Bachelor's Programme in Physics and Astronomy J. Robert Oppenheimer's Neutron Core Trilogy A Search for Full Stellar Collapse in the Late Nineteen Thirties by Lo¨ısBlank Abstract The theoretical physicist J. Robert Oppenheimer wrote three papers on the end state in the life of stars, of which the last described indefinite gravitational contraction, nowadays known as a black hole. These papers concern physics, astron- omy and general relativity: three concepts that were very separated from each other during the nineteen thirties. This thesis gives an overview of the theory of stellar evolution between 1910 and 1940, and the scientific debate of the relation between physics and astronomy, in order to get a deeper understanding of the three papers of interest. It will be shown that Oppenheimer's turn to this more astronomical topic was not fully unexpected, since the path of his career between 1925 and 1930 and Richard C. Tolman played an important role in this. Furthermore this thesis will conclude that Oppenheimer's results can be seen as an anomaly in the theoretical physics paradigm from the nineteen twenties until forties. Report Bachelor Project (size 15 ECTS) Conducted between May 4th 2020 and October 6th 2020 Supervisor: prof. dr. J.A.E.F van Dongen Examiner: dr. M.L. Vonk Institute of Theoretical Physics Faculty of Science, University of Amsterdam Populaire samenvatting De laatste jaren zijn zwarte gaten een aantal keer in het nieuws geweest; bijvoorbeeld in april 2019 toen er voor het eerst een foto was gemaakt van een zwart gat. Zwarte gaten, plekken in de ruimte waar massa naartoe wordt getrokken en waar licht niet uit kan ontsnappen, houden de natuurkunde al enkele tijd bezig. Sinds eind jaren vijftig van de vorige eeuw, is dit een bekend thema binnen de natuurkunde, maar eind jaren dertig van diezelfde eeuw is er ook al onderzoek naar gedaan. J. Robert Oppenheimer was een theoretisch natuurkundige, die vooral bekend is als de 'maker' van de atoombom. Echter heeft hij zich ook bezig gehouden met andere onderwerpen in de natuurkunde, onder andere zwarte gaten. In 1938 en 1939 heeft hij drie artikelen geschreven over neutronensterren, waarbij het laatste artikel beschrijft wat er gebeurt als een ster volledig in elkaar stort; in de jaren zestig is hier de naam zwart gat aan gegeven. In deze scriptie worden deze drie artikelen bekeken, in de hoop deze artikelen beter te begrijpen. Ook wordt er gekeken naar de ontwikkeling van de theorie over sterevolutie tussen 1910 en 1940, om te kijken hoe de artikelen van Oppenheimer zich verhouden ten opzichte hiervan. Uiteindelijk zal blijken dat Oppenheimer zijn tijd vooruit was in het ontwikkelen van de theorie over volledige ineenstorting van neutronensterren. Echter zal ook duidelijk worden dat, kijkend naar zijn carri`erepad,het niet volledig onverwacht is dat hij deze theorie ontwikkelde. 2 Contents 1 Introduction4 2 Stellar Evolution7 2.1 The Rise of White Dwarfs........................7 2.2 Energy Problem of White Dwarfs....................8 2.3 Mass Problem of White Dwarfs.....................9 2.4 Final State of White Dwarfs....................... 11 3 Oppenheimer's Quest for Full Collapse 12 3.1 Prologue................................. 12 3.2 Entering the Stellar Realm........................ 13 3.3 Part One: Unstable Neutron Cores................... 14 3.4 Part Two: Massive Neutron Cores.................... 15 3.5 Part Three: No More Neutron Cores.................. 16 4 Conclusion 18 References 19 3 1 Introduction J. Robert Oppenheimer is best known as the 'father of the atomic bomb', but he was a lot more than that. As Bernstein (1996) states: "Great science sometimes produces a legacy that outstrips not only the imagination of its practitioners, but also their intentions" and three papers Oppen- heimer wrote in 1938 and 1939 turn out to be a great example of this. As was typi- cal of Oppenheimer, he wrote the papers with three of his students: Robert Serber, George M. Volkoff and Hartland S. Snyder. Bernstein(2004) even calls the paper with Snyder one of the great papers in twentieth century physics: The paper is a pioneer in describing how a black hole can be formed. Figure 1: A picture of J. Robert Oppen- 1 This topic would become very popular in heimer. the late nineteen fifties–John A. Wheeler would introduce the term black hole in 1967{and with the first astronomical picture of a black hole fresh in our minds, it is safe to say that this phenomenon is still a very relevant topic in both physics and astronomy. Oppenheimer was born in 1904, which means that his scientific career flourished during the same era as the development of quantum mechanics and Einstein's theory of general relativity. Oppenheimer gained a prominent place in the scientific world: With Harvard as his alma mater and his doctorate from the University of G¨ottingen, he found himself in a perfect position to do physics. His time at Berkeley is even said to play an important role in making this university the centre of theoretical physics in the United States. Bird and Sherwin(2005) describe that, not long after Oppenheimer started teaching, the word started to spread around the country, that if one aspired a career in theoretical physics, Berkeley was the place to do this. 1Picture taken from https://alchetron.com/J-Robert-Oppenheimer on 10 September 2020. 