Preparing for his second trip to the Soviet Union, Dirac read in the British press that Stalin was tightening his grip, forcing through his programme of collective farming, squeezing the peasants in order to pay for a crash programme of industrialisation and persecuting political opponents and religious minorities. Some newspapers were in no doubt of Stalin’s malevolence – the Daily Telegraph wrote regularly of his ‘Reign of Blood’ and his ‘war on religion’ – but others, including the Manchester Guardian, gave him the benefit of the doubt.20 The New Statesman – the house journal of leftist intellectuals in Britain and favourite reading of Kapitza’s in the Trinity common room – insisted that Stalin should be given a fair hearing. Dirac agreed: one of the few things that would draw him into conversation were comments that he perceived to be unfairly hostile to the Soviet Union. Rudolf Peierls later recalled: ‘At a time when everything Russian was anathema, he questioned why each particular item was wrong, and this often caused raised eyebrows.’21 Wanting to see life there for himself, he again ignored the fears of his mother: ‘I do hope it is safe in Russia. One hears dreadful stories about it.’22
During his trip, Dirac felt the arm of the Soviet military on his shoulder: en route to Kharkov, when he attempted to cross the Soviet border at a place not mentioned in the visa that Tamm had obtained for him, border guards held him at the crossing point for three days before releasing him.23 By early July, he had heard that Soviet law forbade foreigners who stayed in the country for more than a month to take out either Soviet money or foreign currency. So he left the USSR in late July, within a month of his arrival, having cancelled his plans to hike in the Caucasus. His vacation foreshortened, he soon returned to England, to what most scientists would regard as the media highlight of their life.
In September, Hardy was praising Bradman’s devastating performances in the Ashes, and Bristol was preparing to host the British Association meeting. Almost three thousand delegates – including George Bernard Shaw – attended, each of them having paid a pound for the privilege.24 Jim Crowther told readers of the Manchester Guardian that the public delegates were young and dressed informally, many of the women in sleeveless and flowered voile frocks, the men in alpaca jackets and grey flannels. The ticket price had not changed since the meetings began almost a century before, when the Association’s leaders were choosing the most appropriate word to describe the participants. They considered ‘savants’, ‘nature peepers’ and ‘nature pokers’, but finally settled on ‘scientists’, coined in 1834 by William Whewell, one of John Stuart Mill’s philosophical adversaries. Though many hated the new word – Michael Faraday disliked it almost as much as the triply sibilant ‘physicist’ – it had caught on by the time Dirac was in junior school.25
The organisers, probably fearing that Dirac would give a technical talk of limited public appeal, scheduled him to speak in a modest room in one of the university’s new physics laboratories, funded by the tobacco manufacturer H. H. Wills. At 11 a.m. on Monday, 8 September, Dirac stood up without fanfare to address a crowded room on the subject of ‘The Proton’.26 Never confident when he spoke at public meetings, he may have been particularly apprehensive at this one: this was the first time he had agreed to address a lay audience and the first time he had spoken to many of the teachers who had seen him flower. If Charles was there, as is likely, he will have had a full heart as he had not heard his son speak in public before: Paul Dirac would now have no choice but to talk about his science to his father.
Dirac entered into the spirit of the British Association. Speaking with his usual directness, in lilting Bristol tones, he talked about his research in a way that might almost have passed as colloquial, though with none of Eddington’s flair. To ensure that he was intelligible to people with no science training, he began with the statement that ‘matter is made from atoms’, and quickly went up the gears, ending with his idea that the proton is a hole in the negative-energy sea of electrons. This implied, he pointed out, that there is only one fundamental particle, the electron, adding that such an economy in nature was ‘the dream of philosophers’. For many in his audience, this will have been an exciting revelation, but not for Gamow and Landau, who were at the back of the room, sitting on wooden benches. The two of them had roared down to Bristol on Gamow’s motorbike, Landau perched behind him on the luggage carrier. They travelled to the meeting, partly as Bohr’s unofficial emissaries, specifically to see if Dirac had anything new to say about his theory. During the talk, Gamow and Landau craned their necks to see the speaker, hanging on his every word, Landau, as usual, unable to resist making snide asides.27 After twenty minutes of reiterating arguments he had already published, often using the same words as he had used in his papers, Dirac drew to a close, and they realised that he had said nothing new. Their trip to Bristol had been a wild goose chase.
