This research project is part of the wider project The Renaissance of General Relativity in the Post-World War II Period, which has involved a high degree of collaboration between members of Department I at the Max Planck Institute for the History of Science and external researchers.

In the first phase of my research, I identified a crucial aspect and a guiding role in the study of dense matter. It plays part in the complex process that began in the late 1920s, eventually converging into the real emergence of the new field of relativistic astrophysics between the late 1950s and early 1960s.

Since the mid-1920s, different strands of research have used stars as physics laboratories to investigate the nature of matter under extreme densities and pressures that are not found on Earth. The evolution of the concept of a dense core in stars was important for an understanding of stellar evolution and as a testing ground for the fast-evolving field of nuclear physics. Precisely, the wide field of nuclear physics—in the deepest and broadest sense it retained up to the mid-1950s—has been the key to unveiling how the problems addressed by different research agendas and their merging and converging threads at the end of the 1950s led to the birth of relativistic astrophysics. The study of the preliminary phase preparing the emergence of relativistic astrophysics (from dense matter studies to gravitational collapse within Einstein’s theory in the 1920s and 1930s) has shown how, in spite of the divide between physicists and astrophysicists, some key actors working in the cross-fertilized soil of overlapping but different scientific cultures formulated models and tentative theories that gradually evolved into more realistic and structured astrophysical objects. These investigations culminated in the first contact with general relativity in 1939, when J. Robert Oppenheimer and his students George Volkoff and Hartland Snyder systematically applied the theory to the dense core of a collapsing neutron star. This pioneering application of Einstein’s theory to an astrophysical compact object can be regarded as a milestone in the path that eventually led to the emergence of relativistic astrophysics in the very early 1960s. These results have been outlined in the paper Stellar Structure and Compact Objects Before 1940: Towards Relativistic Astrophysics.

In parallel, the renaissance process of general relativity has been addressed in two historical case studies. The first—in collaboration with Roberto Lalli and Adele La Rana—is related to the rise of general relativity and gravitational-wave research in Italy. The second case study—in collaboration with Juan Andres Leon—highlights how the interplay between the Renaissance and the growing influence of relativistic astrophysics was instrumental in initiating gravitational-wave research at the Max Planck Institute for Astrophysics in Munich. This eventually led to the construction of increasingly large-scale and innovative prototypes and culminated with the foundation of the Albert Einstein Institute and the construction of the British-German gravitational-wave interferometer GEO600. The latter contributed crucially to all aspects of the first successful direct detection of gravitational waves in 2015 by LIGO.

During the post-World War II period, research related to the design of thermonuclear weapons brought about renewed interest in highly dense stellar matter and in the then abandoned problem of gravitational collapse. This had been tackled in1939 by Oppenheimer and his collaborators, who applied Einstein’s general theory of relativity to the dense core of a neutron star for the first time. Meanwhile, the old problem of nucleosynthesis—now becoming a central issue also for physical cosmology—rekindled interest in neutron stars, which up to that time remained theoretical entities and whose existence had been previously suggested by Fritz Zwicky in 1933. During the 1950s, model-building with computers showed that such exotic objects might actually be formed in nature. Technological progress during the war had opened up new horizons also in the study of astronomy, where the realm of radio stars and very distant radio galaxies had become a subject of investigation. The unveiling of the high-energy universe by radio astronomy and the emerging science of nuclear astrophysics led, at the end of 1950s, to a resurgent awareness of astrophysical processes in which general relativistic effects might play a dominant role. This research has clarified how the dialogue between different subcultures gave a crucial contribution to focusing on the fact that the radio universe was full of high energy as opposed to the quiet optical universe. Such aspects, involving the crucial problem of the synthesis of chemical elements in stars, attracted the attention of theoreticians like Geoffrey Burbidge, Fred Hoyle, and William Fowler. The gradual realization of the existence of violent events, both in stars and in galaxies, and especially the existence of an incredible amount of energy contained in magnetic fields and of the charged particles of some galaxies—which until then required an explanation—marked the entry in the realm of high-energy astrophysics and set the stage for the emergence of relativistic astrophysics in the early 1960s. Apparently triggered by the eventful discovery of quasars and inaugurated by the first Texas conference organized in 1963, the birth of relativistic astrophysics is actually to be understood as the culmination of a complex process including the longstanding tradition of the study of compact stars, revisited within the growing dialogue and co-operation between new astronomies, general relativity, computer simulations, and the nascent fields of high-energy and nuclear astrophysics.  With a new community of researchers in general relativity achieving novel fundamental theoretical insights into Einstein’s equations, the stage was being set up for a dialogue that helped lay the foundations for the establishment of general relativity as a standard working tool of theoretical astrophysics.

