Inside the Centre: The Life of J. Robert Oppenheimer (39 page)

It is difficult to know whether the work that Oppenheimer did in the period after 1932 exemplifies or refutes Rabi’s remarks. On the one hand, this was one of Oppenheimer’s most impressively concentrated periods of work, during which he worked on the most fundamental, and difficult, problems that theoretical physics had to face at that time. Addressing these problems took all of the very considerable talents and energy at Oppenheimer’s disposal. Or, anyway, most of them; it is true that he combined his work in physics with, for example, learning Sanskrit and reading ancient Greek – things that most of us would regard as significant
achievements in their own right – but, for the most part, he approached physics at this time with something of the attitude recommended by Krishna in the Bhagavad Gita: freedom from attachments, seclusion from the ‘noisy multitude’ and, above all perhaps, a ‘constant yearning to know’. If he showed some interest in things of beauty, such as the literature he read, he showed almost none in the social and political upheavals that were happening at that time. Whether one thinks of this as a good or a bad thing, it is certainly hard to see that it hindered his progress in physics, and hard too to regard it as anything but an application of the outlook he had acquired from Hinduism.

On the other hand, despite his extremely hard work and his utter absorption in physics at this time, it is true that he did not accomplish anything remotely comparable to the achievements of Bohr, Heisenberg, Dirac et al. And the reason for that may have something to do with his Hindu-influenced attitudes. Regarding work as being valuable for its own sake, regardless of its results, may have inspired in him a devotion to work that others lacked, but it may also, as Rabi suspected, have made him less single-minded than the very best physicists with regard to
solving
problems, and more accepting of the idea that some problems were simply insoluble. Someone like James Chadwick or Ernest Lawrence, or even Paul Dirac, worked at a problem
in order to solve it
; Oppenheimer took pleasure
in the work itself
.

And yet, if Oppenheimer approached physics from the point of view of someone who saw in it ‘truth, goodness and beauty’, he also had at least
one
practical result in mind – namely, the development and growth of the peculiarly American school of theoretical physics that he had set out to create. In contrast to Paul Dirac, say, who had very few graduate students and spent very little time with those he had, Oppenheimer did almost everything, including his own research, with his students.

For both Oppenheimer and his students, the research agenda was set by the remarkable discoveries of 1932, as Oppenheimer spelled out in his letter to Frank in the autumn of that year. He begins his report with the very distinction that forms the heart of the Bhagavad Gita view of work: ‘The work is fine: not in the fruits but the doing.’ He goes on:

During the year 1932–3, Frank Carlson, having received his PhD in April 1932, was Oppenheimer’s research associate, Melba Phillips was in the final year of her PhD studies, and Leo Nedelsky, who, like Carlson, had completed his PhD in 1932, was still in Berkeley, having had no success in finding an appointment. Harvey Hall, meanwhile, had found a job at Columbia as an instructor in physics. Oppenheimer, perhaps thinking he was helping out, asked Nedelsky to lecture for him during the weeks he left Berkeley to go to Caltech. ‘It won’t be any trouble,’ Oppenheimer told him, ‘it’s all in a book.’ The book, however, turned out to be in Dutch. When Nedelsky reported that this would be a problem, Oppenheimer airily replied: ‘But it’s such easy Dutch.’

In 1932, these graduate students were joined by some postdoctoral students, holders of the coveted National Research Fellowship, who, now that Oppenheimer was there, regarded Berkeley as a serious rival to Cambridge, Copenhagen and Göttingen as a place in which to pursue postdoctoral research. In 1932–3, Oppenheimer worked with two of these NRF postdoctoral students. The first was Wendell Furry, a Methodist minister’s son from Indiana, who had taken his PhD at the University of Illinois. Furry had attended the Ann Arbor summer school in 1931 and had been deeply impressed at seeing Oppenheimer, the only American invited to share the platform with the galaxy of European stars on display, standing up to none other than Wolfgang Pauli. When he went to Berkeley, however (to begin with at any rate), Furry felt hopelessly out of his depth, finding that when he attended Oppenheimer’s lectures he did not understand a word of them. It took him a year to regain his confidence.

