Msc Physics and Astronomy Together We Rise

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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

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. They understood that the fission process produces neutrons, and that slowing down the neutrons could cause a fission chain reaction that released an enormous amount of energy.7 Also other physicists understood that fission was not only exciting; many physicists, especially those who fled fascism in Europe e.g. Enrico Fermi, Eugene Wigner, Leo Szilard, Edward Teller and Albert Einstein, worried that Nazi Germany might develop a nuclear weapon. Therefore, Einstein sent a letter that had been prepared by the three Hungarian physicists Szilard, Teller and Wigner to president Roosevelt in August 1939. This letter warned Roosevelt for a possible nuclear weapon program of Nazi Germany. Although Einstein sent two more letters to Roosevelt, and Roosevelt thanked for the letters and said to acknowledge the importance of the matter, it was not until the 6th of December 1941, the day before the Japanese bombardments on Pearl Harbor that dragged the United States into World War II, that Roosevelt put serious funding in the Manhattan Project, which aimed to produce an American nuclear weapon to balance out the possible future German power. The paper by Bohr and Wheeler that had shown that uranium-235 and plutonium-239 where fissionable, had been the first step in producing a nuclear weapon. Now, the Manhattan scientists had to find out how to enrich uranium and how to produce plutonium, what the critical mass is for a chain reaction, and how to put the fissionable material in a bombshell. To solve these scientific and technological problems before the Nazi’s did, the United States and her allies, Canada and the United Kingdom, called for their brightest scientists. Those scientists were set to work in laboratories in i.a. Chicago, Oak Ridge, Los Alamos and Hanford. Wheeler was one of the physicists that worked on the Manhattan Project. His work with Bohr had made him one of the most established nuclear physicists, so it was not surprising that Arthur Compton, leader of the Chicago division of the Manhattan project, asked Wheeler to join him. In Chicago, Wheeler joined Eugene Wigner’s group that worked on the design of plutonium producing reactors. Wheeler was convinced that building an American nuclear weapon was the best way to restrain Germany and to restrain casualties. He rather wanted to work on theory, but his moral to protect the world from the Nazi’s was stronger. Wheeler felt very strong for the European Jews. He helped the Jewish rooted German-American physicist student Henry Barshall, who emigrated to the US in 1937 during the early Holocaust period. Wheeler “and Janette treated me [Barshall] almost as a member of your family during the difficult war years, and I remain indebted to you not only for your hospitallity while I was a student, but also for willingness to help bring my parents to the U.S.”8 Also after the war, Wheeler kept fulfilling his duty to the American government. For example, he became consultant at Brookhaven National Laboratory, and he worked in different functions for the Atomic Energy Agency (AEC),9 which has been established by congress in 1946 as the successor of the Manhattan program. The objective of the AEC was to monitor the progress in the atomic sciences and technology in peacetime. Wheeler was also one of the founders JASON, an independent group of elite scientist which advise the US government on matters of science and technology, mostly of a sensitive nature. In contrast with the Nazi program, the Manhattan Project made a lot of scientific and technological

7Idem. 8Letter H.H. Barshall to Wheeler, 1st of February 1977, A family gathering volume 2, NBLA 9Contract Brookhaven National Laboratory, 12th of December 1947, JAWP, Job Notebook folder 1, Box 14 & Contract Brookhaven National Laboratory, 1st of July 1948, JAWP, Job Notebook folder 1, Box 14 & Contract reactor safeguard committee, 10th of July 1950, JAWP, Job Notebook folder 2, Box 14

5 1 EARLY CAREER AND WAR EFFORTS

progress, leading to the first successful nuclear test on the 16th of July 1945. In August 1945, president Harry Truman gave the order to bomb Hiroshima and Nagasaki. This heralded the end of World War II. The happiness over the end of the war quickly gave floor to ethical questions concerning the Japanese civil casualties. Many physicists that had warned Roosevelt about the danger of the nuclear weapon program of Nazi Germany, wanted to produce a bomb to balance power, and not to actually use it. The situation became even more painful when it became clear that the Nazi program had made almost no progress. Physicist including Einstein, Bohr and Szilard where now of the opinion that the knowledge needed to build nuclear weapons should be shared with the world. Since the nuclear knowledge of the United States could not be “unlearned”, they saw this as the only option to balance power again, to make sure that the United States would not use nuclear weapons again. However, others, including Teller, were of the opinion that the United States should protect their nuclear secrets, and push the nuclear program even further. They warned for a nuclear arms race with the Soviet Union and wanted to produce a hydrogen bomb to keep the American lead against the “enemies of democracy”. Indeed, a nuclear arms race bursted on the 23th of September 1949, when Harry Truman announced that the Soviet Union had successfully tested a nuclear bomb. The United States feared that Joseph Stalin would use his nuclear powers to reinforce his influence in eastern Europe and spread his power further over the world. Although Wheeler’s attitude was not as aggressive as Teller’s, Wheeler was unquestionably an advocate of nuclear weapons.10 According to Lawrence Wilets, who was a student of Wheeler at that time, “my decisions to join Matterhorn was governed as much by your [Wheeler’s] charisma and the prospect of learning theory from you, as by the political arguments you so forcefully propound.”11 This belief in the hydrogen bomb originated from the death of his brother, who had died as a soldier fighting the Axis powers in Italy in 1944. It had taken Roosevelt two years after Einstein’s letter to start significant funding for the Manhattan Project, and it had costed another four years to produce and test the bomb. Wheeler did not regret the Japanese casualties like most of his former Manhattan Project colleagues, who now distrusted the military with the use of nuclear weapons. Instead, he wished that the project had started two years earlier, so that the war would have ended two years earlier, which would have saved the 15 million deaths that had fallen in the last two years of the war, including that of his brother’s. Also, as Wheeler explained to James Griffin, one of his student: whether the bomb is a good or a bad thing, without it, you wouldn’t be here.12 The need for an American hydrogen bomb was unquestionable to Wheeler. Therefore, Wheeler decided to participate in Project Matterhorn, the program developing the thermonuclear bomb. On the 23th of September 1949, Wheeler was in Paris. After his war effort at the Manhattan Project, he had gone back to his theoretical work. He had a Guggenheim Fellowship to work in peace and quiet in Paris, far away from his obligations at Princeton.13 This trip had started in July 1949 and early, in January 1950. Edward Teller and Henry Smyth, former chair of physics at Princeton and AEC commissioner, asked Wheeler to come back to the United States to serve national security again.14 Following the successful test of a nuclear weapon by Soviet Union in September 1949, the United States accelerated her program to develop a new nuclear weapon. Project Matterhorn was established with the goal to develop a thermonuclear bomb. Teller and Smyth invited Wheeler to Los Alamos to join Project Matterhorn. Wheeler accepted the invitation and worked for Project Matterhorn

10Letter from Richard Bellman, 25th of March 1977, A family gathering volume 1, NBLA 11Letter from Lawrence Wilets, 7th of February 1977, A family gathering volume 1, NBLA 12Personal communication with James Griﬃn on the 21st of January 2020 at Maryland University, College Park. 13Letter Wheeler to Bohr, 13th of June 1949, JAWP, Niels Bohr material folder 2 1949-1956, Box 5 14Letter N.E. Bradbury to Shenstone c.c. Wheeler & Smyth, 18th of March 1950, JAWP, Job Notebook folder 2, Box 14

6 1 EARLY CAREER AND WAR EFFORTS for almost three years. The first year, he worked at Los Alamos, and then he started a satellite campus in Princeton University. Many of Wheeler’s Princeton colleagues found his decision to work for Project Matterhorn “well-meaning but na¨ıve”, and asked: “Why do you let the AEC persuade you to do this sort of things?”15 Wheeler found it important to let his colleagues and the Guggenheim foundation know that “I [Wheeler] have come out here not as a physicist because the work is technically interesting, but as a citizen because the country considers it needs a certain job done”,16 and that he was not “hypnotized by Teller”.17 Wheeler wanted to work on his own theoretical work, and he feared for his reputation, but his fear to make the same mistake again as with Hitler was strong enough to go against his great examples, Einstein and Bohr, and to set aside his theoretical work again. The election of president Dwight Eisenhower, a few days after testing Mike, caused a further mutual distrust between scientists and military. Eisenhower was a high-ranking army general who had fought in Europe during World War II. On one hand, scientists where afraid that the military would be to nonchalant in using nuclear weapons, because Eisenhower prioritized nuclear weapon production over funding of other army divisions. Also, had threatened to use nuclear weapons if China did not want to come to a peace agreement in 1953. On the other hand, the United States government was afraid of espionage from the Soviet Union, and military officials accused nuclear scientists of being incautious. Moreover, the hawks of the Eisenhower administration feared that Soviets secretly ran the international arm control and disarmament organizations of which many American physicists had become members. They suspected those organizations to be a Soviet attempt to establish her hegemony. It was against this tense background that the United States Government accused Oppenheimer, who had been against the development of the hydrogen bomb, of having ties with the communistic party. In a painful process, they repealed his security clearance. On the 1st of January 1953, Wheeler got in trouble with Eisenhower too. He had lost secret documents that contained specific information about nuclear bomb design on a train.18 Wheeler was not supposed to carry those documents in public, but he wanted to use his travel time as efficient as possible. After a nap, Wheeler had found out that the documents were missing, so he called the government and agents searched the whole train, but they never found the documents. Thereafter, FBI started a long investigation on Wheeler, even searching his house, but finding no convincing reasons to recall his security clearance. It was an accident. Therefore, the accident did not have much effect on Wheeler’s career. Wheeler was already planning on leaving the project, so he finished his work and returned to Princeton. The AEC thanked him for his efforts on his leave.19 He could even have stayed in service as consultant of the Reactor Safeguard Committee at the Brookhaven National Laboratory, which was building a graphite reactor.20 Wheeler left Project Matterhorn to return to his theoretical work. He had agreed with Allan Shen- stone, chair of the physics department of Princeton University, that he would teach a course on general relativity already in May 1952.21 But why was Wheeler going to teach relativity? He was one of the best nuclear physicists, and not specialized in relativity. Also, as will be explained in chapter 5, the research on relativity had been stagnated for already almost thirty years. We will answer this question in chapter 6, but not before we look at the status of physics in the United States and the Netherlands, and the scientific relations between both countries in the intermediate period after World War II.

15Letter Wheeler to Harry D. Smyth, April 1950, JAWP, Job Notebook folder 2, Box 14 16Letter Wheeler to Harry D. Smyth, 14th of March 1950, JAWP, Job Notebook folder 2, Box 14 17Letter Wheeler to Harry D. Smyth, April 1950, JAWP, Job Notebook folder 2, Box 14 18This paragraph is also based on Federal Bureau of Investigation ﬁles on John Archibald Wheeler, NBLA 19Letter Gordon Dean to Wheeler, 27th of October 1952, JAWP, Job notebook folder 2, Box 14 20Contract AEC, 10th of July 1950, JAWP, Job Notebook folder 2, Box 14; Letter Frederic H. Williams to Wheeler, 19th of April 1955, JAWP, Job notebook folder 3, Box 14 21Relativity notebook 1, JAWP, p.1

7 2 SCIENTIFIC MANPOWER IN THE UNITED STATES AND NATO

2 Scientiﬁc Manpower in the United States and NATO.22

World War II changed the way the United States valued and shaped physics research. In contrast to World War I, during which chemical weapons had dominated the battlefield, the outcome of World War II had been determined by physics developments.23 During the war, the United States government had realized that scientific and technological development, like microwave radar, proximity fuzes and solid-fuel rockets, could reduce the need of risking ground troops, and that national security depended on it.24 The dropping of the atomic bombs had been the beginning of the end of the war and took away all doubts about the power of scientific, and technological development. The conviction that scientific and technological development was essential for the survival of the nation persisted during the Cold War. The United States had to stay ahead of, or at least keep up with, the developments of the Soviet Union to balance power and secure the bipolar world. World War II had shown the importance of investing in pure science, like the pre-war work of Bohr and Wheeler that had been essential to the success of the Manhattan Project, as well as defense-related research, like nuclear reactor design. Investing in physics research resulted not only in scientific knowledge, but it also put the United States in a position to exercise soft-power in Europe. This soft power could be used to spread the American hegemony in Europe and profit from European science, as will be explained in chapter 3. Investing in physics research became very important for national security. Therefore, the funding for fundamental research ballooned. This was strengthened because of the American military-industrial complex. A military-industrial complex is an informal alliance between a nation’s military and the defense industry, which share the interest in a large and technologically developed army: the industry is paid to produce weapons, and the army obtains them. President Dwight Eisenhower, who had roots in the military himself, warned in his farewell speech for the “unwarranted influence [...] by the military-industrial complex”, because “the potential for the disastrous rise of mis- placed power exists and will persist.”25 Under the influence of this group, the amount of governmental spending on physics research inflated: between 1938 and 1953, the total funding for fundamental physics research within the United States increased by a factor 20 to 25, taking inflation into account.26 Un- surprisingly, the vast majority of funding had come from defense-related agencies within the federal government.27 But not only the amount of and the source of governmental funding for physics research changed. Also the size and nature of physics research projects changed drastically during and after the war. Little Science projects, where individual authors may have significant impact, became largely outnum- bered by Big Science projects, which are characterized by big governmental budgets, big laboratories, big apparatus and big research groups.28 The Manhattan project is a perfect example of this new style of physics research: the project costed 22 billion American dollars,29 occupied multiple laboratories, made use of big equipment, and employed more than 125 thousand people by 1944.30 But with Big Science, a big problem arose: there were not enough physicists to fulfill all the new positions at the new projects. Due to the growing interest in physics, the consensus grew that the United States now dealt with

22This chapter is primarily based on (Kaiser, 2002), and (Krige, 2000, p. 84-91) 23(Kaiser, 2015), p.523 24(Kevles, 1990), p239-240 & (Kaiser, 2014), p.154 25Eisenhower’s farewell address, 17th of January1961 26(Kaiser, 2002), p.132 27(Forman, 1987), p.190 28The popularization of the term “Big Science” is often attributed to Alvin Weinberg (1961) 29With an equivalent today equivalent of 22 milliard American dollar (Stine, 2008), p.1 30(US. Department of Energy, 1946), p.124-125

