The Strangest Man

by Graham Farmelo

  7 Interview with Klein, AHQP, 28 February 1963, p. 18. Klein recalled that ‘Heisenberg once told me that when Dirac got the Nobel Prize some years later – in 1933 – he asked Dirac if he believed in his own theory. Dirac answered, in his very precise way, that a year before the positive electron was discovered he had ceased to believe in the theory’ (interview with Klein, AHQP, 28 February 1963, p. 18).

  8 Cathcart (2004: Chapters 12 and 13).

  9 Reynolds’s Illustrated News, 1 May 1932.

  10 Daily Mirror, 3 May 1932.

  11 Cathcart (2004: 252). Einstein’s lecture took place on 6 May; see the Cambridge Review, 13 May 1932, p. 382.

  12 Howarth (1978: 187).

  13 Howarth (1978: 224).

  14 Report in Sunday Dispatch on 19 November 1933.

  15 Interview with von Weizsächer, AHQP, 9 June 1963, p. 19.

  16 Note from P. H. Winfield to Dirac, Dirac Papers, 2/2/5 (FSU).

  17 Letter from Sir Joseph Larmor to Terrot Reaveley Glover (1869–1943), the classical scholar and historian, 20 February 1934, STJOHN.

  18 Infeld (1941: 170).

  19 Letter to Dirac from his mother, 27 July 1932, Dirac Papers, 2/2/6 (FSU).

  20 Letter to Dirac from his sister, 14 October 1932, Dirac Papers, 2/2/6 (FSU).

  21 Letter to Dirac from his sister, 11 July 1932, Dirac Papers, 2/2/6 (FSU).

  22 Letter to Dirac from his sister, 15 October 1932, Dirac Papers, 2/2/6 (FSU).

  23 Letter to Dirac from his mother, 21 April 1932, Dirac Papers, 2/2/6 (FSU). See also the letter of 1 June 1932.

  24 Letter to Dirac from his father, Dirac Papers, 1/1/10 (FSU).

  25 The paper combined several parts, one mostly from Dirac, the other mostly from Fock and Podolsky, and also a part that developed during the process of writing in correspondence between the three authors. One snapshot of the collaboration is in the letter written to Dirac by Podolsky in Kharkov on 16 November 1932, Dirac Papers, 2/2/6 (FSU). I thank Alexei Kojevnikov for this information.

  26 Weisskopf (1990: 72–3).

  27 Infeld (1941: 172).

  28 Article in the Los Angeles Times by Harry Carr, 30 July 1932.

  29 For more detail on the discovery of the anti-electron, see Anderson (1983: 139–40), and Darrow (1934).

  30 Interview with Louis Alvarez by Charles Weiner, 14–15 February 1967, American Institute of Physics, p. 10.

  31 Von Kármán (1967: 150).

  32 Von Kármán (1967: 150).

  33 Interview with Carl Anderson, 11 January 1979, available online at http://oralhistories. library.caltech.edu/89 (accessed 13 May 2008).

  34 Galison (1987: 90).

  35 New York Times, 2 October 1932.

  36 Letter from Robert Oppenheimer to Frank Oppenheimer, autumn 1932, in Smith and Weiner (1980: 159).

  37 Nye (2004: 54). The incident, recalled by Blackett’s student Frank Champion, probably took place during the 1931–2 academic year. I am grateful to Mary Jo Nye for this information.

  38 See http://www.aps-pub.com/proceedings/1462/207.pdf (accessed 13 May 2008).

  39 De Maria and Russo (1985: 254).

  40 Contribution of Occhialini to the Memorial Meeting for Lord Blackett, Notes and Records of the Royal Society, 29 (2) (1975).

  41 Dalitz and Peierls (1986: 167). The anecdote is due to Maurice Pryce.

  42 Dirac’s notes on Fowler’s lectures on ‘Analytic Dynamics’ are in Dirac Papers, 2/32/1 (FSU).

  43 Letter from Dirac to Fock, 11 November 1932, passed to me by Alexei Kojevnikov.

  44 Greenspan (2005: 170).

  45 Bristol Evening Post, 28 October 1932.

  46 Letter to Dirac from his mother, 26 October 1932, Dirac Papers, 2/2/7 (FSU).

  47 Letter to Dirac from his mother, 9 January 1933, Dirac Papers, 2/2/8 (FSU).

  Seventeen

  Einstein says that he considers Dirac the best possible choice for another chair in the Institute [for Advanced Study]. He would like to see us try for D[irac] even if the chance of getting him is very small. He rates him ahead of everyone else in their field. He places Pauli of Zurich second, apparently.

