Inside the Centre: The Life of J. Robert Oppenheimer (37 page)

Anderson had started his research on cosmic rays in the autumn of 1930, after he had completed his PhD. Though he worked with Millikan, he regarded Millikan’s theological view of cosmic rays as mere wishful thinking, and certainly did not feel himself obliged to provide evidence for it. Rather, he wanted to gather hard evidence about the nature of cosmic rays, and so developed a method of photographing their activity inside a cloud chamber, which allowed him to make visual records of the paths of charged particles emitted from cosmic-ray collisions. By the autumn of 1931, Anderson had about 1,000 such photographs and, in November, he wrote to Millikan, who was then in Cambridge in order to give a paper at the Cavendish, sending him some photographs that puzzled him. What the photographs appeared to show were collisions that resulted in the simultaneous emission of a negatively charged particle, which was surely an electron, and a positively charged particle, which Anderson assumed to be a proton.

Millikan could shed little light on these photographs, but exhibited them at the Cavendish anyway, presenting them merely as evidence of the tremendous energies of cosmic rays, which he thought explicable only by adopting his own theological interpretation of them. Among Millikan’s audience at the Cavendish, however, was Patrick Blackett, who was deeply intrigued by Anderson’s photographs and resolved to find an explanation for them. In fact, the explanation for the phenomenon Anderson had photographed had already been given by Dirac in his lecture at Princeton in October, in which he had said that it should be possible to detect positrons – or, as he was calling them at this time, ‘anti-electrons’ – experimentally. In collisions between pairs of ultra-energy photons, Dirac explained, sometimes the photons should disappear and in their place should appear a pair of particles: an electron and an ‘anti-electron’, a process subsequently named ‘pair production’.

Clearly, this is what had happened inside Anderson’s cloud chamber, but when Millikan addressed the Cavendish in November, Dirac was still in Princeton, and nobody else seems to have made the connection between Dirac’s prediction and Anderson’s photographs. Why did the connection not occur to Oppenheimer? Or, if it did, why did he not mention it to Anderson? Late in life, Anderson recalled that around this time he ‘talked to Oppenheimer quite a bit’, but also that ‘I found it hard to talk to Oppenheimer because his answers were usually, at least to me, encased in some sort of mysticism. I couldn’t understand what he was saying, but the idea of pair production, if he had said that, I would have understood.’
As Farmelo remarks: ‘It beggars belief that Oppenheimer never pointed out the connection between Dirac’s theory and Anderson’s experiment to Dirac, to Anderson or to anyone else. Yet that appears to be what happened.’

Several possibilities suggest themselves. One is that the idea of pair production simply did not occur to Oppenheimer. After all, he was not at Dirac’s Princeton lecture and the lecture had not been published, so he might well have remained ignorant of Dirac’s latest thoughts on the question. But even if the specific notion of pair production did not occur to him, it still seems odd that he did not mention what he had already said in print – namely that Dirac’s theory demanded the existence of a positively charged particle with the same mass as the electron. Another thought is that he was reluctant to help someone working on cosmic rays with Millikan, because he assumed that the point of his research was to lend support to an analysis of cosmic rays that he thought was mistaken. Most likely, though, is that he was still so convinced that Dirac’s theory was wrong that the last thing he thought Anderson could possibly have photographed was evidence that it was right. This does not entirely resolve the puzzle, since it raises the question: if Oppenheimer did not think Anderson had photographed the positron, what did he think he had photographed? A proton?

In any case, after he had shown his photographs to Millikan, it was to be another nine months before Anderson’s further experiments allowed him to summon up the confidence to go into print with the claim that he had discovered a new particle. In that time, Urey discovered deuterium, Chadwick discovered the neutron, and Cockcroft and Walton split the atom. Meanwhile, Dirac himself was losing faith in his own theory. In April 1932, shortly before the dramatic announcement of Cockcroft and Walton’s achievement, Dirac was in Copenhagen, attending the meeting at which the pastiche of Goethe’s
Faust
mentioned above was performed. Dirac, of course, appears as a character in the play, which pokes fun at his ‘hole’ theory of quantum electrodynamics. Throughout the meeting, in fact, Dirac had to put up with a great deal of scepticism about his theory. Nobody, it seems, believed it, least of all Bohr, who is recorded as saying: ‘Tell us, Dirac, do you really believe in that stuff?’ Dirac did not say so publicly, but a couple of years later he told Heisenberg that he had, privately, ceased to believe in his theory in the months before the discovery of the positron was made public. In July 1932, a month before Dirac’s thirtieth birthday, it was announced that he was to succeed Sir Joseph Larmor as the Lucasian Professor of Mathematics at Cambridge, the chair that had previously been held by Isaac Newton and was subsequently to be held by Stephen Hawking. The appointment made Dirac financially secure, but it also came with expectations. It was thus a bad time to be associated with a discredited theory.

