extensive Air Shower Experiment at Kiel. Proceedings of the 9th International Cosmic Ray Conference, London, UK. London New York Paris Los Angeles: Institute of Physics and the Physical Society 1965, 738–741. https://ui.adsabs.harvard.edu/#abs/1965ICRC....2..738B. Last accessed 11/23/2018. See fig. 5, showing a cross section of the laboratory building. In order to use the necessary sufficiently broad class of models for nuclear interactions, the group used Monte Carlo calculations, but at that time it was not yet possible to do Monte Carlo simulations of the cascade. They were able to measure the hadron distribution in the air shower cores at energies above 800 GeV. This was about two orders of magnitude higher than the energy of the cern Proton Synchrotron, or of the Brookhaven Alternating Gradient Synchrotron, two powerful accelerators put into operation between 1959–60. The work was supported by the Land Schleswig-Holstein and the Bundesministerium für Wissenschaftliche Forschung.
Ludwig Biermann, and Reimar Lüst: Max-Planck-Institut für Physik und Astrophysik. Institut für Astrophysik und Institut für extraterrestrische Physik. Mitteilungen der Astronomischen Gesellschaft 29 (1971), 86–112, 86. http://adsabs.harvard.edu/abs/1971MitAG..29...86B. Last accessed 10/30/2018.
Trümper, Joachim: interview by Luisa Bonolis and Juan-Andres Leon, Munich, August, 7–8, 2017. DA GMPG, BC 601036.
As evident from his publications, Allkofer’s interests continued to be especially focused on muons, also later, during the 1980s, in connection with high-energy physics at cern accelerators. See, for example, Otto Claus Allkofer et al.: The Kiel Cosmic Ray Muon Spectograph. Nuclear Instruments and Methods 83/2 (1970), 317–325. doi:
Erich Bagge, Manfred Samorski, and Wilhelm Stamm: A New Air Shower Experiment at Kiel. 15th International Cosmic Ray Conference, Plovdiv, Bulgaria, August 13–26, 1977, Conference Papers. Budapest: Dept. of Cosmic Rays, Central Research Institute for Physics of the Hungarian Academy of Sciences 1977, 24–29. https://ui.adsabs.harvard.edu/#abs/1977ICRC...12...24B. Last accessed 11/13/2018. The new eas experiment was designed to investigate the structure of extensive showers initiated in the atmosphere by primary cosmic rays having an energy range from
Trevor C. Weekes: Very High Energy Gamma-Ray Astronomy. Physics Reports 160 (1988), 1–121, 3. doi:
Manfred Samorski, and Wilhelm Stamm: Detection of
Alan A. Watson: Ultra High Energy Cosmic Rays and Ultra High Energy γ-Rays. Advances in Space Research 4/2 (1984), 35–44, 41. doi:
J. Lloyd-Evans et al.: Observation of γ Rays
Donald H. Perkins: Proton Decay Experiments. Annual Review of Nuclear and Particle Science 34 (1984), 1–52. doi:
Hypothetical ‘exotic particles’ might be responsible for the observed showers. A. Michael Hillas: Why Is Cygnus X-3 (with Related Sources) a Highlight of Cosmic-Ray Astrophysics? Proceedings from the 19th International Cosmic Ray Conference, La Jolla, USA, August 11–12, 1985. 1985, 407–414. http://adsabs.harvard.edu/abs/1985ICRC....9..407H. Last accessed 11/16/2018.
Aleksandr E. Chudakov: Is the Signal from CYG X-3, as Recorded in Some Underground Experiments, Real? Proceedings from the 19th International Cosmic Ray Conference, La Jolla, USA, August 11–12, 1985 9 (1985), 441–444. https://ui.adsabs.harvard.edu/#abs/1985ICRC....9..441C/abstract. Last accessed 3/23/2019.