  Letter from OSWALD VEBLEN to ABRAHAM FLEXNER, 17 March 19331

  It seems that it was not until mid-December 1932 that Dirac was confident that the anti-electron exists. Later, memories were too hazy for the date to be made precise: Dirac recalled that he ‘probably’ heard the news from Blackett, who never said publicly when he was sure of the new particle’s existence. It may be that he discovered it independently of Anderson, though Blackett was always careful to give credit to his American rival for being the first to put his observation into print. Blackett and Occhialini probably learned of Anderson’s photographs in the autumn through the grapevine, but they read his article on ‘easily deflectable positives’ only in January, three months after its publication, when they were taking cosmic-ray photographs by the dozen every day.2 In this bitterly cold Cambridge winter, Blackett and Occhialini had to trudge each morning to the entrance of the Cavendish through snow, slush and ice; inside, the laboratory was buzzing with the thrill of the new cosmic-ray photographs. It seemed that another success was in the offing, but there was a problem: no one was sure precisely what the images were showing.

  The photographs featured a ‘shower’ of cosmic rays, with tracks that curved both to the left and to the right, emanating from a single location. In several of the snaps it was plain that Blackett and Occhialini had observed positively and negatively charged particles of about the same mass as they zipped through the cloud chamber: these appeared to be electrons and anti-electrons. Blackett asked Dirac to help interpret the data, and soon he was in the laboratory, doing detailed calculations using his hole theory. The most likely explanation was, they concluded, that incoming cosmic rays were breaking up nuclei and that in the vicinity of some of these breakups, pairs of positive and negative electrons were being created. It was a classic application of Einstein’s equation E = mc2: the energy of the collision was converted into the masses of the particles. Dirac’s calculations persuaded the hyper-cautious Blackett that the photographs were strong evidence for anti-electrons that behaved just as the Dirac equation predicted.

  When Blackett and Occhialini were preparing to make their results public, Dirac was also reading about events in Berlin. In the November election, the Nazis had lost over two million votes and had seen their representation in the Reichstag fall, but on 30 January, after weeks of chicanery by Hitler and his supporters, he was appointed Chancellor. The following night, Göttingen was ablaze with torchlight as a procession of uniformed Nazis wended its way through the streets of the old town, singing patriotic songs at the tops of their voices, waving their swastikas and making anti-Semitic jokes. Hitler dashed naive hopes that he would moderate his policies on coming to power, swiftly implementing a dictatorship. On 6 May, the Nazis announced a purge of non-Aryan academics from universities, and, four days later, book-burning ceremonies were held all over Germany, including Göttingen and Berlin. Even before Hitler rose to power, Einstein had left Germany, and he quickly announced that he would not return.

  Hundreds of other Jewish scientists were desperate to emigrate. Dozens were rescued by Frederick Lindemann, Rutherford’s counterpart at Oxford University, a prickly and sarcastic snob who had toured universities in Germany in his chauffeur-driven Rolls Royce offering threatened academics a safe haven in his laboratory. Cambridge University did not openly recruit potential refugees but waited for them to apply: from scientists, it received thirty such applications every day.3 One of them was Max Born, who was given a short-term academic appointment and – partly as a result of Dirac’s support – an honorary position at St John’s. In November, his colleague Pascual Jordan became one of three million storm troopers and proudly wore his brown uniform, his jackboots and his swastika armband.4

  Although Heisenberg never joined the Party, he remained in Germany and was pleased that Hitler had come to power, if an anecdote rela
ted by Bohr’s Belgian student Léon Rosenfeld is correct. Soon after Hitler became Chancellor, Bohr commented to Rosenfeld that the events in Germany might bring peace and tranquillity, insisting that the situation ‘with those Communists’ was ‘untenable’. When pressed by Rosenfeld, Bohr remarked: ‘I have just seen Heisenberg and you should have seen how happy [he] was. Now we have at least order, an end is put to the unrest, and we have a strong hand governing Germany which will be to the good of Europe.’5

  Although Dirac was privately appalled by Hitler’s appointment, his outward response was so discreet as to pass unnoticed except by a few colleagues, including Heisenberg: Dirac vowed never again to talk in German.6 He had learned two foreign languages but now wanted to speak neither of them.

