by Ray Monk
While Truman was busy formulating the Truman Doctrine, he told the AEC members that he was too busy to meet them and instead they met his aide, Clark Clifford, who, to their relief, did not seem very concerned about Oppenheimer’s FBI file either. By the end of March the AEC had testimonials vouching for Oppenheimer’s loyalty from an impressive array of people, including the Secretary of War, Robert Patterson, and General Groves. On 11 August, the AEC were ready to agree unanimously to approve Oppenheimer for clearance, by which time the FBI, for the time being anyway, had decided to cease their ‘technical surveillance’ of him. He was officially no longer regarded as a security risk. On the contrary, he was now the man most responsible for framing US policy on the development of atomic energy.
What makes the doubts about Oppenheimer’s loyalty seem so perverse is that, from this distance anyway, one of his most striking characteristics is his deep, and sometimes fierce, devotion to his country. It is one of the very few things that remained constant throughout his life and is clearly evident in almost everything he did. It was behind both the extraordinary energy and effort that he put into directing Los Alamos and his determination to play a leading role in the formation and execution of America’s atomic policies. It is also evident – and had been from the very beginning of his academic career – in his concern to establish the US as the world’s centre of theoretical physics.
In the post-war period Oppenheimer was to see that dream he had had in the 1920s – of America replacing Germany as the country where the most fundamental developments in physics took place – become a reality. Moreover, he himself was able to play a leading part in making it a reality, not (as he had done before the war) through his publications and his teaching, but rather through the influence that he wielded at a series of important conferences.
The first, and most important, of these was the Shelter Island Conference, which took place in June 1947 and has gone down in history as one of the most important conferences in the development of physics in the twentieth century. Rabi said it ‘would be remembered as the 1911 Solvay Congress is remembered, for having been the starting-point of remarkable new developments’, while Richard Feynman has said: ‘There have been many conferences in the world since, but I’ve never felt any to be as important as this.’
Shelter Island was the conference at which Willis Lamb introduced the discoveries about hydrogen spectra – the so-called ‘Lamb shift’ – for which he won the Nobel Prize. It was at Shelter Island, too, that Rabi reported on experiments conducted in his laboratory at Columbia, which measured, with an unprecedented degree of accuracy, the magnetic interactions between the protons and electrons in hydrogen and found that the measurements obtained disagreed, slightly but significantly (by about 0.22 per cent), with those derived from the then-accepted theory. The conference was also the occasion at which Robert Marshak first proposed that the puzzles about the meson, to which Oppenheimer himself had devoted so much thought over the previous decade, could be solved by what became known as the ‘two-meson hypothesis’. Moreover, it was at this conference that Richard Feynman gave the first public presentation of what became known as ‘Feynman diagrams’fn61 and, in the attempt to understand the startling series of experimental observations that had been made in 1947, the seeds were sown for the major advances in quantum electrodynamics that Feynman and Schwinger were to make in the coming years.
Not only were all these young physicists American, but, unlike Oppenheimer and most of his generation, all of them had been graduate students at American universities: Lamb at Berkeley, Marshak at Cornell, Feynman at Princeton and Schwinger at Columbia. For a long time the US had been the country in which the best physics was being done (much of it by refugees, emigrants and people trained overseas), but now it was also the country producing the best physicists.
In terms of the number of participants, the Shelter Island Conference was not large. Just twenty-three people took part, but every one of them was either a world-renowned scientist (like Bethe, Fermi, Rabi, Teller, Uhlenbeck and Wheeler) or widely identified as an up-and-coming star (for example, Feynman, Pais and Schwinger). Together with Kramers and Weisskopf, Oppenheimer was asked to act as a ‘discussion leader’, each of whom was asked to draw up an outline of what they thought ought to be discussed under the general heading ‘Foundations of Quantum Mechanics’. Weisskopf’s outline was divided into three: 1. problems in quantum electrodynamics; 2. problems in understanding nuclear and meson phenomena; and 3. proposed experiments using high-energy particles. Kramers concentrated entirely on issues in quantum electrodynamics, while Oppenheimer’s outline was focused solely on the problems of understanding mesons and, in particular, the discrepancy between the currently accepted theory and experimental results.