Weekes, Very High Energy Gamma-Ray Astronomy, 1988, 1–121, Appendix B. For a review of controversies and claims about the unusual signals from Cygnus X-3, see also Trevor C. Weekes: TeV Radiation from Galactic Sources. Space Science Reviews 59/3 (1992), 315–364. doi:
Eckart Lorenz, and Robert Wagner: Very-High Energy Gamma-Ray Astronomy. A 23-Year Success Story in High-Energy Astroparticle Physics. European Physical Journal H 37/3 (2012), 459–513, 470. doi:
M. L. Marshak et al.: Evidence for Muon Production by Particles from Cygnus X-3. Physical Review Letters 54/19 (1985), 2079–2082. doi:
The experiment, aiming at the search for point sources, consisted of an array of scintillation detectors and an associated muon detector, which underwent expansion in several stages after going into operation in 1986. Todd J. Haines et al.: The Status of the CYGNUS Experiment: Past, Present, and Future. Nuclear Physics B—Proceedings Supplements 14/1 (1990), 244–249. doi:
For a book providing an excellent orientation in the field of astroparticle physics, bridging the gap between a presentation of the field at a simple level and a textbook for more expert readers, see Claus Grupen: Astroparticle Physics. 2nd ed. Cham: Springer 2020. doi:
Abdus Salam: Astro-Particle-Physics. In: Remo Ruffini (ed.): Proceedings of the Fourth Marcel Grossmann Meeting on General Relativity, Held at the University of Rome La Sapienza, 17–21 June, 1985. Elsevier Science 1986, 3–7. Abdus Salam had recently been awarded the Nobel Prize in Physics 1979, jointly with Sheldon Lee Glashow and Steven Weinberg, for his contribution to the theory of the unified weak and electromagnetic interactions, and was still working on the extension of unification of fundamental forces, including articles on magnetic monopoles and supersymmetry. It appears that the first to use the term in a printed source was Gary Steigman, as early as 1984, mentioning the ‘astro-particle physics community’ in a review article on a book on the very early Universe. Gary Steigman: Inflationary Cosmology. Nature 309/5967 (1984), 473–474. doi:
At this latitude, Cygnus X-3 and Hercules X-1 could be detected and, in addition, three further candidate sources were in the observation field: the Crab Nebula, Geminga (whose nature was still quite unknown), and the pulsar PSR 1937, all three known as gamma-ray emitters around
As in the old Kiel experiment, the detectors were scintillation counters of 1 m2 area each, with two photomultipliers, one for particle density and the other for fast-timing measurements.
Razmik Mirzoyan, and Christian Spiering: Nachruf auf Eckart Lorenz. Physik Journal 13/12 (2014), 50–50. www.pro-physik.de/details/articlePdf/7074441/issue.html. Last accessed 8/14/2018. Razmik Mirzoyan: Eckart Lorenz 1938–2014. CERN Courier 55/1 (2015), 40–40. https://cds.cern.ch/record/1983144. Last accessed 12/18/2018.
Dirk H. Lorenzen: Stimmen zum Gammateleskop MAGIC. Physik der Welt, 1/14/2004. https://www.weltderphysik.de/gebiet/universum/kosmische-strahlung/detektoren/magic/stimmen-zu-magic/. Last accessed 8/14/2018.
See Annual Report for 1988 of the Institute for Physics and Astrophysics, where the hegra experiment is mentioned for the first time (“Suche nach kosmischen gamma-Quellen oberhalb
Otto Claus Allkofer et al.: Results of the HEGRA Experiment. In: M. Nagano, and F. Takahara (eds.): Astrophysical Aspects of the Most Energetic Cosmic Rays. Proceedings of the
ICRR
International Symposium, Kofu, Japan, 26–29 November 1990. Singapore: World Scientific 1991, 200–211, 203. See in particular table 1 and table 2. For data collected from summer 1989 to late spring 1991, see Victoria Fonseca: The HEGRA Experiment (High Energy Gamma Ray Array). Nuclear Physics B—Proceedings Supplements 28/1 (1992), 409–412. doi:
A. Borione et al.: High Statistics Search for Ultrahigh Energy Gamma-Ray Emission from Cygnus X-3 and Hercules X-1. Physical Review D 55/4 (1997), 1714–1731. doi:
Cronin’s decision to build the shower array casa-mia was directly related to Samorski and Stamm’s claim, as confirmed by Joachim Trümper to the author L.B., 01/10/2021).
