by Ray Monk
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 = mc2 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 o
f 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 Yaquifn34 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.’
He reassured Frank that their father, who was by now sixty years old, ‘looks well, better than in months’. Julius, in fact, was enjoying himself at Pasadena, learning French, attending concerts, taking driving lessons and even joining in some academic seminars. Every morning, Oppenheimer reported, Julius and he were served breakfast by the Tolmans’ maid, Moline, who ‘after I am gone listens with enchanting patience to F[ather]’s reports on high finance’. On 18 January 1932, Julius himself wrote to Frank, telling him: ‘I am meeting lots of Robert’s friends and yet I believe that I have not interfered with his activities.’ Julius, impressed both with his son and with Caltech for having such distinguished connections, reported to Frank that Robert ‘has had a couple of short talks with Einstein’.
These talks would probably have taken place during the second of Einstein’s three visits to Caltech. During the first, in the New Year of 1930, he came to love Pasadena so much that he took to calling it ‘paradise’. In between discussing cosmic rays with Millikan and relativity with Tolman, Einstein had toured the movie studios of Hollywood, had dinner at Charlie Chaplin’s Beverly Hills home and attended a banquet in his honour, at which there had been 200 guests. So much in demand was he that a millionairess gave Caltech $10,000 for the privilege of meeting him. Evidently hoping to recruit him permanently, Millikan invited him back for the New Year of 1932 – a visit that, at Einstein’s request, was rather more low-key. Though he loved California, Einstein was less impressed with Millikan, whose political conservatism clashed with his own determination to speak out on behalf of the poor, the dispossessed and the persecuted.
More to Einstein’s taste was the educator Abraham Flexner, who, having secured funding of $5 million, was in the process of establishing an Institute for Advanced Study. During Einstein’s second visit to California, Flexner took the opportunity to sound him out about the possibility of joining his proposed new institute. The reply was encouraging enough for Flexner to visit Einstein in Germany during the summer of 1932, where he told Einstein that the new institute would be based in Princeton and asked him to name his own price and conditions. Einstein initially declined, but the rapid growth in the power and influence of the Nazis in Germany forced him to reconsider. When he left Germany for his third visit to Caltech in December 1932, his ostensible plan was to return to Germany two months later before taking up his position at Flexner’s new institute, but in reality he probably knew that he would not be returning.
While Einstein was in Pasadena in January 1933, the news came that Hitler had been made Chancellor. He was still there on 5 March when he heard that the Nazi Party had received the most votes (44 per cent) of any party in Germany’s general election. Einstein returned to Europe at the end of March, but sensibly did not step foot in Germany, where his home had been seized, his books burned and his theories officially repudiated as ‘Jewish science’. Throughout the new ‘Reich’ scientists who were not, like the physicist Philipp Lenard, active Nazis or, like Max Planck or Werner Heisenberg, prepared to work under the Nazis, were making plans to leave Germany. The many Jewish scientists, of course, had no choice. Max Born, having been thrown out of Göttingen because he was Jewish, prepared to move to England, where Cambridge had offered to take him. Leo Szilard, meanwhile, left Germany with his life savings hidden in his shoes. After a few months in England, Einstein returned to the United States and, with much fanfare, took up his appointment at the Institute for Advanced Study. He never once returned to Europe.
Of all this turmoil in Germany – the home of his ancestors and some of his not-very-distant relations, as well as of many of the scientists for whom he had the greatest regard – there is not a single word in Oppenheimer’s letters, even when he touches on subjects that relate to it. For example, in a letter to Frank written on 12 March 1932, he tells him that their father, his health having been restored by his time in California, is now returning to New York. ‘I have urged him very strongly not to go to Europe alone this summer,’ Oppenheimer writes. One might think this advice was prompted by Oppenheimer’s concern at his father placing himself at the mercy of the violent anti-Semitism that had erupted in Germany. The rest of the letter suggests, however, that his concern was not about the conditions in Germany, but merely about his father’s physical condition. ‘Only if things should break unexpectedly well,’ he writes, ‘e.g. should he find a very good person to travel with, ought he, or will he, go abroad.’ He adds: ‘I have said that next summer I should consider going myself, that in that case we could at least cross both ways together.’
Again, in October 1933, he wrote to his brother about Frank’s plans to study at Cambridge. ‘The theoretical physics should be awfully good in Cambridge,’ he told him, ‘with Dirac there, and Born.’ But nowhere does he reflect on, or even mention, why Born was in Cambridge. In March 1934, he responded to an appeal for financial support for dismissed German physicists by pledging 3 per cent of his salary for two years. Apart from that, he remained silent until his interest in political and social questions was finally aroused in 1936. Until then, his attitude is summed up by a remark he once made to Leo Nedelsky: ‘Tell me, what has politics to do with truth, goodness and beauty?’
Oppenheimer’s concern with truth, goodness and beauty led him in the early 1930s to a serious study of ancient Hindu literature; so serious, indeed, that he took lessons in Sanskrit so that he could read the Hindu texts in their original language. The first mention of this comes in his letter to Frank of 10 August 1931, in which he writes: ‘I am learning Sanskrit, enjoying it very much, and enjoying again the sweet luxury of being taught.’
His teacher was Arthur Ryder, who was professor of Sanskrit at Berkeley. Harold Cherniss has described Ryder as ‘a friend half divine in his great h
umanity’. In his views on education, he was a curious mixture of the ultra-traditionalist and the iconoclast. He believed on the one hand that a university education ought to consist primarily of Latin, Greek and mathematics (with the other sciences and humanities given as a reward to good students and the social sciences ignored altogether). On the other hand, his approach to the teaching of Sanskrit was refreshingly free from the deadening hand of dry scholarship. He regarded the learning of Sanskrit as the opening of a door onto great literature, not as an academic discipline. Perhaps for that reason he was the ideal teacher for Oppenheimer, who held him in enormously high regard. ‘Ryder felt and thought and talked as a stoic,’ Oppenheimer once told a journalist, extolling him as ‘a special subclass of the people who have a tragic sense of life, in that they attribute to human actions the completely decisive role in the difference between salvation and damnation. Ryder knew that a man could commit irretrievable error, and that in the face of this fact, all others were secondary.’
Oppenheimer gave few details of his learning of Sanskrit or of his reading of the Hindu classics. In a letter to Frank of January 1932, he alludes very briefly to the Hindu god Shiva; the following autumn he mentions that he is reading ‘the Cakuntala’ (more usually spelled Shakuntala, a verse play written by the great Sanskrit poet and dramatist Kalidasa) and promises Frank that at their next meeting he will afflict him ‘with clumsy translations of the superb poems’; and a year later that he is reading the Bhagavad Gita, which ‘is very easy and quite marvellous’. Then, in June 1934, he writes to Frank, thanking him for ‘the precious Meghaduta and rather too learned Veda’, which were presumably birthday presents. ‘The Meghaduta I read with Ryder, with delight, some ease, and great enchantment,’ Oppenheimer told his brother. ‘The Veda lies on my shelf, a reproach to my indolence.’ Otherwise known as ‘The Cloud Messenger’, the Meghaduta is a poem by Kalidasa that tells how a cloud is used to take a message from an exiled subject of Kubera, the god of wealth, to his wife in the Himalayan Mountains. The Vedas are the most ancient of Hindu scriptures, consisting of hymns, poems and mantras.