One such result (mentioned by name in Weisskopf’s outline and alluded to in Oppenheimer’s) was an experiment carried out in Italy during the war, of which Oppenheimer, in a non-technical lecture he gave later in the year, gave an excited and colourful account. Now confident of America’s unquestioned position at the forefront of physics, he could afford to be generous in his assessment of work in Europe, and told his audience that ‘of the two or three important experimental discoveries of the last two years, two at least come from Europe’:
One was carried out long before its publication in the cellar of an old house in Rome by three Italians who were under sentence of death from the Germans because they belonged to the Italian Resistance. They were rescued by an uncle of one of the men from a labor squad at Cassino, and smuggled into a cellar in Rome. They got bored there, and they started to do experiments. These experiments were published last spring; and in the field of fundamental physics they created a real revolution in our thinking.
The Italian scientists in question were Marcello Conversi, Ettore Pancini and Oreste Piccioni (when Fermi gave a seminar explaining the importance of their experiment, he remarked jokingly that he ‘would not dare to pronounce those names’). In February 1947, the Physical Review published a letter of theirs, ‘On the Disintegration of Negative Mesons’, reporting on experiments they had conducted in 1945, which showed conclusively that something was fundamentally wrong with meson theory as it then stood. According to that theory, the mesons found in cosmic rays were also the particles that Yukawa had suggested as carriers for the nuclear force that binds protons and neutrons together in a nucleus. Mesons are found with both negative and positive charges, and, if that theory is correct, then negative mesons should always be absorbed by surrounding nuclei, whereas positive mesons should not. Because every nucleus is positively charged, positively charged mesons should be repelled, and instead of being absorbed will decay very quickly (mesons have a life of only a few microseconds) into electrons and neutrinos. What Conversi, Pancini and Piccioni found was that, contrary to the theory, negatively charged mesons – though absorbed by the nucleus of the relatively heavy element of iron – decayed in carbon, which is a much lighter element. What Weisskopf and Oppenheimer wanted the illustrious scientists gathering at Shelter Island to discuss was: what is going on? Why do the carbon nuclei not absorb the negatively charged mesons?
The conference started on Monday 2 June at the Ram’s Head Inn, Shelter Island, right on the tip of Long Island. Reflecting the post-war celebrity of nuclear physicists, the event was reported in gushing terms by the New York Herald Tribune:
Twenty-three of the country’s best known theoretical physicists – the men who made the atom bomb – gathered today in a rural inn to begin three days of discussion and study, during which they hope to straighten out a few of the difficulties that beset modern physics.
It is doubtful there has ever been a conference quite like this one. The physicists, backed by the National Academy of Science, have taken over the Ram’s Head Inn . . . The conference is taking place with almost complete informality, aided by the fact that the scientists have the inn all to themselves and feel that there is no one to mind if they take off their coats and get to work.
The organiser of the conference, Duncan MacInnes, recorded in his diary that ‘it was immediately evident that Oppenheimer was the moving spirit of the affair’, while the chairman of the conference, Karl Darrow, has recorded:
As the conference went on the ascendancy of Oppenheimer became more evident – the analysis (often caustic) of nearly every argument, that magnificent English never marred by hesitation or groping for words (I never heard ‘catharsis’ used in a discourse on [physics], or the clever word ‘mesoniferous’, which is probably O’s invention), the dry humour, the perpetually-recurring comment that one idea or another was certainly wrong, and the respect with which he was heard.
Abraham Pais says his recollections confirm these impressions:
I had heard Oppenheimer speak before but had never yet seen him in action directing a group of physicists during their scientific deliberations. At that he was simply masterful, interrupting with leading questions (at physics gatherings interruptions are standard procedure), summarizing the main points just discussed, and suggesting how to proceed from there.
