Inside the Centre: The Life of J. Robert Oppenheimer

by Ray Monk

  In the early summer of 1934, Oppenheimer and Lauritsen wrote a short paper together about the scattering of gamma rays produced by Thorium C". It was the only paper they ever wrote together, but they continued to have a great influence on each other’s work. Lauritsen, like Lawrence in Berkeley, would look to Oppenheimer to keep him informed about the latest developments in theory, while Oppenheimer kept a close eye on Lauritsen’s laboratory work, looking for things that needed explaining and that might provide the subject matter for papers written by himself and his students.

  Another avenue for collaborative work opened up in the summer of 1934 with the arrival at Stanford University of Felix Bloch. Bloch was a Jewish physicist from Switzerland, whom Oppenheimer had known and liked in Zurich. After leaving Zurich, Bloch had worked with Bohr in Copenhagen and with Enrico Fermi in Rome before accepting a post as a lecturer at Leipzig. He was driven out of his job by the Nazi regime and, like many others, came to the United States. Along with (to mention only the most prominent) Einstein at Princeton, Hans Bethe at Cornell and James Franck at Johns Hopkins, Bloch thus became part of the extraordinary enrichment of American physics that was brought about through the absorption of Jewish émigrés. Indeed, within a few years the United States had replaced Germany as the world’s leading centre for the study of physics, partly because many of the people who had made Germany pre-eminent in the field were now working in American universities. As the relentlessly patriotic Oppenheimer was quick to point out, these refugees would not have had the impact they did had there not been ‘a rather sturdy indigenous effort in physics’, but Oppenheimer, of all people, knew the influence that world-leading physicists could have.

  For this reason, no doubt, as well as for the reason that he happened to like and respect him, Oppenheimer helped to find Bloch a position at Stanford, which is about thirty miles south of Berkeley, on the other side of the San Francisco Bay. Every week, after Bloch’s arrival in California, there would be a joint seminar open to both his students and Oppenheimer’s: one week at Stanford, the next at Berkeley. As Bloch later remembered them: ‘One of us would go up and tell about something he had thought about and read about, and then there would be discussions. It was very stimulating for me. I did not feel quite as isolated as I would have felt otherwise.’

  After the seminar, Oppenheimer would treat the entire group (which would vary in size between twelve and twenty people) to dinner at Jack’s, his favourite restaurant in San Francisco, ‘a fish place down in the harbour’, as Bloch remembered it. ‘These were post-depression days,’ Serber recalls, ‘and students were poor. The world of good food and good wines and gracious living was far from the experience of many of them, and Oppie was introducing them to an unfamiliar way of life.’ On one occasion, Serber says, ‘Bloch grew expansive, and leaned over and picked up the check. He looked at it, blinked, leaned over again and put it back down.’

  Wendell Furry was no longer at Berkeley, as Oppenheimer had succeeded in finding him a job at Harvard, starting in the autumn of 1934. The series of Oppenheimer/Furry papers therefore came to an end, and Oppenheimer worked instead on a joint paper with Melba Phillips, who since completing her thesis in 1933 had been unable to find a full-time academic post and so had stayed at Berkeley. ‘There were no jobs,’ she remembered, ‘but one could get enough part-time work, part-time teaching, to live; and we stayed and did work, grading papers and so forth. There were several of us who did that. I stayed there for two more years, and it was during that period that I taught practically everything that was thrown my way, filling in for everybody, it felt like.’

  In the spring of 1935, a promising topic for Oppenheimer and Phillips to work on together was provided by Lawrence’s cyclotron experiments. After the debacle of the Solvay Congress at the end of October 1933, Lawrence’s work had received fresh impetus in January 1934, with the startling discovery that it was possible to create radioactive materials artificially. The discovery had been made in Paris by Frédéric and Irène Joliot-Curie (the pair combined surnames after their marriage in 1926), who showed that, by bombarding boron with alpha particles, it was possible to create a radioactive isotope of nitrogen, and by bombarding aluminium, radioactive phosphorus was produced. As the medical applications for radioactive materials were by then being explored and the demand for them was therefore increasing, the discovery attracted a great deal of excitement because it promised a cheap and plentiful supply. Laboratories all over Europe and America began to turn their attention to the possibilities opened up by this discovery. In Rome, most notably, Enrico Fermi decided to see what happens when one bombards elements with neutrons rather than alpha particles, and discovered that it was possible to create radioactive materials in that way too.