8 2 SCIENTIFIC MANPOWER IN THE UNITED STATES AND NATO a shortage of “scientific manpower”.31 Physicists were needed to execute fundamental research in big science projects, and to translate this fundamental knowledge to military use. Furthermore, physicists had to train the people who were going to use the new technology, and good physicists were needed at universities to train more physicists. Together with the growing interest in physics, the training of physicists became priority. The threat of the Soviet Union made that the United States not only wanted to fill the jobs for the big science projects, but that she wanted to create an overcapacity of scientific manpower. The United States needed to train an army of potential weapon builders for when it would come to war with the Soviet Union. When the United States entered the Korean war in June 1950, the training of physicists became top priority of the government: a year after the United States entered the Korean war, the government had increased her demand by a factor of ten.32 According to Henry Smyth, no one knew if war would “come soon or [. . . ] not for years”, but “scientific manpower [is] a major war asset of this country” which needed to be “stockpiled” and “rationed”. Smyth was “speaking of scientists”, a group he belonged to himself, “not as men who enrich our culture but as tools of war needed for the preservation of our freedom.” He used this objective and impersonal way of speaking about scientists because he faced with “courage and intelligence” the reality that the survival of the society of “free man and woman of the United States” was at stake, and that the United States needed to use “men of science, traditionally peaceful, internationally minded, and nonpolitical,” to the greatest advantage for national security. President Truman agreed that students in “essential topics”, i.e. being science, engineering and medicine, must be exempted of active military service because of their value to the nation. Most physicists, although being funded with defense-related money from the government, were primarily concerned with training potential weapon makers, i.e. creating “scientific manpower.”33 Also, they performed fundamental research, which was justified by their education task. They were not directly involved in weapon development, but with building an army of potential weapon builders. The United States needed research facilities to educate this “scientific manpower”. Mid 1950s, the number of students could not multiply more than three times compared to the pre-war numbers because universities could not educate with more students.34 This increase in students had already caused overcrowded offices and laboratories. The AEC now played an important role in the training of physicists. The AEC took over most of the Army and Navy research installations, created an extensive national laboratory system and supported many university-based laboratories. Not only the number of research faculties increased by the need of producing scientific manpower, but also the choice of research projects partly depended on the educative value. An example can be found at Princeton University. Wheeler wanted to set up a satellite facility of Project Matterhorn, the AEC’s H-bom project, at Princeton University.35 To convince Allen Shenstone, chair of the department of physics, Wheeler argued that Project Matterhorn was of great educative value to the students of the university. Wheeler reasoned that every student had to take part in university-sponsored war projects during their education, and that no project could be more all-around and interesting than Project Matterhorn. Because of this argument, Shenstone allowed Wheeler to set up a satellite facility of Project Matterhorn at Princeton University. The educative value was of great importance for the number of big science projects and the choice of projects. Not only were physicists needed to work on the big science projects, but also were big science projects needed to train even more physicists that could produce weapon in case of war. One could even

31(Smyth, 1951) 32(Kaiser, 2002), p.142 33(Smyth, 1951) 34(Powers, 1950), p.165 35(Galison & Bernstein, 1989), p.320

9 2 SCIENTIFIC MANPOWER IN THE UNITED STATES AND NATO

argue many big science projects were primary set up to train scientific manpower, instead of the other way around. Cold War events, i.e. the United States entering the Vietnam War in 1955, led to an ever growing request for scientific manpower. A few months after the United States entered the Vietnam War, the Academy-Research Council constituted an Advisory Committee on Scientific Manpower to advice and review her policy on the scientific manpower problem.36 Wheeler was a member of this committee. Be- fore, the Academy had asked individual members for advice, but now the problem of scientific manpower had become so urgent that an advisory committee was established to help solve this problem.

The United States worried not only about her own scientific manpower shortage but also about the shortages in other NATO countries.37 In the early 1950s, the Organization for European Economic Cooperation (OEEC)38 had started to keep track of the number of trained scientists and engineers in her member countries.39 In 1955, they concluded that European NATO countries combined had only trained 27,000 scientists or engineers to a bachelor or equivalent degree, which was lower than the “already problematically low” number of 45,500 trained men and women in the United States.40 Therefore, NATO took an initiative to solve the manpower problem by establishing the Special Committee on Scientific and Technological Personnel in November 1956.41 The committee had a clear mandate: to survey the manpower situation in NATO countries and to compare it with the manpower situation in the Soviet Union; to propose ways to use the manpower resources in a more effective way; to prepare a long-term program to increase the scientific manpower in NATO; and to develop a plan for the individual countries and NATO as a whole to resist an armed attack by the Soviet Union.42 United States senator Henry M. Jackson was nominated chair of the committee. He established the Amerian Advisory Group to assist him in preparing recommendations for the Special Committee on Scientific and Technological personnel. Wheeler, who had been working for United States governmental organizations since the Manhattan Project,43 was asked to be the chairman of this group that existed of educators, scientists and industrialists. On the 5th of September 1957, the American Advisory Group submitted their report Trained Man- power for Freedom to the Committee on Scientific and Technical Personnel.44 The group recommended to increase the quality and quantity of scientific manpower in NATO, and specifically in the European NATO countries, to prevent that the scientific imbalance between NATO and the Soviet Union would lead to a military imbalance. Besides providing grants for students and universities, the group focused

36Brief Detlev W. Bronk to Wheeler, 10th of April 1956, JAWP, Job Notebook folder 3, Box 14 37(Krige, 2000), p.84-91 38Established in April 1948 to allocate and distribute aids from the Marshall Plan 39Austria, Belgium, Denmark, Greece, Ireland, Iceland, Italy, Luxembourg, Netherlands, Norway, Portugal, Sweden, Switzerland, Turkey, United Kingdom, and Western Germany 40(Krige, 2000), p.85 41Resolution on ‘The Establishment of a Special Committee on Scientific and Technical Personnel’, November 1956 & Trained Manpower for Freedom, 5th of September 1957, JAWP, American Advisory Group on NATO Scientific and Technical personnel folder 1, Box 19. Those documents are also described by John Krige (2000), 84-91. 42Letter Jackson to Wheeler, 24th of January 1957, JAWP, American Advisory Group on NATO Scientific and Technical personnel folder 1, Box 19 43Wheeler had for example been consultant at Brookhaven National Laboratory (Contract Brookhaven National Labo- ratory, 12th of December 1947, JAWP, Job Notebook folder 1, Box 14 & Contract Brookhaven National Laboratory, 1st of July 1948, JAWP, Job Notebook folder 1, Box 14) and at the AEC (Contract reactor safeguard committee, 10th of July 1950, JAWP, Job Notebook folder 2, Box 14). He also contributed to Project Matterhorn (Letter Shenstone to Wheeler, April 14th, 1952, JAWP, Job Notebook folder 2, Box 14) 44Trained Manpower for Freedom, 5th of September 1957, JAWP, American Advisory Group on NATO Scientific and Technical personnel folder 3, Box 19

10 2 SCIENTIFIC MANPOWER IN THE UNITED STATES AND NATO on international collaboration, including establishing summer study institutions and expanding international exchanges of scientific and technical personnel. Because respecting national sovereignty in science and technology policy made it hard to establish international rules, the group preferred international collaboration as an instrument to increase the quality and quantify of NATO scientific manpower. But feasibility was not the only reason why the advisory group preferred international collaboration. Also, international collaboration had the potential to create an international elite that would be held together by professional respect, friendship and political and ideological consensus.45 The United States understood that military aggression against Western Europe by the Soviet Union had become unlikely in the climate of assured mutual destruction, putting up the question if NATO would stay together without fear of nuclear weapons. This question was strengthened because the United States was clearly not prepared to provide her allies in the Middle East with military support during the Suez Crisis. Also, Eisenhower’s disapproval of the British-French actions in Egypt showed the fragility of consensus within NATO. According to the advisory group, NATO needed to strengthen her non-military connectivity, like shared culture, traditions, norms and values, to create a community feeling with roots deeper than shared defense. The United States policy makers perceived the exchange of Americans, like leading members of the scientific and technological establishment, as a mechanism to spread American norms and values. Building a community of trained researchers whose knowledge and skills in basic science would not only bring military strength but also economic growth and political harmony within NATO. The case of the scientific manpower problem in NATO was not the first where the United States used international scientific collaboration as a tool to strengthen her position and the position of NATO. We will see in the next chapter how the United States used scientific exchange to establish her hegemony in Europe by sharing norms and values, and by creating an international elite that was focused on the United States.

45(Krige, 2000), p.84-91

11 3 REBUILDING DUTCH PHYSICS AND CO-PRODUCING AMERICAN HEGEMONY

3 Rebuilding Dutch Physics and co-producing American hege- mony46

During World War II, a discrepancy in the development of physical research emerged between warring and occupied countries. The United States, the United Kingdom and the Soviet Union invested largely in Big Science projects and made enormous scientific and technological developments. In contrast, scientific research in occupied countries, like the Netherlands,47 stagnated because of a lack of financial resources, a lack of scientific personnel caused by the holocaust and “arbeitsanzats”,48 and because of laboratory plundering by the Germans. Despite the poor economic situation, the Dutch government decided to invest in physics research right after the war. One research subject in which the Dutch government invested was nuclear science.49 The nuclear bombs on Hiroshima and Nagasaki had made clear the power and possibilities of nuclear technology, and the Netherlands perceived nuclear energy as a big economical promise.50 At the same time, the bombs showed how big the technological gap between the United States and the Netherlands was. The Dutch needed to substantially invest in nuclear physics to make use of economical promise. Therefore, prime-minister Wim Schermerhorn established the commission for nuclear physics, which had to investigate the problem of nuclear energy from a theoretical perspective. This was an initiative of physicists Hendrik Kramers and Dirk Coster. In 1946, the commission was transformed into the Stichting voor Fundamenteel Onderzoek der Materie (Foundation for Fundamental Research on Matter or FOM ) which aimed to build a nuclear power plant. Besides nuclear physics, the Dutch government invested in defense-related research.51 This was in sharp contrast to the pre-war situation, where the infrastructure for defense research had existed of only two small laboratories, one for physical and one for chemical research, which had been part of the Ministry of Defense but had only a small budget, few relations with politics and the military, and no significant international collaboration. In 1939, a new laboratory, the Centraal Laboratorium (Central laboratory) had been opened, but it had only employed two researchers, J. van Ormondth and J.H. de Boer, and lacked significant funding. However, this changed after the German occupation. During the war, the two physicists of the Central laboratory had fled to Great Britain and planned a Dutch defense strategy, which, like the British approach, considered scientific research essential to national defense. After the war, the Schermerhorn administration took over those ideas and they significantly increased the budget for defense-related research. The bulk of the defense related budget was spent by the Rijksverdedigingsorganisatie (National Defense Organisation or RVO), a new organization that combined multiple small laboratories, which grew quickly to fifty employees working in five relatively well-equipped laboratories mid-1950s. Fundamental research and international collaboration characterized FOM and RVO. Gerardus Sizoo, a professor in physics at the Vrije Universiteit (Free University or VU) who was involved in the establishment of FOM and RVO, was the first to explain clearly the importance of fundamental research. He started his explanation by arguing that fundamental knowledge was the foundation of technological development, which has a positive effect on economic growth. The Netherlands was lacking the funda-

46This chapter is primarily based on Out of a clear blue sky? FOM, the bomb, and the boost in Dutch physics funding after World War II by Jeroen van Dongen and Friso Hoeneveld, 2013, p.264&270-276&284, Quid pro quo: Dutch defense research during the early Cold War by Jeroen van Dongen and Friso Hoeneveld, 2015, and The fulbright program in the netherlands: An example of science diplomacy by Gilles Scott-Smith, 2015 47The occupation lasted from May 1940 until May 1945 48Forced labor in the war economy of the occupier of inhabitants of occupied areas 49(Hoeneveld & van Dongen, 2013), p.264 & 270-276 & 284 50(Hoeneveld, 2011), p.89 51(van Dongen & Hoeneveld, 2015)

12 3 REBUILDING DUTCH PHYSICS AND CO-PRODUCING AMERICAN HEGEMONY mental knowledge to develop promising technology, i.e. nuclear energy, so the Dutch had to invest in fundamental research for economical recovery. Therefore, the FOM divided financial sources over fundamental research projects, focusing on nuclear science. He continued by explaining that fundamental research was a field in which the Netherlands could be strong, and that the Netherlands needed to invest in fundamental research to enforce international collaboration in defense and technology, which was ab- solutely necessary because those topics were to big for the Netherlands to solve by themselves. Scientific and technological development were essential to national security, international status and economical recovery, but the Netherlands was too small and too far behind to overtake their arrears alone. There- fore, the Netherlands needed to find international collaboration partners to catch up with international science before being excluded from major technological developments, most importantly nuclear energy. Also, the Dutch sought collaboration on defense-related research, since the Netherlands was too small to independently do defense research, and defense without research was undesirable. To be included in international collaborations, the Dutch needed to offer something to be recognized as a scientific partner. Since the pre-war structure for fundamental research was still intact, the Dutch strategy was to produce fundamental knowledge in a few specific fields to “trade” this knowledge for international partnership and knowledge. The Netherlands was good at doing fundamental physics research, and this was the only way to become an actor in international science again. The establishment of RVO under the Nederlandse Organisatie voor Toegepast Natuurkundig On- derzoek (Dutch Organization for Applied Physics Research or TNO) was essential in giving RVO the freedom to focus on fundamental research. The reason to establish RVO under TNO, and not as a part of the Ministry of Defense, was primarily based on cost efficiency. Placed under TNO, the RVO would more easily profit from the expertise of TNO, and accidental duplication in research could be prevented. Furthermore, it also minimized the influence by the military that was pushing for technological oriented research. Sizoo became the director of RVO because he was strong enough to resist pressure from the military, and the RVO focused on fundamental defense research under his direction. Immediately after the war, the RVO had been seeking international collaboration. Initially, the Dutch, including Ormondt and de Boer, had established ties between the RVO and the British Defence Research Policy Committee. The contacts increased and a significant number of researchers traveled to the United Kingdom. However, this collaboration declined after 1949, because the British did not want to endanger their collaboration with the United States by sharing joint work with the Dutch. At this moment, the United States started to show interest in collaboration with the Dutch. Directly after World War II, the United States had turned down collaboration with the Dutch, but because of the high level of fundamental defense-related research at the RVO, the United States changed her mind. RVO’s work, especially on fire control and digital radar-image communication, drew the interest of the Americans who even wanted to take part in this work. The Dutch strategy to become an attractive international partner had worked. The Dutch investment in fundamental defense-related research and international collaboration led to a big payoff in 1955 when NATO established SHAPE TC in The Hague. SHAPE TC was a NATO institute for air-defense research. Since the beginning of the Cold War, the United States and western European countries wanted to collaborate on air-defense to protect NATO countries positioned close to the Soviet Union. The problem was that some countries, i.e. France and Great-Britain, did not want to give up sovereignty over their air force to the NATO. Also, it was technologically complicated to integrate the different national air force systems. Therefore, the NATO established SHAPE TC, an independent air-defense agency to protect western Europe, paid by the United States. NATO placed SHAPE TC in The Hague, under daily supervision of RVO - meaning that RVO did the payment

13 3 REBUILDING DUTCH PHYSICS AND CO-PRODUCING AMERICAN HEGEMONY of salary, administrative processes, security etc.- , housed in the same building, and with Sizoo as chairman. Although SHAPE TC was not a part of RVO - SHAPE TC internationally recruited her own employees and the research at SHAPE TC was more technologically focused than the research of RVO - the placement of SHAPE TC in The Hague was a big success for the Netherlands. The choice to establish SHAPE TC in The Hague was partly because of RVO’s neutral placement under TNO. The establishment of SHAPE TC was relatively easy to explain to the American taxpayer since RVO was not a commercial company; RVO being independent from the Ministry of Defense and relatively independent from the Dutch government made it easier for other European member to accept the establishment of SHAPE TC. But more important in the decision to place SHAPE TC in The Hague was RVO’s high level of fundamental research. This case clearly shows that the Dutch plan to invest in fundamental physics to reinforce international collaboration worked. The Dutch were also seeking international collaboration in nuclear physics. Ties between the Nether- lands, Great-Britain, Norway and France were established.52 Although no nuclear collaboration with the United States had been formed in the early 1950s - the United States barely supported nuclear science in Europe -, the Netherlands bore in mind what kind of nuclear knowledge the United States might value, hoping on future collaboration. After president Eisenhower’s Atoms for peace speech in 1953, in which he had pleaded further spreading of nuclear knowledge for peaceful purposes like nuclear energy, nuclear ties between the Netherlands and the United States were established.53 This new program opened the way for a bilateral agreement about sharing classified information in the future between the United States and the Dutch government and FOM in 1955. With this prospect in mind, a research reactor in Petten was proposed, and the Technical college in Delft launched a new master program on nuclear reactor technology. Those steps created a new nuclear cluster that involved Delft, FOM, Petten, and the experts from other Dutch universities. Both RVO and FOM had been successful in finding international cooperation, especially with the United States, by investing in fundamental science in crucial fields.

