The Strangest Man

by Graham Farmelo

  The debate about the positron rumbled on for months. Bohr thought the particle might not be real but caused ‘by air current drift’ in the cloud chamber. Only after Heisenberg and colleagues went on a skiing vacation in Bavaria with Bohr and took one of Anderson’s cloud-chamber photographs did Bohr begin to believe that the positron existed. In California, Anderson wavered and Millikan refused to believe that electrons and positrons were produced in pairs, because the observations did not agree with his theory of cosmic rays. Even in Cambridge, the question was controversial for several months. Rutherford, uncomfortable with the idea that abstract theory could predict a new particle, liked his physics done bottom-up: ‘I would have liked it better if the theory had arrived after the experimental facts had been established.’18

  Although few theoreticians accepted Dirac’s hole theory, many interpreted the positron’s detection as another personal triumph, some once again wearily despairing that it was impossible to compete with him.19 Tamm, writing to Dirac from Moscow, was unstinting in his praise and even implied that Dirac had given up hope that his prediction would be verified: ‘your prediction of the existence of the [positron] […] seemed so extravagant and totally new that you yourself dared not cling to it and preferred to abandon the theory.’20 Dirac, privately pleased that his controversial theory had been vindicated by experiments, showed no emotion. He remarked thirty years later, with a detachment that went beyond the Olympian, that he derived his greatest satisfaction not from the discovery of the positrons but from getting the original equations right.21 In case Dirac should be in the least pleased with himself, Pauli was as ready as ever to bring him down to earth: ‘I do not believe in your perception of “holes” even if the anti-electron is proved.’22

  It was only by the end of 1933 that the majority of quantum physicists accepted that the positron existed, that electron–positron pairs could be created out of the vacuum that the positron had figured in Dirac’s hole theory before its detection. Only Millikan, almost alone in standing by his ‘birth cry’ theory of cosmic rays, held out against the pair-creation idea.23 But by early 1934, the evidence for the new particle was incontrovertible: the number of positrons detected annually had risen, owing mainly to Blackett and Occhialini’s technique, from about four in the previous year to a new annual total of thirty thousand.24 More importantly, experimenters at the Cavendish and at other laboratories had demonstrated that positrons could be produced at will using radioactive sources on the laboratory bench rather than only as a consequence of showers of cosmic rays bombarding the Earth.25 Again, Dirac monitored the experimenters’ results to see if they agreed with his theory’s predictions.

  In hindsight, it was clear that if physicists had taken the Dirac hole theory seriously, the positron would have been detected several months earlier. Anderson later remarked that any experimenter who took the theory at face value and who was working in a well-equipped laboratory ‘could have discovered the positron in a single afternoon’ using radioactive sources.26 Blackett agreed.27 As Dirac appeared to realise later, he must shoulder most of the responsibility for this, as he never advocated strongly that experimenters should hunt for the anti-electron or suggested how they might detect it using apparatus readily available to them. Thirty-three years later, when asked why he did not speak out plainly and predict the anti-electron, Dirac replied: ‘Pure cowardice.’28

  Although Dirac believed he had predicted the positron, and talked about it publicly from 1933 onwards, some commentators have objected that ‘prediction’ is too strong a word.29 Even Blackett wrote in 1969 that ‘Dirac nearly but not quite predicted the positron,’ words that will probably have stung Dirac if he read them.30 The consensus among today’s scientists, however, is that Dirac’s role in foreseeing the existence of the positron is one of the greatest achievements in science. In 2002, shortly after the centenary of Dirac’s birth, the theoretical physicist Kurt Gottfried went further: ‘Physics has produced other far-fetched predictions that have subsequently been confirmed by experiment. But Dirac’s prediction of anti-matter stands alone in being motivated solely by faith in pure theory, without any hint from data, and yet revealing a deep and universal property of nature.’31

  During the past seven years, theoreticians had driven most of the progress in physics, but there were now clear signs – particularly from the Cavendish and Caltech discoveries – that experimenters were in the driving seat. Disillusioned with quantum field theory, and having worked for two years without coming up with what he regarded as a strong new idea, Dirac joined Kapitza in his laboratory. It was another unlikely pairing: the most reserved, cerebral theoretician working with the most outgoing, practically minded experimenter. Yet they were like brothers at play.

