This conference was shaping up to be a highlight in Dirac’s career, until he heard some appalling news from Amsterdam. Lunchtime in the city’s Vondelpark on the last Monday in September had been like any other on an early autumn weekday: the mothers teaching their little children to feed the ducks, the cyclists whooshing past the strolling pedestrians, a few picnickers in the last of the bright afternoon light. But suddenly the calm was shattered by gunshots. A few onlookers gathered round a horrifyingly violent scene: a young boy with Down’s syndrome, fatally wounded but still breathing, lying next to a man in his fifties, dead, part of his head blown away. The man was Paul Ehrenfest. Moments before, he had shot his son Wassik but had not quite summoned the will to kill him. Two hours later, the boy died.55
In countless confused seminars on the new quantum ideas, he had done more than anyone else to pick out the diamonds from the mud. He had now been drowned by the wave he had helped to create. Dirac, needing to clarify his own thoughts and feelings, wrote Bohr a four-page letter, describing his last moments with Ehrenfest.56 Of all Dirac’s surviving letters, this is among the longest and most emotionally direct. With the fluency of a novelist, he recalls every detail of his last meeting with Ehrenfest, more sensitive to emotional nuance than most of his colleagues would have believed. He lamented to Mrs Bohr that he should have taken Ehrenfest’s last words to him more literally – a shortcoming of which no one thought Dirac capable – and that he should have advised her husband to keep Ehrenfest in Copenhagen. Dirac concluded that he ‘could not help blaming himself for what happened’. Mrs Bohr replied with consoling words, thanking him for doing ‘so much to make Ehrenfest’s last days here as happy as his sad mood allowed’. She added, ‘he loved you very much.’57
Ehrenfest had written a suicide note a month before the Copenhagen meeting – to Bohr, Einstein and a few other close colleagues, though not to Dirac. After declaring that his life had become ‘unbearable’, he concluded:
In recent years it has become ever more difficult for me to follow developments [in physics] with understanding. After trying, ever more enervated and torn, I have finally given up in DESPERATION […] This made me completely ‘weary of life’ […] I did feel ‘condemned to live on’ mainly because of economic cares for the children […] Therefore I concentrated more and more on ever more precise details of suicide […] I have no other ‘practical’ possibility than suicide, and that after having killed Wassik. Forgive me.58
Ehrenfest never sent this terrible note. It was tragic that he did not live to take his place a few weeks later at the Solvay Conference, the climax of almost a decade of research into matter at its most elementary level. Originally scheduled to be about the applications of quantum mechanics to chemistry, the organisers had decided in July 1932 – in the wake of the Cavendish discoveries that year – to switch the theme to the atomic nucleus. It was probably expected that Rutherford would be the cock of the walk at the meeting, but by autumn 1933 nuclear physics had moved on and was aflame with new discoveries, new ideas, new techniques. Rutherford, never one to avoid the limelight, may well have felt eclipsed as he saw the focus of attention turn to others: to America’s most flamboyant young experimenter, Ernest Lawrence, and his invention of a high-energy particle accelerator so compact that it fitted on a desktop; to Enrico Fermi and his discovery that slow neutrons could induce some nuclei to undergo radioactive decay artificially; to Heisenberg and his new picture of the typical atomic nucleus as a combination of protons and neutrons, but no electrons.
Dirac’s intuition was not as sure-footed in this subatomic realm: he disagreed with Heisenberg’s view of the nucleus – soon to be in textbooks – just as he did not believe in the existence of Pauli’s neutrino. Dirac was most at home when he was teasing out the implications of quantum mechanics, and he was able to do so at the conference, but only after the organisers had been pressed to give him a slot by Pauli.59
This was to be another of Dirac’s seminal talks. Having pointed out that the discovery of the positron had renewed interest in the existence of a sea of negative-energy electrons, he argued that the presence of these background particles forces physicists to rethink the concepts of the vacuum and of electrical charge. As Oppenheimer and one of his students had independently suggested, the vacuum was not completely empty but was seething with activity, vast numbers of particle–antiparticle pairs continually bubbling up out of nothing and then annihilating each other, in fractions of a billionth of a second. These processes of creation and destruction are so brief that there is no hope of detecting them directly, but their existence should cause measurable changes in the energies of atomic electrons. Likewise, Dirac suggested that the charge of an ordinary positive-energy electron should be affected by the presence of the negative-energy sea: the electrical charge of an ordinary electron should be slightly less than the value it would have if the background were absent.