The United States decided to invest in collaborate with the Dutch for two reasons. First, and most ob- vious, the United States wanted to add the Dutch knowledge to their European “arsenal of knowledge”. Once the Dutch had produced knowledge in crucial fields, the United states wanted to secure her access to this knowledge in order to keep her scientific lead in the bipolar world. Second, and more importantly, the United States used her scientific lead relative to Europe to exercise soft-power for spreading norms and values and securing the American hegemony in Europe.54 One way to do this was by strengthening Europe’s science to strengthen her economic situation and make it less susceptible for communist influences, in the same line of thinking as the Marshall Plan. Furthermore, scientific exchange would help to create a scientific elite, held together by professional respect, friendship and shared ideas, norms and values, creating a strong bond between the socio-economical and political elites of Europe and Amer- ica. In other words: The United States used scientific exchange as a tool to establish her hegemony in Europe. The Netherlands used scientific exchange as a tool to rebuild her scientific infrastructure. Both countries acted out of self-interest in this cooperation that established the American hegemony in Europe, a process that is dubbed the “co-producing American hegemony” by John Krige.55

52Secrecy and the Early Dutch-Norwegian Nuclear Collaboration by Machiel Kleemans, presentation on the History of Science Society Annual Meeting 2019, Utrecht, the Netherlands & (van Splunter, 1994a), p.273 & (van Splunter, 1994b), p.283 & (van Dongen & Hoeneveld, 2015), p.281. & Eisenhower’s Atoms for peace can be found at https://www.iaea.org/about/history/atoms-for-peace-speech, visited on the 27th of May 2020. 53(Scott-Smith, 2015), p.201 & (Hoeneveld & van Dongen, 2013), p.274 54(Krige, 2006, 2008, 2010, 2016) 55(Krige, 2010), chap.1

14 3 REBUILDING DUTCH PHYSICS AND CO-PRODUCING AMERICAN HEGEMONY

The Fulbright program is a clear example of the above.56 In 1945, senator James William Fulbright established the Fulbright program to sell surplus war material and use that money to fund support international educative exchanges between the United States and other countries “[...] to bring a little more knowledge, a little more reason, and a little more compassion into world affairs and thereby increase the chance that nations will learn at last to live in peace and friendship.”57 In May 1949, the Netherlands and the United States signed a Fulbright agreement. The program was established to reduce international tension and to spread peace, but also to spread American’s influence in other countries, to make sure that Americans going on an exchange would better understand their position and the position of the United States in the world, and to make sure that future leaders would understand and focus on the United States. The program became a basic mechanism to send ideas, wrapped up in a person, from the United States to Europe. During the beginning years of the program in the Netherlands, the vast part of the Fulbright budget was spent on American lecturers, researchers and graduate students visiting the Netherlands.58 Those Americans were influential: They spread their knowledge, norms, values and techniques; They gave lectures at universities and professional organizations; They were included in prominent research groups, published in Dutch and international journals, and were asked for advice on educative and professional matters. Also, they stimulated Dutch students to study at American universities and they stayed in contact with their Dutch colleagues once back home. In numbers, Dutch graduate students going to the United States were the majority.59 The program and Dutch professors selected the most intelligent and promising Dutch students to study at American universities. In this way, the United States could culturally imbue the future socio-political and economic elites of the Netherlands. After those future elites went back home, the United States stayed in contact with them via the Dutch alumni club De Halve Maen. The Fulbright program is a classical example of exercising hegemonic power in many aspects, whereby the hegemon provided opportunities for allies to integrate themselves into the systems of the hegemon. Although the Fulbright program was clearly an asymmetrical enterprise and the Americans had more influence in the Netherlands than the other way around, the Dutch also profited from the program. Es- pecially nuclear physics profited since the program made scientific exchange possible when FOM did not have the financial resources to pay for scientific exchange. The Fulbright program played an important role in strengthening the Dutch and American nuclear physics connection by scientific exchange of nuclear physicists, like John Wheeler. Wheeler got Fulbright scholarship to travel to Leiden, where he mixed with the Dutch physics community at Leiden University and the nuclear conference and other meetings. This exchange is a clear example of how the United States used her scientific lead in order to establish her hegemony in Europe by spreading her norms and values via scientific exchange. However, the Dutch did not seem to worry about facilitating American hegemony. As we will see in the next chapter, Wheeler’s visit to Leiden was much needed for physics in Leiden.

56(Scott-Smith, 2015) 57J. William Fulbright Quotes, published by the United States department of state, the Bureau of Educational and cul- tural aﬀairs, https://eca.state.gov/fulbright/about-fulbright/history/j-william-fulbright/j-william-fulbright-quotes visited on 27th of May 2020 58(Scott-Smith, 2015) 59Idem.

15 4 PHYSICS IN LEIDEN IN THE FIRST HALF OF THE 1900S

4 Physics in Leiden in the ﬁrst half of the 1900s

At the beginning of the twentieth-century, Dutch physics was centered around Leiden and of an out- standing quality.60 Hendrik Lorentz, Heike Kamerlingh Onnes, Paul Ehrenfest and Willem de Sitter made Leiden a pivot in international physics.61 In 1870, Lorentz started his study mathematics, physics and astronomy at Leiden University. The university appointed him professor in theoretical physics after his promotion, and he quickly became one of Europe’s most prestigious physicists. In 1902, Lorentz and his Amsterdam colleague Pieter Zeeman won a Nobel Prize for the discovery of the Zeeman effect. In 1912, Lorentz retired early from the university to become a curator at the Teylers Museum. He kept a special appointment at the university. Paul Ehrenfest was Lorentz successor. With his enthusiasm, humanity and kindness, he established a big school containing e.g. Hendrik Kramers and Hendrik Casimir. Ehrenfest often visited international colleagues and enthousiastically participated in national and international conferences.62 Therefore, he often had prominent international physicists visiting him in Leiden. Furthermore, he made sure that his most promising students would study abroad for a while. Under influence of Ehrenfest, theoretical physics in Leiden became more out-looking. One guest that visited regularly was Einstein; he was a great admirer of Lorentz and he tested his ideas on a benev- olent audience existing of Lorentz, Ehrenfest and de Sitter -63 the promising Frizian astronomer and cosmologist who had been professor of astronomy at Leiden University since 1908.64 Also experimental physics flourished. Heike Kamerlingh Onnes, who had been professor of experimental physics since 1882, made Leiden “the coldest place on earth”.65 This attracted other famous physicists, like double Nobel Prize winner Marie Currie, who came to Leiden in 1911 to study radioactivity at low temperatures. In 1914, Onnes won a Nobel Prize for making helium fluid at a temperature of -269◦C. Theoretical and experimental physics flourished, attracting important international visitors. Because of Einstein’s frequent visits to Leiden, Leiden had become one of the early leading research centers on relativity.66 In 1913, Einstein and Grossman published a paper in which they made the first generalizations from special to general relativity.67 In response, a group of physicists and mathematicians started to study the theory extensively, and cooperation emerged between Einstein and Lorentz and his students Johannes Droste and Adriaan Fokker. In 1915, Einstein published the final version of his theory of General Relativity. Between 1915 and 1920, a group of nine Leiden physicists and mathematicians, i.a. Lorentz, Droste, Fokker, Jan Schouten, de Sitter and Hendrik Kramers,68 focused on the theory and published over thirty articles. Leiden also fulfilled a pivotal role in the theory’s development during World War I.69 The war secluded German physicists from their French, American and British colleagues, and the Netherlands functioned as a bridge to carry Einstein’s ideas over the sea to England, since it remained neutral. Especially de

60(Hoeneveld & van Dongen, 2013), p.268-299 61(A. Kox, 2010), p.21-47 62(A. Kox, 2010), p.52 63(Rispens, 2009), p.53-56 & 93 & 150 64https://hoogleraren.leidenuniv.nl/search?keyword=de%20sitter;docsPerPage=1;startDoc=2 visited on the 9th of May 2020 65https://lorentz.leidenuniv.nl/history/cold/cold.html visited on the 9th of May 2020 66(A. J. Kox, 1992) 67(Einstein & Grossmann, 1913) 68Between 1916 and 1926, Kramers worked in Niels Bohr’s group in Copenhagen. He was a student of Ehrenfest until 1919 and had intensive contact with Leiden, so he is included in this list. Memorium Niels Bohr by Hendrik Kramers, 18th of July 1952, published in the Nederlands Tijdschrift voor Natuurkunde, NHA, 769 prof. H.B.G. Casimir, Inv.nr 46 & Memorium Hendrik Antony Kramers by John Wheeler, 1953, Reprint from Year Book Of The American Philosiphical Society, NHA, 769 prof. H.B.G. Casimir, Inv.nr 46 & (Dresden, 1987) 69(A. J. Kox, 1992) & (Rispens, 2009), p.23

16 4 PHYSICS IN LEIDEN IN THE FIRST HALF OF THE 1900S

Sitter was crucial in this process; because he had published three relativity papers in the Monthly Notice of the Royal Astronomical society,70 the publicity of the theory in England signiﬁcantly increased. This led to the undertaking of Eddington’s famous solar eclipse expedition, which they found the empirical proof for Einstein’s theory that made him world famous.71 But the interest in relativity in Leiden and the rest of academia declined during the 1920s, as we will see in the next chapter.72 Most Leiden physicists now focused their gaze on quantum mechanics and solid state physics. Only Fokker and de Sitter kept researching relativity. Fokker, who worked at the Technical University Delft from 1928, never published inﬂuential work; however, de Sitter became one of world most famous cosmologists.73

During the ten years before World War II, physics in Leiden started to decline. Towards the end of the thirties, the Netherlands was no longer an important physics center.74 The Dutch government had stopped the direct funding of science and instead founded the Dutch organization for applied research (Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek, or ‘TNO’) in 1932. The research of TNO was to immediate benefit the public. Industrial physics flourished, while fundamental science, and especially fundamental physics, had to work with the modest funding that universities received. In the 1930s, state sponsoring for ‘la science pour la science’, and in particular fundamental physics, barely rose above the modest funding that universities received. A steady decline had set in, and towards the end of the thirties, the Netherlands was no longer an important hub in the network of innovative physics centers. Furthermore, physics in Leiden suffered from the loss of Lorentz passing away in 1928.75 Only five years later, Paul Ehrenfest took his own life. There were multiple aspects that had made his life unbearable, of which was the worries about the growing antisemitism in Germany - Ehrenfest was Jewish.76 In the same year - the year that Adolf Hitler became Germany’s chancellor - Einstein, who still had a special appointment at Leiden University,77 fled to the United States. In 1934, Willem de Sitter passed away. In a relatively short time, Leiden lost many of her important physicists. Casimir and Kramers, two former students of Ehrenfest and Niels Bohr, returned to Leiden to strengthen the theoretical physics department. Casimir was in Leiden because Ehrenfest had strongly asked him to return - in retrospect because he knew what was coming. Casimir took over Ehrenfest’s tasks after he passed away until 1934, when the university appointed Kramers as Ehrenfests successor. Kramers was a suitable candidate because he had shown to be an influential physicist with many international relations, e.g. Werner Heisenberg, Erwin Schrodinger, Louis de Broglie and, in particular, Niels Bohr. Kramers led the already weakened theoretical physics department into World War II. During World War II, theoretical physics in Leiden almost completely stagnated. At the beginning, the war did not have much influence, but a few months after the start of the occupation in May 1940, universities got in trouble.78 The Nazi’s and their Dutch followers wanted to make Leiden, the oldest university of the Netherlands, a nationalistic-socialistic bastion. The university awaited an insecure future. On December 26, 1940 the Nazi’s dismissed all Jewish professors. Students and professors

70(De Sitter, 1916a, 1916b, 1917) 71(Rispens, 2009), p.130 72(A. J. Kox, 1992) 73(Casimir, 1983), p.152 74(Hoeneveld & van Dongen, 2013), p.269 75(A. Kox, 2010) 76(A. Kox, 2010), p.52-54 77https://hoogleraren.leidenuniv.nl/search?keyword=Einstein;docsPerPage=1;startDoc=3 visited on the 9th of May 2020 78(Casimir, 1983), p.234-244

17 4 PHYSICS IN LEIDEN IN THE FIRST HALF OF THE 1900S responded with massive protests, so to the Nazi’s responded by closing the university. After a short reopening, the university definitively closed in 1942. Casimir started working for Philips in Eindhoven. Despite his special appointment in Leiden, he could barely do anything for the university.79 Kramers did his best to keep the theoretical research going. Some students were hiding in the basement of the Kamerlingh Onnes Laboratory, so Kramers moved his work space to the lab to work with his students.80 He felt strongly for his Jewish students and colleagues and tried his best to help them. Besides the stress of the war, he managed to publish multiple articles,81 although they can hardly be called groundbreaking. Kramers was responsible for the little theoretical research that had been done during the war. Despite his efforts, theoretical physics in Leiden almost completely stagnated.82 Meanwhile, the Kamerling Onnes Laboratory stayed open, and experimental physics was relatively unaffected by the war.83 The directors of the laboratory, Wander De Haas and Willem Keesom, kept the laboratory open to prevent it from pillage. They urged their students to make sure not to stand out to the Nazi’s. Only in the summer of 1944, the occupiers came to collect some instruments. The lab became isolated and lost a few instruments, but the laboratory was overall relatively untouched. The strategy of de Haas had worked. In 1946, Cor Gorter replaced Keesom and became professor experimental physics at the university. Due to Gorter’s energy, commitment, organizational skills and many international relations, the laboratory recovered even further.84 According to Dirk van Delft, the lab even quickly recovered to her pre-war status (2012). In contrast, theoretical physics in Leiden did not recover during the first years after the war. This was partly because Kramers was too busy with matters of international scientific physics collaboration and non-proliferation to spend time and energy on the recovery of theoretical physics in Leiden. After being involved in the establishment of FOM, as explained in chapter 3),85 Kramers left to the United States from the summer of 1946 until august 1947.86 He moved to New York for his function as the first Dutch delegate at the AEC. Although he highly valued the development of nuclear energy and non-proliferation, mounting frustrations about the internal structure of the AEC caused Kramers to quit. He lengthened his leave from Leiden and spent a few months at the Institute for Advanced Study in Princeton, where he had multiple old friends, i.a. Wheeler.87 After this period, Kramers returned to Leiden in August 1947. He was also chair of the International Union of Pure and Applied Physics from 1946 until 1951. 88 The war seemed to have tired out Kramers; he failed in recovering Leiden physics. After his death in 1952, the university offered Casimir to succeed Kramers, but he declined the

