MSc Physics and Astronomy GRavitation AstroParticle Physics Amsterdam Master Thesis Together we rise: The Renaissance of General Relativity, rebuilding Dutch physics, and co-producing American hegemony in the post-war period A study of John A. Wheeler's guest professorship at Leiden University in 1956 by Eline V.A. van den Heuvel 10616357 July 2020 60 EC October 2018 – July 2020 Supervisor/Examiner: Examiner: Prof. dr. J.A.E.F van Dongen Prof. dr. D.D. Baumann Institute for Theoretical Physics Together we rise: The Renaissance of General Relativity, rebuilding Dutch physics, and co-producing American hegemony in the post-war period A study of John A. Wheeler's guest professorship at Leiden University in 1956 Eline V.A. van den Heuvel July 8, 2020 Abstract - The post-war period was a crucial time in international and Dutch history of science. The United States used her scientific superiority to influence international relations and nuclear proliferation by scientific exchange, thereby influencing the Dutch, who were rebuilding their scientific infrastructure and economy and were looking to the US for guidance, focusing on nuclear aspects. At the same time, theoretical physics flourished under her new war-induced status and money inflow, being a starting condition for the revival interest in relativity. The connection between the above is hard to understand while looking at the events separately. Instead, this research focuses on John A. Wheeler's Visiting Lorentz professorship at Leiden University in the Netherlands in 1956. Wheeler, a former student of Niels Bohr, was a leading nuclear and theoretical physicist. He participated in the Manhattan Project and Project Matterhorn, and was largely responsible for the revived interest in relativity in the late 1950s. This research explains Wheeler's visit to Leiden in the historical context, thereby linking the rebuilding of Dutch physics, American scientific diplomacy and the revived of relativity, and clarifying the Dutch scientific orientation to the United States, while finding quite divergent expectations by Wheeler and the Dutch. 1 CONTENTS CONTENTS Contents Introduction 3 Part I 4 1 Early career and war efforts 4 2 Scientific Manpower in the United States and NATO 8 3 Rebuilding Dutch Physics and co-producing American hegemony 12 4 Physics in Leiden in the first half of the 1900s 16 Part II 19 5 Renaissance of General Relativity 20 5.1 The low-water-mark period . 20 5.2 The Renaissance period . 22 6 Wheeler entering relativity 24 6.1 Prelude to relativity: \Everything as Electrons" . 24 6.2 Teaching general relativity . 28 6.3 Relativity: particles in terms of space-time . 29 6.4 Radical reductionism & daring conservatism . 31 Part III 33 7 Wheeler in Leiden 33 Conclusion 41 Acknowledgments 43 Consulted archives 43 References 43 2 CONTENTS CONTENTS Introduction The post-war period was a crucial time in the history of international and Dutch physics. World War II had shown the power of physical and technological development. Warring countries, like the United States, had made major physical and technological progress, which had further advanced under the pressure of the emerging cold war. At the same time, the scientific structure of occupied countries, like the Netherlands, had been heavily damaged. This had led to the situation in which the United States used her scientific superiority to influence the Dutch, who were looking for international collaboration to rebuild their scientific infrastructure. At the same time, theoretical physics had flourished under her new war-induced status and money inflow, setting of a process that has later been dubbed the renaissance of general relativity. John Archibald Wheeler (1911-2008) was one of the key figures in international physics in the post- war period. Wheeler was a former student and collaborator of Niels Bohr, and a former participant in the Manhattan project. He was one of worlds most prominent nuclear physics. In the early 1950s, Wheeler started to change the focus of his research agenda. He switched from a successful and thoroughly mainstream career in nuclear physics to his groundbreaking research of general relativity and became one of the key figures in the renaissance of general relativity. He established one of the world's most important hubs of the renaissance of relativity at Princeton, and he trained a generation of leading American relativists. In the spring semester of 1956, Wheeler visited Leiden, the Netherlands, as a Lorentz professor. He took a small team of students with him. With his students, Wheeler worked on his new research interest: formulating particles in terms of spacetime. Furthermore, he taught a course on relativity, quantum mechanics and the connection between those two theories. Also, he collaborated with the Dutch and other European scientists on his work on formulating particles in terms on spacetime, as well as on nuclear