  In the Radiation Laboratory at Berkeley, work was dramatically interrupted by Lawrence on the day he saw the Joliot-Curies’ article in Comptes rendus. Running through the door waving a copy of the article, Lawrence translated for the benefit of his staff some key sentences, including one that made direct reference to the power of the cyclotron. Noting that their own apparatus was puny by comparison, the Joliot-Curies speculated what might be achievable with something like the cyclotron. For example, they said, nitrogen-13, which should be radioactive, might be produced by bombarding carbon with deuterons – that is, deuterium nuclei, which, because they have only half the atomic mass of alpha particles, should be roughly twice as penetrative. Immediately the cyclotron was set up to fire a beam of deuterons at a sample of carbon and a Geiger counter wired up to record any radioactivity produced. ‘Click . . . click . . . click . . . went the Geiger counter,’ recalled Milton Livingston. ‘It was a sound that no one who was there would ever forget.’

  Throughout 1934, Lawrence’s cyclotron was put to use making radioactive materials, many of which had never been seen before. ‘It was a wonderful time,’ one of Lawrence’s assistants later said. ‘Radioactive elements fell in our laps as though we were shaking apples off a tree.’ The New York Times ran an editorial on Lawrence, in which it said: ‘Transmutation [and] the release of atomic energy are no longer mere romantic possibilities.’ In the wake of this excitement, Lawrence was courted by rival universities even more assiduously than Oppenheimer had been, and to keep him the University of California increased his salary so that he became by far the best-paid scientist there. The Radiation Laboratory was made independent from the physics department, given its own budget and its own director: Lawrence.

  Meanwhile, relations between the theoretical physicists and the ‘Rad Lab’ grew ever closer. One of the new generation of physicists appointed to positions in the lab, Ed McMillan, became an accepted member of the Oppenheimer group and often joined them on their trips to San Francisco. Likewise, Oppenheimer and his students became familiar faces in the laboratory. The topic of Oppenheimer’s joint paper with Melba Phillips was provided by experiments conducted by Ed McMillan, Lawrence and a postdoctoral student at the Rad Lab called Robert Thornton. What Lawrence, McMillan and Thornton had discovered was that radioactive isotopes could be created by the bombardment of various elements with deuterons with less energy than the prevailing theory predicted.

  In their paper, ‘Note on the Transmutation Function for Deuterons’, Oppenheimer and Phillips gave an explanation for this that was quickly accepted – the ‘Oppenheimer–Phillips process’ becoming an accepted part of nuclear physics and finding its way into the textbooks. Together with the Born–Oppenheimer approximation, the Oppenheimer–Phillips process became Oppenheimer’s best-known piece of work among students and experimental physicists. The process in question is this: when an element, for example carbon, is bombarded with deuterons, the neutron in the deuteron binds with the carbon atom to form an isotope, in this case carbon-13, while the proton is emitted. The reason this process happens at lower energies than one would expect, Oppenheimer and Phillips explain, is that the deuteron is less stable than the target nucleus and, as it moves towards the target, it does so, so to speak, ‘ne
utron-first’, so that the neutron is able to overcome the electrostatic barrier that then repels the proton.

