Eur. Phys. J. H 42, 311–393 (2017) DOI: 10.1140/epjh/e2017-80014-4 THE EUROPEAN PHYSICAL JOURNAL H Stellar structure and compact objects before 1940: Towards relativistic astrophysics Luisa Bonolisa MaxPlanckInstitutf¨ur Wissenschaftsgeschichte, Boltzmannstraße 22, 14195 Berlin, Germany Received 8 March 2017 / Accepted 8 March 2017 Published online 28 April 2017 c The Author(s) 2017. This article is published with open access at Springerlink.com Abstract. Since the mid-1920s, different strands of research used stars as “physics laboratories” for investigating the nature of matter under extreme densities and pressures, impossible to realize on Earth. To trace this process this paper is following the evolution of the concept of a dense core in stars, which was important both for an understand- ing of stellar evolution and as a testing ground for the fast-evolving field of nuclear physics. 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 ten- tative theories that gradually evolved into more realistic and structured astrophysical objects. These investigations culminated in the first con- tact with general relativity in 1939, when J. Robert Oppenheimer and his students George Volkoff and Hartland Snyder systematically ap- plied 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 eventually leading to the emergence of relativistic astrophysics in the early 1960s. 1 Introduction Despite its enormous influence on scientific thought in its early years, general relativ- ity experienced a so-called ‘low-watermark period’, going roughly from the mid-1920s to the mid-1950s (Eisenstaedt 1986, 1987a, 2006), during which it remained cut off from the mainstream of physics and was perceived as a sterile, highly formalistic sub- ject. Accompanied by a series of major astrophysical discoveries, the status of General Relativity definitely changed in the 1960s, when it became an extremely vital research stream of theoretical physics. Quasars, the cosmic microwave background radiation, and pulsars – soon identified as rotating neutron stars – led to the recognition that physical processes and astrophysical objects exist in the universe that are understand- able only in terms of the general theory of relativity. In providing definitive proof of the existence of neutron stars, the discovery of pulsars and binary X-ray sources, made a e-mail: [email protected] 312 The European Physical Journal H even plausible the possibility of black holes, entities that had previously existed only in the minds of a few theorists. In raising new challenges to the emerging relativity community, these had of course an important role in strengthening the process which turned general relativity into a “subdiscipline of physics” (Blum et al. 2015, 2016). However, the view of a community of relativists magically awakened from its slum- ber by the new astrophysical discoveries is too one-dimensional. As Alexander Blum, Roberto Lalli, and J¨urgen Renn have outlined in their historiographical framework exploring the main factors underlying the return of general relativity into the main- stream of physics, a complex series of elements underlying such process must be taken into account: intellectual developments, epistemological problems, technological ad- vances, the characteristics of post-World War II and Cold-War science, as well as the newly emerging institutional settings. Starting from the mid 1950s, further im- plications began to be explored and general relativity gradually came into focus as a physical theory. This framework, in which they propose to speak of a reinvention of general relativity, rather than a renewal, is leading to an understanding of the rein- vention as a result of two main factors: the recognition of the untapped potential of general relativity and an explicit effort at community-building. These two factors al- lowed this formerly dispersed field to benefit from the postwar changes in the science landscape. The dynamics underlined in (Blum et al. 2015) is actually independent from – and prior to – the major astrophysical discoveries of the 1960s. Up to that time, the view prevailed that general relativistic effects were significant only for cosmology. However, the violent events that seemed to occur in the core of radio galaxies involv- 6 ing enormous energies corresponding to a rest-mass energy of 10 solar masses (M) (Burbidge 1959), the growing field of nuclear astrophysics (Burbidge 1962; Burbidge et al. 1957; Cameron 1958), and the eventful discovery of quasars, had prepared the stage for the emerging awareness at the beginning of the 1960s of physical processes in which general relativistic effects are dominant and that could release much larger fractions of the rest mass as energy than the small fraction provided by the binding energies of nuclei. Such processes that did seem possible in the framework of gen- eral relativity suggested the actual existence of astrophysical objects in the universe satisfying