This chapter will explore the interplay between the renaissance of general relativity and the advent of relativistic astrophysics following German involvement in gravitational-wave research through the window of the Max Planck Society, from the very first interests of its scientists, to the point when gravitational-wave detection became established by the appearance of full-scale detectors and international collaborations. On the background of the spectacular astrophysical discoveries of the 1960s and the growing role of relativistic astrophysics, Ludwig Biermann and his collaborators at the Max Planck Institute for Astrophysics in Munich became deeply involved in research related to such widening horizons, already unveiled by radio astronomy during the 1950s. At the end of the 1960s, Joseph Weber's announcements claiming detection of gravitational waves sparked the decisive entry of this group into the field, in parallel with the appointment of the renowned relativist Juergen Ehlers. The Munich area group of Max Planck institutes provided the fertile ground for acquiring a leading position in the 1970s, facilitating the transition from resonant bars towards laser interferometry and its innovation at increasingly large scales, eventually finding a dedicated site in Hannover in the early 1990s. An early pan-European initiative broke up into two major projects: the British-German GEO600, and the French-Italian Virgo. The German approach emphasized perfecting experimental systems at pilot scales, and never developed into a fully-scaled interferometer, rather joining the LIGO collaboration at the end of the century. 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.

This second chapter follows the enormous expansion of the space sciences around the world after the launch of Sputnik, as well as the uniquely constrained West German response; and it focuses on how the Max Planck Society maneuvered itself into a role of predominance in the space sciences, under these circumstances. Thanks to its strong scientific traditions and political backers, the Max Planck Society was singularly well placed to take advantage of the rising interest in the study and conquest of outer space: while guaranteeing a concerted emphasis on 'fundamental research' and international collaboration, it mobilized existing projects in plasma physics, cosmochemistry, and balloon-based cosmic rays, and joined in diverse space activities with the United States and various European countries. This entry into the space age paved the way to the Society's subsequent expansion into astronomy (the subject of the next chapter), and also allowed the scientific traditions of the early postwar era to diversify: dependency on 'nuclear' sociopolitical interests and funding was now succeeded by a focus on astrophysical subjects proper. As we will see in subsequent chapters, this reorientation ultimately became one of the vehicles propelling these longstanding traditions towards the most effervescent topics of 21st-century astrophysics. 'Sputnik Shocks'Within only a few months of the launch of the Soviet satellite, the status of disciplines such as astronomy and astrophysics changed dramatically, as they now became integrated into the Cold War apparatus, just as experimental physics had been in 1945. Key players in this radical shift were those scientists around the world who had preexisting strengths and interests in the cosmic sciences, but had formulated their research in terms of 'nuclear' topics during the postwar years. Space exploration initiatives in the United States, Soviet Union, France, Britain, and other European countries would now become the model for the German MPI scientists described in the previous chapter, and, eventually, their collaboration counterparts, too. We describe this transition, from the predominantly 'nuclear' period up to 1957 to the nascent space age.

International collaboration was always key to Max Planck leadership; in the first postwar decade, astrophysicists and cosmochemists were frequent guests within much larger projects based in Allied countries. During the post-Sputnik boom, one of the objectives of the vast expansion was to be able to mobilize national strengths to obtain a stronger voice in international collaborations. The chapter takes up this process of internationalization as it matures, from the 1970s on, and becomes the main mode of research in the Max Planck Society, which it still is to this day. Unexpectedly, this was thanks not so much to German-owned large infrastructures but, rather, to the weight of longstanding scientific and technical traditions which brought to the global table the Germans' theoretical insights, innovative experimentation, and superior instrument-making capacities. German reunification and the end of the Cold War further accelerated the Max Planck Society's transition toward this 21stcentury mode of scientific production. Reform in the 1990s coincided with the geopolitical shifts, as well as with the retirement of many of those leading Max Planck Institute directors who had led the wavelength expansion in the previous 30 years. Their successors de-emphasized the construction and ownership of observatories, focusing instead on scientific research within large collaborations, secure in the knowledge that their institutes' instrumental expertise would provide political leverage and a comparative advantage over their partners. Political pressures to relocate institutes to the former East Germany, or even to close them down, were successfully turned into opportunities for expansion, and ultimately, even the one most seriously under threat from these reforms, Biermann's original (theoretical) Institute for Astrophysics, found a reinvigorated mission within the cluster of Max Planck Institutes dedicated to cosmic research, as well as in the, by then, global powerhouse of Garching.

Until the 1960s, observational astronomy was not considered a field of interest by the Max Planck Society, whose astrophysical pioneers were strongly oriented toward topics intersecting with the nuclear age. In West Germany, astronomy retained an aura of antiquatedness, and was based largely in observatories dating from previous centuries and still the purview of individual federal states. This changed radically after Sputnik, when astronomy underwent a revival around the world. Even before 1957, an astronomical revolution had been spearheaded by radio astronomy. This was the case also in Germany, where radar pioneers had built the first radio telescopes and forged an international reputation during the first postwar decade. The Max Planck Society, in its moment of most radical expansion, now absorbed these scientists and turned their projects into national infrastructures. This model was then repeated, with the absorption of the most promising observatory project in the traditional optically visible wavelengths, and, simultaneously, a major drive toward space-based astronomy in wavelengths inaccessible from the ground. In all these fields, the Max Planck Society grew by attracting external experts who, in addition to their flagship projects, continued to expand into adjacent wavelengths in subsequent decades, at their respective institutes. This absorption of astronomy led to a significant shift within the Max Planck Society itself, an institution where astrophysics had hitherto been dominated by theoretical plasma physicists in Munich, and experimental nuclear and particle physicists in Heidelberg. The growth of astronomy and its corresponding political influence led to a major reconfiguration of the disciplinary focus of several Max Planck Institutes in the 1970s, and this also signaled a transition from the space sciences of the early post-Sputnik era to the more differentiated astronomy, astrophysics, and planetary sciences of the coming decades.

The focus of this first chapter is astrophysics in the first decade following World War II, when the discipline was embedded in the promises of the 'nuclear age' and, in West Germany, the hardship of reconstruction and Allied restrictions. A key element at the time was the ambiguity of the term 'nuclear' physics, which tacitly implied the potential of nuclear energy and weapons, while also connoting a disparate array of scientific subjects, in addition to the atomic nucleus, such as fusion and plasmas, cosmic rays, and subatomic particles. This ambiguous overlap enabled astrophysicists to obtain financial and political support for any research signaling future expansion into the relevant applied fields, while yet respecting postwar prohibitions. The backdrop to this support was, in essence, the regionally focused competition between the Allies, which played out in the various occupation zones and nascent states of West Germany. Scientists rooted in diverse scientific traditions and political orientations adapted to the different regional interests and the priorities of the Allied occupiers. It was the strength acquired in this decade, partly as a result of competition among these different factions, that assured the Max Planck Society a good headstart after Sputnik, when the 'space age' took center stage. Postwar Scientific Traditions in GöttingenA community of scientists converged in Göttingen, in the aftermath of World War II, to become part of what was to become-at the initiative of the famous physicist Werner Heisenberg-the Max Planck Institute for Physics and Astrophysics, the primary hub and powerhouse of the young Max Planck Society. This chapter focuses on its trajectory during the early post-war nuclear age, until the move to Munich in 1958. Göttingen was the birthplace of the research tradition rooted in the theoretical plasma astrophysics led by the astrophysicist Ludwig Biermann and his disciples, who were able to make contact and collaborate closely with scientists working on nuclear fusion in the United States.