79Aanstelling bijzonder hoogleraarschap Casimir door het Leids Universiteitsfonds, 25th of July 1945, NHA, 769 prof. H.B.G. Casimir, Inv.nr 1 & https://hoogleraren.leidenuniv.nl/search?keyword=casimir;docsPerPage=1;startDoc=1 visited on the 9th of May 2020 80Memorium Hendrik Antony Kramers by John Wheeler, 1953, Reprint from Year Book Of The American Philosiphical Society, NHA, 769 prof. H.B.G. Casimir, Inv.nr 46 & (Dresden, 1987) 81(H. A. Kramers & Wannier, 1941a, 1941b) (H. Kramers, Belinfante, & Luba´nski,1941) (H. Kramers & Haar, 1942) (H. Kramers & Kistemaker, 1943) (H. Kramers, 1943, 1944) 82(Casimir, 1983) p.234-237 & (Dresden, 1987) 83(van Delft, 2012) & (Casimir, 1983) 84http://resources.huygens.knaw.nl/bwn1880-2000/lemmata/bwn2/gortercj visited on the 9th of May 2020 & https://www.lorentz.leidenuniv.nl/history/gorter/biography.html visited on the 9th of May 2020 85(Dresden, 1987), p.3&379&501 86(Dresden, 1987), p.501-505 & Memorium Niels Bohr by Hendrik Kramers, 18th of July 1952, published in the Nederlands Tijdschrift voor Natuurkunde, NHA, 769 prof. H.B.G. Casimir, Inv.nr 46 87Wheeler and Kramers knew each other most likely because of their connections with Copenhagen. (Dresden, 1987), p.382 & Memorium Hendrik Antony Kramers by John Wheeler, 1953, Reprint from Year Book Of The American Philosiphical Society, NHA, 769 prof. H.B.G. Casimir, Inv.nr 46 88Memorium Hendrik Antony Kramers by John Wheeler, 1953, Reprint from Year Book Of The American Philosiphical Society, NHA, 769 prof. H.B.G. Casimir, Inv.nr 46

18 4 PHYSICS IN LEIDEN IN THE FIRST HALF OF THE 1900S

offer to become chair of the “dejected” theory institute. Casimir found that “the old rooms, [and] the old library are almost exactly the same as back in the days, without clear signs of growth. There [is] almost no administrative help, there is only a little bit of funding for travel and for inviting international speakers.”89 Theoretical physics in Leiden had been declining from the ten years before the war until now. Sybren de Groot succeeded Kramers and changed the status of the theoretical physics department. De Groot and his graduate student Peter Mazur started a series of innovations to rebuild the theory department.90 They gave Instituut-Lorentz her name, attracted a larger scientific staff, and housed the institute at a new location.91 Furthermore, they established the Lorentz Chair: a special guest professorship to attract prominent international physicists. A committee with experimentalists, theorists and astronomers decided every year who would be invited. In 1954, the first Lorentz professor was George Uhlenbeck. A year later, it was John Wheeler’s turn. Many more followed, i.a. Nobel Prize winners Eugene Wigner in 1957 and John Van Vleck in 1960. In 1961, when de Groot resigned and Mazur took over, the institute was world famous again. With the arrival of de Groot, the institute recovered. But why did the Leiden decide to invite Wheeler as their second Lorentz professor? And why did Wheeler teach a course about relativity, quantum mechanics and the relations between those two theories? We will answer this question in chapter 7. But first, we will look at the history of relativity and a Wheeler’s work around the time he came to Leiden.

89(Casimir, 1983), p.279. Translated from Dutch to English 90Levensberichten en herdenkingen Koninklijke Nederlandse Akademie van Wetenschappen, Sybren de Groot by Peter Mazur (1995) & https://hoogleraren.leidenuniv.nl/search?keyword=de%20groot;docsPerPage=1;startDoc=3 visited on the 9th of May 2020 91https://www.lorentz.leidenuniv.nl/research/mazur/mazur dut.html visited on the 9th of May 2020

19 5 RENAISSANCE OF GENERAL RELATIVITY

5 Renaissance of General Relativity

In 1915, Einstein finished the final version of his theory of general relativity. Only four years later, during the solar eclipse on the 29th of May 1919, Arthur Eddington and Frank Dyson undertook a solar eclipse expedition. This expedition measured one prediction of the theory: the gravitational deflection of light. Eddington presented their findings to the Royal Society of London, and the deflection of light became widely accepted. This sparked a burst of excitement about the theory, and Einstein became a scientific superstar.92 Today, general relativity is still the standard theory of gravitation and the foundation of cosmology, and it determines how we think about space and time. However, the theory of general relativity has not always been a flourishing one.

5.1 The low-water-mark period93

In the mid-1920s, after the initial burst of excitement, a thirty-year stagnation period emerged. During this period of stagnation, which is dubbed the “low-water-mark” period by physicist Jean Eisensteadt,94 only a few scientists worked on the theory. The theory was considered to be a mathematically complex and highly formal theory that only provided minor corrections to Newtonian predictions, without providing far-reaching physical implications. Therefore, the majority of the physics community lost its interest in the theory. Instead, the majority focused on more promising branches of physics, like nuclear physics, quantum electrodynamics and quantum mechanics. Especially the last was popular, since it had a strong link to experimental activities and a lot of possible technological applications in micro physics and solid-state physics. The career prospects in general relativity were bad, so the vast majority of physicists discouraged their students to focus on the theory. Even Robert Oppenheimer, who had done research on relativity himself, discouraged his students to focus on relativity. The few that worked on general relativity aimed on going beyond the theory; they saw general relativity as a stepping stone towards a larger theoretical framework. One can distinguish three traditions in the low-water-mark relativity efforts. The first tradition aimed to go beyond general relativity by attempting to unify general relativity and electrodynamics. This tradition was dominated by purely mathematical approaches that tried to repeat Einstein’s successful strategy of constructing a new physical theory by changing the notions of space and time. This includes Einstein’s work of that time.95 However, those attempts lacked the input of physical experience and thought experiments that had been crucial to Einstein’s earlier success. The second tradition aimed to unify general relativity and quantum mechanics, i.e. to create a theory of quantum gravity. Most attempts were made by following the method that had been used to quantize electrodynamics, which had resulted in the first quantum field theory: quantum electrodynamics. Although some progress was made in the 1920’s, the low-water-mark attempts never resulted in a full theory of quantum gravitation that could replace general relativity. Both research traditions stuck to Einstein’s methodology, but not to his theory. Instead, physicists tried to use elements from the theory to construct other theories or to solve problems in other fields of physics, like particle physics, nuclear physics and quantum field theory. Wheeler’s geon project is a clear example of the above:96 Wheeler tried to solve a problem in particle physics by unifying general relativity and electrodynamics and then taking an element of general relativity, i.e. space-time curvature.

92(Kaiser & Rickles, 2018), p.340 93This section is primary based on (A. Blum, Lalli, & Renn, 2015, 2016), p.599&602-612, p. 344-346 & (A. Blum, Giulini, Lalli, & Renn, 2017), p.2 & (A. S. Blum, 2017), p.96 & (Lalli, 2017), p.13-16 94(Eisenstaedt, 1986) 95(Van Dongen, 2010) & (Misner, 2010), p.23 96(A. Blum et al., 2015)

20 5.1 The low-water-mark period 5 RENAISSANCE OF GENERAL RELATIVITY

The third research tradition aimed to go beyond general relativity by establishing the first ties between astronomy and general relativity. This was the only subject where relativity was expected to make major, empirically observable corrections to Newtonian theory, instead of formal corrections. A few astronomers, i.a. Willem de Sitter, Edwin Hubble, and Arthur Eddington, pushed this field. From the beginning, there was a separation into two branches. The first branch focused on the three classical tests of general relativity: the perihelion precession of Mercury’s orbit, the deflection of light by the Sun and the gravitational red shift of light. Einstein had already envisioned these tests in 1907. The astronomers concerned with the three classical tests had a broad interest; they were not merely focusing on general relativity and its foundations. Of the classical tests, both the bending of light and the gravitational red-shift remained controversial matters until the renaissance, since research focused on the development of better measurement techniques and higher precision. The second branch focused on the question of how to apply general relativity to cosmological problems. This cosmological research was performed by a group of mathematicians and astronomers with strong mathematical training. They focused on understanding cosmic dynamics and interpreting the cosmological solution to the Einstein equations. In particular the separation of space - to which sim- plified assumptions concerning the structure of the universe, such as homogeneity and isotrophy, were applied - and time - which determines the the evolution of the universe - got attention. Once they established the research field of relativistic cosmology, a philosophical debate concerning the fundaments of cosmology emerged. Founders of relativistic cosmology and opponents with different theories argued about the epistemology of the theory, and therefore also the methodology of how to develop the field. Besides cosmology, applications of general relativity beyond the three classical tests were even more frail. Gravitational waves remained a controversial issue until the 1950s; the question whether gravitational waves are physically real, e.g. carrying energy away from the source, stayed unanswered during the renaissance.97In the early 1930s, Subrahmanyan Chandrasekhar had started the research on gravitational collapse by studying white dwarfs. In 1939, Robbert Oppenheimer and Hartland Snyder had been the first to use general relativity to solve this problem. They had generalized Schwarzschild’s theory to allow for non-stationary gravitational collapse and calculated that a thermonuclear burned-out star will collapse to a singularity. Very little physicists - with some exceptions like Lev Landau, and ten years later, John Wheeler - had paid attention to the physical meaning of this work. Their work had been regarded as an unwarranted extrapolation of general relativity which was limited by its empirical domain. Philosophical disagreement and controversies formed the field of relativistic cosmology. Because of the disagreements concerning epistemology, the majority of the physicists received the developments made in relativistic cosmology with skepticism.

Epistemological disagreement characterized not only cosmology, but the whole general relativity research field. Some found general relativity belonged to the field of pure mathematics, while others found it belonged to physics, astronomy or astrophysics. Therefore, research activities had very different goals and methods, and there was no common way to evaluate research. Generally, mathematical work was ignored or distrusted, with far-reaching consequences. This caused some important research to stay unrecognized for a long time. There was a strong connection between this epistemological dispersion and social dispersion of the researchers during the low-water-mark period. National and interdisciplinary boundaries prevented a

97(A. Blum, Lalli, & Renn, 2018)

21 5.2 The Renaissance period 5 RENAISSANCE OF GENERAL RELATIVITY

smooth communication and knowledge transfer between the scholars. General relativity papers were spread out over publications in disciplines like mathematics, physics, astrophysics and astronomy. Fur- thermore, there were no conferences specially dedicated to general relativity and there was no common research agenda. In other words: there was no coherent community of relativity researchers. Therefore, some gained insights stayed unrecognized because of this social dispersion. By the mid-1950s, few research centers had been established. Those centers were typically led by one principal researcher with a stable position, surrounded by a group of young researchers. John Wheeler and his group at Princeton University was one of those centers.98 He was one of the few authoritative theoretical physicists that focused on relativity. Wheeler’s group focused on going beyond the theory as explained earlier, instead of studying the foundations and the physical implications of the theory itself. The few researchers that focused on relativity made little progress on the theory, and potential of the theory stayed largely untouched. But, despite its shortcomings, this research kept the interest in the theory alive.

5.2 The Renaissance period99

General relativity re-entered the mainstream of physics during the mid-1950s. This process, in which the theory regained attention after a thirty-year period of stagnation, is dubbed the “Renaissance of General Relativity” by physicist Clifford Will.100 During the Renaissance, the potential of the theory was discovered and the theory by itself became worth studying. Furthermore, a coherent research community was formed. The theory became the standard theory of gravitation and the basis for cosmology that it still is today. How did this happen? The first important factor in this process was World War II. As we saw in chapter 2, physics had been of major importance during World War II and remained essential during the global arms race. This caused a rise in the status of theoretical physics and an increase in funding. Also, as discussed in the chapter 2, the number of students and graduate students rose dramatically. The above-mentioned research centers profited from this increase in status, money and PhD students. This resulted in a productivity rise. Furthermore, the number of PhD students, and therefore also the number of postdocs, rose so fast that it resulted in the “Postdoc cascade”: The academic system could not absorb the large number of graduated graduate students, so a long postdoctoral period was established. During these two or three years lasting postdoc periods, the postdoc typically worked in three or four different research institutes. Therefore, the communication between different research institutes improved drastically. This communication provided problems, concepts and theoretical tools to be shared between different institutes. This was essential for the Renaissance to happen. The second essential factor to the Renaissance were the explicit attempts of community building. This started with the Bern conference in 1955, that was organized to celebrate the fiftieth birthday of Einstein’s special relativity theory.101 However, the celebratory meeting turned into a conference concerning current problems and research on general relativity. For the first time, researchers recognized that there were several active research agendas. Although those programs were very diverse, scholars saw opportunities to bring them closer together. This led to a stable tradition of conferences that still

98(Goenner, 2004) 99This section is primary based on (A. Blum et al., 2015, 2016, 2018), p.599 & 612-620, p. 347-348, p.3-5 & (Lalli, 2017), p.16-17&37-58 100(Will, 1986), p.3-18 101At this time, it was easier to get funding for a celebratory meeting than for a substantive conference, what clearly illustrates that status of the theory in the mid-1950s

22 5.2 The Renaissance period 5 RENAISSANCE OF GENERAL RELATIVITY lasts today. In 1962, the Bulletin on General Relativity and Gravitation was established. In 1970, it got absorbed into the journal General Relativity and Gravitation, the first journal devoted to the field. Because of the improvement in communication and community building, research topics got more adjusted to each other. Furthermore, the research field of general relativity was taken more seriously. Therefore, the field attracted more scientists, including physicists with a strong interest in physical implications. Because of those members, the new community paid more attention to the fundamental questions considering the theory, i.e. they discovered the fundamental and physical potential of the theory. In the beginning, the consensus had been that a deeper understanding of the theory would be needed in order to go beyond the theory; later, the community considered the theory worth studying for its own sake. By the late 1950s, the view on the epistemology of the theory had dramatically been changed: during the low-water-mark period, the theory had been regarded as a formal, mathematical theory that could be used to go beyond, while during the Renaissance, the theory was expected to have physical implications worth experimental research. Gravitational waves was the first problem that the new community dealt with. It had been a marginal subject during the low-water-mark period, but it became a central matter during the Renaissance. Some researchers made gravitational waves their main research, resulting in fundamental theoretical development, what in turn encouraged experimental activities. One example is Joseph Weber. During his work with Wheeler in 1956/1957, his theoretical considerations on geons led him to research gravitational waves. Later, he started a long-term project to detect gravitational waves. He even developed an instrument to measure them: the Weber bar. The turn towards physical questions prepared for, and was in turn re-enforced by, the astrophysical discoveries of the 1960s. Technological developments, provided during World War II and the direct postwar period, allowed for better observations. When in 1963 a new astrophysical object was discovered, the new relativity community was well repaired to declare this observation. Within the same year, the Kerr solution to the Einstein equations, which describes a rotating black hole, was formulated. This quick theoretical response was possible because the tools needed to solve this problem had already been developed by theoretical physicists and mathematicians. The community organized a series of large conferences dedicated to this new object, dubbed quasars, to investigate the theoretical explanation based on general relativity. Even tough general relativity was not immediately capable of giving a realistic and detailed physical description of the dynamics of a quasar, it was widely accepted that further research relied on the quickly development of the theoretical tools provided by general relativity. Therefore, a new field was born: relativistic astrophysics. The field of general relativity had become a highly active field of research in which theoretical explorations went hand in hand with new astrophysical discoveries.102 This would have been unthinkable before the Renaissance. During the Renaissance, the theory of general relativity became the standard theory of gravitation and the basis for cosmology. The theory became trusted down to the Planck length. By the 1970s, it was empirically well tested by a coherent research community. Wheeler had been one of the leaders of the Renaissance. But why Wheeler? How did one of the most influential nuclear physicists become interested in relativity? This question will be answered in the next chapter.

