Inside the Centre: The Life of J. Robert Oppenheimer (41 page)

After the Berkeley spring semester ended in April, and it was time for Oppenheimer to leave for Pasadena, Serber recalls: ‘Many of his students made the annual trek with him.’

Danish by birth, Lauritsen had been in the United States since 1916, when he emigrated with his wife and small baby, and in Pasadena since 1926. Before he came to Caltech and started an academic career, he had been a radio engineer – a background he put to good use in his work in experimental physics. At Caltech’s high-voltage laboratory Lauritsen worked on developing ‘super-voltage’ X-rays for use in medicine. Then, after Cockcroft and Walton succeeded in splitting the atom, Lauritsen, now working at the new Kellogg Radiation Laboratory, converted one of the X-ray tubes into a particle accelerator, and began work on the artificial production of neutrons and the bombardment of deuterium.

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, ‘neutron-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 Weyl
^fn36
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’.