  They were among the first users of the state-of-the-art facilities in the Mond Laboratory, which Rutherford had arranged to be built for Kapitza in the courtyard of the Cavendish, with funds from the Royal Society. Its opening in early February 1933 was a grand occasion, dozens of trilby-hatted journalists scribbling on their notepads as the procession passed, adding flashes of colour to the grey midwinter afternoon. Dirac was there, in his scarlet gown, watching the proceedings led by Stanley Baldwin, the university’s Vice Chancellor and Deputy to the Prime Minister Ramsay MacDonald. During one of the ceremonies, Kapitza pointed to the body of a crocodile carved into the brickwork of the laboratory’s main entrance by the modernist sculptor and typographer Eric Gill. Inside the laboratory foyer there was another Gill commission, a bas-relief of Rutherford, a carving that exaggerated the size of Rutherford’s nose, making him look like a brother of Einstein. Some artistically conservative authorities in Cambridge were so upset by Gill’s depiction that they spent three months trying to have it removed; their anger was diffused only after Bohr declared the carving to be ‘most excellent, being at the same time thoughtful and powerful’.32 During the furore, Rutherford remained indifferent, claiming that he did ‘not understand anything about art’.33

  Dirac and Kapitza conceived a new and potentially revealing experiment to probe the nature of light and electrons. As Dirac had seen for himself in Davisson’s Manhattan laboratory, when a crystal is struck by a beam of electrons, their paths are bent, demonstrating that electrons can behave as waves. Thus, electrons and light resemble one another in that both behave sometimes as waves, sometimes as particles. Dirac and Kapitza hit on the idea of replacing the crystal with light. Their idea was to reflect light back and forth between two mirrors so that only a whole number of half-wavelengths of light can exist between the mirrors, analogous to the number of half-wavelengths on a rope that is held down at one end and swung at the other. Just as the crystal consists of a regular three-dimensional arrangement of atoms, the reflected light has a regular pattern of allowed wavelengths, so both should be able to bend the path of a beam of electrons. Such an experiment should be a unique probe of the wave-like and particle-like behaviour of both electrons and light. Dirac’s calculations showed that it should be possible to detect the electron beam’s bending but only if the reflected light is extremely bright, brighter than the best-available lamps. So the state of lighting technology had thwarted the first plans of Dirac and Kapitza to do experiments together. It would not be long, however, before they were back in the laboratory.

  In spring 1933, the Cambridge Review, sober chronicler of the university’s affairs, published an anonymous article pointing out that ‘the young are now more concerned [with politics] than they have been for a long time past’.34 The hedonism of the late 1920s had all but disappeared, giving way to alarm about the national economic malaise and the threat of war. Hitler, Mussolini and Stalin were shaking the English out of their indifference to political extremes. Winston Churchill, in the political wilderness, repeatedly warned of the need to rearm, but he was ignored.

  At the Cambridge Union in late February, despite a barnstorming performance from the Fascist Sir Oswald Moseley, the motion ‘This House Prefers Fascism to Socialism’ was heav
ily defeated, another sign that the students favoured Stalin over Hitler.35 The dons were also turning left, many of them dissatisfied with the unscientific approach taken by politicians to social issues and revolted by the harsh treatment meted out to the unemployed. A few political leaders emerged among the academics, egged on by Jim Crowther, who cleverly promoted his Marxist views without ruffling the feathers of the many scientists who were wary of political commitment. The ones who emerged as the socialist leaders were all workaholic males, able to combine high-flying academic careers with an energetic commitment to politics and, in some cases, effective popularisation. Quietest among them was Blackett, not a Communist but a firm supporter of the Labour Party. He was horrified to see that ‘the whole structure of liberalism and free trade is collapsing all over the world’, and was struck by ‘the paradoxical situation in which so many starve in the midst of so much plenty’. Scientists and engineers had, in Blackett’s view, ‘produced the technical revolution which has led to this situation’, and so ‘must therefore be directly concerned with the great political struggles of the day’.36