102(Will, 1986), p.3-18 & (Thorne, 1995), chap. 7

23 6 WHEELER ENTERING RELATIVITY

6 Wheeler entering relativity: solving “the elementary particle problem”

6.1 Prelude to relativity: “Everything as Electrons”

AAD electrodynamics or the Wheeler-Feynman absorption theory

The roots of Wheeler’s shift from nuclear physics to General Relativity can be traced back to a program that had nothing to do with gravity in the 1930s.103 During the 1920s, nuclei were thought to exist of the two elementary particles known by that time: the proton and the electron. However, that model was theoretically imperfect and not in line with experiments.104 In 1932, James Chadwick discovered the neutron.105 He determined that it is a new particle, distinct from the proton, and established the picture that nuclei exist of protons and neutrons. In the same year, the positron was discovered. Dirac, Heisenberg, Oppenheimer and Furry worked out “pair theory”, an analysis of electrons and positrons based on the Dirac equation. Although Wheeler knew there were strong indications that electrons could not be the primary particles in the atomic core, like the range of electromagnetic interaction being too large to match the short range of nuclear interactions, he thought pair theory might offer a mechanism to bind electrons and positrons close together. Therefore, he thought that he could use electrons and their electromagnetic interactions as a substitute for the speculative theories of nuclear interactions,106in which electrons and positrons are the fundamental particles of which nuclei consist. To achieve this, Wheeler aimed to formulate a direct action-at-a-distance (AAD) theory for electrodynamics. AAD theories explain how bodies that are separated in space influence each other, i.e. how bodies influence each other without mechanical interaction. There are two types of AAD theories. First, there are direct AAD theories - simply referred to as AAD theories - in which bodies influence each other without an intermediate substance that propagates the force, i.e. there is a “spooky” force between the two bodies. Second, there are field AAD theories - referred to as field theories - in which the force between two bodies propagates through an intermediate field. Wheeler wanted to replace Maxwell’s field equations by an AAD theory for electrodynamics. In this theory, electrodynamics is build of fundamental particles, i.e. electrons, and the interactions between them. Wheeler was not the first to consider a direct AAD theory for electrodynamics. In fact, the discussion about AAD and field theories is as old as theoretical physics itself, going back to the formulation of Newton’s law of gravitation in 1687, and Coulomb’s formulation of the electromagnetic force between charged particles in 1785. In 1845, Carl Gauss wrote down his concerns about Coulomb’s law. In this AAD law, Coulomb did not consider that it takes time for an electromagnetic signal to travel from one body to another. According to Gauss, this was not in accordance with experiments. Therefore, he wondered how retardation effects could be included in AAD electrodynamics theories. Since the physics community generally focused on field theories - which naturally include retardation effects - and the Maxwell’s field theory won popularity, Gauss’ question remained unanswered for a long time. In the beginning of the 20th century, the concept of AAD electrodynamics was adressed by Karl Schwarzschild, Hugo Tetrode and Adriaan Fokker.107 Independently, they attempted to formulate a

103Information about Wheeler’s motivation to work on an AAD theory for electrodynamics is primarily based on (A. Blum & Brill, 2019), p.4-11 & (Wheeler & Ford, 2000), p.164-165 104(Pais, 1986) 105(Chadwick, 1932) 106At this time, re-normalization methods had not yet been developed and quantum electrodynamics was still still far from ﬁnished 107(Schwarzschild, 1903) & (Tetrode, 1922) & (Fokker, 1929)

24 6.1 Prelude to relativity: “Everything as Electrons” 6 WHEELER ENTERING RELATIVITY theory where the electromagnetic interaction propagates with the speed of light, i.e. they tried to formulate a relativistic retarded AAD electrodynamics theory. Fokker found a formula for delayed action-at-a-distance in a system of electromagnetically charged particles a, b, c,..., which describes the dynamics of this system. The action (J) is given by Z J = Ldt (1) where L is the Lagrangian. L is deﬁned as the kinetic energy minus the potential energy of the system. Every particle has an inertial term, which contributes to the potential energy of the system. The inertial term for particle a is given by Z ma da (2) where ma is the mass of particle a, and da is an element of proper time of particle a. In this notation, the speed of light c is taken as unity. Every particle also has a potential energy term because of its electromagnetic interaction with the other particles. Particle a interacts with particles b, c, ....., and the interaction term is given by ZZ X 2 i k eaeb δ(sAB) ηik da db (3) aworld line of particle a and the world line of particle b. When an electromagnetic signal is send from point A to world line b, there are two points on the world line of 2 particle b, B+ and B−, for which sAB = 0 (see ﬁgure 2). Point B+ is later in time than point A, and point B−is earlier in time than point A. The solution B− implies that an electromagnetic signal can go backwards in time, which seems to be problematic with our day-to-day experiences.

Wheeler and his former student Richard Feynman adressed those problems.110 First, they deﬁned

108(Hoyle & Narlikar, 1995), p. 114 & (Narlikar, 2003), p.171 109(Wheeler & Ford, 2000), p.165 110The information about the Wheeler-Feynman absorption theory is from (Wheeler & Feynman, 1945) & (Hoyle & Narlikar, 1974) & (Hoyle & Narlikar, 1995), p.116-119 & (Narlikar, 2003), p.171-174

25 6.1 Prelude to relativity: “Everything as Electrons” 6 WHEELER ENTERING RELATIVITY

Figure 2: Point A on world line a and point B−(earlier in time) and point B+ (later in time) on world line b are connected by a null-ray (the dotted line). Figure adjusted from (Hoyle & Narlikar, 1995), p.115 the four-potential at point X due to charged particle b to be Z (b) 2 k Ai = eb δ(SXB)ηikdb (5) where particle b only influences point X if they are light-like separated. The corresponding “direct particle field” is given by (b) (b) (b) Fik = Ak;i − Ai;k . (6) The direct particle field is not a normal field: it has no independent degrees of freedom; it is not an entity by itself. Wheeler and Feynman calculated the direct particle field in the future and past light-cone of particle b to be 1 F (b) = [F (b) + F (b) ] (7) 2 ret adv (a) (a) where Fret is the advances solution and Fadv is the retarded solution. The indices i and k are suppressed. Wheeler and Feynman adopted an idea of Tetrode, namely that the medium that absorbs radiation is essential to the mechanism of radiation, i.e. radiation can not exist without absorbance. This means that a charge can only radiate electromagnetic energy if the rest of space absorbs this radiation. Wheeler and Feynman applied the absorber boundary condition, which means there are enough charged particles present in the universe to absorb all radiation. In the future light-cone of particle b, the universe acts as an absorber. In the process of absorbing, the future half of the universe sends back a reaction R(b). Although the influence of an individual particle, like b, is time-symmetric, the influence of all particles in the universe is not. Wheeler and Feynman calculated the reaction of the universe to the movement of b to be 1 R(b) = [F (b) − F (b) ]. (8) 2 ret adv Therefore, a test particle close to particle b experiences the direct field

(b) (a) (a) (b) Ftot = F + R = Fret (9) i.e. a test particle close to b experiences only the retarded part of the direct particle ﬁeld. This ﬁts with our day-to-day experience. The universe as an absorber is essential to the Wheeler-Feynman theory of relativistic retarded AAD electrodynamics.

26 6.1 Prelude to relativity: “Everything as Electrons” 6 WHEELER ENTERING RELATIVITY

AAD gravitation111

Wheeler tried to apply the AAD concept to gravity in the same way as he had done to electrodynamics. With that, he aimed at constructing a direct AAD theory that would be equivalent to General Relativity in the same way the Wheeler-Feynman absorption theory was a direct AAD equivalent to Maxwell’s theory. The inspiration for this comes probably from a seminar at Princeton in 1941 where Wheeler and Feynman had presented their work on the absorption theory. Einstein had remarked that he had found it difficult to make a corresponding theory for gravity, what seems to have triggered Wheeler to construct a AAD gravitation theory. The implications of this idea were far-reaching: the space-time between particles had to be removed. The only entities in this theory where to be world lines of individual particles, a, b etc. On those world lines, one can define points α, β etc., independent of the parameterization. Points that lie on each other’s light-cone can influence each other. The functions that describe the interaction between two world lines are called “liaison functions”,112 and they describe the dynamics of the system. The liaison function α+(β) describes the path forwards in time from point β to point α. The liaison function

α−(β) is the inverse of α+(β), so it describes the path backwards in time from point β to point α. The functions β−(α) and β+(α) are defined in the same manner. Wheeler attempted to describe gravitation by finding non-trivial,113 parametrization-independent liaison functions. Wheeler suggested the expression 0 0 α = α−(γ+(β+(α))) where α is the point on world line a that is reached when one travels from (1) point α over the future light-cone to point β on world line b, and then continues (2) over the future light-cone to point γ on world line c, and then (3) on the past light-cone of γ back to world line a. If 0 0 α = α, the points α, β and γ are on one line in three dimensional space. If α 6= α, the three points are not on a line in three dimensional space, but they form a triangle. So this method can show with only one-dimensional world lines and liaison functions the difference between a line and a triangle, i.e. the position of particles relative to each other, independent of the 0 parametrization of α and α . More complex systems with more particles can be configured in the same manner. With only world lines and liaison functions, Wheeler hoped to construct a theory of gravity that was equivalent to general relativity like the Wheeler-Feynman absorption theory was equivalent to Maxwell’s. However, Einstein and Wheeler found an unsolvable problem with the most simple case: the two particle universe. In a two particle universe, there are only two world lines a and b, which means that the only equations to study are α−(β+(α)) and β+(α−(β)). But those equations are each other’s inverse, so there seems to be no physics in a two body universe. Since General Relativity give a description for a two body universe, there was a discrepancy between Wheeler’s AAD gravitation theory and General Relativity. In defense of his world line theory, Wheeler adopted a viewpoint of space and time in the tradition of Ernst Mach. In the 19th century, Ernst Mach stated that rotation is being determined by the large-scale matter distribution in the universe. This was opposing Newton’s notion of absolute space and time. Wheeler argued that if Mach’s principle is true, the two-body problem is non-dynamical, because the distance between two bodies can not change since there is no way to measure this distance. Mach’s Principle had played an important role in Einsteins applications of General Relativity in Cosmology; therefore, Wheeler argued, the two-body solution in general relativity is without physical meaning. Then,

111This section is fully based on (A. Blum & Brill, 2019), p.11-15 112In the beginning, Wheeler called them “light-cones”, because electrically charged particles send signals with the speed of light 113 0 Functions like α = α−(β+(α)) = α are trivial

27 6.2 Teaching general relativity 6 WHEELER ENTERING RELATIVITY

since there exists no two-body solution to general relativity, there is no discrepancy with Wheeler’s AAD gravitation.

6.2 Teaching general relativity

From 1950 to 1953, a new war effort, the development of the hydrogen bomb, separated Wheeler from his theoretical work once again. During this project, he kept thinking about the correspondence between his AAD gravity and general relativity.114 Wheeler, who had hoped to construct an AAD gravity from scratch, started to realize that he needed to take general relativity as a starting point. To do so, he needed a better understanding of general relativity. To Wheeler, the best way to learn was by teaching; therefore, Wheeler asked Allen Shenstone, head of the physics department at Princeton University, if he could teach a course on general relativity on his return to Princeton in the academic year 1952/1953. Shenstone said “yes”, and so it happened that Wheeler thought the first course on general relativity at Princeton University, starting in the beginning of 1953.115 Also, and more important, the research on general relativity was largely stagnated from the mid-1920s to the mid 1950s, as is explained in chapter 5.1 The theory was considered to be a mathematically complex and highly formal theory, on which was seen as a theory for mathematicians instead of for physicists. Up until Wheeler’s initiative general relativity was only taught in the mathematics department at Princeton. When Wheeler asked Shenstone to let him teach a course about General Relativity, Shenstone could not really decline, because Wheeler was widely recognized as being important because of his work on nuclear physics.116 During this course, Wheeler’s personal interest in gravitation and general relativity can easily be recognized:117 he spent time on Mach’s principle in general relativity, his own AAD theory for gravity, the Wheeler-Feynman absorption theory, and “the elementary particle problem”. With “the elementary particle problem”, Wheeler meant the problem of giving a consistent description of the internal structure of elementary particles. In the paper Geons - the first paper in which Wheeler tries to solve this problem by formulating classical particles in terms of curved space-time, as we will discuss in the next section - Wheeler made clear what problems he observed with the notion of classical particles.118 According to Wheeler, “the position of the concept of body in the general theory of relativity has always been interesting.” With a body he means an object of mass and three position coordinates, that has both externally and internally a classical description, in contrast to a particle which has at least the interior with which quantum properties are associated. He continues: “Either one sticks to general relativity as it is, and treats the object as a singularity in the metric, or one postulates that the field is regular everywhere, and counts on quantum theory somehow to explain how this can be so even in the region of the body. Both attempts, Wheeler considered unsatisfactory. At this time, Wheeler did not believe in the physical existence of singularities. This led him to spend time on gravitational collapse during the course. Reading the papers by Robert Oppenheimer co-authored by Hartland Snyder and George Volkoff about gravitational collapse had made big impact on Wheeler’s thinking about general relativity.119 The article with Snyder caught Wheeler’s eye by

114(A. Blum & Brill, 2019), p.15 & (Rickles, 2018), p.251 115It might sound strange that Wheeler was the ﬁrst one to teach a relativity at the physics department since Princeton was the American center for relativity research. However, those relativists, including Einstein, worked at the Institute for Advanced Studies 116Interview Charles Misner by the author, 14th of January 2020. The transcript is archived at the Niels Bohr Library & Archives, American Institute of Physics, College Park, United States 117Information about the content of the course is manly based on (A. Blum & Brill, 2019), p.16-17 118(Wheeler, 1955), p.511 119(Wheeler & Ford, 2000), p.228-229 & (Oppenheimer & Volkoﬀ, 1939) & (Oppenheimer & Snyder, 1939)

28 6.3 Relativity: particles in terms of space-time 6 WHEELER ENTERING RELATIVITY coincidence in 1952, since it had been published in the same number of Physical review as The mechanism of nuclear fission. Oppenheimer and Snyder concluded the end state of the gravitational collapse of a thermonuclear burned out star to be a singularity. Wheeler was immediately considering an escape route from this conclusion; he was wondering if, at small distances, quantum mechanical forces would grow bigger than gravitational forces, and prevent gravitational collapse. In turn, this thinking about singularities led him to think about gravitation at small distances. Wheeler was convinced that quantum mechanics could provide more fundamental insights in general relativity and gravitation. For example, Wheeler thought that quantum mechanics might illuminate why the (+ + + −) is the signature of four-dimensional spacetime. Here we can find the origin of Wheeler’s later work on quantum gravity.120 Wheeler’s interest in general relativity went hand in hand with physical problems that would later become central subjects in the field of general relativity. As said, Wheeler spend time on gravitational collapse. Also, Wheeler found it essential to think about gravitational radiation, because this might tell if the universe in general relativity is “absorbing”. Since the Wheeler-Feynman absorption theory is built on the assumption that the universe meets the absorption boundary condition, understanding if the universe in general relativity is absorbing was of major importance for constructing AAD gravitation. In the course, we clearly see how Wheeler’s interest in constructing an AAD gravitation theory leads him to study fundamental questions of Einstein’s general relativity theory.