In the meantime, Oppenheimer began to work closely with Milton Plesset, his second National Research Fellow, though he had arrived not at Berkeley but at Caltech, initially hoping to work with Paul Epstein, the Russian physicist who had been there since 1921. Plesset had done his PhD at Yale on a subject close to Oppenheimer’s heart, Dirac’s theory of the electron, and so it was natural that Oppenheimer would take some interest in his work and that they should start working together. After all, people who understood Dirac’s quantum electrodynamics were few and far between. As Plesset later recalled: ‘The state of theoretical physics in this country at that time was not very advanced, except for Oppenheimer.’ When Oppenheimer arrived in Pasadena, Plesset remembers, ‘things really started to move’. He and Oppenheimer got interested in what Plesset
describes as ‘a problem with the Dirac electron’ and together they wrote a short paper that ‘put a new light on the Dirac theory’.

That paper, entitled ‘On the Production of the Positive Electron’, was published as a letter to the editor in the
Physical Review
in the summer of 1933 and is the first of a series of papers in which Oppenheimer, usually together with one of his students, attempted to address what he saw as the problems in Dirac’s theory of the electron. This had been an ongoing preoccupation of his since Dirac had first shown him the theory in 1928, but, after the discovery of the positron in 1932, this preoccupation took on a rather different form. He could no longer, as he had before, point to the negative energy states in the theory as evidence of a problem; these states were filled, as Dirac had predicted, with positively charged particles. Indeed, the paper jointly written by Oppenheimer and Plesset begins with the following acknowledgement: ‘The experimental discovery of the positive electron gives us a striking confirmation of Dirac’s theory of the electron.’ It had quickly been confirmed that positrons could be created not only by the impact of cosmic rays, but also in the laboratory. In their paper, Oppenheimer and Plesset discuss, in particular, the experiments at Caltech conducted by Carl Anderson and his colleague Seth Neddermeyer, in which they showed that pairs of electrons and positrons are created when the very energetic gamma radiation from thorium C" passes through lead. From Dirac’s theory, they point out, one can make predictions about the frequency of pair production that are confirmed by the experimental evidence, but only – and this is where Oppenheimer thought major changes in the theory were needed and where his interest really lay – up to energies of a certain limit. Beyond that limit, Oppenheimer and Plesset claim, the theory fails.

In the penultimate paragraph of their short paper, Oppenheimer and Plesset make what Abraham Pais has described as a ‘fundamental observation’, namely that ‘fast electrons and positives [positrons] . . . will themselves tend to produce further pairs’ – a prescient anticipation of the phenomenon of
showering
that would later be studied intensely by physicists, including Oppenheimer himself. Pais is less complimentary about the mathematics in the paper, pointing out: ‘Their final formula was wrong, as usual, as others rapidly noted.’

These two aspects of Oppenheimer’s work – his originality and his mathematical carelessness – are neatly captured in a recommendation that Ralph Fowler wrote to Edwin Kemble of Harvard, when Harvard expressed an interest in trying to lure Oppenheimer away from California. Kemble knew Oppenheimer himself, of course, but he also knew that Fowler had seen him on many occasions during his rise to prominence, the latest of which was a visit Fowler made to Berkeley in the autumn of 1932. Dated 30 November 1933, Fowler’s report reads:

Nevertheless, despite his unreliable mathematics, Oppenheimer was successfully putting both Berkeley and Caltech on the international map. When distinguished physicists came to California, the man who made the most impression on them was usually Oppenheimer. This was certainly the case when God himself, Niels Bohr, came to Pasadena in the summer of 1933. This was the only opportunity Oppenheimer had had to discuss physics with his greatest scientific idol since their initial meeting at Cambridge in 1926, and it seems to have been remembered by both as a pleasant and instructive occasion. At the very end of their 1933 paper, Oppenheimer and Plesset extend their ‘profound thanks’ to Niels Bohr, ‘who has helped us to understand the essential consistency of the theory which we have here applied’. For his part, Bohr was very pleased to be able to discuss physics with Oppenheimer, his meeting with whom seems to have been the highlight of his visit. He was much less enchanted with Millikan, but nevertheless agreed to meet the trustees of Caltech to tell them how well the school of physics was doing. On 14 June 1933, Oppenheimer wrote to Bohr to thank him for his visit, and to send him a copy of the paper he had written with Plesset, who was about to leave Pasadena to spend a year at Bohr’s institute.

