Blackett was in the audience at a special Cavendish seminar on Monday, 23 November, when Millikan presented the latest photographs of cosmic rays to be taken at the California Institute of Technology (Caltech). The photographer was Carl Anderson, until recently Millikan’s Ph.D. student, only twenty-six years old and already touted as one of the brightest experimenters in the United States. Three weeks earlier, he had pointed out to his boss that the new photographs showed ‘Very frequent occurrence of simultaneous ejection of electron and positive particle’.43 Anderson was trying to take images of the charged particles produced by cosmic rays using a cloud chamber, which enables the tracks of electrically charged particles to be photographed as they travel through a cloud of water vapour. Anderson had built his own cloud chamber and, at Millikan’s suggestion, arranged for the entire chamber to be bathed in a strong and uniform magnetic field, which would deflect the paths of the charged particles as they hurtled through it. Each track contained crucial information: from the density of droplets along each track, Anderson could determine the particle’s electric charge, and he could calculate the particle’s momentum from the deflection caused by the magnetic field.44
It required great skill for Anderson to take any photographs at all. Most of his images were blank, but by early November he had obtained some ‘dramatic and completely unexpected’ images, which he sent to Millikan in Europe.45 The photographs made no sense in terms of the theory they were using. In a puzzled letter to Millikan, Anderson remarked that many of the photographs featured the track of a negatively charged electron with a positively charged particle, two particles appearing at the same time, presumably when a cosmic ray strikes an atomic nucleus in the chamber.
When Millikan presented Anderson’s inexplicable subatomic images in his seminar at the Cavendish, Blackett was fascinated. Here was a cloud-chamber expert with a talent that everyone knew was great but unfulfilled. Here was a new field in a mess. And here was the perfect opportunity for him to make his name.
Millikan’s audience in the Cavendish seminar did not include Dirac, who was still in Princeton. Many of his colleagues, including Martin Charlesworth in St John’s, feared they were about to lose him to one of the higher-paying American universities. Charlesworth wrote to Dirac saying how much he missed his ‘kindly irony’, imploring him ‘Don’t let them persuade you to stay in the USA. Here is your home.’46 Charlesworth was right to be concerned, for Veblen was energetically wooing Dirac. Even before the carpenters and decorators had put the finishing touches to Fine Hall, Veblen had begun to work with the educator Abraham Flexner, who was trying to set up an institute for advanced study, where world-class thinkers could study in peace, free of all distractions. Einstein was at the top of their wish list, but they were competing with others, including the wily Millikan, at Caltech.47
Charlesworth may have worried, too, that Dirac might not be looking forward to returning home. From newspaper and radio reports, Dirac knew that his homeland was plunging into difficult times. On 21 September, the Government removed the pound from the gold standard and allowed the currency to settle down to whatever price the money-market dealers were prepared to pay for it. It was a national humiliation. The economy plunged deeper into crisis: unemployment continued to escalate, and soon the pound had been devalued by 30 per cent, making Dirac’s $5,000 fee for his single-term stay look even more generous. The inevitable General Election returned a stabilising coalition government, but the economic privations continued: that year, one in every two British industrial workers had been unemployed for over four months.