But the theory was still replete with infinities. Dirac suggested ways of coping with this, using special mathematical techniques to make testable predictions. The audience could see that this was the work of a master, if one who was too clever by half. Pauli despaired of the theory (‘so artificial’), while for Heisenberg it was ‘erudite trash’.60
Dirac probably agreed with Pauli and Heisenberg more than he let on, for he knew as well as anyone that his techniques involved the sort of procedures results-hungry engineers would be happy to use but that would make any self-respecting mathematician blanch. Convinced that any fundamental theory worth its salt must make perfect mathematical sense, he was becoming seriously disenchanted with quantum field theory. This Solvay talk would be the last time he used the theory to probe the inner workings of the atom: he would go on to make other fundamental contributions to science, but this presentation marks the end of his golden creative streak, which he had sustained for eight years.
Midway through the autumn term in Cambridge, on Thursday, 9 November, Dirac received the telephone call that most first-rate physicists hope for, if only in secret. A voice from Stockholm told him that he was to share the 1933 Nobel Prize for physics with Schrödinger for ‘the discovery of new and productive forms of atomic theory’; the deferred 1932 prize went to Heisenberg. Dirac was surprised by his own award but not by the other two, certainly not by the one given to Heisenberg – the principal discoverer of quantum mechanics, in Dirac’s opinion.61 Nervous of the inevitable press attention, Dirac considered refusing the prize, but he soon took Rutherford’s advice: ‘A refusal will get you more publicity.’62 The Dirac family first heard the news on the day of the announcement, soon after ten at night, when a note was slipped through their letter-box by Charles’s friend Mrs Fisher.
The Nobel Prize for physics had been instituted in 1901, when it was awarded to the German experimenter Wilhelm Röntgen for his discovery of X-rays. The institution of the prize for physics – and also for chemistry, literature and physiology – was the idea of the Swedish inventor, Alfred Nobel, whose legacy funded the prize in perpetuity. Since the first year, the status of the prizes had grown, and, by 1933, the annual announcements of the winners were featured in newspapers all over the world. As some of the reports noted, Dirac was a special winner: at thirty-one, he was the youngest theoretician ever to win the prize for physics.63
Most English national newspapers mentioned Dirac’s prize on the day after it was announced.64 The Daily Mail squeezed in a short report about the award to the ‘silent celebrity’ next to a long article on ‘Hitler’s homage to fallen Nazis’. Readers of The Times also read of Dirac’s award alongside a report from Germany, where Hitler’s deputy, Rudolf Hess, had issued regulations to ensure that electioneering is ‘conducted in a dignified manner’. None of the hurriedly prepared articles mentioned the discovery of the positron or captured Dirac’s personality; it was left to the Sunday Dispatch later in the month to publish an overheated but insightful description of Britain’s newest Nobel laureate. The anonymous author noted that ‘more than publici
ty, [Dirac] fears women. He has no interest in them, and even after being introduced to them, cannot remember whether they are pretty or plain.’ Dirac was ‘as shy as a gazelle and modest as a Victorian maid’.65
The first congratulatory note to arrive in Dirac’s pigeonhole was a telegram from Bohr. Dirac replied with forgivable sentimentality:
I feel that all my deepest ideas have been very greatly and favourably influenced by the talks I have had with you, more than with anyone else. Even if this influence does not show itself very clearly in my writings, it governs the plan of all my attempts at research.66
In the Cavendish, the announcement of the prizes was welcomed by everyone except Max Born, bitter that he had been passed over in favour of Dirac.67 Others in Cambridge were preoccupied with the most dramatic event to take place in the town for years: on Armistice Day, three days after Dirac heard from Stockholm, the Socialist Society organised a march of hundreds of students through the centre of Cambridge, seeking ‘to provoke clashes, to make a stir […] to put politics on the map and into university conversation; to bounce, startle, or shock people into being interested’.68 In a normal Armistice Day march, a carnival of several hundred undergraduates walked through the city centre, selling blood-red paper poppies to passers-by in order to raise money for survivors of recent wars and to commemorate the lives of soldiers who had fallen in battle. The tragic aspect of the proceedings was often lost in hilarity, making the occasion ripe for subversion. On that grey Sunday afternoon, the pavements of Cambridge were lined with crowds, jeering as they were passed by marchers, some of them holding the banner pole of the Socialist Society, others bearing a wreath inscribed ‘To the victims of the Great War, from those who are determined to prevent similar crimes of imperialism’. The second phrase should be removed, the police escorts insisted, as it might provoke a breach of the peace. By the time the marchers reached the entrance to Peterhouse College, an eruption was inevitable. Onlookers threw flour and white feathers over the students and pelted them with rotten eggs, tomatoes and fish; the marchers retaliated by using a car as a battering ram to push back their tormentors.