Within this project I have also investigated how the long quest for the supermassive black hole at the center of our galaxy became one of the central research themes at the Max Planck Institute for Extraterrestrial Physics in Garching—near Munich—with the appointment of Reinhard Genzel as director and head of the infrared astronomy department. Based on the Max Planck Society’s unique technical expertise in instrument building, Genzel’s team entered the era of observational black hole physics with a dedicated long-term effort in infrared observations of radiation phenomena, gas dynamics, and especially of the proper motions and velocities of a group of stars tightly orbiting the supermassive black hole candidate at the center of our galaxy. Owing to the advanced technologies of their very sensitive instruments, the international collaboration led by Genzel’s group was able to observe matter circling the “event horizon”—the boundary of the black hole beyond which nothing, including light, can escape. Additionally, for the first time, effects predicted by Einstein’s theory of general relativity, such as the precession of the orbit and the redshift of the light of a star orbiting very close to the central mass, were observed in the strong gravitational field of a black hole of about four million times the mass of the sun. The existence of these predicted effects was demonstrated by such observations beyond any reasonable doubt. The 2020 Nobel Prize in Physics was awarded jointly to Roger Penrose for the theoretical proof that black holes must in fact exist and to Reinhard Genzel and Andrea Ghez for providing the observational evidence that there is a supermassive black hole at the center of our galaxy. This story is part of the section Question-Oriented Integration of Theory and Observation in Astronomy, Chapter 4, in the forthcoming volume with Juan Andres Leon Astronomy, Astrophysics and Space Science in the Max Planck Society (Brill 2022).


Bonolis, Luisa (2017). “Stellar Structure and Compact Objects before 1940: Towards Relativistic Astrophysics.” The European Physical Journal. H 42 (2): 311–393.

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Bonolis, Luisa, Adele La Rana, and Roberto Lalli (2017). “The Renaissance of General Relativity in Rome: Main Actors, Research Programs and Institutional Structures.” In The Fourteenth Marcel Grossmann Meeting: On Recent Developments in Theoretical and Experimental General Relativity, Astrophysics, and Relativistic Field Theories. Part D, ed. M. Bianchi and R. T. Jantzen, 3372–3377. Singapore: World Scientific.

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La Rana, Adele, and Luisa Bonolis (2017). “The Beginning of Edoardo Amaldi’s Interest in Gravitation Experiments and in Gravitational Wave Detection.” In The Fourteenth Marcel Grossmann Meeting: On Recent Developments in Theoretical and Experimental General Relativity, Astrophysics, and Relativistic Field Theories. Part D, ed. M. Bianchi and R. T. Jantzen, 3378–3383. Singapore: World Scientific. .

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Bonolis, Luisa, and Juan Andres León Gómez (2020). “Gravitational-Wave Research as an Emerging Field in the Max Planck Society: The Long Roots of GEO600 and of the Albert Einstein Institute.” In The Renaissance of General Relativity in Context, ed. A. S. Blum, R. Lalli, and J. Renn, 285–361. Basel: Birkhäuser.

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Bonolis, Luisa (2020). “How Relativistic Astrophysics Bubbled up from Post-WWII Science: A Preliminary Survey.” In Atti del XXXIX Convegno annuale = Proceedings of the 39th Annual Conference, Pisa, 9–12 Settembre 2019, Società Italiana degli Storici della Fisica e dell’Astronomia (SISFA), ed. A. La Rana and P. Rossi, 159–170. Pisa: Pisa University Press.

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