Plesset remembers that Bohr’s institute that summer was ‘swarming’ with refugees from Nazi Germany, mostly Jewish scientists. Shortly after Plesset’s arrival, the institute held its annual seminar, which for a number of reasons is remembered as a melancholy occasion. Dirac, Heisenberg and Ehrenfest were among the attendees, and Plesset recalls ‘a lot of discussion over the validity of Dirac’s theory. People were groping still.’

In especially poor spirits at Bohr’s seminar was Paul Ehrenfest, who was described as looking ‘pudgy-faced and overweight’ and ‘losing his grip on physics’. When the time came to leave, Dirac, who had grown close to Ehrenfest, saw him waiting for a taxi, looking flustered and unhappy, and thanked him for his contributions to the discussions. This elicited from Ehrenfest an extreme and, for Dirac, extremely worrying, reaction: ‘What you have said, coming from a young man like you, means very much to me because, maybe, a man such as I feels he has no force to live.’ A few days later, the dreadful news came that Ehrenfest had shot himself, prompting Dirac to write a four-page letter to Bohr describing in detail his last moments with Ehrenfest and telling him that he could not help blaming himself for what had happened.

News of Ehrenfest’s death seems to have reached Oppenheimer rather
slowly. On 7 October 1933, nearly two weeks after the suicide, he wrote to Frank, apparently still unaware of it. What was more on his mind was what he described as the ‘work with pairs’, which he told Frank ‘has gone along nicely’. He was now more certain than ever that Dirac’s ‘theory gives the wrong answer for the production of very high energy pairs’ and seemed confident that he and his students were making progress in ‘cleaning up the formalism’.

Oppenheimer’s close attention to experimental work in this period is illustrated in the letter to Frank, as are some of the problems that it caused him. Lawrence, he writes, ‘has definitely established the instability of the H2 [deuterium] nucleus. It decomposes upon collision into neutron and proton, to the tune of about six million volts.’ This, he adds, with a discernible note of triumph, makes a ‘hopeless obstacle’ to Heisenberg’s theory of the nucleus. As it turned out, Heisenberg’s theory was a good deal more trustworthy than Lawrence’s observations.

Lawrence that autumn was given the honour of being invited to the seventh Solvay Congress in Brussels, which was held in the week of 22–9 October 1933. The theme originally chosen was the application of quantum mechanics to chemistry, but, in the light of the momentous discoveries of 1932, this was changed to the nucleus. Lawrence’s invitation was the source of great pride at Berkeley. It was, his PhD student Robert Thornton has said, ‘Lawrence’s first European recognition’. When the time came for Lawrence to leave for Belgium: ‘The whole staff went down to the train to see him off. Next morning they got together in the lab and then took off for a kind of two-day picnic climbing Mount Lassen. They were so happy you’d have thought they were all going to talk at the Solvay Congress.’

Unfortunately for Lawrence, his appearance at the Solvay Congress turned out to be something of a humiliation. Heisenberg, Bohr, Chadwick and Irène Curie all used the occasion to express scepticism about his results, unimpressed by his claim that they had been obtained (and could only be obtained) with a machine capable of 800,000 volts. No matter how many volts he had at his command, they insisted, from a theoretical point of view, his interpretation of his results did not make sense. According to Nuel Pharr Davis: ‘Lawrence left the conference feeling bad.’ A colleague at Berkeley described it as ‘one of Lawrence’s saddest experiences’. About a month later, it was shown by scientists at the Carnegie Institution in Washington that Lawrence’s results had been skewed by impurities.