6.3 Relativity: particles in terms of space-time121

So far, Wheeler had been focusing on formulating a gravitation theory based on fundamental particles, i.e. electrons. This changed in the spring of 1953, when Wheeler abandoned the idea that electrons could be the fundamental particle. He had two reasons to do so:122 The first reason was that electrons did not have enough degrees of freedom to describe quantum characteristics, e.g. spin, of particles; The second reason was that electrons turned out to be not as simple as Wheeler had thought. The experimental confirmation of the Lamb shift had confirmed that the electron undergoes interaction with vacuum energy. Therefore, Wheeler concluded that the field approach fitted the electron better than the particle picture, and he abandoned his “Everything as Electrons” program. Wheeler kept pushing his direct AAD theory of gravitation during the 1950s, but now with another view on ‘particles’. He set aside his goal to formulate space-time in terms of world lines and liaison functions and he started a new project, later called geometrodynamics. Now, Wheeler wanted to describe physics in purely geometrical terms, starting with particles. Particle characteristics, like mass and charge, were no longer fundamental entities, but constructs of space and time. Space-time was not just the arena of physics, it was physics. Wheeler’s first geometrodynamics project was the construction of the geon (see figure 3a).123 The principle behind the geon is the deflection of a light wave or a gravitational wave in a strong gravitational field. When an object is massive enough, it can catch a beam of radiation in an orbit around it. If the energy of the radiation is increased until it is equivalent to the mass of the original object, and we take away the original object, we are left with “a model of something particle-like and yet not a particle: a geon, an [electromagnetic or] gravitational wave going around in a circle held in orbit by the mass-energy of that wave itself”. The geon seems to be a massive object, because it is a centering of energy,but there

120(Rickles, 2018), p.256 121This section is primarily based on (Rickles, 2018), p.257-264 122(A. Blum & Brill, 2019), p.20 & (Rickles, 2018), p.253 123This paragraph, including the citations, is based on (Wheeler, 1955)

29 6.3 Relativity: particles in terms of space-time 6 WHEELER ENTERING RELATIVITY

(b) A wormhole. Lines of force in a doubly connected (a) A geon. Two waves of equal strength run around space. One of the mounts appears to be a positive the torus in opposite direction to produce a standing charge, where the other appears to be an equal and wave. opposite charge.

Figure 3: Figures taken from (Wheeler, 1955), p.512 & 535 is no “real” mass involved: “mass without mass”. The geon, which Wheeler published in his article Geons in 1955, was clearly meant to be a classical model for a particle. It is a description of a classical particle based on classical general relativity, singularity-free and without any call on quantum theory. It is a model for particles built out of relativistic space-time. Furthermore, Wheeler used wormholes to give a geometrical description of charge. He was clearly inspired by the work of Einstein and Rosen on wormholes. 124 Wheeler’s idea was that when lines of force go trough doubly connected space-time, one of the mounts appears to be a positive charge, where the other appears to be an equal and opposite charge (see figure 3b). In this way, Wheeler wanted to describe charge in termes of space-time geometry; he wanted to create “charge without charge”. Although the Geon paper was focused on geons, his work on wormholes turned out to be more fruitful for the rest of Wheeler’s career. Wheeler not only wanted to formulate how wormholes are a model for charges; instead of taking wormholes to be a priory multiply connected spacetime, he wanted to understand the origin of wormholes. This led Wheeler to study the application of Feynman quantization, i.e. a sum over field histories or path integrals, on general relativity. Path integrals replace the classical notion of a single classical trajectory for a system with a sum over an infinite amount of possible quantum possibilities which all have a certain chance. This gave the basic insight that small scale fluctuations in the space-time geometry and topology occur. Wheeler thought that might be the origin of wormholes. The idea that space-time fluctuates on a very small scale due to quantum mechanics is called quantum foam of space-time foam. This led Wheeler further into quantum gravity.

In the spring of 1956, Wheeler fulﬁlled a guest professorship at Leiden University in the Netherlands. At this moment, the problem of quantum gravity, and especially the status of space-time concepts at small distances, was on his mind. Wheeler worked on this problem with Charles Misner, one of the students that accompanied him to Leiden. He was also in contact with Bohr about this matter.125 Misner and Wheeler saw Feynman path integrals as a “starting point for a quantum theory of gravity”,126 and they were expecting to ﬁnd that quantum numbers, like strangeness, isospin etc., would come out of

124Wormholes were first theoretized by Hermann Weyl (Weyl, 1921). Einstein and Rosen had attempted to derive an atomistic theory of mater and charge from the gravitational field of general relativity and the electromagnetic field of Maxwell’s theory (Einstein & Rosen, 1935). They did not tackle this problem, but they drew attention to the possibility to derive quantum phenomena from those two fields. They formulated the Einstein-Rosen bridge, which was later dubbed a “wormhole”, a term popularized by Wheeler 125Letter Wheeler to Bohr, 24th of April 1956, JAWP, Niels Bor file 2, box 5 126Report dated 1st of January 1956, JAWP, Charles Misner file 1, Box 18

30 6.4 Radical reductionism & daring conservatism 6 WHEELER ENTERING RELATIVITY it.127 Since the Planck length is the limit where general relativity seems to break down and quantum fluctuations take over, it got a central position in their thinking. Although Wheeler was working on fluctuations in spacetime, primarily he was still working on explaining the properties of elementary particles. It was not until after 1957 that Wheeler began to seriously look at general relativity and quantum gravity independent from concerns in particle physics. In Leiden, Wheeler and Misner also worked on the “already unified field theory”.128 This theory concerns a universe only made of Einstein’s gravity and Maxwell’s electromagnetism.129 The essence of the “already unified field theory” is to show that in this universe, the gravitational field alone contains all the necessary information to deduce the values of the Maxwell field. The combined Einstein-Maxwell equations can be written in terms of the metric field. The Maxwell equations can be extracted from geometry; They are emerging concepts. Wheeler and Misner “secured from the standard theory of Maxwell and Einstein an already unified field theory”.130 Because this suggests the possibility that geometry and topology can be used to describe classical physics, i.e. classical gravity and electromagnetism, it was an important step in the development of geometrodynamics. Although they figured out that George Rainich had already done this work in 1924, it was important work for their article Classical physics as geometry, in which they described classical physics in terms of curved empty space and nothing more, which they wrote in Leiden. Wheeler was also accompanied by his student Peter Putnam and by Professor Joseph Weber. Weber was a professor at the engineering faculty of the University of Maryland, who had done important work on the development of lasers. Weber joined Wheeler’s group during a sabbatical year in 1955/1956. Think- ing about geons sparked a strong interest in gravitational waves, which were not generally accepted to exist at that time. During his stay in Leiden, Weber started working on gravitational radiation. Wheeler and Weber wrote Reality of the cylindrical gravitational waves of Einstein and Rosen in Leiden.131 Fur- thermore, Wheeler had invited Tullio Regge to come over for a week.132 Together, they wrote the article about the Stability of a Schwarzschild Singularity.133 This article was full of errors, but it had developed the necessary tools to show that the Schwarzschild metric is stable. Later, C. Misner and his students would show that this metric described a black hole.134

6.4 Radical reductionism & daring conservatism135

The research goal that led Wheeler into general relativity was a radically reductionist one: reducing the growing number of elementary particles to one fundamental particle, while still being able to explain and reproduce the multitude of observed particles, their properties and nuclear interactions. Wheeler was not alone in this goal, but his methodology was special. Instead of proposing a new fundamental substitute that was yet unobserved, Wheeler wanted to use a known entity from a well-established theory whose properties were already empirically proven. Later, Wheeler called this methodology daring conservatism: taking (an element of) a well-established theory (conservative) and applying it outside of their traditional,

127Letter Misner to Wheeler, 10th of March 1957, JAWP, Charles Misner ﬁle 1, Box 18 128Personal communication with Charles Misner on the 3th of January 2020 & (Rickles, 2018) 129(Misner, n.d.) 130(Misner & Wheeler, 1957) 131(Weber & Wheeler, 1957) 132Letter Wheeler to Regge, 18th of May 1956, JAWP, Relativity notebook IV, p.85 133(Regge & Wheeler, 1957) 134Interview Charles Misner by the author, 14th of January 2020 135This section is based on Blum and Brill (2019) p.3 & 33 & 34

31 6.4 Radical reductionism & daring conservatism 6 WHEELER ENTERING RELATIVITY well-tested domain (daring).136 We clearly see this goal of radical reductionism and the methodology of daring conservatism in Wheeler’s work on AAD electrodynamics. First, his goal was to formulate a theory where all of physics could be described in terms of the fundamental particle: the electrons. He was specifically interested in building a theory were the nucleus was made out of electrons. To do so, he started by formulating an AAD theory for electrodynamics, where electrodynamics could be described in terms of electrons and the interactions between them. Wheeler extended this work to attempts to formulate an AAD gravitation theory. The goal and the methodology were the same as with his AAD theory for electrodynamics. In pursuing this goal, he had to reflect on the fundamental questions concerning general relativity. This intellectual trajectory was typical for the Renaissance. In the early 1950s, Wheeler had to give up his hope of “Everything as Electrons”. The theory that Wheeler ultimately selected as the key-element in his reductionist program was general relativity. On his entrance to relativity, Wheeler attempted to solve the “elementary particle problem” by reducing “particles” and particle characteristics to curved space- time. After 1957, Wheeler started to more extensively explore other possibilities of general relativity too. Now, he tried to formulate all of physics in terms of curved space-time. Wheeler has been one of the leaders of the Renaissance and his work was influential. This was not because of the specific attempts with which he tried to solve the elementary particle problem: his geons were physically impossible, his world lines and liaison functions are not considered to be fruitful for the way we think about particles today, and Wheeler’s path integral method for quantizing gravity does not seem to be successful. Still, Wheeler has had a large influence on the Renaissance because he understood the potential of the theory: not only did he understand that general relativity had a potential to solve problems and to predict new phenomena, but he also understood what kind of problems and phenomena relativity could solve and predict. Already in his first relativity course in 1953, Wheeler had spent time on problems like gravitational collapse, gravitational waves and empirical cosmology, which would later become central elements in the Renaissance. He had connected the isolated field of general relativity to other sub-disciplines of physics, like particle physics, quantum theory, and astrophysics. By doing so, Wheeler paved the pad for many others.

136According to Stefano Furlan (sfurlan@mpiwg-berlin.mpg.de), this daring conservatism might originate in Wheeler’s respect for Einstein and Bohr (personal communication on the 18th of November 2919). Even though Wheeler was a respected physicist himself, he was always behaving like a “freshman” in relation to professor Einstein and professor Bohr. When testing out new ideas, their opinion was of major importance to Wheeler. Because of his respect for Einstein and Bohr, he wanted to explore the potential of their theories instead of formulating new theories; he used their theories to explore phenomena in other ﬁelds of physics.

32 7 WHEELER IN LEIDEN

7 Wheeler in Leiden

On 31st of May 1954, Sybren de Groot and Cor Gorter sent Wheeler a letter to invite him to fulfill the Lorentz professorship in Leiden in academic year 1955/1956.137 Wheeler enthusiastically responded that he was honored and that “[he had] always been a great admirer of Lorentz not only for his work but for his character, his modesty and his concern for the fundamentals” and that “[he] would look forward with much pleasure to associate and discus with you [de Groot] and other colleagues.”138 After discussing the offer with Shenstone, Wheeler let de Groot know that he would fulfill the position.139 De Groot and Gorter put the nomination machinery to work and prepared the paperwork. Because the position at Leiden would pay poorly,140 Wheeler had to find supplementary income. Therefore, he applied for a Fulbright scholarship.141 The Guggenheim fellowship allowed him to use the part of the scholarship that he had not been able to use in 1950 because he had to go back to the United States for his H-bomb work.142 Finally, all the preconditions were met, and Wheeler left to Leiden in the winter of 1955/1956. Now the question arises why Leiden decided to invite Wheeler. As we have seen in chapter 4, De Groot had established the Lorentz chair to attract influential international physicists. Wheeler fitted this picture. It is unclear if de Groot was aiming for nuclear physicists.143 It is imaginable that Leiden was looking for someone to teach courses in nuclear physics after losing Kramers. Nuclear physics was an important subject, so Leiden could have wanted to teach this. Why then did they give Wheeler free choice for a course to teach? First, they may have wanted to make the position more attractive by giving freedom, since the pay was rather low. Second, they probably did not expect a nuclear physicist like Wheeler to choose a topic within the field of nuclear physics either way. Whether Leiden invited Wheeler not only because he was an international influential physicists but also specifically for his nuclear expertise, stays unclear. However, it does not matter for this decision, since there was a second actor that influenced the choice for Wheeler, who was definitely interested in him for his nuclear expertise. The second actor involved in the choice for Wheeler was the Dutch physics community as a whole. In making this decision, Leiden physicists took into account the need of Leiden and the needs of the Netherlands as a whole. Physicists in the Netherlands, including multiple Leiden physicists, had organized themselves in a discussion on the future of physics in the Netherlands. They shared a vision on about the rebuilding of physics and the Netherlands, and the role physics could play in Dutch economic recovery. Therefore, Leiden physicists took not only the needs of Leiden but also the needs of the Netherlands as a whole into account when they invited Wheeler for the Lorentz chair. As we have seen in chapter 3, Dutch physicists were of the opinion that the Netherlands had to focus on both cooperation with the United States and on nuclear knowledge. The interest of the Netherlands as a whole on the choice of the Lorentz professor becomes clear when we analyze the similarities between the first six Lorentz professors (it shows what we already expected): George E. Uhlenbeck (1955), John A. Wheeler (1956), Eugene P. Wigner (1957), Walter H. Heitler (1958), John G. Kirkwood (1959) and John H. Van Vleck (1960). First, all those Lorentz professors, except Heitler, were born or naturalized American, so inviting those physicists promoted the American-Dutch cooperation. Uhlenbeck had strong ties with Leiden, since he was born in the Netherlands and had studied in Leiden under the supervision

137Letter de Groot to Wheeler, 31st of May 1954, JAWP, Leiden Preparation, Box 148 138Letter Wheeler to de Groot, 5th of June 1954, JAWP, Job Notebook folder 2, Box 14 139Letter Wheeler to de Groot, 27th of October 1954, JAWP, Job Notebook folder 2, Box 14 140Letter Shenstone to Wheeler, 11th of June 1954, JAWP, Job Notebook folder 2, Box 14 141Motivation letter Fulbright application, JAWP, Relativity III, p.274 & Fulbright form Wheeler, of October 17th, 1956, JAWP, Leiden Preparation, Box 148 142Letter Henry Allen Moe to Wheeler, 20th of December 1955, JAWP Leiden Preparation, Box 148 143The author has not been able to ﬁnd archive material concerning the appointment of Lorentz professors

33 7 WHEELER IN LEIDEN of Ehrenfest. After Kramers, he had been professor of theoretical physics in Utrecht in 1935, until he fled to the United States due to raising Nazism. In 1952, he became official American. Uhlenbeck had stong ties with Leiden, so it was a logical choice to start by inviting him. From there on, other American physicists could be invited. Gorter and Wheeler had been in contact after Kramer’s death.144 Wheeler and Kramers knew each other well, so Wheeler might also have been in contact with Kramers collegues in Leiden. This might have made it easier to approach Wheeler for the position. Second, Uhlenbeck, Wheeler, Wigner and van Vleck were known for their nuclear expertise. Wheeler, Wigner and van Vleck had all played an important role in the Manhattan project. Wheeler was one of the most prominent American nuclear physicists of that time. He was a former student of Niels Bohr and had made important contributions to the Manhattan Project. Therefore, he not only fit the Leiden picture - influential and international- but also the picture of the Netherlands as a whole. The motivation to invite Wheeler to Leiden was thus to strengthen physics collaborations with the United States and to learn from Wheeler’s expertise in nuclear physics. Not only academical Dutch physicists but also the Dutch industry saw the importance of Wheeler’s visit for the rebuilding of Dutch physics. The Dutch industry too understood the important of Wheeler visit to Leiden for the rebuilding of Dutch physics. Dutch industries financially supported Wheeler’s visit to Leiden. Industries paid Wheeler a total of 4500,- Dutch guilders in addition to the total salary of 94444,66 Dutch guilders that University Leiden paid him.145 Wheeler’s visit to Leiden was clearly not only a matter of rebuilding physics in Leiden, but a matter of rebuilding Dutch physics as a whole.