The collaboration included the Department of Atomic Physics and Astrophysics of the Universidad Complutense of Madrid, the University of Hamburg II (Institute for Experimental Physics), Institute for Pure and Applied Nuclear Physics of Kiel University, the University of Wuppertal, and the Max Planck Institute for Physics and Astrophysics in Munich. See the hegra detector arrangement with Kiel’s detectors already in operation since July 1988, and the Hamburg-Munich-Madrid set-up under construction in Fig. 1, on p. 346 of Otto Claus Allkofer et al.: The HEGRA Project for UHE Gamma Ray Astronomy. Nuclear Physics B—Proceedings Supplements 14/1 (1990), 345–347. doi:
Hofmann’s call was related in part to Peter Brix’s retirement (CPTS meeting minutes of 27.06.1984, 18.09.1984, AMPG, II. Abt., Rep. 62, No. 1802, 1803), but also to plans for the future developments at the Max Planck Institute for Nuclear Physics, which were to tackle new fundamental questions in the realm between nuclear and elementary particle physics (CPTS meeting minutes of 04.02.1987, AMPG, II. Abt., Rep. 62, No. 1810). Hofmann, who had been working since 1982 at Lawrence Berkeley Laboratory in the US, with a focus on heavy quark phenomena, appeared to be the right person for reorienting experimental research, definitely shifting the focus from the more ‘classical’ nuclear physics, moving up in energy towards the boundary between nuclear physics and high-energy physics, and to the next stage, the quark level.
J. Heintze et al.: Measuring the Chemical Composition of Cosmic Rays at
The proposal for an Extensive Shower Array appeared as Heidelberg Report HD-PY 88/05 (related to the parallel proposal to the Bundesministerium fur Forschung und Technologie “Cosmic Ray Tracking—ein neuer Weg für die gamma-Astronomie bei höhen Energien”). See also, Annual Report of the Max-Planck-Institut für Physik und Astrophysik. Werner-Heisenberg-Institut für Physik. Jahresbericht 1989, 65–69 (AMPG, IX Abt. Rep. 5, No. 632). The Cosmic Ray Tracking project was based on the measurement of individual cosmic ray tracks and muon identification using large-area drift chambers, applying an electronic system suitable for the read-out of several thousand channels, techniques similar to those they had successfully used in track-detectors at electron-positron storage rings (jade at petra/desy, tpc at pep/slac, opal at lep/cern).
J. Heintze et al.: Cosmic Ray Tracking—A New Approach to High-Energy γ-Astronomy. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 277/1 (1989), 29–41. doi:
Michael Riordan, Lillian Hoddeson, and Adrienne W. Kolb: Tunnel Visions: The Rise and Fall of the Superconducting Super Collider. Chicago, IL: University of Chicago Press 2015.
Taubes, Astronomers, 1993, 177–179, 179.
Eric Hand: Particle Physics: Sam Ting’s Last Fling. 7215. Nature 455/7215 (2008), 854–857. doi:
Trevor C. Weekes et al.: Observation of TeV Gamma Rays from the Crab Nebula Using the Atmospheric Cerenkov Imaging Technique. Astrophysical Journal 342 (1989), 379–395. doi:
Gamma-ray showers can be discriminated from the overwhelming background due to cosmic ray-initiated air showers based on the image shape and orientation. Gamma-ray images from purely electromagnetic cascades appear as narrow, elongated ellipses in the camera plane. The long axis of the ellipse corresponds to the vertical extension of the air shower and points back towards the source position in the field of view.
In this regard, as recalled by Lorenz and Wagner later, “The community followed a suggestion of Trevor Weekes that observed sources were accepted as discoveries only if their significance exceeded 5 sigma and all sources on the sky map were at least confirmed by one other experiment.” Lorenz, and Wagner, Very-High Energy Gamma-Ray Astronomy, 2012, 459–513, 492.
Lorenz, and Wagner, Very-High Energy Gamma-Ray Astronomy, 2012, 459–513, 462.
Trevor C. Weekes: A Fast Large Aperture Camera for Very High Energy Gamma-Ray Astronomy. 17th International Cosmic Ray Conference (ICRC 1981), 13–25 Jul 1981. Paris, France. Gif-sur-Yvette, France: Centre d’Etudes Nucleaires, Saclay 1981, 34–37. https://ui.adsabs.harvard.edu/#abs/1981ICRC....8...34W. Last accessed 12/2/2018. For the coming of age of the IACT technique, after many years of investigation and instrumentation development at Whipple Observatory, see Fegan, Cherenkov Reflections, 2019.
A. Michael Hillas: Cerenkov Light Images of EAS Produced by Primary Gamma Rays and by Nuclei. Proceedings from the 19th International Cosmic Ray Conference, La Jolla, USA, August 11–12, 1985. 1985, 445–448. https://ui.adsabs.harvard.edu/#abs/1985ICRC....3..445H. Last accessed 11/28/2018. Michael Hillas had started to work on simulations in the mid-1970s, when advances in the performance of computers had facilitated the Monte Carlo simulation of Extensive Air Showers on a new scale, improving significantly the signal-to-noise ratio.