Dirac’s theory of negative-energy electrons nevertheless captured the imagination of journalists, and the British newspaper reports gave him more publicity than he had ever known. After his presentation, the representative from the American Science News Service wired Washington: ‘This new theory may prove to be as important and interesting to the public as Einstein’s theories have been.’28 The New York Times picked up the story and reported that Dirac’s ‘acclaimed’ theory ‘upset all present conceptions of space and matter’, adding that ‘These physical scientists have a more exciting life than Columbus.’29 But Dirac’s peers were unimpressed. On the way back to Cambridge, Landau and Gamow stopped at a post office. Landau sent Bohr a telegram consisting of a single word: ‘Crap’.30
The telegram reached Bohr soon after he received from Dirac a copy of his textbook, The Principles of Quantum Mechanics. Even if the author’s name were not on the cover, his identity would have been obvious to Bohr from a quick flick through: the unadorned presentation, the logical construction of the subject from first principles and the complete absence of historical perspective, philosophical niceties and illustrative calculations. This was the vision of a mathematically minded physicist, not an engineer. Dirac’s peers marvelled at its elegance and at the deceptively plain language, which somehow seemed to reveal new insights on each reading, like a great poem. Many of the students – especially the less able ones – were bemused, dissatisfied and sometimes even dispirited.31 The book had been written with no regard for his readers’ intellectual shortcomings, without the slightest sign of emotion, with not a single leavening metaphor or simile. For Dirac, the quantum world was not like anything else people experience, so it would have been misleading to include comparisons with everyday behaviour. He scarcely mentioned empirical observations except at the beginning, where he described an experiment that demonstrates the failure of classical theory to account for matter on the atomic scale and, hence, motivates the need for quantum mechanics. In its 357 pages, The Principles of Quantum Mechanics featured neither a single diagram, nor an index, nor a list of references, nor suggestions for further reading. This was, above all, a personal view of quantum mechanics, which is why Dirac – usually someone who abjured personal pronouns – always referred to it as ‘my book’.
Physicists immediately hailed it a classic. Nature published a rhapsodic review by an anonymous reviewer who – to judge by the eloquence and sharp turn of phrase – may well have been Eddington. The author made it clear that this was no ordinary account of quantum mechanics:
[Dirac] bids us throw aside preconceived ideas regarding the nature of phenomena and admit the existence of a substratum of which it is impossible to form a picture. We may describe this as the application of ‘pure thought’ to physics, and it is this which makes Dirac’s method more profound than that of other writers.32
The book eclipsed all the other texts on quantum mechanics written at about the same time – one by Born, another by Jordan – and became the canonical text on the subject in the 1930s. Pauli warmly praised it as a triumph and, although he worried that its abstractions rendered the theory too distant from experiment, descri
bed the book as ‘an indispensable standard work’.33 Einstein was another admirer, writing that the book was ‘the most logically perfect presentation of quantum theory’.34 The Principles of Quantum Mechanics later became Einstein’s constant companion: he often took it on vacation for leisure reading and, when he came across a difficult quantum problem, would mutter to himself, ‘Where’s my Dirac?’35
But some of Dirac’s undergraduate students were not pleased to find that the book was largely a transcript of his lectures: why, these students wondered, was it worth bothering to go and listen to him? Yet others found the course uniquely compelling.36 He would enter the lecture theatre punctually and in full academic garb, wearing the traditional uniform of gown and mortarboard. Otherwise, there was nothing else theatrical about him. He would clear his throat, wait for silence, then begin. For most of the lecture, he would stand still and erect, enunciating each word, addressing what one of his students described as his ‘personal unseen world’.37 At the blackboard, he was an artist, writing calmly and clearly, beginning at the top left-hand corner, then methodically working downwards, writing every letter and symbol so that someone at the back of the room could see it clearly. The audience was usually quiescent. If a student asked a question, he would dispatch it with the economy of a great batsman and then move on, as if nothing had disturbed his flow. After precisely fifty-five minutes he would draw his presentation to a close and then, unceremoniously, gather his papers together and walk out.
One of the new students who were impressed by Dirac’s course in the autumn of 1930 was Subrahmanyan Chandrasekhar, later a leading astrophysicist but then a wide-eyed student just arrived from Bombay. For him, the course was ‘just like a piece of music you want to hear over and over again’.38 During his time in Cambridge, he attended the entire course four times.
Dirac probably knew he had disappointed his colleagues at the British Association meeting by failing to say anything new. He was about to go to his second Solvay Conference, aware that few of the physicists took seriously his unified theory of electrons and protons; his proposal that protons were holes in the negative-energy sea was beginning to look not just implausible but untenable. One of the blows he suffered came shortly after the Bristol meeting when Tamm wrote to tell him that Pauli had proved that the holes have the same mass as the electron. Experimenters had not detected such a particle, which is probably why Tamm added a sympathetic comment: ‘I would be very much pleased to hear that Pauli is wrong.’39
This Solvay meeting was later remembered for being the one where leadership of the community of theoreticians passed from Einstein to Bohr. Einstein was looking out of touch, downcast after Bohr had bested him in one of their tussles about quantum mechanics and its meaning. For Einstein, the theory was fundamentally unsatisfactory as it did not even claim to describe physical reality, only the probabilities for the appearance of a particular physical reality on which an observing experimenter’s attention is fixed. Such a theory may be good at explaining experimental results, but it is certainly not complete, Einstein argued.40 Disillusioned, and uninterested in much of what his colleagues had to say, he consoled himself by playing after-dinner violin duets with the Queen of Belgium, one of his new friends.