Of course, Dirac’s theory was soon to be confirmed, but it took an extraordinarily long time for anyone to realise or admit that it
had
been confirmed. On 2 August 1932, Anderson obtained a photograph of a track that seemed to have been left by an electron except that, from the direction of its curvature, he could see that it was
positively
charged. Still knowing nothing of Dirac’s ‘anti-electron’, Anderson thought he had discovered a previously unknown and unsuspected particle. The discovery of a new particle, however, was such a rare and unexpected event that he took his time to consider all other possibilities before he committed himself in print to the claim that that is what had happened. Not until the beginning of September did he send a short report of his discovery, with the tentative title ‘The Apparent Existence of Easily Deflectable Positives’, to the journal
Science
. The two-page article ended with the statement: ‘It seems necessary to call upon a positively charged particle having a mass comparable with that of an electron.’

Unlike the previous major breakthroughs of 1932, the discovery of the positron was not immediately heralded as an important achievement. Very few people seem to have even read Anderson’s report and, of those who did, most seem not to have believed it. Anderson did not publish his fully worked-out follow-up article in the
Physical Review
until March 1933. Astonishingly, in the intervening period, even now that the discovery of the positron had been announced in print, Oppenheimer still did not mention to Anderson that his discovery confirmed Dirac’s prediction, nor did he tell him the explanation of how positrons appear that Dirac’s theory provides. ‘It is surprising to me,’ Anderson later said, with admirable restraint, ‘that Oppenheimer during the six months after I first published the paper on the positron – I had no idea, even though I’d searched my mind and gone nuts trying to figure out how these things could be – it’s very surprising to me that Oppie didn’t think of that idea. It’s the sort of thing you would have expected him to think of.’ It is all the more surprising because in a letter to Frank, undated but almost certainly written in the autumn of 1932, Oppenheimer mentions ‘Anderson’s positively charged electrons’ as one of the things he and his students were thinking about.

On 17 February 1933, before Anderson had sent off his detailed paper to the
Physical Review
, he was shocked to read in the newspapers that the discovery of the ‘positive electron’ had been announced in London
by someone else
. The person in question was Patrick Blackett, Oppenheimer’s old laboratory supervisor, who, since Millikan’s presentation of Anderson’s photographs at the Cavendish in November 1931, had been conducting his own researches into cosmic rays and taking his own, even more impressive photographs. In this he had been helped by an Italian visitor to the Cavendish, Giuseppe Occhialini, whom everyone knew as ‘Beppo’.
Occhialini had arrived at the Cavendish already having had some experience in investigating cosmic rays using Geiger counters. Together, Blackett and Occhialini devised an ingenious method of getting cosmic rays to, as it were, take photographs of themselves. They did this by placing Geiger counters above and below a cloud chamber, in such a way that when a cosmic ray was detected, a photograph was taken.

Blackett and Occhialini did not read Anderson’s report in
Science
until January 1933, by which time they had amassed an impressive collection of photographs that showed, even more clearly than Anderson’s pictures, the paths of positively charged particles. Where they had a huge advantage over Anderson was in having the time, the goodwill and the active interest of Paul Dirac, who realised that their photographs confirmed his prediction of the ‘anti-electron’ and was thus able to overcome his previous doubts about his own theory. ‘I was quite intimate with Blackett at the time,’ Dirac later remembered, ‘and told him about my relativistic theory of the electron.’