  International politics were not Dirac’s only distraction. He was also turning his attention to moral philosophy, probably as a result of talking with the formidable Isabel Whitehead. ‘Don’t despise philosophers too much,’ she had counselled him after one of his visits, ‘a great deal that they say may be useless, but they are after something which matters.’7 Mrs Whitehead had been on the receiving end of one of Dirac’s tirades against the only academic discipline he openly disdained. One of his bêtes noires was the internationally admired Trinity College philosopher Ludwig Wittgenstein, regarded by many as one of the cleverest academics in Cambridge. Several decades later, Dirac remarked that he was an ‘Awful fellow. Never stopped talking.’8

  Dirac’s disenchantment with philosophers had degenerated into hostility when he read the ignorant comments several of them made on quantum mechanics; in a book review, he had already noted that it had taken the Heisenberg uncertainty principle to awaken the dozy philosophers to the revolutionary implications of quantum mechanics. 9 The philosophers who least offended Dirac and other theoretical physicists were the logical positivists, who held that a statement had meaning only if it could be verified by observation.10 There are traces of this philosophy in three pages of notes Dirac wrote out by hand in mid-January 1933, the raw and unpretentious jottings of a young man who wants to take stock and clarify his thinking about religion, belief and faith.11 He had recently told Isabel Whitehead, ‘I am mainly guided in my philosophical belief by Niels Bohr’, but these notes indicate that mainstream philosophers influenced Dirac more than he knew.12

  Dirac begins by considering belief. Some of the things a person believes in, he remarks, are not based on evidence but simply because they promote happiness, peace of mind or moral welfare. Such things constitute a person’s faith or religion. In the only example he gives to illustrate this, he considers suicide, pointing out that most people believe that it ‘is not a good thing, although there is no logical reason against it’. He was still haunted by Felix’s demise and by the feeble purchase of logic on grief.

  When Dirac focuses on the transience of life, he is driven to an important moral conclusion: ‘A termination of one’s life is necessary in the scheme of things to provide a logical reason for unselfishness. […] The fact that there is an end to one’s life compels one to take an interest in things that will continue to live after one is dead.’

  This, he says, is quite different from the unselfishness preached by orthodox religion, which he characterises as sacrificing one’s interests in this life for one’s interests in the next. Although he regards such a sacrifice as wrong-headed, he concedes – with uncharacteristic condescension – the argument made by many an imperial missionary that ‘Orthodox religion would be very suitable for a primitive community whose members are not sufficiently developed normally to be taught true unselfishness.’

  Although Dirac rejects religious faith, he accepts that another faith is needed to replace it, something to make human life, effort and perseverance worthwhile. This leads him to his credo, one that would later influence his thinking on cosmology:

  In my case this article of faith is that the human race will continue to live for ever and will develop and progress without limit. This is an assumption that I must make for my peace of mind. Living is worthwhile if one can contribute in some small way to this endless chain of progress.

  At the end of his notes, Dirac turns to belief in God. This notion is so vague and ill defined, he says, that it is hard to discuss with any rigour. He first gave his views on the subject in his diatribe at the 1927 Solvay Conference, and is no less scathing here: ‘The object of this belief is to cheer one up and give one courage to face the future after a misfortune or catastrophe. It does this by leading one to think that the catastrophe is necessary for the ultimate good of the people.’

  Perhaps Dirac had at least partly in mind his father’s rediscovery of his childhood Catholicism after the death of Felix. Dirac himself had no such solace and had to try to cope with the tragedy entirely without a spiritual crutch. Unable to fathom what he takes to be the religious justification for how a benevolent deity could condone natural disasters – they are part of God’s plan, ultimately to the good of humanity – Dirac concludes by dismissing the idea that religion has any place in modern life: ‘Any further assumption implied by belief in a God which one may have in one’s faith is inadmissible from the point of view of modern science, and should not be needed in a well-organized society.’