  In the spring of 1935, Oppenheimer wrote to Lawrence from Pasadena to say that he was sending Melba Phillips ‘an outline of the calculations & plots I have made for the deuteron transmutation functions’. The analysis, he reported, ‘turned out pretty complicated, & I have spent most of the nights of this week with slide rule & graph paper.’ The results, he stressed, needed to be checked by Melba very carefully: ‘You must give M time to work it over.’ As this suggests, Melba Phillips was a more competent and more careful mathematician than Oppenheimer, and was often turned to when difficult calculations needed to be made. In fact, many of his students were better mathematicians than he was. Willis Lamb remembers: ‘Oppenheimer’s lectures were a revelation. The equations he wrote on the board were not always reliable. We learned to apply correction-factor operators to allow for incorrect signs and numerical coefficients.’ However, if Oppenheimer benefited from Melba Phillips’s mathematical skills, she benefited from his intuitions into the nature of physical phenomena and his reputation. After their joint paper was published in the summer of 1935, she suddenly found jobs coming her way: first a teaching post at Bryn Mawr and then, more prestigiously, a research fellowship at the Institute for Advanced Study in Princeton.

  Because of the nature of the experimental work going on at both Berkeley and Pasadena, involving as it did much bombardment of nuclei and many transmutations and disintegrations to explain, Oppenheimer was drawn into the area of nuclear physics, where his contributions, such as his joint paper with Melba Phillips, were accepted readily and warmly applauded. However, it was not where his heart was. ‘I never found nuclear physics so beautiful,’ he was once quoted as saying. He much preferred to think about electrodynamics and field theory. He never spelled out why this was, but his interest in Hinduism and the remarks by Rabi quoted earlier perhaps provide a clue: he preferred to think about what connected things than what disintegrated them. Dirac’s relativistic quantum electrodynamics excited him because it promised to bring together relativity theory and quantum theory. His disappointment with it, I suspect, was not fundamentally to do with the troublesome infinities, but rather had to do with the fact that, in its talk of particles, anti-particles and ‘holes’, it presented a vision of discrete and separate things, rather than one of the interconnectedness of everything.

  Oppenheimer wrote little on quantum electrodynamics after 1935, but he kept up with the literature on it and his students continued to work on it and, in some cases, make important contributions to it. One suspects that his disengagement from it – as well as having to do with his interest in other rapidly developing areas, such as cosmic-ray research and nuclear physics – had something to do, like his initial engagement with it, with his relations with Paul Dirac.

  Dirac spent the year 1934–5 at the Institute for Advanced Study in Princeton, where he worked on the second edition of his classic text, The Principles of Quantum Mechanics. Remarkably, Dirac, then thirty-two years old, found love in Princeton, when he met Eugene Wigner’s sister, Margit, whom he married in 1937. Even after their marriage, according to the many Dirac stories that circulate among physicists, he was in the habit of introducing her as ‘Wigner’s sister’ rather than as ‘my wife, Margit’. Oppenheimer visited Princeton in the new year of 1935, but Dirac was away. He did, however, see Einstein and visit the Institute for Advanced Study, but, as he wrote to Frank, his impressions were not favourable: ‘Princeton is a madhouse: its solipsistic luminaries shining in separate & helpless desolation. Einstein is completely cuckoo; Dirac was still in Georgia. I could be of absolutely no use at such a place, but it took a lot of conversation & arm waving to get Weylfn36 to take a no.’

  It would evidently take something more connected to the real world than the Institute for Advanced Study to tempt Oppenheimer away from the school of physics that he had so successfully built up.

  * * *

  fn26 One electron volt is the energy of an electron when it has experienced the potential of one volt. In the context of discussing the energies of particles, physicists frequently abbreviate ‘electron volts’ to simply ‘volts’.

  fn27 The advantage of publishing a short paper as a letter to the editor is that it can appear in print within a very brief time – Oppenheimer’s letter was dated 14 February 1930 and appeared in the 1 March issue of the journal. The disadvantage is that it has less authority than if it has gone through the normal peer-review procedure.

  fn28 A full understanding of beta decay was not arrived at until a few years after Pauli’s postulation of the neutrino, and therefore many years after Rutherford’s original identification and naming of it. What Rutherford knew was that there was a form of radioactive decay different from alpha decay, in which the radiation consisted not of positively charged helium nuclei, but of much smaller, negatively charged particles, which he correctly identified as electrons. What was subsequently discovered is that these electrons are being emitted from neutrons that are decaying into protons.