The first day was dominated by the reports of Lamb and Rabi of the startling experimental results mentioned above. Lamb’s experiments, conducted like Rabi’s at Columbia, and, again like Rabi’s experiments, using radar technology developed during the war, measured the energy of electrons far more precisely than had previously been possible and established that electrons in hydrogen atoms do not behave as Paul Dirac’s theory would predict. Electrons at one level, Lamb discovered, have a higher energy than those at another, rather than (as Dirac’s theory would suggest) all of them having the same energy. An explanation of this ‘shift’ would, as Oppenheimer suggested in the discussion that followed Lamb’s presentation, require a new understanding of quantum electrodynamics (QED). The results reported by Rabi of experiments conducted by two students of his, John Nafe and Edward Nelson, also seemed to call for adjustments to QED, since they gave accurate and reliable measures of the ‘magnetic moments’ of the electrons in hydrogen that contradicted what the Dirac theory would predict. During the second day papers by Kramers and Weisskopf addressed the theoretical issues raised by what became known respectively as the Lamb shift and the ‘anomalous magnetic moment’, and Schwinger indicated during the discussion of Kramers’ paper what shape might be taken by the new understanding of QED that these observational results seemed to demand.
On the final day of the conference, 4 June, Oppenheimer led an extended discussion of the problems physicists faced in understanding mesons. In the ensuing discussion Robert Marshak made his now-famous suggestion that these puzzles might be solved by distinguishing two kinds of meson, one bigger than the other. The bigger of the two would be the Yukawa particle, responsible for the strong nuclear force, which decays into the smaller of the two – that is, the mesons found in cosmic rays – which in turn decay into electrons. In fact, though the participants at the Shelter Island Conference did not yet know this, experimental evidence confirming Marshak’s hypothesis had already been published.
In the issue of the British journal Nature published on 24 May 1947 (and therefore not available in the States until a few days after the Shelter Island Conference) a group of experimental physicists based at Bristol and led by Cecil Powell reported on some investigations they had conducted, which demonstrated the existence of a process whereby what they called a ‘primary’ meson could decay into a ‘secondary’ meson. The authors of this report pointed out that the existence of this process resolved many of the puzzles about mesons, including those presented by the experiments of the Italian group. These experiments by Powell and his group form the second example that Oppenheimer gave in his lecture, mentioned earlier, of important experimental discoveries coming from Europe.
In a letter he wrote to Frank Jewett, the president of the National Academy of Sciences, Oppenheimer described the Shelter Island Conference as ‘unexpectedly fruitful’. ‘The three days were a joy to us,’ he told Jewett, adding that the participants ‘came away a good deal more certain of the directions in which progress may lie.’ A few months later, when, in the aftermath of the conference, several fundamentally important papers had been published by those present, Oppenheimer was even more effusive, saying that the conference was, for most of the participants, ‘the most successful conference we had ever attended’. Out of it, he claimed, had come ‘a new understanding of the probable role of the meson in physical theory, and the beginnings of a resolution of the long outstanding paradoxes of the quantum electrodynamics’. By the end of the year, he was circulating plans for a second meeting to be held the following spring.
When the conference finished, Oppenheimer did not return to California, but went on instead to Harvard, where he was to receive an honorary degree. To avoid the usual difficulties of travelling to or from Shelter Island, he arranged for a private seaplane to fly him from Port Jefferson to Bridgeport, Connecticut, where he could catch a train to Boston. As Schwinger, who taught at Harvard, and Rossi and Weisskopf, who were both at MIT, also had to return to Boston, Oppenheimer invited them to join him. On the way they flew into a storm, so the pilot decided to land at the only available place, which happened to be a naval base, which civilian aircraft were not supposed to use. They disembarked to find an angry naval officer waiting to give them a dressing-down. ‘Don’t worry,’ Oppenheimer said to the pilot. ‘Let me handle this.’ As he stepped off the plane he offered his outstretched hand to the officer and said calmly: ‘My name is Oppenheimer.’ ‘The Oppenheimer?’ gasped the officer. Upon being reassured that he was indeed in the presence of the most famous physicist in the country, the officer changed his attitude completely, welcomed Oppenheimer and his companions to the officers’ club where they were served tea and biscuits, and then arranged for them to be driven to the local railway station, from where they were able to take a train to Boston.