  Most influential of all was Bernal, ‘the Saint Paul of the science and society movement of the thirties’, as one of his colleagues later described him.37 He later remembered how he was inspired by the Soviet experiment:

  [T]here was no mistaking the sense of purpose and achievement in the Soviet Union in those days of trial. It was grim but great. Our hardships in England were less; theirs were deliberate and undergone in an assurance of building a better future. Their hardships were compensated by a reasonable hope.38

  Although Dirac talked politics with Kapitza and Blackett, he seems to have been one of the fellow travellers with the socialist and Communist scientists, never in the vanguard. The political activists were becoming impatient with Dirac’s indifference to sharing new knowledge with people outside science: in a short article ‘Quantum Mechanics and Bolshevism’ in the Cambridge Review, the anonymous author reported on Soviet displeasure with the ‘completely non-political character of his work, and its detached tone, divorced from problems and questions of the present day’.39 In the summer, Bernal included Dirac in his list of intellectual ‘culprits’ – including Joyce, Picasso and Eliot – who were ‘tending to a private dream world’, indifferent to the popular accessibility of their work.40 Dirac would have pleaded guilty as charged as he regarded it as his job to seek better theories of fundamental particles, not to inform the public about the search. Although he did not attend the annual meeting of the British Association for the Advancement of Science in September 1933, he agreed with its conclusion: scientists have a duty to contribute to public debate and should promote the importance of science and technology in getting the country back on its feet.41 The community was leaning on Dirac and other scientists of his soloist ilk to speak out.

  Dirac appears not to have bothered to tell his parents about his success with the positron. Their first excitement that year was a spring visit to Paris, where Betty was studying for her degree. She did not write to her mother but sent regular letters to her father, who was so thrilled when he heard that she might be heading for Geneva that he decided to drop everything and join her. Soon after 5 a. m., on the day after Betty’s letter arrived, Charles and Flo headed down to the railway station via the tram, Flo carrying her husband’s laden suitcase.42 She returned home to receive a letter from Dirac inviting her to spend a day with him in Cambridge, and he later paid for her to take a ten-day cruise round the Mediterranean. ‘Won’t it be funny’, she wrote to him from her cabin like a truant schoolgirl, ‘if I get home and Pa doesn’t know anything about it?’43 So it turned out: Charles and Betty arrived back at 6 Julius Road in the middle of September, having cabled her in advance, the first communication Flo had received from her husband in eight weeks. This act of abandonment seems to have annoyed Dirac. For at least eight years, he had addressed his postcards home to both parents but, from then on, he addressed them only to his mother.44

  Dirac had spent the summer in Cambridge, trying to understand the infinities that plagued his field theory of photons and electrons and reflecting on the work he had done during the previous year. He had proved the equivalence of his theory to Heisenberg and Pauli’s, had discovered the action principle in quantum mechanics, had seen his prediction of the positron verified and had begun a promising laboratory project with Kapitza. This was one of the most distinguished years of work by any scientist in modern times, but Dirac was disappointed. He wrote to Tamm, who had complained that he was going through lean times: ‘I am like you in feeling dissatisfied with my research work during the past year, but unlike you in having no external reasons to blame it on.’45 He needed a vacation.

  After hiking and climbing in Norway, Dirac was to attend a conference at Bohr’s institute before moving on to Leningrad for the first Soviet Conference on Nuclear Physics, where he was sure to be feted as a star. But it turned out that he would be in no mood to savour the acclaim.