Weekes et al., Observation of TeV Gamma Rays from the Crab, 1989, 379–395. Weekes, TeV Radiation, 1992, 315–364. As emphasized by Lorenz and Wagner, a main ingredient of the ‘discovery’ was the use, for the first time, of a camera allowing an efficient gamma/hadron separation of the data. The “third and most important achievement” was “the introduction of a refined gamma/hadron separation method based on the calculation of image moments,” an analysis developed by the Whipple collaboration in the mid-1980s, based on the combination of the measurement of the shower image orientation proposed by Trevor Weekes in 1981, with an analysis to evaluate the difference in images between gamma-ray showers and hadron showers originally proposed by Stepanian and his group in 1983. Lorenz, and Wagner, Very-High Energy Gamma-Ray Astronomy, 2012, 459–513, 474.
B. M. Vladimirskii et al.: Some Results of a Search for Point Sources of High-Energy Gamma Rays. Soviet Astronomy 16/1 (1972), 1–5. https://ui.adsabs.harvard.edu/#abs/1972SvA....16....1V. Last accessed 12/11/2018. Vladimirsky, Stepanian, and Fomin, High-Energy Gamma-Ray Outburst, 1973, 456–460. A. A. Stepanian et al.: A Search for Discrete Gamma-Ray Sources of Energy Greater than
With two or more images of the same shower, a 3-dimensional reconstruction of the shower axis becomes possible. The first page of the proposal presented by the Yerevan Physics Institute in February 1985 and the scheme for the five Imaging Cherenkov Telescopes are reproduced in Razmik Mirzoyan: Early Days of Cherenkov Emission and Milestones. Presented at the SPSAS School, San Paulo, 5/23/2017, 44. http://www.astro.iag.usp.br/~highenastro/Talks/Lecture_III_Razmik_Mirzoyan_1.pdf. Last accessed 12/9/2018.
Their original idea had been to build a system of five imaging Cherenkov telescopes surrounded by an array of Cherenkov detectors. The planned array comprised about 50 scintillation detectors of 1 m2 area each (to which further 150 detectors should be added) dislocated on an area of about
“We had special photomultipliers, at that time they were produced only a few hundred pieces per year, most of them disappeared for military purposes. We prepared very high-quality mirrors by ourselves… We prepared everything for building five telescopes. While we were commissioning the first telescope, the country decayed, Soviet Union decayed. At some moment, the situation became obsolete. I was working in a powerful institute and you ordered this and that and then suddenly there was nothing, we had a cut of electricity, and then there was confusion everywhere, supermarkets became empty. Everything was decaying, we could not continue like that. We remembered we had met Prof. Allkofer from Kiel, at the Institute for Nuclear Physics; we got in contact with him. He told us they were building an array at La Palmas, hegra: ‘Why don’t you join us?’” Razmik Mirzoyan: interview by Juan-Andres Leon, Munich, August 13–14, 2018. DA GMPG, BC 601021. In 1992, the level of state support decreased by two times that of the previous years and by 1994, the level of financing of Russian science was almost six times lower than in developed countries of the West. On the breakup of the Soviet Union and crisis in Russian science, see Loren Graham, and Irina Dezhina: Science in the New Russia. Crisis, Aid, Reform. Bloomington, IN: Indiana University Press 2008.
See a brief summary of the physics program in Eckart Lorenz: The HEGRA Experiment. In: P.C. Bosetti (ed.): Trends in Astroparticle-Physics. Wiesbaden: Vieweg+Teubner Verlag 1994, 139–151, 139. A substantial completion would occur between 1991–92, with 49 muon detector stations, allowing an effective suppression of the background of hadron-induced showers, thus helping to clarify whether the gamma-induced showers really had a much lower muon content than hadron-induced showers, as predicted by theory.
An official letter of invitation was sent from the Institute for Nuclear Physics in Kiel, dated October 24, 1990, and signed by Manfred Samorski (Spokesman of the hegra Collaboration). As specified in the document, participants in the collaboration at the time were the Max Planck Institute in Munich and the Universities of Hamburg, Kiel, Madrid, Nottingham, Wuppertal. Courtesy of Razmik Mirzoyan.