  The entire document reveals that Dirac’s thinking about morality and religion is suffused with two principal concerns: how these types of knowledge square with scientific observations and how they can be used as a guide to living. This is consistent with the approach of John Stuart Mill, who would have applauded Dirac’s suggestion that a personally rewarding faith was sometimes needed to replace the untenable belief in eternal life and for everyone to feel that they are contributing in some way to human progress. Some of Dirac’s turns of phrase – his reference to ‘a well-organized society’ in particular – might be a result of the influence of Mill’s French colleague and friend Auguste Comte, the founder of positivism.13 More likely, Dirac was taking the Marxist line that religion is ‘the opium of the people’.

  On Thursday 16 February, dozens of scientists made their way through the London fog in the fast-fading light of the late afternoon. They were heading for the grand Piccadilly home of the Royal Society, in the East Wing of Burlington House, on the site of today’s Royal Academy of Arts. This was the headquarters of British science, a stone’s throw from many of the city’s finest shops and restaurants, a few minutes’ walk from the West End theatres.14 The audience, including Cockcroft and Walton, probably hoped that the first of the five talks that they would hear would be more exciting than its title: ‘Some Results of the Photography of the Tracks of Penetrating Radiation’. Unusually for formal presentations like this, the audience included a posse of journalists – no doubt tipped off by Crowther – most of them probably wondering whether they were wasting their time. If there really was a good story here, why announce it so close to their deadline? It is likely that the newshounds hoped, too, that the handsome speaker at the front of the room was more excited than he looked. Shortly after four-thirty, Blackett rose.

  His talk was sensational.15 He described his experiment and showed vivid photographs of the showers of charged particles that continually rain down on the planet and yet, until these experiments, had never been recorded on film. Blackett had almost no sense of theatre, but when he projected the photographs of cosmic-ray showers – revealing the hitherto unnoticed showers of particles bombarding the planet from outer space – mouths fell open in disbelief. Although cautious in his interpretation of his pairs of positive and negative particles, Blackett said that they fitted ‘extraordinarily well’ with the Dirac hole theory. Here, in front of the audience’s eyes, was plain evidence for particles emerging out of nothing and for the opposite process, in which electrons and anti-electrons annihilate one another as soon as they meet. Blackett described this as their ‘death compact’.

  After the talk, when the applause had faded, Blackett agreed to give interviews to journalists. Always the perfect gentleman, he stressed tha
t the discoverer of the positive electron was Carl Anderson and that the best theoretical interpretation of the photographs had been given by Dirac. Where, then, was Dirac? He was giving a seminar in another part of Burlington House, unavailable for comment.16

  The newspaper reports reflected the excitement of the briefing. Of all the London newspapers, the Daily Herald featured the story most prominently: the headline ‘Science Shaken by Young Man’s Researches’ and ‘Greatest Atom Discovery of the Century’ was followed by a breathless account of the experiment. It made no mention of Dirac’s theory. The anonymous writer excised Occhialini from the story, as did Crowther in the same morning’s Manchester Guardian, where he interpreted the discovery using Dirac’s theory and used Millikan’s colourful term ‘cosmic rays’. The New York Times also featured the story on the Friday morning and included a wary quote from Rutherford: ‘there seems to be strong evidence of the existence of a light positive particle corresponding to the electron. But the whole phenomenon is exceedingly complex and a great deal of work will have to be done on it.’ The reporter did well to extract this quote, as Rutherford did not attend the meeting, having made clear that he mistrusted Blackett and Occhialini’s use of Dirac’s ideas, which Rutherford believed were nonsense.

  Not since Eddington’s solar-eclipse announcement thirteen years before had a talk at the Society made such a splash in the international press. Eddington’s shrewd handling of the press had made Einstein an international star, but Blackett’s presentation was never going to do the same for Dirac. He had no wish at all to be a celebrity; the very thought of it would have revolted him. And, after Rutherford’s guarded comments, few journalists will have been motivated to draw Dirac out of his carapace.

  After the press reported Blackett’s announcement, Anderson was on edge. Most physicists had not read or even heard of his paper on the ‘easily deflectable positives’, and he had not yet published his photographs in a professional journal. He had not even given the new particle a name. For several months, he and his Caltech colleagues had considered contracting the term ‘positive electron’ to ‘positron’ and, at the same time, suggested that the ordinary, negatively charged electron might be renamed the negatron. Other names were forthcoming, too: the astrophysicist Herbert Dingle in London recalled that Electra in Greek mythology had a brother Orestes and so suggested that the positive electron should be called the oreston. It was Anderson, hurriedly completing a long paper on his discovery, who chose the name that stuck: the positron.17