Unlike the previous Solvay Conference in 1927, the atmosphere at this one was heavy with forebodings about the world outside physics, where the recession was ravaging most industrialised nations and providing fertile ground for political extremists. A month before the conference, Hitler’s National Socialists had taken second place in Germany’s election, followed by the Communists. Göttingen was now bedecked with Nazi flags, many of its shops displaying trinkets decorated with swastikas. Einstein was sick of the anti-Semitism in Berlin and despised Germany’s emerging leader: ‘If the stomach of Germany was not empty, Hitler would not be where he is.’41
As Dirac kept his politics almost exclusively to himself, most of his Cambridge colleagues mistakenly believed he had no interests at all, that he was as one-dimensional as the lines in his projective geometry. He was privately alarmed by the rise of Hitler and broadly supportive of Stalin’s project in the USSR, especially its commitment to mass literacy and education. Aware of Dirac’s interest, Tamm wrote to him about the radical experiment in ‘brigade education’, in which students studied intensively, alone and in groups, with no lectures, but with a professor on standby for consultation:
I never thought it possible for a large body of students to work as hard as our students do now. Our [brigades, each of five students, work and study together] 9 days out of 10 […] from 9am to 9pm with a 2-hour interruption for a meal (research work included, which is of course conducted individually by each student). Yesterday, speaking with a brigade, I found them troubled by the fact that they have ‘lost without cause’ six out of 270 working hours of the last month!42
Although Dirac was interested in the Soviet experiment, it was of only marginal interest to him compared with theoretical physics. By late autumn, he had every reason to be dissatisfied with his progress as his hole theory was in deep trouble. Oppenheimer and Weyl had independently come to the same conclusion as Pauli – that Dirac had no theoretical justification for believing that his holes were protons. The implication was that the theory was incorrect; something was amiss with the Dirac equation. But he was convinced that it was correct – what was needed was the correct interpretation of its mathematics. The American theoretician Edwin Kemble later put his finger on the kind of faith Dirac had in his equation: ‘[He] has always seemed to me a good deal of a mystic […] he thinks every formula has a meaning if properly understood.’43
Towards the end of term, Dirac went through his annual chore of refusing most invitations to Christmas parties, though he did occasionally attend the annual dinner of the Cavendish Physical Society, a boisterous evening of eating, drinking and singing.44 After Kapitza attended the dinner for the first time in December 1921, he wrote incredulously to his mother, observing how quickly even a moderate amount of alcohol freed the inhibitions of his English colleagues and made their faces ‘lose their stiffness and become lively and animated’. 45 By the end of the meal, after the cheeseboard and port had been passed round, the air was thick with cigar smoke and everyone was shouting to be heard above the din. The ritual was not yet over: the next stage was a series of facetious toasts (one had been ‘To the electron: may it never be of any use to anybody’46) alternating with off-key renditions of popular tunes such as ‘I Love a Lassie’, their lyrics rewritten as a jokey commentary on the past year at the laboratory. 47 At the climax, the portly Rutherford, Thomson and everyone else stood on their chairs, linked together with arms crossed and belted out ‘Auld Lang Syne’ and then, finally, the National Anthem ‘God Save the King’. After the bacchanalia ended, usually well after midnight, it was up to those left standing to take their drunken colleagues to their homes.
In 1930, Dirac did not attend the dinner but will probably have heard later that Kapitza was the focus of attention that night. Rutherford, then President of the Royal Society, had secured a professorship for his favourite colleague and funding for the construction of a new building to accommodate him and his laboratories. At the end of the seven-course dinner, while the sixty guests were chewing their mince pies, Darwin reminded them of the experience of entering Kapitza’s laboratory: ‘you had to ring to be admitted by a “flunkey” and became confronted not with men working in their shirt sleeves, but with Prof Kapitza seated at a table, like the arch criminal in a detective story, only having to press a button to do a gigantic experiment’.48
The laughter at this image of Kapitza, apparently a forerunner of a James Bond villain, will have been hearty, and it is safe to guess that knowing glances will have passed among his colleagues, many of them envious of his relationship with their laboratory’s director. Blackett was not there. Rutherford had no time for petty jealousy but was not above making a thinly disguised attack on his recently retired colleague Sir James Jeans, whose The Myst
erious Universe had been a best-seller since it first appeared in the bookstores the month before. Rutherford was as down to earth and, at the same time, as snobbish as anyone in science. As the recorder of the dinner wrote: Sir Ernest Rutherford ‘deplored the writing of popular books by men who had been serious scientists, to satisfy the craving for the mysterious exhibited by the public’.49 This was a common opinion in Cambridge. A few months later, his idoliser C. P. Snow – a scientist about to become a writer – sneered at science popularisers for doing a job that was just too easy: ‘there is no argument and no appeal, just worshipper and worshipped’. The result was, Snow declared, a ‘great evil’.50 Within three years, Snow published his semi-autobiographical novel The Search, the first fiction to bring to a wide audience the atmosphere of Rutherford’s laboratory, and to feature Paul Dirac.51
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