requirements that appeared to be beyond the scope of nuclear physics. The problem of finding the source of the tremendous energy stored in cosmic rays and magnetic fields of some powerful radio galaxies, led to a theory put forward by William Fowler and Fred Hoyle in January 1963. They suggested that exceedingly massive star-like objects probably could exist with masses up to 108 times that of the sun at the center of those galaxies. The gravitational collapse of such supermassive stars could be the driving force behind the great amount of energy emitted by those strong radio sources (Fowler and Hoyle 1963a). Their opinion was that in the process 7 8 of contraction of a mass of 10 − 10 M “general relativity must be used” in order to obtain the energies of the strongest “stellar-type” sources (Fowler and Hoyle 1963b, p. 535). A few months after this proposal, new objects were discovered, having apparently masses of this order of magnitude, dimensions of about a light week, and having a luminosity two orders of magnitude larger than the luminosity of a large galaxy having dimensions a million times larger and containing something like 1011 stars. In particular, the crucial identification of the high redshift of the already known radio source 3C273 (Hazard et al. 1963; Oke 1963; Schmidt 1963) and of the source 3C48 (Greenstein and Matthews 1963), made now even more pressing the problem to explain the mechanism whereby these and other sources that were masquerading as a star and were thus identified as “quasi-stellar” objects, managed to radiate away the energy equivalent of five hundred thousand suns at a very fast rate. L. Bonolis: Stellar structure and compact objects before 1940 313 The “supermassive stars” suggested by Fowler and Hoyle immediately became an attractive explanation for these new peculiar astrophysical objects, that appeared to be farther away than most known galaxies but were luminous enough to be observed by optical telescopes. Their enormous luminosity could also sharply change in the course of one week, as analysis of historical plate material of Harvard Observatory showed (Smith and Hoffleit 1963). As such enormous energies must be emitted by regions less than one light-week across, collapsed objects became candidates for the engine of quasi-stellar radio sources. The intriguing discovery of quasi-stellar radio sources – soon renamed quasars (Chiu 1964, p. 21) – with their large red-shifts and corresponding unprecedented- large radio and optical luminosities, opened up the discussion on a series of exciting questions. Among the problems raised were the following: Were these objects the debris of a gravitational implosion? By what machinery could gravitational energy be converted into radio waves? Would gravitational collapse lead to indefinite contraction and a singularity in space time? If so, how should theoretical assumptions be changed to avoid this catastrophe? (Robinson et al. 1965, Preface). “The topic was just right for reporting and sorting out observations as well as for theoretical analysis” (Schucking 1989, p. 51): during the summer 1963, three rel- ativists in Dallas, Ivor Robinson, Alfred Schild, Engelbert Schucking, realized that a conference bridging the gap between the still exotic world of general relativity and the realm of astrophysics, might be well timed, and it would be a perfect occasion to make known the recently created Southwest Center for Advanced Studies. They immediately involved Peter Bergmann, an influential relativist who had been asso- ciated with Einstein at the Institute for Advanced Study in Princeton since 1936, and sent out letters of invitation. Three hundred relativists, optical and radio as- tronomers, and theoretical astrophysicists attended the International Symposium on Quasi-Stellar Sources and Gravitational Collapse (Robinson et al. 1965), the first of the long series of Texas Symposia, which set up the stage merging two seemingly distant fields: general relativity and astrophysics, so distant that the organizers had to invent a new label for this brand new field: “The suspicion existed that quasars might have something to do with relativity and thus might fit into an imaginary dis- cipline combining astronomy with relativity. One of us – Alfred, Ivor or I? – invented a catch phrase for this new field of science: relativistic astrophysics [emphasis added]” (Schucking 1989, p. 50). Robert Oppenheimer was asked to chair the first session, a most natural choice, because of his involvement in the first systematic application of Einstein’s general theory of relativity to a compact astrophysical object. Oppenheimer’s three papers published between 1938 and 1939, each with a different collaborator (Oppenheimer and Serber 1938; Oppenheimer and Volkoff 1939; Oppenheimer and Snyder 1939), are regarded as a milestone both in his scientific production and in the path eventually leading to the emergence of relativistic astrophysics in the early 1960s.