  fn29 To understand the problem, it might help to give an example. A nucleus of Cobalt (with atomic number 27) undergoes beta decay and so gains a proton, thus transforming into Nickel (atomic number 28). In this process, an electron is emitted. What puzzled Pauli and other physicists at this time was that, when this happens, the figures often do not add up: the total energy of (in this case) the Nickel nucleus plus the electron sometimes does, and sometimes does not, equal the energy of the original Cobalt nucleus, depending on the energy of the electron, which varies along a continuous spectrum.

  fn30 Pauli was at the time going through an emotionally draining divorce from his first wife.

  fn31 The notion of an isotope originated in 1912, when the chemist Frederick Soddy coined the word to describe two or more atoms that occupy the same place in the periodic table, but have different radioactive properties. After the discovery of the neutron in 1932, it was realised that two isotopes of the same element differ with respect to the number of neutrons in their nuclei.

  fn32 C.P. Snow reports that, during this period, a ‘dialogue passed into Cavendish tradition: “Tired, Chadwick?” “Not too tired to work.”’

  fn33 When an alpha particle hits a screen made of a suitable substance (zinc sulphide was the most commonly used), it emits a tiny flash of light known as a ‘scintillation’. The experiments of Rutherford and his team at the Cavendish – and, indeed, the work pursued at most advanced physics laboratories – made use of this fact to detect the presence of alpha particles.

  fn34 I am rather puzzled as to what Oppenheimer might mean by this. The Yaqui are a Native American tribe, whose original lands were in what is now Mexico, California and Arizona. Presumably, further up the hill on which Oppenheimer’s house stood, there lived a group of Yaqui people.

  fn35 It would not be until the summer of the following year that Oppenheimer became resigned to the word ‘positron’, which he regarded as a barbaric mixture of Latin (posi-) and Greek (-tron).

  fn36 The great German mathematician Hermann Weyl had been at the institute in Princeton since 1933.

  9

  Unstable Cores

  UNTIL THE SUMMER of 1935, the longest, most intimate, most revealing letters that Oppenheimer wrote were to his brother, Frank. In that summer, however, the series of letters came to a temporary end when Frank moved to California. He did so to begin a PhD at Caltech with Charles Lauritsen (‘Charlie’ to both Oppenheimers and to most people who knew him). Frank was then twenty-three years old. Since graduating from Johns Hopkins two years earlier, he had spent about eighteen months at the Cavendish in Cambridge and another six months at the University of Florence. He had also spent some time in Germany. Though he always felt himself to be under the shadow of his accomplished older brother, there was one respect in which, by the time he returned to the US, he had succeeded where Robert had failed: he had mastered the skills needed for laboratory work and to
become an experimental physicist.

  Another way in which Frank differed from his brother was that, throughout his school and university education, he had taken an active interest in politics. From the first, his political sympathies were with the downtrodden. ‘I remember once,’ he laughingly said in an interview, recalling an incident during his school days, ‘I went with some friends to hear a concert at Carnegie Hall that didn’t have a conductor. It was a kind of “down with the bosses” movement.’ In the 1928 presidential election, Frank, while still at school, had taken part in the campaign to elect the Democratic Party candidate, Al Smith, who famously aroused the fierce and frightening antagonism of the Ku Klux Klan, both for his liberal politics and for being a Roman Catholic. The campaign was unsuccessful – Smith was beaten by Herbert Hoover – but it provided a focus for liberal politics in the US that paved the way for Franklin D. Roosevelt’s victory in 1932 and the ‘New Deal’ that followed.

  In the light of what was to occur in the 1950s, one interesting aspect of the 1928 Smith campaign was the candidate’s use of the word ‘un-American’ to characterise not those on the left of American politics, but those on the right. When he arrived in Oklahoma City to be greeted by the Ku Klux Klan burning crosses, Smith said: ‘To inject bigotry, hatred, intolerance and un-American sectarian division into a campaign. Nothing could be so out of line with the spirit of America. Nothing could be so foreign to the teachings of Jefferson. Nothing could be so contradictory of our whole history.’ He went on: ‘The best way to kill anything un-American is to drag it out into the open, because anything un-American cannot live in the sunlight.’