Yet the depression was still more serious in the United States, even in affluent Princeton. At the university, many students struggled to pay their fees. Around the town, young vagrants were walking the streets, some of the two million roaming the country in search of work. About thirty million Americans, a quarter of the population, had no income at all. Many people who had money were so frightened of losing it that they hoarded their dollars under mattresses or buried it in the garden. Even President Hoover – long in denial about the extent of the depression – realised that ordinary people were losing faith in the American way of life.48
As Dirac will have been aware, unemployment was said to be zero in the USSR. The admirers of Stalin’s Five Year Plan in the press included the New York Times’s Moscow correspondent Walter Duranty, who called the plan a ‘stroke of genius’ and won the Pulitzer Prize the next year for his reports.49 Yet Dirac’s friends in the Soviet Union suffered terribly when Stalin’s attitude towards science changed abruptly, from a subject worthy of study for its own sake to a weapon for fighting capitalism. Tamm and Kapitza supported the new Soviet line, at least in public, but Dirac heard the other side of the story from Gamow, who had been exasperated by the change in the Government’s attitude when he returned to Russia in the spring of 1931. The Communist Academy had declared Heisenberg’s version of quantum mechanics anti-materialistic, incompatible with the state’s increasingly rigid version of Marxist philosophy. During a public lecture at the university on the uncertainty principle, Gamow experienced the full force of state censorship when a commissar, responsible for supervising moral standards, interrupted him and told the audience to leave. A week later, Gamow was forbidden to speak again about the principle in public.50
Since the mid-1920s, Gamow and Landau had been two leaders of the informal group of young Soviet theorists nicknamed the ‘Jazz Band’.51 In its seminars, the group discussed new physics, the Bolshoi Ballet, Kipling’s poetry, Freudian psychology and any other subject that took their fancy. The Jazz Band was mastering the new quantum physics much more quickly than their professors – ‘the bisons’ – whom they teased unmercifully, while taking care to remain within the bounds of decorum. The Band overstepped the mark in 1931, however, when they ridiculed a new encyclopedia article on relativity theory, edited to conform to the Party’s views on the subject. The butt of the Jazz Band’s barbs was the Director of the Physics Institute in Moscow, Boris Hessen, a thoughtful Marxist who had fended off several of the Government’s attempts to make orthodox theories of physics conform to ‘dialectical materialist’ principles, the philosophical basis of Stalinist Marxism, which accords much higher priority to concrete matters than to abstractions. Hessen had only a meagre knowledge of quantum mechanics and general relativity, so he was ill equipped to defend them against ideological interference from Stalin’s officials.52 This ignorance led him to write a ludicrous article in the Greater Soviet Encyclopedia about the ether, declaring it to be ‘an objective reality together with other material bodies’, contrary to Einstein’s teaching. Gamow, Landau and three colleagues sent a mocking note to Comrade Hessen and were put on trial as saboteurs of Soviet science. Landau was temporarily banned from teaching at the Moscow Polytechnic, and the miscreants were banned from living in the five largest cities of the USSR, though the ban was not enforced. According to Gamow, the offending physicists had been found guilty by a jury of machine-shop workers.
Even Dirac fell foul of the censors when the Russian translation of his book was being edited, when his publishers objected that his quantum mechanics was in conflict with dialectical materialism. The book eventually appeared in bookstores after an uneasy deal between the publisher and the editor, Dmitry ‘Dimus’ Ivanenko, a Jazz Band leader and another of Dirac’s effervescent Russian friends. In the awkward opening to the book, it is easy to see reflections of the delicacy of the deal: Ivanenko’s preface is conventionally laudatory, but it is preceded by an apologetic note from the ‘Publishing House’, arguing feebly that although the material in the book is ideologically unsound, Soviet scientists need to use its methods to advance dialectical materialism.53 A ‘counterflow’ of ideologically correct science will then follow, the publishers hoped.54 In a simpering conclusion, Ivanenko thanked Dirac, ‘a sincere friend of Soviet science’.
Censors were also scrutinising science in Germany, where the Depression was wreaking economic mayhem. Scruffy buskers,
match-sellers and bootlace salesmen walked the streets in the hope of being paid a few pfennig to buy a loaf; tens of thousands of the unemployed queued outside Nazi offices, waiting for the storm troopers to reward them with a mug of hot soup. The once-peaceful Göttingen, where Born was Dean of his faculty, was now seething with political tensions: in the physics library he saw Communist leaflets, while outside the Nazis greeted each other ostentatiously with a click of their heels and a ‘Heil Hitler’ salute.55 The Nazis, the majority party in the local government and student congress, were insisting that Einstein’s ‘Jewish physics’ was wrong and pernicious. Born was beginning to think that he had no alternative but to emigrate.
To most people who came across Dirac, he seemed to be no more engaged with world affairs than an automaton. With no need to share his thoughts with others, unless they were close friends, he gave the impression that he was indifferent to the fate of others. He appeared to have none of the usual need to be warmed by the good opinion of other human beings.