The university authorities panicked. Away from the public posturing, students and dons debated round college firesides whether the marchers had desecrated the day of remembrance or had restored seriousness to what had become a maudlin carnival. The event had marked the beginning of a militant student socialist movement in Cambridge.
In his rooms in St John’s, the Lucasian Professor probably watched the events carefully and pondered how he could make his feelings heard.
Notes - Chapter seventeen
1 IAS Archives Faculty Series, Box 32, Folder: ‘Veblen, 1933’.
2 De Maria and Russo (1985: 266 and 266 n.). Anderson’s paper had been available in the university library from the mid-autumn of 1932.
3 Archie Clow, contributing to Radio 3 programme Science and Society in the Thirties (1965). Script stored in the library of Trinity College, Cambridge.
4 Schücking (1999: 27).
5 Interview with Léon Rosenfeld, AHQP, 22 July 1963, p. 8.
6 Halpern (1988: 467).
7 Letter to Dirac from Isabel Whitehead, 20 July 1932, Dirac Papers, 2/2/6 (FSU).
8 Taylor Sen (1986).
9 Dirac, book review in the Cambridge Review, 6 February 1931.
10 Interview with von Weizsächer, AHQP, 9 June 1963, p. 19.
11 Private papers of Mary Dirac. Dirac wrote the notes on 17 January 1933.
12 Letter from Dirac to Isabel Whitehead, 6 December 1936, STJOHN.
13 Compte remarked that ‘The greatest problem, then, is to raise social feeling by artificial effort to the position which in the natural condition is held by selfish feeling.’ See http://www.blupete.com/Literature/Biographies/Philosophy/Comte.htm (accessed 14 May 2008).
14 The headquarters of the Royal Society were then at Burlington House.
15 Bertha Swirles, Dirac’s former student colleague, described the talk as ‘sensational’ in her letter of 20 February 1933 to Dirac’s colleague Douglas Hartree. Hartree archive, 157, CHRIST’S.
16 Dirac was giving a technical talk at the London Mathematical Society on his favourite topic, ‘The Relation Between Classical and Quantum Mechanics’, at the Royal Astronomical Society in Burlington House, Dirac Papers, 2/26/18 (FSU).
17 The word was used in the 15 March issue of the Physical Review.
18 Quoted in Pais (1986: 363).
19 Interview with von Weizsächer, AHQP, 9 July 1963, p. 14.
20 Letter from Tamm to Dirac, 5 June 1933, in Kojevnikov (1996: 64–5).
21 Interview with Dirac, AHQP, 14 May 1963, p. 31.
22 Letter from Pauli to Dirac, 1 May 1933, see Pais (1986: 360).
23 Galison (1994: 96).
24 Darrow (1934: 14).
25 Roqué (1997: 89–91).
26 Brown and Hoddeson (1983: 141).
27 Blackett (1955: 16).
28 Gell-Mann (1994: 179).
29 See the lecture Dirac gave in Leningrad on 27 September 1933 (Dalitz 1995: 721), Dirac’s Nobel Prize lecture in December 1933 and most of Dirac’s subsequent lectures on the positron.
30 Blackett (1969: xxxvii).
31 Gottfried (2002: 117).
32 Bohr’s support was sought by Kapitza. See the correspondence quoted in Kedrov (1984: 63–7).
33 The quote from Rutherford is from Kapitza’s letter to Bohr of 10 March 1933, quoted in Kedrov (1984: 63–4).
34 Anon., ‘Conservatism and the Young’, Cambridge Review, 28 April 1933, pp. 353–4.
35 The debate was held on 21 February 1933 and was reported in the Cambridge Evening News on the following day. See also Howarth (1978: 224–5).