The agreement implied the construction of a system of 5 imaging Cherenkov light receivers on La Palma, which might work as a standalone system, with the potential to be operated simultaneously with the hegra particle array: “By this unique combination of an extended particle array with detectors for atmospheric Cherenkov light, the observation of cosmic gamma-ray sources will be possible over an extended energy range from
A proposal for “Imaging Air Cherenkov Telescopes in the HEGRA Particle Array,” dated May 31, 1991, was signed by F. A. Aharonian, A. G. Akhperjanian, A. S. Kankanian, R. G. Mirzoyan (Yerevan Physics Institute), A. A. Stepanian (Crimean Astrophysical Observatory), M. Samorski, W. Stamm (Institute for Nuclear Physics, University of Kiel), M. Bott-Bodenhausen, E. Lorenz, P. Sawalisch (Max Planck Institute for Physics and Astrophysics, Munich). This document was kindly shown by Mirzoyan to one of us (J.-A. L). On the presentation of the new set-up at the HEGRA site, see Felix A. Aharonian et al.: Status and Extensions of the HEGRA Detector on La Palma. Proceedings of the 22nd International Cosmic Ray Conference. 11–23 August, 1991. Dublin, Ireland 4 (1991), 452–455. https://ui.adsabs.harvard.edu/#abs/1991ICRC....4..452A. Last accessed 5/18/2018. Felix A. Aharonian et al.: A System of Air Cherenkov Telescopes in the HEGRA Array. Proceedings of the 22nd International Cosmic Ray Conference. 11–23 August, 1991. Dublin, Ireland 2 (1991), 615–617. https://ui.adsabs.harvard.edu/#abs/1991ICRC....2..615A. Last accessed 5/18/2018. Each telescope was planned to have a 3 m diameter tessellated mirror of 5 m2 area, to be equipped with a 37-pixel imaging camera in the focal plane at 5 m.
Razmik Mirzoyan: interview by Juan-Andres Leon, Munich, August 13–14, 2018. DA GMPG, BC 601021. The MPIP Annual Report for 1990 mentioned that five Cherenkov Telescopes would be added at the hegra site, the first of which was under construction (Annual Report of the Max-Planck-Institut für Physik und Astrophysik. Werner-Heisenberg-Institut für Physik. Jahresbericht 1990, 80. AMPG, IX Abt. Rep. 5, No. 632). Unfortunately, Eckart Lorenz passed away on June 20, 2014. Mirzoyan, and Spiering, Nachruf auf Eckart Lorenz, 2014, 50–50.
M. Bott-Bodenhausen et al.: Airobicc–a New Array of Angle Integrating Cerenkov Counters for Improved γ/Hadron Separation in Extended Air Showers. AIP
Conference Proceedings 220/1 (1991), 305–309. doi:
See status and planned extensions of the hegra detector array, showing the different detectors, in Fig. 1 of Allkofer et al., Results of the HEGRA Experiment, 1991, 200–211, 402.
Felix A. Aharonian et al.: The System of Imaging Atmospheric Cherenkov Telescopes. The New Prospects for VHE Gamma Ray Astronomy. Experimental Astronomy 2/6 (1992), 331–344. doi:
M. Bott-Bodenhausen et al.: A New Air Cherenkov Counter Concept for the Observation of Extended Air Showers. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 315/1–3 (1992), 236–251. doi:
General results were based on data recorded from September 1992 to February 1993. F. Krennrich et al.: Observation of VHE γ-Emission from the Crab Nebula with the Prototype of the HEGRA Air Cerenkov Telescope Array. In: D. A. Leahy, R. B. Hicks, and D. Venkatesan (eds.): 23rd International Cosmic Ray Conference, Vol. 1, Held 19–30 July, 1993 at University of Calgary, Alberta, Canada. 1993, 251–254. https://ui.adsabs.harvard.edu/#abs/1993ICRC....1..251K. Last accessed 12/11/2018. A preliminary estimate of the flux was in agreement with extrapolations from the Whipple data. Razmik Mirzoyan et al.: The First Telescope of the HEGRA Air Cherenkov Imaging Telescope Array. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 351/2 (1994), 513–526. doi:
The roots of this interest go back to the 1960s, when Völk was still based at the Institute for Extraterrestrial Physics, before moving in 1975, as Director, to the Heidelberg Institute for Nuclear Physics, and he applied his plasma physics background to cosmic-ray physics to understand the transport of cosmic rays in turbulent interstellar medium or in solar wind. In 1982, the ‘Kosmochemie’ section was renamed ‘Kosmophysik’ in the Annual Report. The origin of cosmic rays as well as the sources of gamma rays were widely discussed: Generalverwaltung der Max-Planck Gesellschaft (ed.): Max-Planck-Gesellschaft Jahrbuch 1982. Göttingen: Vandenhoeck & Ruprecht 1982, 534. All these developments were going on in parallel with the development of the gallex project for the detection of solar neutrinos. At the same time, Völk’s interest in gamma-ray astronomy had been aroused because of Pinkau’s involvement at the Institute for Extraterrestrial Physics in the satellite COS-B. Völk, Heinrich: interview by Luisa Bonolis and Juan-Andres Leon, Heidelberg, October 9–10, 2017. DA GMPG, BC 601037.