Now we understand why Wheeler was invited to Leiden. But why did Wheeler accept this invitation? Wheeler had a well-established career in nuclear physics and got many short and long-term offers from multiple universities,146 so he could have chosen another position that did not pay as poorly as Leiden. Why did Wheeler still spend half of his sabbatical year in Leiden? The reason Wheeler gave in his letter to de Groot is that he had always been a great admirer of Hendrik Lorentz. Leiden had of course been a prominent center for theoretical physics, often visited by Einstein, and one of the first research centers on general relativity. However, Wheeler knew about the backwater position Leiden had ended up in. Therefore, this was not his primary motivation to come to Leiden. Then what was his motivation? The primary reason for Wheeler to visit Leiden was that it offered him the possibility to focus on his efforts to formulate elementary particles in terms of curved space-time.147 As explained in chapter 6, Wheeler was focusing on quantum gravity, and the status of space-time concepts at small distances in particular, to understand elementary particles in terms of curved space-time. He was looking to spend his sabbatical year “in quiet work on problems of elementary particle theory at a place where [he] would

144Letter Cor Gorter to Wheeler, 4th of September 1952, JAWP, Cor Gorter, Box 40 145Salary specification, send by F.Y. Verploegh Case to Wheeler, 14th of January 1957, JAWP, Income tax records/receipts, Box 246 The author thinks that the money from industries has been paid by the Dutch company Koninklijke Philips. Philips is a technology company that would have benefited from a revival in Dutch physics. Furthermore, it was large and successful enough to pay this money. Also, Casimir, who had strong ties to Leiden physicists, was first one of the three directors and later member of the board of directors of the company. However, the author has not been able to check this suspicion, since Philips does not allow students in their archives. 146Wheeler was offered to become a William Phyle Philips Visiting Lecturer at Haverford College (Letter Richard M. Sutton to Wheeler, 3th of January 1955, JAWP, Job Notebook folder 3, Box 14), he was offered three weeks at the Verenna in Italy (Letter Wheeler to Polvani, 17th of February 1955, JAWP, Job Notebook folder 3, Box 14) and he was offered a position at Yale University (Letter Wheeler to Breit, 17th of February 1955, JAWP, Job Notebook folder 3, Box 14) 147Interview Charles Misner by the author, 14th of January 2020. The transcript is archived at the Niels Bohr Library & Archives, Amercan Institute of Physics, College Park, United States

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have stimulating colleagues and freedom from obligations.”148 The “nature of the position [in Leiden had] a very great appeal” to Wheeler,149 since the duties consisted of giving two or three hours of lecture per week on a subject of the lecturer’s choice.150 Wheeler learned by teaching, so the opportunity to teach relativity - a field still not many universities were interested in - was a great benefit of the position in Leiden. Furthermore, de Groot allowed Wheeler to bring his own relativity team, existing of Joe Weber, Charles Misner and Peter Putnam. Therefore, he could continue his promising work with his group, while being free of his Princeton obligations. Wheeler also invited Tullio Regge for a week. Leiden seemed an ideal opportunity to work in quiet on the elementary particle problem and quantum gravity at small distances with his own team while sharpening his mind by teaching. A second reason that made the position in Leiden attractive was that it offered Wheeler the opportunity to collaborate more closely with Bohr.151 They discussed a broad range of topics via letters, and Wheeler visited Bohr in Copen- hagen in May 1956.152 Among them were the question of how to combine several fields form- ing a vacuum with zero mass density, the effort to recover elementary particles from zero rest mass fields, and the quantization of gravity, centering the notion of the Planck Length.153 Furthermore, Wheeler and Bohr discussed Ev- erett’s interpretation of quantum theory, that later became known as the “many world” interpretation.154 Wheeler was interested in Everett’s interpretation of quantum theory, since it could help him with a problem he was working on: formulating a quantum wave function for the Uni- verse. The Copenhagen interpretation takes the view that a quantum wave function is only mean- ingful to an observer outside the system under in- vestegation. Such an observer seemed out of the question when the system was the whole universe. How then can a wave function of the entire universe be found? Hugh Everett had formulated an Figure 4: Charles Misner in front of Institute idea in which the Schrödingerequation was always Lorentz in Leiden. Picture from personal collection valid:155 wave functions do not collapse under ob- Charles Misner servation. In his idea, the observer was included in the system being analyzed. Instead of a collapse

148Letter from Wheeler to Kenneth Bainbridge c.c. Shenstone, 28th of October 1954, JAWP, Leiden Preparation, Box 148 149Letter Wheeler to de Groot, 5th of June 1954, JAWP, Job Notebook folder 2, Box 14 150Fulbright application and motivation letter, JAWP, Leiden Preparation, Box 148 and Relativity notebook III p. 274 151Interview Charles Misner by the author, 14th of January 2020 152Letter Bohr to Wheeler, 24th of April 1957, JAWP, Relativity IV, p.64 153Letter Wheeler to Bohr, 24th of April 1956, JAWP, ﬁle Niels Bohr, Box 5 154Personal communication with Charles Misner on the 3th of January 2020 155(Misner, 2015)

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of the wave function of the universe, there are multiple universes that together describe the quantum system. Then, if there is no collapse, no observer is needed, so the wave function of the universe could be defined. In 1955, Everett had started his PhD under supervision of Wheeler concerning this multiple universe interpretation. However, Bohr and the rest of the defense committee of Everett’s thesis had been less enthusiastic about this idea. Wheeler was enthusiastic about Everett’s work, but he also placed great value in the opinion of his respected and beloved mentor Bohr; his enthusiasm was constrained by Bohr’s opinion.156 Therefore, Wheeler tried to get a tolerant view from Niels Bohr on Everett’s work during his visit to Copenhagen.157 Wheeler was clearly focused on formulating elementary particles in terms of curved space-time, and valued the collaboration with his own team and with Bohr. Was the collaboration with Dutch and other European physicists on this subject also of importance to travel to Leiden? During his stay in Leiden, Wheeler collaborated with a few physicists and mathematicians on his effords to formulate elementary particles in terms of spacetime. One of them was mathematician Jan Schouten, with whom Wheeler discussed his work with Misner.158 Wheeler was interested in Schouten’s mathematical insights: he had already ordered Schouten’s book Ricci calculus before going to Leiden.159 During their stay in Leiden, Schouten helped Wheeler and Misner with “illumination on the mathematics of geometrodynamics”, for which he has been acknowledged in the paper Classical physics as geometry. A second mathematician with whom Wheeler and Misner collaborated, and that is thanked in Classical physics as geometry, was the Belgian Jules Géhéniau.160 Those collaborations were not of primary interest in Wheeler’s decision to come to Leiden, but they were useful and added to Wheeler’s papers. Furthermore, Wheeler had more discussions witch Dutch and other European physicists that did not end up in the acknowledgment of his papers. One of then was the Belgium theoretical physicist Léonvan Hove. From 1949 to 1955, van Hove had worked at the Institute for Advanced Study in Princeton, Wheeler knew him. In 1956, when Wheeler was in Leiden, they had an appointment at University Utrecht. Wheeler and van Hove were in contact again, discussing multiple topics like the Fermi field - a quantum mechanical field whose quanta are fermions - in space time161 and the analogue between elementary particles and superconductivity. Furthermore, Wheeler discussed cosmology with Jan Oort,162 and he had contact with Fokker and Casimir, with whose work Wheeler had been familiar before he came to Leiden.163 However, he did not make notes of their discussions in his notebook, nor did he thank them in his papers.164 Wheeler also made some short visits to European colleagues to discussed his relativity work. On his way to Leiden, Wheeler was to meet Alexandru Proca in Paris.165 Unfortunately, Proca, who had already been ill, died before Wheeler could see him. Wheeler still gave the planned seminar on geons at

156(Misner, 2015) 157Wheeler was not able to change Bohr’s view. Everett’s thesis, published in Modern Physics in 1957, was a compromise between Everett and Wheeler and was accompanied with a supporting discussion by Wheeler (Wheeler, 1957) 158Letter Schouten to Wheeler, 13th of March 1956, JAWP, Relativity notebook IV, p.16 & Notes by Wheeler, March 20th, 1956, JAWP, Relativity notebook IV, p.30 159Letter Wheeler to Maxwell and Co., 6th of January 1956, JAWP, file Leiden Preparation, Box 148 & Receip Ricci calculus, December 1955, JAWP, Income tax records/receipts, box 246 160Notes by Wheeler, 11th of March 1956, JAWP, Relativity notebook IV, p.74 161Notes by Wheeler, 1th of May 1953, JAWP, Relativity notebook I, p.152 & Notes by Wheeler, 16th of May 1956, JAWP, Relativity notebook IV, p.82&84 & Notes by Wheeler, 15th of March 1954, JAWP, Relativity notebook II, p.137 162Notes by Wheeler, 24th of March 1956, JAWP, Relativity notebook IV, p.26 163Casimir 1954 retarded van der Waals forces, JAWP, Box 88 & Notes by Wheeler, 11th of May 1953, JAWP, Relativity notebook I, p.152 & Notes by Wheeler, June 24th, 1956, JAWP, Relativity notebook IV, p.119 164Articles checked from 1956 and 1957 165Letter Wheeler to Proca, 11th of November 1955, JAWP, file Leiden Preparation, Box 148 & Letter Maurice Jean to Wheeler, 15th of January 1956, JAWP, file Leiden Preparation, Box 148

36 7 WHEELER IN LEIDEN the Henri Poincare Institute. During his stay in Leiden, Wheeler was also in contact with Herman Bondi, who was interested in Wheeler’s geon project.166 Bondi, who worked at King’s college Londen, had just moved his focus to General relativity and was interested in Wheeler’s geons. On the 24th of March 1956, Wheeler and Oort went to Amsterdam to meet with Bondi and to discuss cosmology and Mach’s principle. Furthermore, in the beginning of June 1956, Wheeler made a trip to Cambridge to give a talk at the Cavendisch Physical Society on invitation of Nevill Francis Mott.167 During this trip, Wheeler discussed how to present the Fermi field in space time with Abdus Salam and James Hamilton.168 Although all those meetings seem to have been interesting additions to Wheeler’s considerations on his relativity work, they were not a big motivation for Wheeler’s Leiden trip. Wheeler discusses topics related to his work on elementary particle problem and quantum gravity at small distances with his European colleagues, but those discussions were not of major influence on Wheeler’s work in Leiden. Besides the two mathematicians Schouten and Géhéniau,non of the above-mentioned scientists were acknowledged in the papers Wheeler wrote in Leiden. Wheeler’s notebooks in Leiden mostly concern discussions and letters with people from his American cycle, i.e. Weber, Misner, Everett and Komar, and Putnam. Although the meetings with the Dutch physicists and mathematicians and Wheeler’s short trips to other scientists in Europe seem to have been interesting, Wheeler was clearly focused on his own team. Collaboration with the Dutch and the Europeans was clearly not a primary interest of his visit to Leiden.