36 Anon (1935); essay by Blackett (based on a radio broadcast in March 1934), pp. 129–44, see p. 130.
37 Werskey (1978: 168).
38 Werskey (1978: 148).
39 The Cambridge Review, 20 January 1933. The article alerted the Cambridge University community to the reservations expressed by the translators of Dirac’s book into Russian.
40 Anon. (1933) ‘The End of a Political Delusion’, Cambridge Left, 1 (1): 10–15; p. 12.
41 Daily Herald, 15 September 1933, p. 10. McGucken (1984: 40–1).
42 Letters to Dirac from his mother, 20 July and 22 July 1933, Dirac Papers, 1/4/3 (FSU).
43 Letter to Dirac from his mother, 8 August 1933, Dirac Papers 1/4/3 (FSU).
44 Postcards from Dirac to his mother, from September 1933 (DDOCS).
45 Letter from Dirac to Tamm, 19 June 1933, in Kojevnikov (1993: 67); see also the letter from Tamm to Dirac on 5 June 1933 (Kojevnikov 1993: 64).
46 Interview with Beck, AHQP, 22 April 1967, APS, p. 23.
47 The mansion was awarded to Bohr in December 1931, whereupon Bohr and his family moved in during the summer of 1932. The Bohrs’ first sleeping-over guests were Ernest Rutherford and his wife, who stayed there from 12 September to 22 September 1932. I thank Finn Aaserud and Felicity Pors for this information.
48 Parry (1968: 117).
49 Casimir (1983: 73–4). Letter from Dirac to Margrethe Bohr, 24 September 1933, NBA.
50 Letter from Dirac to Margrethe Bohr, 24 September 1933, NBA.
51 Letter from Dirac to Bohr, 20 August 1933, NBA.
52 Fitzpatrick (1999: 40–1).
53 Conquest (1986: Epilogue).
54 M. Dirac (1987: 4).
55 Anne Kox, ‘Een kwikkolom in de Westertoren: De Amsterdamse natuurkunde in de jaren dertig’, available online at http://soliton.science.uva. nl/~kox/jaarboek.html (14 May 2008).
56 Letter from Dirac to Bohr, 28 September 1933, NBA.
57 Letter from Margrethe Bohr to Dirac, 3 October 1933, NBA.
58 Letter from Ehrenfest to Bohr, Einstein and the physicists James Franck, Gustave Herglotz, Abram Joffé, Philipp Kohnstamm and Richard Tolman, 14 August 1933, NBA. Another suicide note, written on the day before Ehrenfest killed himself was unearthed in 2008: see Phys
ics Today, June 2008, p. 26–7.
59 Roqué (1997: 101–2).
60 Letter from Heisenberg to Pauli, 6 February 1934, in Hermann et al. (1979).
61 Dirac mentioned his surprise to a reporter from the Daily Mirror. See the article on 13 November 1933.
62 Taylor (1987: 37).
63 The youngest experimenter to win the prize was, and remains, Lawrence Bragg, who won it when he was twenty-five. Dirac’s record as the youngest theoretician to win the prize was broken (by a margin of three months) in 1957 by T. D. Lee.
64 Reports on 10 November 1933 included the Daily Mail, Daily Telegraph, Manchester Guardian; the Daily Mirror reported on the following day.
65 Sunday Dispatch, 19 November 1933.
66 Letter from Dirac to Bohr, 28 November 1933, NBA.
67 Greenspan (2005: 242). Maurice Goldhaber remembers that when he remarked that Dirac’s award was ‘great news’, Born scowled. Interview with Maurice Goldhaber, 5 July 2006.
68 Cambridge Review, 17 November 1933; Brown (2005: 120). See also Stansky and Abrahams (1966: 210–13). A few days before the march, a few socialists and pacifists clashed with audiences leaving the Cambridge cinema Tivoli, after an evening showing of the patriotic movie Our Fighting Navy. The fight was the talk of the town and therefore guaranteed interest in the Armistice Day march.
Eighteen
Few misfortunes can befall a boy which bring worse consequences than to have a really affectionate mother.
W. SOMERSET MAUGHAM, A Writer’s Notebook, 1896
The Strangest Man Page 34