4 As stated, only three papers of Oppenheimer's oeuvre are on the topic of neutron cores and today physicists agree that this part of his career was groundbreaking. The last paper of this trilogy was published on the day Nazi Germany invaded Poland and drew little attention at the time: in fact the trilogy as a whole experienced a period of neglect after its publication.2 However, nowadays we know that Oppen- heimer's three papers opened the door to twenty-first century black hole physics (Bird and Sherwin, 2005, 89). If Oppenheimer was moved to work on this topic, why did he write only three papers about it? This question carries a lot of other questions with it. One can search for the answer by looking at Oppenheimer and the paradigm of theoretical physics during the nineteen twenties and thirties.3 One could also broaden the scope of research by looking at Oppenheimer, other physi- cists who worked on this topic and the physics paradigm of the nineteen twenties and thirties. In this thesis we will focus on Oppenheimer and try to better understand the position of the trilogy on neutron cores in his scientific career. This will be done by looking at the development of his career and physics during the nineteen twenties and thirties, concerning the topic of neutron cores, up to and including the publica- tion of Oppenheimer's third article. In order to do this, section2 gives an overview of a history of stellar evolution in the making, focusing on what eventually let to the hypothesis of neutron cores. In the third section, we will look at Oppenheimer and the three papers of interest. We will place Oppenheimer within the historical context from section two, to get a deeper understanding of the three papers and try to place these within his career. By the end of this thesis we will compare the posi- tion of Oppenheimer's neutron core trilogy in his career and the theoretical physics paradigm from the nineteen twenties and thirties, with the evolution of science as described by Thomas S. Kuhn. Before we start our discussion about neutron cores and Oppenheimer, a few general remarks, should be made. To do historical research about the early twentieth century, it is important to know what the scientific world was like during this time. Since some call this era a golden age of physics, a lot can be said: we will try to point out the things, relevant for this thesis. First of all it is important to realise that the landscape of scientific disciplines was not like it is nowadays: fields as cosmology or astrophysics were not known in the way we know them now. A good anecdote on this is given by Bonolis(2017), who explains that the first Texas Symposium in 19634 was actually set up with the idea of merging general relativity and astrophysics. The organisers of the symposium thought this was about time, since the suspicion existed that relativity had something to do with quasars and they came up with the name relativistic astrophysics for this new discipline. An important note is that this event took place almost thirty years after Oppenheimer published his work on full stellar collapse. So a clear example of a deviation from physics today, is that physics and astronomy were a lot more separate 2See Hufbauer(2005) for an extensive reading on the possible reasons for this. 3Where paradigm is the common intellectual framework shared by members of a certain com- munity for whom there are enough puzzles left to resolve (Kuhn, 1962, 10). 4It was called the International Symposium on Quasi-stellar Sources and Gravitational Collapse and chaired by Oppenheimer. The Texas Symposia evolved into a long series, from (Bonolis, 2017, 313). 5 from each other in the nineteen twenties and thirties. A second issue, which is connected to what is just said, is that Einstein's theory of general relativity experienced a so-called low-watermark period from 1920 until 1955: During this period only some specialists worked on it, putting it in a state of little progress and definitely not in the position of a conventional theory (Blum et al., 2016, 344). The same Texas Symposium as just mentioned, can be seen as the moment when general relativity started to get more attention{after a period in which is was attributed to mathematicians{and in doing so blurred the lines between physics and astronomy (De Swart et al., 2017, 6).