Heinrich J. Völk, and M. Forman: Cosmic Rays and Gamma-Rays from OB Stars. The Astrophysical Journal 253 (1982), 188–198. doi:
Catherine Cesarsky, and Heinrich Völk: Cosmic Ray Penetration into Molecular Clouds. Astronomy and Astrophysics 70 (1978), 367–377. https://ui.adsabs.harvard.edu/#abs/1978A&A....70..367C. Last accessed 5/16/2018.
Heinrich Völk: interview by Luisa Bonolis and Juan-Andres Leon, Heidelberg, August 9–10, 2017. DA GMPG, BC 601037.
Felix A. Aharonian: Very High Energy Gamma-Ray Astronomy and the Origin of Cosmic Rays. Nuclear Physics B—Proceedings Supplements 39/1 (1995), 193–206. doi:
See, for example, Felix A. Aharonian, and H. J. Völk: Very High Energy Gamma-Ray Astronomy with Ground-Based Imaging Cherenkov Telescopes. In: W. Wamsteker, M.S. Longair, and Y. Kondo (eds.): Frontiers of Space And Ground-Based Astronomy. Dordrecht: Springer Science & Business Media 1994, 705–706. doi:
Krennrich et al., Observation of VHE γ-Emission from the Crab Nebula, 1993, 251–254.
J. Heintze et al.: The Heidelberg Cosmic Ray Project—Aims and Status. Nuclear Physics B—Proceedings Supplements 14A/1 (1990), 148–152. doi:
Successful operation of this prototype would lead to a detailed proposal for the funding agencies by mid-1992. M. Feuerstack et al.: Cosmic Ray Tracking—a New Approach to High-Energy γ-Astronomy. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 315/1 (1992), 257–259. doi:
A preliminary analysis of data obtained during the first weeks of operation was reported at the 23rd International Cosmic Ray Conference held in July 1993 in Canada. Konrad Bernlöhr et al.: Tracking Detectors for High Energy γ-Astronomy. In: D. A. Leahy, R. B. Hicks, and D. Venkatesan (eds.): 23rd International Cosmic Ray Conference, Vol. 4, Held 19–30 July, 1993 at University of Calgary, Alberta, Canada. 1993, 199–202. https://ui.adsabs.harvard.edu/#abs/1993ICRC....4..199B. Last accessed 12/11/2018. A 10-detectors crt test array at La Palma, although being too small for a search for cosmic gamma-ray sources, would enable them to test shower reconstruction algorithms under realistic conditions and to evaluate the performance that could be expected from the planned full-sized crt array consisting of almost 400 modules.
Krennrich et al., Observation of VHE γ-Emission from the Crab Nebula, 1993, 251–254.
A. Konopelko et al.: Detection of Gamma Rays above 1 TeV from the Crab Nebula by the Second HEGRA Imaging Atmospheric Cherenkov Telescope at La Palma. Astroparticle Physics 4/3 (1996), 199–215. doi:
A. Daum et al.: First Results on the Performance of the HEGRA IACT Array. Astroparticle Physics 8/1 (1997), 1–11. doi:
Heinrich J. Völk: Max-Planck-Institut für Kernphysik–Astrophysik. Jahresbericht für 1997. Mitteilungen der Astronomischen Gesellschaft Hamburg 81 (1998), 469–484, 469. https://ui.adsabs.harvard.edu/#abs/1998MitAG..81..469V. Last accessed 9/26/2018. Werner Hofmann, and HESS Collaboration: The High Energy Stereoscopic System (HESS) Project. AIP
Conference Proceedings 515/1 (2000), 500–509. doi:
See March 1998 version of the design study (supported in part by a contract of the Bundesministerium für Bildung und Forschung, the Spanish Inter-ministerial Commission for Science and Technology, and the European Union), and related early publications. MAGIC Collaboration: The
MAGIC
Telescope. Design Study for the Construction of a 17 m Čerenkov Telescope for Gamma-Astronomy above 10 GeV, 1998. Eckart Lorenz: The MAGIC Telescope Project for Gamma Ray Astronomy in the 15 to 300 GeV Energy Range. Nuclear Physics B Proceedings Supplements 48 (1996), 494–496. doi:
In fact, there are reminiscences that one of the early pioneers of ground-based gamma astronomy, Arnold Stepanian, had been setting up a pioneering system in Chile in the early 1970s, but this was interrupted by the right-wing military coup, after which all Soviet astronomers had to swiftly leave the country. Razmik Mirzoyan: interview by Juan-Andres Leon, Munich, August 13–14, 2018. DA GMPG, BC 601031.