At work in his office in the new Fine Hall, he was putting into practice the philosophy that he had preached earlier in the year, learning advanced topics in pure mathematics in the hope that they would find application in theoretical physics.56 He had also returned to field theory, a subject he had co-founded four years before. The theory seemed fated to generate predictions that were not ordinary numbers but infinitely large. While Dirac was preoccupied with his ideas, Heisenberg and Pauli had been developing a full-blown theory of how electrons and photons interact with one another, a quantum theory that accounted for the spontaneous creation and destruction of particles, consistent with the special theory of relativity. Heisenberg and Pauli’s theory was also consistent with both quantum theory and experiment, but it was ugly and unwieldy. Oppenheimer later described it as ‘a monstrous boo-boo’.57 Unconvinced that this was the right way to describe nature at a fundamental level, Dirac sought a superior description, one that was logically sound and not plagued with infinities. The more Dirac looked into the Heisenberg–Pauli theory, the more he disliked it. In his view, it was not even consistent with the special theory of relativity because it describes processes throughout space using time measured by a single observer, whereas Einstein had taught that no single time could suffice for all observers, as they make different measurements of time. Dirac spent hours in Fine Hall examining the Heisenberg–Pauli theory and coming to terms with the problem of curing the sickness of field theory. The challenge would obsess him for the rest of his life.
By the end of the autumn, as Dirac’s sabbatical was ending, it was clear that the industrialised world was sliding into its worst-ever economic crisis, and there was a disturbing new militarism in Germany, Japan, Italy and throughout much of east-central Europe. In Britain, everyone was talking about the possibility of another war. The spirit of the age was no longer caught in the freewheeling, life-affirming bravura of Rhapsody in Blue but in the headlong, ominous prelude to Die Walküre.
In Bristol, it had been a sombre autumn at 6 Julius Road. In her letters, Dirac’s mother told him that she and his father had recovered from their climactic row and were back to their routine: she waited on him almost full-time, feeding him his vegetarian meals, washing his clothes and spending hours helping him dress. Each Sunday, she would give him – in silence – the ‘ninety-degree’ bath that he insisted was good for his rheumatism. After one of them, he had a heart attack. The family doctor told her soon afterwards that her husband ‘is a man accustomed to his own way & will not take advice […] He may live 20 years or he may go suddenly.’58
By September, the family were feeling the pinch of the economic crisis: Charles cut his tuition fees and insisted that they could no longer afford to run the car. When Betty told the family’s bank manager this, he laughed, Flo told her son. She believed Charles had plenty of money stashed away, although he was spending virtually nothing. Earlier, when Flo tried to claim the small amount of money Felix had left six years before, the authorities sent her a form for her husband to sign as the law specified that the funds must be paid to him. She told Dirac: ‘I tore up the form.’59
Dirac did not return in time for Christmas. Three days before the holiday, his mother wrote to him: ‘I am always so grateful that you broke away from our narrow little life.’60
Dirac was about to have one of his most exhilarating years. The word on the physicists’ street was that Chadwick was on to something important at the Cavendish Laboratory.61 Chadwick – a lean, severe figure – was usually busy overseeing his colleagues’ work, dispensing the paltry annual budget for equipment. But he had temporarily put administration to one side. Soon after the Christmas vacation, Chadwick had read an article that he suspected might lead to the neutron, a particle whose existence Rutherford had predicted.62 In the article, two French experimenters – Frédéric Joliot and Madame Curie’s daughter Irène – reported from their Paris laboratory that they had fired helium nuclei at a target made of the chemical element beryllium and found that particles with no electrical charge were ejected. They argued that these particles were photons, but Chadwick believed they were wrong and that the particles were Rutherford’s elusive neutrons. Rutherford agreed. Having just turned forty, Chadwick may have sensed that this could be the last chance for him to make his name, to emerge from the shadow of his imperious leader. He hungrily grabbed the opportunity, working alone night and day, borrowing apparatus and radioactive samples from colleagues all over the laboratory, making new equipment, filling his notebook with data and calculations. Oblivious of the freezing Cambridge midwinter, he was in a world of his own, as his colleagues saw. After three exhausting weeks, he had nailed the neutron. He proved to his satisfaction, and Rutherford’s, that his results made sense only if a particle with no charge and about the same mass as a proton is ejected in the nuclear collisions he observed. But when he wrote a report on his work for the journal Nature, he gave it the cautious title ‘Possible Existence of the Neutron’.