  The atmosphere at Bohr’s annual meeting in 1933 was tense and uneasy. It hardly felt right to enjoy a spirited debate about the positron or a cathartic game of ping-pong while Jewish colleagues in Germany were being hounded out of the country. But, with most physicists now convinced of the existence of the positron, Dirac could feel that his confidence in hole theory had been rewarded. Pauli, not wanting to be there to see it, skipped the meeting and went on vacation to the south of France.46

  Bohr organised the usual week-long programme, combining talks at the institute and gatherings at his new home, a mid-nineteenth-century mansion in the south-west of Copenhagen, in the grounds of the local Carlsberg brewery.47 Set in hundreds of acres of immaculate gardens, this was a grace-and-favour residence, a gift of the Government, who offered it, whenever it became vacant, to the person considered the most distinguished living Dane.

  The physicists at the meeting were in buoyant mood, though Ehrenfest was in poor spirits. Pudgy-faced and overweight, he was losing his grip on physics; for him, the succession of research reports were now a dispiriting agglomeration of detail. Convinced that his own work was worthless, he was looking for a new, less prominent academic position where he could motor in the slow lane.48 But he had not given up completely: during the discussions, he was still the unselfconscious inquisitor, pressing every speaker towards complete clarity, helping to draw attention away from irrelevancies and towards the saliencies of the new ideas. He was especially close to Dirac at this meeting, and they spent hours talking, keeping a few breaths away from the smokers’ fug.49

  After the closing speeches in Bohr’s home, the physicists put their luggage in the entrance hall and said their goodbyes.50 It was the usual bitter-sweet parting, but one delegate seemed especially out of sorts: Ehrenfest, about to catch a waiting taxi, looked flustered and unhappy. When Dirac thanked him for his contributions to the meeting, he was speechless and, apparently to avoid responding, hurried over to Bohr to say farewell. When he returned, Ehrenfest was bowing and sobbing: ‘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.’ Ehrenfest should not be allowed to travel home alone, Dirac thought, but he changed his mind. Abandoning his usual assumption that people mean exactly what they say, he concluded that Ehrenfest meant to say not ‘maybe’ but ‘sometimes’ – he sometimes felt that life is not worth living. Trying to say the right thing, Dirac stressed that his compliment was sincere. Still weeping, Ehrenfest held on to Dirac’s arm, struggling for words. But none came. He climbed into the taxi, which speedily made its way round the small grassy roundabout in front of the mansion, through the gardens, under the arch of the Carlsberg building and on towards the railway station.

  A few days later, Dirac was sailing to Helsinki, playing deck games and relaxing in the sun, en route to the Soviet Union. Since Hitler came to power, the attitude of the USSR towards scientists from other countries had changed: Stalin no longer encouraged his own scientist
s to mix with foreign colleagues, and such liaisons became a crime, except for Dirac and a small number of other friends of the Soviet Union. Dirac was keen to make light of this when he wrote an ambassadorial letter to Bohr a month before, assuring him of a ‘warm welcome from Russian physicists’ and noting that the economy there was not depressed: ‘the economic situation there is completely different from everywhere else’.51 Like many other gullible guests, Dirac had virtually no idea of the extent of the starvation and economic tribulations in the Soviet Union since the beginning of the Five Year Plan and the adoption of the collectivisation programme: people went round with string bags in their pockets on the off chance that they should come across a queue.52 In 1933, the privations were at their worst: the Soviet diet included little milk and fruit, and only a fifth of the meat and fish consumed thirty years before. Almost the only people to eat well were state officials and visiting dignitaries, such as Dirac, who was almost certainly unaware of the cost of the collectivisation programme: about 14.5 million lives during the previous four years, a higher death toll than the Great War.53 But Dirac knew that times were hard and that even basic items of clothing were not in the shops: when Tamm said that he would not be able to buy a heavy coat he needed for the coming months of freezing cold, Dirac gave his own coat to him and spent the next winter in England without one.54