The ambition was to collect enough photoelectrons to lower the threshold energy to about 20 GeV, thus giving overlap with the satellite detectors that had discovered a great number of point sources, but with much higher sensitivity to faint sources. magic started normal operations, with the first telescope in 2004, and stereo observations with both telescopes in 2009. The twin ultra-large magic telescopes, with 17 m diameter mirrors, were built incorporating several novel features. In this regard, it has been emphasized that “As it was obvious that a stereo system of such telescopes with so many new features would never have been funded in the first round, a second telescope was built only after the new items of the first one proved to work.” Lorenz, and Wagner, Very-High Energy Gamma-Ray Astronomy, 2012, 459–513, 494.
Felix A. Aharonian et al.: The Potential of Ground Based Arrays of Imaging Atmospheric Cherenkov Telescopes. I. Determination of Shower Parameters. Astroparticle Physics 6/3 (1997), 343–368. doi:
Heinrich Völk: interview by Luisa Bonolis and Juan-Andres Leon, Heidelberg, August 9–10, 2017. DA GMPG, BC 601037; Werner Hofmann: interview by Luisa Bonolis and Juan-Andres Leon, Heidelberg, August 10–11, 2017. DA GMPG, BC 601010.
Heinrich Völk: interview by Luisa Bonolis and Juan-Andres Leon, Heidelberg, August 9–10, 2017. DA GMPG, BC 601037.
The first of the four telescopes of the first phase of the h.e.s.s. project went into operation in summer 2002 and started stereoscopic observations in 2003. These detectors were actually located next to the mountain that had been acquired by the Max Planck Society for the southern observatory of the Max Planck Institute for Astronomy in Heidelberg. When the Spanish in La Palma, loyal to Munich, obstructed the placement of h.e.s.s. on their island, Völk was advised by Hans Elsässer, Director of the Astronomy Institute, to use their site in Namibia instead. In the end, for cost-saving reasons, the detectors were placed next to the mountain, not on top of it as originally intended (Heinrich Völk: interview by Luisa Bonolis and Juan-Andres Leon, Heidelberg, August 9–10, 2017. DA GMPG, BC 601037).
The progress report 2001/2002, providing an overview of scientific work conducted at the Institute for Nuclear Physics, had a new general heading for fields such as high-energy, theoretical and infrared astrophysics, neutrino physics, and heavy flavor physics: “Crossroads of Particle Physics and Astrophysics,” officially inaugurating one of the two new research directions formulated at the end of the 1990s (the second one being “Many Particle Dynamics of Atoms and Molecules”) and mirrored in the new structure of the Annual Report (AMPG, IX Abt., Rep. 5, No. 413).
h.e.s.s. has surveyed the Milky Way in gamma-ray light for the last 15 years. To celebrate this anniversary, the h.e.s.s. collaboration has published its largest set of results in a series of papers, in a special issue of the journal Astronomy & Astrophysics: Thierry Forveille, Sergio Campana, and Steve Shore: H.E.S.S. Phase-I Observations of the Plane of the Milky Way. Astronomy & Astrophysics 612 (2018), E1. doi:
Science New Staff: Fire Damages Gamma-Ray Observatory. Science | AAAS, 10/24/1997. https://www.sciencemag.org/news/1997/10/fire-damages-gamma-ray-observatory. Last accessed 3/27/2021.
Teshima had led the Japanese efforts in gamma-ray astronomy in the 1980s in Japan, and 1990s in Utah. The proposal for such a position had been specifically motivated by the need for a director who could lead research in experimental astroparticle physics, a field which was viewed as having strong potential for future developments at the Institute for Physics. In particular, it was stressed how the rising costs for instruments at accelerators favored the development of promising areas, such as astroparticle physics, which could be operated on a relatively small financial basis (CPTS meeting minutes of 18/19.10.2001, AMPG, II. Abt., Rep. 62, No. 1855, p. 19).