Thus, with Dirac’s help, when Blackett and Occhialini presented their results in public, which they did on 16 February 1933 at the Royal Society in London, they were able, unlike Anderson, not only to announce a new particle, but also to
explain
how that particle was produced. And it was the explanation that made the new particle so interesting. For this was an even more astonishing illustration of the Einsteinian formula E = mc
²
than the splitting of the atom had been. The formula asserts the equivalence of mass and energy, and what Cockcroft and Walton had demonstrated was an example of mass being converted into energy and, in the process, they had shown just
how much
energy could be released from a small amount of mass – as Einstein’s formula asserts. But what Patrick Blackett was able to show – using dramatic photographs of rays from outer space, no less – was the equivalence going in the other direction: energy becoming mass! Whereas Anderson had ‘gone nuts’ trying to work out how positrons could possibly exist, Blackett knew perfectly well from his discussions with Dirac how they could be: they had been created by the conversion of energy into mass, in accordance with the ‘pair production’ that was predicted by Dirac’s theory. In presenting his photographs of the ‘positive electron’ (as he called it at this time), Blackett was scrupulous in spelling out its connections with Dirac’s theory, showing on the one hand how it provided evidence for that theory, and on the other hand how the theory helped to explain things about the particle that might be puzzling. For not only could Dirac’s theory explain how the particle came into being, but it could also explain why the positron had remained undetected for so long. The answer is that, as an ‘anti-particle’, it has a very short life because, as soon as it comes into contact with its opposite number – in this case, an electron – it is annihilated.

In the starkest contrast to Anderson’s announcement the previous September, Blackett and Occhialini’s results were immediately hailed as an important, indeed sensational, breakthrough. The morning after Blackett’s presentation at the Royal Society, their achievement was reported in the
New York Times
, the
Manchester Guardian
and the London
Daily Herald
, which described it as the ‘Greatest Atom Discovery of the Century’. Whenever he was interviewed by reporters, however, Blackett was careful to stress that he had been anticipated, and that the real discoverer of this new positive particle was Anderson. When Anderson’s own detailed treatment of the particle appeared in the
Physical Review
, however, it was already old news, except for one thing. In place of Dirac’s ‘anti-electron’ and Blackett’s ‘positive electron’, Anderson introduced the name that subsequently stuck: the positron.

The astonishing series of breakthroughs in 1932 occupied Nobel Prize committees for many years to come: Harold Urey won the Nobel Prize in Chemistry in 1934 for his discovery of deuterium, while the Nobel Prize in Physics went to Paul Dirac in 1933, partly, at least, for his prediction of the positron; James Chadwick in 1935, for discovering the neutron; Carl Anderson in 1936, for his discovery of the positron; Ernest Lawrence in 1939, for inventing the cyclotron; Patrick Blackett in 1948, for his work on nuclear physics and cosmic rays (chief among which was his identification of the positron as Dirac’s ‘anti-electron’); and Cockcroft and Walton in 1951, for splitting the atomic nucleus.

These breakthroughs also provided the topics for research pursued by Oppenheimer and his students for the following few years, concentrating as they did on the investigation of deuterium, cosmic rays, the positron and the phenomenon of pair production. From the point of view of American physics, the encouraging thing about the list of Nobel laureates created by the breakthroughs of 1932 was that three of them (Urey, Anderson and Lawrence) were American. All three of them, however, were experimentalists. In theory, the Americans still lagged behind the Europeans, though they were catching up. Oppenheimer’s contributions to the theoretical issues of that day may have been a step or two behind the leading Europeans, and he may have made some glaring errors here and there, and, in the case of Anderson, shown an inexplicable reticence, but he had at least
made
contributions, some of which were discussed at the forefront of physical theory. Moreover, he had done this without once, since the start of his appointments in California, setting foot in Europe.

By this time, Oppenheimer was settled in California. At Berkeley, he had moved out of the faculty club at the start of the 1931–2 academic year, and into what he described to Frank as ‘a little house up on the hill with a view of the cities and of the most beautiful harbor in the world
. . . There is a sleeping porch; and I sleep under the Yaqui
^fn34
and the stars and imagine I am on the porch at Perro Caliente.’ After the family holiday in New Orleans following Ella’s death, Oppenheimer brought his father with him when he returned to California. For a few weeks in the New Year of 1932 they lived together; not, however, in Berkeley, but in Pasadena, which Julius preferred. Julius, Oppenheimer told Frank, ‘is very much pleased with this place, liking the cottage – which is in fact excruciatingly ugly – and not I think sorry to have me under the same roof.’