Although Wheeler’s primary interest was general relativity and elementary particles, he was still working on a few nuclear physics projects. The Dutch were clearly more interested in Wheeler’s nuclear knowledge than in his relativity work. In Leiden, he got two offices: one large room with four desks for Wheeler and his relativity trio, and a small office for his nuclear work.169 Joe Weber recalls that one day, “Dr. Tolhoek asked Johnny to discuss some of the problems of physics and society with the Dutch Physicists. They left the office early in the afternoon and did not return before the end of the day.”170 According to Misner, the people that came talking to Wheeler came mostly to talk about nuclear matters.171 Moreover, Leiden physicist Hendrik Tolhoek showed great interest in collaboration with Wheeler on nuclear matters. Tolhoek researched the formulation of collective flow in nuclei, phenomena at the nuclear surface and nuclear saturation, and the theory of α− disintegration.172 Since Wheeler had been working in those fields, Tolhoek would have “extremely appreciate[d] to have the opportunity to discuss with you [Wheeler] those topics.” An exchange of letters and articles formed the prelude to a fruitful collaboration period during Wheeler’s time in Leiden.173 One of Wheeler’s projects was an article about the collective motion in nuclei with Jim Griffin, one of his former graduate students who now worked in Bohr’s group.174 During his Copenhagen visit, Wheeler discussed this subject with Jim Griffin and Aage Bohr, nuclear physicist and son of Niels Bohr. They worked on the paper Collective Motions in Nuclei by the Method of Generator Coordinates, which was published in 1957. Tolhoek helped Wheeler and Griffin with “several helpful discussions” for their paper Collective Motions in Nuclei by the Method

166Letter Bondi to Wheeler, 12th of July 1954, JAWP, Bondi 1954, Box 5 & Notes by Wheeler, JAWP, Relativity notebook III, p.58 & Notes by Wheeler, 24th of March 1956, JAWP, Relativity notebook IV, p.26 167Letter Nevill F. Mott to Wheeler, 22th of November 1955, JAWP, file Leiden Preparation, Box 148 168Cambridge Visit summary notes by Wheeler, 9th of June 1956, JAWP, Relativity notebook IV, p.101 169Letter from Charles Misner, June 1977, A family gathering volume 1, NBLA 170Letter Joe Weber, A family gathering, NBLA 171Interview Charles Misner by the author, 14th of January 2020. 172Letter Hendrik A. Tolhoek to Wheeler, 22th of November 1955, JAWP, file Leiden Preparation, Box 148 173Letter Hendrik A. Tolhoek to John A. Wheeler, 22th of December 1955, JAWP, file Leiden Preparation, Box 148 174Letter Griffin to Wheeler, 3th of May 1955, JAWP, Griffin 1954-1976, Box 11

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of Generator Coordinates,175 while Wheeler helped Tolhoek with “valuable discussions” for his article A theoretical estimate of the influence of the character of the four-fermion interaction on the muon capture rate.176 Clearly, there was collaboration and discussion between Wheeler and Tolhoek, although Wheeler’s primary focus was general relativity. Furthermore, Siegfried Wouthuysen, secretary of the organizing committee of the International Con- ference on Nuclear Reactions in Amsterdam in July 1956, invited Wheeler to give a talk on fission and to give a final resume of the conference.177 De Groot, van Hove, Casimir and Sizoo were part of the organization committee of the conference.178 They were familiar with the status and the needs of physics in the Netherlands, and inviting Wheeler to this conference matched the motivation to invite Wheeler to the Netherlands to learn from his expertise on nuclear physics. Wheeler was “very happy to participate and speak on the subject of fission” and he “appreciate[d] very much your [Wouthuysen] giving me the honor to ask me to undertake that responsibility.”179 Therefore, preparing his talk Fission was one of his occupations in Leiden. Moreover, Wheeler gave a seminar on nuclear models. As said before, it is unclear if de Groot had hoped Wheeler to chose nuclear physics as subject for his lecture series. When Wheeler proposed to teach a course on “fields, space-time and particles” this seemed “perfect” to de Groot. However, de Groot much appreciated if Wheeler could also give one or more seminars that were suitable not only for theoretical physicists but also for experimentalists.180 Although de Groot had not suggested a subject for the seminars, Wheeler naturally chose the subject of nuclear models, since that was his expertise. Whatever de Groots intentions might have been, students in Leiden profited from Wheeler’s nuclear knowledge during the seminar. Clearly, the Dutch showed more interest in Wheeler’s nuclear expertise than in his efforts to formulate elementary particles in terms of spacetime. This clearly shows the backwater position general relativity was in, since at the doorstep of the Renaissance, Wheeler, one of the causers of the Renaissance, was doing innovative work on relativity under the nose of Leiden, one of the former central institutions for relativity research, but Leiden did not recognized the potential of Wheeler’s work. Although Wheeler liked to go speak on the nuclear conference in Amsterdam181 and the collaboration with Tolhoek was fruitful, Wheeler actually wanted to focus on his elementary particle work with his own team. One might wonder if the situation in Leiden was the “quite peace” Wheeler had been hoping for. However, later, Wheeler described his stay in Leiden as “one of the most productive and pleasant periods of my life”.182 And indeed, he wrote an article with Jim Griffin, with Charles Misner, with Tulio Regge and with Joseph Weber.183

In Leiden, there was shove, something more on Wheeler’s mind than theoretical physics. Wheeler was involved in the scientific manpower shortage in the United States as a member of the Advisory Committee on Scientific Manpower, as explained in chapter 2. He had learned about the scientific manpower shortage in NATO during his duties as Vice President of the International Union of Physics

175(Griffin & Wheeler, 1957) 176(Tolhoek & Luyten, 1957) 177Letter Wouthuysen to Wheeler, 15th of July 1955 & Letter Wouthuysen to Wheeler, 8th of October 1955 & Letter Wheeler to Wouthuysen, 13th of October 1955, JAWP, file Leiden Preparation, Box 148 & Letter Wouthuysen to Wheeler, 29th of November 1955, JAWP, file Leiden Preparation, Box 148 178Proceedings of the international conference on nuclear reactions: Amsterdam, 2-7 July 1956 179Letter Wheeler to Wouthuysen, 13th of October 1955, JAWP, file Leiden Preparation, Box 148 180Letter de Groot to Wheeler, 25th of May 1955, JAWP, Leiden preparation, Box 148 181Interview Charles Misner by the author, 14th of January 2020. 182(Wheeler & Ford, 2000), p.248 183(Griffin & Wheeler, 1957), (Misner & Wheeler, 1957), (Regge & Wheeler, 1957), (Weber & Wheeler, 1957)

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in 1951-1954, and he “saw it even more vividly as Lorentz Professor at the university of Leiden.”184 At the same time, Wheeler had been asked to become part of the Advisory Committee on Scientific Manpower, a committee to advice the Academy-Research Council of the United States, as explained in chapter 2. This had attracted his attention even more to the shortage of scientific manpower in the United States and NATO as a whole. Wheeler had been much concerned about the scientific manpower problem in NATO, and especially in Europe. He had found that “[in] the struggle to preserve a free world, [. . . ] Europe’s weaknesses [is] our deepest concern” and that “[. . . ] Europe’s weaknesses stem from shortage of scientific and technical manpower”.185 The harsh contrast between the former grandeur of Leiden and the situation during his stay strengthened his worries about the backwater position of European science. After his return to the United States, Wheeler became officially involved in the scientific manpower problem of NATO: as explained in chapter 2, he became chair of the American Advisory group on invitation of senator Jackson.186 In this function, he often contacted the Dutch physicists he met during his period in Leiden. For example, in the summer of 1957, Wheeler sent the draft of the American Advisory group rapport, titled Trained Manpower for Freedom, to multiple physicists, people in industry and politicians for review, among whom Gorter. Jackson handed the rapport over to the third conference of parliamentarians from NATO countries in the week of the 11th of November 1957. In the rapport, the group recommended NATO to increase the quality and quantity of scientific manpower in NATO, and specifically in the European NATO countries, to prevent that the scientific imbalance between NATO and the Soviet Union would lead to a military imbalance. Besides providing grants for students and universities, the group focused on international collaboration, including establishing summer study institutions and expanding international exchanges of scientific and technical personnel. Wheeler worked on the execution of those recommendation. Therefore, he wrote multiple proposals for support for European summer schools to the Ford Foundation, which supported international exchange to promote American values and interests in the 1950s.187 Remarkably often, Wheeler asked the trio Casimir, Gorter and van Hove, whom he had got to know (better) during his time in Leiden, to review those reports. All five proposals that have been preserved as part of the John A. Wheeler Papers at the Philosophical Society Library in Philadelphia had been reviewed by four to six people, including Gorter, Casimir or van Hove.188 It is notable that those three Dutch physicists are the only Dutch Wheeler had asked to review the applications. Also, only a few Europeans had given a review of those proposals, so the reviews of the trio is extra remarkable. Furthermore, Wheeler made new recommendations for investments for the Ford Foundation in the end of 1959. In those recommendations we see back the influence of Wheeler’s stay in Leiden again: he recommended to invest in thermodynamics of irreversible processes, which was studied only in a few European institutions, like Leiden University; furthermore, he suggested to invest in radio astronomy, of which Jan Oort was

184Letter Wheeler to Jackson, 12th of February 1957, JAWP, American Advisory Group on NATO Scientiﬁc and Technical Personal folder 1, Box 19 185Notes from the JAWP as cited in (Krige, 2000), p.85 186Letter Jackson to Wheeler, 24th of January 1957 & Letter Wheeler to Jackson, 12th of February 1957, JAWP, American Advisory Group on NATO Scientiﬁc and Technical Personal folder 1, Box 19 187(Krige, 1999) 188Proposal to the Ford Foundation, Organization of the Atlantic Community Foundation, 11th of November 1957 & Proposal to the Ford Foundation, Support of a Summer Institution in Solid State Physics [...], November or December 1957 & Proposal to the Ford Foundation, Assistance to the French Summer School of Physics of the University of Grenoble [...], 11th of November 1957 & Proposal to the Ford Foundation, Assistance to the Italian Summer School at Varenna for the summer of 1958, 22th of November 1957 & Proposal to the Ford foundation - Assistance to the school of the Italian Society at Verenna for the summer of 1958, JAWP, NATO folder 4, Box 19

39 7 WHEELER IN LEIDEN a great advocate. Interestingly, it seems like Wheeler might have been actively trying to create chances for Leiden. Wheeler used the connections that he build in Leiden in his eﬀorts to strengthen scientiﬁc manpower in the NATO. The relationships that Wheeler established during his stay in Leiden concerned the future of physics in Europe and in the Netherlands. Wheeler kept contacting a few physicists that he had got to know during his time in Leiden concerning how the Ford Foundation best could spend money in Europe in order to strengthen the European manpower. Furthermore, it does not seem that Wheeler stayd in close contact with his Dutch colleagues. There was some friendly contact between Wheeler and i.e. Sybren de Groot and Casimir 189, and they used to see each other at conferences, but there does not seem to have been further collaboration.

189Letter Wheeler to de Groot, 3th of July 1979, JAWP, Sybren de Groot, Box 40 & Letter Wheeler to Casimir, 23th of August 1978, JAWP, Casimir 1978, 1985, Box 38 & Letter Casimir to Wheeler, 26th of August 1978, JAWP, Casimir 1978, 1985, Box 38 & Letter Wheeler to Casimir, 2th of September 1985, JAWP, Casimir 1978, 1985, Box 38

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Conclusion

From studying the historical background, we have learned that three actors were involved in this scientific exchange: the United States governmental circles, and especially the Fulbright program; Dutch physicists, and especially Leiden physicists; and, of course, John Wheeler. These actors had very different objectives and motivations for supporting Wheeler’s visit to Leiden. First of all, The United States wanted to be the global hegemony. Therefore, she wanted to establish and strengthen her hegemony in Europe. Furthermore, the United States wanted to strengthen her international position by establishing a European arsenal of knowledge. The United States used her scientific lead to meet those goals, using scientific exchange as her primary tool. The Fulbright Program became the principal mechanism for promoting educational exchange to and from the United States. During the early years of the program, the major part of the Fulbright budget was spent on American lecturers and researchers visiting the Netherlands who spread the American norms and values. Wheeler is a clear example of the above. He got a Fulbright scholarship to travel to Leiden, where he mixed with the Dutch physics community at Leiden University, at the International Conference on Nuclear Reactions in Amsterdam, and at other meetings. This exchange is a clear example of how the United States used her scientific lead in order to establish her hegemony in Europe by spreading her norms and values via scientific exchange. The Dutch chose to invited Wheeler so that the Netherlands could profit from his expertise in nuclear physics. Physicists in the Netherlands were organized and shared the opinion that the Netherlands needed to invest in nuclear physics while focusing on the United States. The choice for the Lorentz professor was a superposition of the needs of the Lorentz institute, namely an influential international theoretical physicist, and the Netherlands as a whole. The choice for Wheeler clearly reflects those needs, since Wheeler was an influential American theoretical nuclear physicist. The salary offered Wheeler was quite low due to limited resources of the Lorentz institute and FOM. Therefore, the Fulbright program supplemented Wheeler’s income. Of all scientific fields in the Netherlands, nuclear physics profited most from the Fulbright program. This underlines again that the Netherlands was looking for nuclear knowledge and American collaboration, for which they utilized the Fulbright program. Wheeler’s exchange was not an exception but part of a bigger trend. With this collaboration, the United States could secure her hegemony in the Netherlands, while the Netherlands could rebuild her scientific infrastructure. Together, they co-produced the American hegemony in the Netherlands. In contrast to the Netherlands’ motivation to invite Wheeler to profit from his nuclear expertise, Wheeler wanted to focus on his efforts to formulate elementary particles in terms of spacetime. He had changed his gaze from nuclear physics to general relativity. His primary motivation to accept Leiden’s offer had been that it offered him a calm place to focus on this recent research interest with his team while sharpening his mind by teaching a course on relativity and quantum mechanics. However, the switch from nuclear to relativity had not been as absolute as is sometimes depicted, and Wheeler had also still been working on nuclear physics. He cooperated on some nuclear matters, and he spoke at the nuclear conference in Amsterdam. In other words: he gave the Dutch what they wanted. At the same time, he had a very productive time working on his efforts to formulate elementary particles in terms of curved spacetime with his own team. Although the three actors had very different motivations for supporting this exchange, they did not prohibit each other. They all got what they wanted: Wheeler had a productive time working on his new research interest and the Dutch could still profit from his nuclear knowledge, while Wheeler was at the same time fulfilling the needs of the United States by making Dutch connections and by spreading

41 7 WHEELER IN LEIDEN

American norms and values. Wheeler used his Dutch contacts for advice about solving the scientiﬁc manpower problem in Europe, while he was at the same time creating opportunities for Leiden University.

Now that we understand the motivation of Wheeler and the Netherlands for Wheeler’s stay in Leiden, we clearly see that this event is an example of the backwater position relativity was still in halfway the 1950s. Leiden University, one of the former central hubs for relativity research, did not recognize the potential of the innovative relativity work of one of the leaders of what would soon become the Renaissance. Leiden was not interested in Wheeler’s relativity work, and they did not utilize Wheeler’s knowledge of relativity. They were double behind: they were behind the United States in science research in general, and they did not recognize the potential of Wheeler’s work. This exchange did not contributed to the building of the relativity community, since Wheeler did not establish strong connections with Dutch physicists that were interested in relativity, because the Dutch were generally not interested in relativity. The trip also did not spread much relativity knowledge or interest in the theory, since the Dutch were mostly interested in Wheeler’s nuclear knowledge. Therefore, the Netherlands was not dragged into the Renaissance. That even in Leiden the potential of Wheeler’s work was not recognized, again shows the backwater position relativity was in. Instead of establishing relativity relations, relations that were concerning the future of science in Europe and particularly the Netherlands were established. Wheeler’s exchange is a clear example of how the United States helped the Netherlands to rebuild her scientific infrastructure in such a way where both the United States and the Netherlands profited. Wheeler’s exchange in Leiden shows how two events in which he was involved, namely the renaissance of general relativity and the rebuilding of Dutch physics with American help, and which both have roots in World War II, are two separated events. The renaissance of general relativity does not seem to have influenced American-Dutch scientific diplomacy nor vice versa.

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Acknowledgments - We are very grateful to prof. J.A.E.F. (Jeroen) van Dongen for supervising this project. We also wish to thank J.G. (Jaco) de Swart for writing advise. Furthermore, we wish to thank Stefano Furlan and prof. J. (James) Griﬃn for insightful conversations about Wheeler. We are gratefull to prof. C.W. (Charles) Misner for the interview, of which the transcript is archived at the Niels Bohr Library & Archives, American Institute of Physics, College Park. The research trip to the United States has been made possible by the grant-in-aid from the Friends of the Center for History of Physics of the American Institute of Physics, the individual travel grant for students from the Amsterdams Universiteitsfonds, and the travel grants for young researchers of the Genootschap ter bevordering van Natuur-, Genees- en Heelkunde.

Consulted archives

JAWP NBLA John Archibald Wheeler Papers, 1880-2008 Niels Bohr Library & Archives American Philosophical Society Library American Instite of Physics 105 S 5th St, Philadelphia 1 Physics Ellipse Dr, College Park PA 19106 United States MD 20740 United States https://search.amphilsoc.org/collections/ https://www.aip.org/history-programs/niels-bohr- view?docId=ead/Mss.B.W564-ead.xml library

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