Eckart Lorenz, and Manel Martinez: The Magic Telescope. Astronomy & Geophysics 46/6 (2005), 6.21-6.25. doi:
The author L. B. is grateful to Alessandro de Angelis for this specific remark and for emphasizing the relevance of this major innovation that increasingly results in a successful specialty for magic (personal communication, November 27, 2019).
The MAGIC Collaboration: Observation of Pulsed γ-Rays Above 25 GeV from the Crab Pulsar with MAGIC. Science 322/5905 (2008), 1221–1224. doi:
On January 14, 2019, both the Fermi and Swift satellites detected a spike of gamma rays from the constellation Fornax. The missions transmitted to the astronomical community the location of the burst, dubbed GRB 190114C, 30 seconds after the event. The two magic 64-ton telescopes automatically turned to the direction of the burst. They began observing it just 50 seconds after the explosion, and captured the most energetic gamma rays yet seen from these events. MAGIC Collaboration: Teraelectronvolt Emission from the γ-Ray Burst GRB 190114C. Nature 575/7783 (2019), 455–458. doi:
In 2018, for the first time, dedicated observations of the microquasar SS 433, taken from 2006 to 2011, from both magic and h.e.s.s., were combined, accounting for a total effective observation time of 16.5 hours. Such data were used to place constraints on the particle acceleration fraction at the inner jet regions and on the physics of the jet/medium interactions, so providing hints on the behavior of relativistic particles in the source. MAGIC Collaboration et al.: Constraints on Particle Acceleration in SS433/W50 from MAGIC and H.E.S.S. Observations. Astronomy & Astrophysics 612 (2018), A14. doi:
IceCube Collaboration: Neutrino Emission from the Direction of the Blazar TXS 0506+056 Prior to the IceCube-170922A Alert. Science 361 (2018), 147–151. doi:
Felix A. Aharonian et al.: 5@5—a 5 GeV Energy Threshold Array of Imaging Atmospheric Cherenkov Telescopes at 5 Km Altitude. Astroparticle Physics 15/4 (2001), 335–356. doi:
A special collection of review and commentary articles on multi-messenger astrophysics published in Nature Review Physics was highlighted on September 2, 2020 (https://astronomycommunity.nature.com/posts/a-collection-on-multi-messenger-astrophysics. Last accessed 7/19/2021). See, in particular, Mészáros et al., Multi-Messenger, 2019, 585–599.
Heinrich Völk: interview by Luisa Bonolis and Juan-Andres Leon, Heidelberg, August 9–10, 2017. DA GMPG, BC 601037. Razmik Mirzoyan: interview by Juan-Andres Leon, Munich, August 13–14, 2018. DA GMPG, BC 601031. Juan-Andres Leon, conversation with Masahiro Teshima, Munich, August 14, 2018.
In 2015–16, the cameras of the four h.e.s.s. i telescopes were fully refurbished using state-of-the-art electronics and, in particular, the newly developed NECTAr readout chip especially designed for the next big experimental Cherenkov Telescope Array. S. Vorobiov et al.: NECTAr: New Electronics for the Cherenkov Telescope Array. Nuclear Instruments and Methods in Physics Research Section A 639/1 (2011), 62–64. doi:
See a short account of this merging in Frank Grotelüschen: Insight Starts Here. 50 Years of DESY. Hamburg: Deutsches Elektronen-Synchrotron DESY 2009.
Americans are notably reluctant to be minority partners in international collaborations. Even in alma, their current participation (2018) is 37.5 percent, exactly the same as in eso, and above the remaining 25 percent of “East Asia.”
This is again very similar to alma. See Chapter 4.
The CTA Consortium: Design Concepts for the Cherenkov Telescope Array CTA. An Advanced Facility for Ground-Based High-Energy Gamma-Ray Astronomy. Experimental Astronomy 32/3 (2011), 193–316. doi:
A whole issue of the journal Astroparticle Physics has been dedicated to the science explored with the CTA: Jim Hinton et al. (eds.): A New Era in Gamma-Ray Astronomy with the Cherenkov Telescope Array. Astroparticle Physics 43 (2013), 1–2. doi:
Cherenkov Telescope Array Observatory: CTA Prototype LST-1 Detects Very High-Energy Emission from the Crab Pulsar, 6/22/2020. https://www.cta-observatory.org/lst1-detects-vhe-emission-from-crab-pulsar/. Last accessed 3/27/2021.