physics. We aim at understanding this scientific exchange in her historical context, and at using this understanding to gain more insight in the historical context. To understand the actors involved and their motivations and interests, we study the different facets of the historical background and look at the details of Wheeler's visit to Leiden. Once we understand how this scientific exchange came to be, it can give us more insight in the different aspects of the historical background and of the connection between the separate aspects. To do so, we will start by looking at Wheeler's career in nuclear physics, and the influence of World War II on physics in the Unites States, the Netherlands, and the scientific collaboration between the two countries. Then, we will look at the history of relativity and at why Wheeler changed his career from nuclear physics to relativity. Last, we will analyze his guest professorship at Leiden University in 1956. 3 1 EARLY CAREER AND WAR EFFORTS 1 Early career and war efforts1 John Archibald Wheeler was born in Jacksonville, Florida, on the 9th of July 1911.3 He was the first of four children of librarian Joseph and homemaker Mabel (\Archie") Wheeler. John was an inquisitive child, who devoured Sir John Arthur Thomson's classic \Introduction to Science" and Franklin Jones' \Mechanisms and Mechanical Movements." After going to high school in Baltimore, Maryland, he went to Johns Hopkins University on a scholarship at the age of 16. He started his career focusing on nuclear physics, with a graduate research on Theory of dispersion and absorption of helium under supervision of Karl Herzfeld.4 At the age of 21, he earned his doctorate in physics, making him the first in his family to become a scientist. After a postdoc year with Gregory Breit at New York University, Wheeler went on to work at the University of Copenhagen with Niels Bohr from the autumn of 1934 until June 1935. He focused on the interaction of cosmic rays with nuclei. Then, he got a three-year position at the University of North Carolina at Chapel Hill, and in 1938 he was appointed assistant professor at Princeton University. On the 16th of January 1939, Niels Bohr and Leon Rosenfeld Figure 1: A picture of John A. 2 arrived in New York carrying some exciting news across the At- Wheeler. lantic. They were on their way to a three-month stay at Princeton to discuss the fundaments of Quantum Mechanics with Albert Einstein.5 However, not much came of this since the news would attract all attention. On January 3, shortly before departing from Copen- hagen, Bohr had learned from Otto Frisch, a German physicist working in Copenhagen, that he and his aunt Lise Meitner had postulated fission. They had done so after seeing the results of an experiment by the German physicists Otto Hahn and Fritz Strassman, that had proved that bombarding uranium with neutrons led to barium production. Bohr wanted to keep the news secret to make sure Meitner and Frisch would get the credits they deserved, but Wheeler persuaded Rosenfeld to tell him the news. Now that the secret had been spoiled, Bohr excitedly started to work on fission, helped by his former student Wheeler. Bohr stayed at Princeton from January until May 1939 to explain the process of fission with Wheeler.6 Bohr and Wheeler worked on the Liquid Drop Model of atomic nuclei and showed that the excitation of a nucleus results in a sequence of distortions. They calculated the energy barrier that needs to be overcome to undergo fission, which depended on the mass-charge-ratio of the isotope. Bohr and Wheeler described the conditions in which uranium can undergo fission, finding that working with low-energy neutrons was more effective in making uranium with high-energy neutrons. Furthermore, they found that the energy barrier of the rare isotope uranium-235 is lower than for the common 238 isotope, which meant that the rare isotope is most fissionable. Also, they calculated that a new, artificial isotope 1This chapter is primarily based on chaper 1 to 9 from (Wheeler & Ford, 2000) and on (Halpern, 2017) 2Figure taken from https://www.manhattanprojectvoices.org/people/john-wheeler-0 on the 6th of June 2020. 3https://www.princeton.edu/news/2008/04/14/leading-physicist-john-wheeler-dies-age-96 visited on 7th of May 2020 4(Wheeler, 1933) 5(Wheeler, 1979), p.272-282 & Intervieuw Wheeler by Marcia Bartusiak, conducted for her book Einstein's Unfinished Symphony, 3th of March 1998, NBLA & Letter Wheeler to Mrs. Rosenfeld on the 28th of February 1982, NBLA 6Idem. & (Kleemans, 2009) 4 1 EARLY CAREER AND WAR EFFORTS plutonium-239 should be able to undergo fission with slow neutrons. This knowledge would become of fundamental importance in the development of the American atomic bomb. Bohr and Wheeler published their findings on the 1st of September 1939, coincidentally the same day that Adolf Hitler invaded Poland and started World War II in Europa.