Having at last reached Harvard, Oppenheimer was awarded his honorary degree at the graduation ceremony on 5 June. The ceremony turned out to be a historic occasion, because, in a speech that he gave to the graduates, General Marshall announced a major new policy initiative: the European Recovery Program, or Marshall Plan as it became known, which offered billions of US dollars to European countries, on the condition of closer cooperation.
Oppenheimer, as his evidence to Congress in support of the May–Johnson Bill had shown, had a deep admiration for General Marshall. Haakon Chevalier tells a revealing story that illustrates not only Oppenheimer’s warm regard for Marshall, but also his delight at finding himself moving in the same circles as the esteemed Secretary of State, whom Time magazine would that year name as ‘Man of the Year’. Chevalier recalls meeting Phil Morrison in New York during this period and, in the course of catching up and reminiscing, asking him about Oppenheimer. ‘I hardly see him any more,’ Morrison replied. ‘We no longer speak the same language . . . He moves in a different circle.’ To illustrate what he meant, Morrison told Chevalier that at one of his most recent meetings with him, Oppenheimer kept saying, ‘George thinks this . . .’ and ‘George says that. . . .’ Eventually Morrison felt compelled to ask who this ‘George’ was. ‘You understand,’ he told Chevalier, ‘General Marshall to me is General Marshall, or the Secretary of State – not George.’ Oppenheimer, he remarked, had changed profoundly: ‘He thinks he’s God.’
In the summer of 1947, as he, Kitty and the children prepared to leave California to move into Olden Manor, the splendid residence reserved for the director of Princeton’s Institute for Advanced Study, Oppenheimer was at the very height of his reputation, among scientists, politicians and the general public. As the sociologist Philip Rieff has written, during these years ‘Oppenheimer became a symbol of the new status of science in American society. His thin handsome face and figure replaced Einstein’s as the public image of genius.’ Anne Wilson’s concern that he would get into ‘terrible trouble’ in the east seemed, for the moment at least, to have been misplaced.
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fn61 A Feynman diagram is a pictorial representation of the interactions of subatomic particles. A typical diagram might show, for example, an electron and a positron annihilating each other, emitting waves of electromagnetic energy.
16
The Booming Years
‘THIS IS AN unreal place,’ Abraham Pais wrote, after he had been at Princeton’s Institute for Advanced Study for a few months. ‘Bohr comes into my office to talk, I look out at the window and see Einstein walking home with his assistant. Two offices away sits Dirac. Downstairs sits Oppenheimer.’
Apart from Einstein, the two other great physicists Pais mentions, Bohr and Dirac, were brought to the institute by Oppenheimer during his very first year there. Both were figures who had for Oppenheimer great symbolic importance: Bohr, the physicist from the previous generation for whom Oppenheimer had the greatest respect and the man whom he revered above all others; and Dirac, the greatest physicist of Oppenheimer’s own generation, whose career had been closely watched by Oppenheimer, sometimes with rivalry, but always with enormous admiration. It was no surprise that Oppenheimer wanted to attract Bohr and Dirac to the institute, but Pais himself was, in fact, more representative of Oppenheimer’s ambitions for the place. As Oppenheimer well knew, the next big steps in physics would not be taken by men of Bohr and Einstein’s generation, or even by those of Oppenheimer and Dirac’s generation; they would be taken by people the age of Pais, Schwinger, Feynman, and so on. It was those young people, above all, whom he wanted to come to Princeton to be ‘directed’ by him.