On 17 February, Chadwick sent off his paper to Nature, which rushed it into print. Six days later, after a good dinner in Trinity College with Kapitza, he presented his results to his colleagues at the Kapitza Club. Relaxed and emboldened by a few glasses of wine, Chadwick confidently described his experiments, giving appropriate credit to his colleagues, and finally set out the powerful arguments for the existence of the neutron. It was a coup for Chadwick and for the Cavendish Laboratory, which had at last come up with the kind of ground-breaking result that Rutherford longed for – one that put nature into fresh focus, clarifying the very nature of matter. The audience gave him the unusual accolade of a spontaneous ovation. After the meeting, he asked ‘to be chloroformed and put to bed for a fortnight’.63
The discovery gave fresh impetus to the notion that new types of subatomic particle might be predicted before they were detected. The ability to foresee the different types of grain in nature’s fabric was a challenge to even the greatest scientists: Einstein had, in effect, predicted the existence of the photon but occasionally lost confidence in his idea before he was proved right; Rutherford – the experimenter’s experimenter – had actually been more consistent, never wavering in his belief in the reality of neutrons. Perhaps Dirac’s anti-electron and Pauli’s neutrino were worth taking seriously, after all?
Notes - Chapter fifteen
1 Letter to Dirac from his mother, 27 April 1931, Dirac Papers, 1/4/1 (FSU). Dirac appears to have left Bristol on 15 April (postcard from Dirac to his parents, 15 April 1931, DDOCS).
2 Letter from Dirac to Van Vleck, 24 April 1931, AHQP.
3 Kapitza Club, 21 July 1931. See the Kapitza Club notebook in CHURCHILL.
4 Dirac (1982: 604); Dirac (1978).
5The size of the force between two attracting monopoles separated by a millionth of a millimetre – roughly the distance between the electron and the proton in a hydrogen atom – is about a ten-thousandth of the weight of a medium-sized apple.
6 Heilbron (1979: 87–96).
7Sherlock Holmes used these words in the novel The Adventure of the Blanched Soldier (1926), and used extremely similar words in several other stories.
8 The phrase ‘theorist’s theorist’ is often applied to Dirac. See, for example, Galison (2000).
9 Tamm arrived in Cambridge on 9 May and left on 25 June.
10 Fen (1976: 181).
11 Crowther (1970: 103).
12 Letter from Tamm to his wife, undated c. May 1931, in Kojevnikov (1993: 54).
13 Letter to Dirac from Tamm, 18 May 1931, in Kojevnikov (1993: 54–5).
14 Werskey (1978: 92).
15 Annan (1992: 181).
16James Bell (1896–1975) was one of Scotland’s leading climbers and was fascinated by the Soviet Union. He stayed in contact with Dirac for decades.
17 Wersley (1978: 138–49).
18 Bukharin (1931).
19 Brown (2005: 107).
20 Letter to Dirac from Tamm, 11 July 1931, Dirac Papers, 2/2/4 (FSU).
21 Home Office Warrant 4081, 27 January 1931, KV 2/777, UKNATARCHI.
22 Postcard from Dirac to his parents, 13 July 1931 (DDOCS).
23 Letter to Dirac from his mother, 8 July 1931, Dirac Papers, 2/2/4 (FSU).
24 The most direct comment on this from Dirac was reported by his mother in her letter to Betty from Stockholm in December 1933: ‘[Dirac] says it is awful and time we made an improvement.’ In her letters to Dirac, she often mentions the disrepair of the family home.
25 Letter to Dirac from his mother, 19 July 1931, Dirac Papers, 2/2/4 (FSU).
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