The rest of Dirac

 Farmelo

Dirac and Einstein were poles apart, and neither was comfortable speaking the other’s language. Dirac was twenty-three years younger, and his awe rendered him even more shy than usual. But probably the main reason why they did not engage was that their approaches to science contrasted so sharply, partly because they responded so differently to philosophical matters. They agreed that science was fundamentally about explaining more and more phenomena in terms of fewer and fewer theories, a view they had read in Mill’s A System of Logic. Yet, whereas Einstein remained interested in philosophy, for Dirac it was a waste of time. What Dirac had retained from his reading of Mill, bolstered by his studies of engineering, was a utilitarian approach to science: the salient question to ask about a theory is not ‘Does it appeal to my beliefs about how the world behaves?’ but ‘Does it work?’ 

At the conference, Dirac made his first recorded outburst on topics outside physics religion and politics. Some four decades later, Heisenberg described the event, which took place one evening in the hotel’s smoky lounge, where some of the younger physicists were lying around on the chairs and sofas. Dirac’s youthful outspokenness needed to be indulged, the elderly Heisenberg said: ‘Dirac was a very young man and in some way was interested in Communistic ideas, which of course was perfectly all right at that time.’*” Most vivid in Heisenberg’s memory was a rant from Dirac about religion, triggered by a comment about Einstein’s habit of referring to God during dis. cussions about fundamental physics. Like many of Heisenberg’s accounts of incidents in the 1920s, this one is implausibly detailed ~ it consists of two speeches of several hundred words, quoted as if his memory were word perfect but it is consistent with other accounts of Dirac’s views. According to Heisenberg, Dirac thought religion was just ‘a jumble of false assertions, with no basis in reality. The very idea of God is a product of the human imagination.’ For Dirac, ‘the postulate of an Almighty God’ is unhelpful and unnecessary, taught only ‘because some of us want to keep the lower classes quiet’. Heisenberg wrote that he objected to Dirac’s judgement of religion because ‘most things in this world can be abused -~ even the Communist ideology which you recently propounded’. Dirac was not to be deflected. He disliked ‘religious myths on principle’ and believed that the way to decide what was right was ‘to deduce it by reason alone from the situation in which I find myself: I live in a society with others, to whom, on principle, I must grant the same rights I claim for myself. I must simply try to strike a fair balance.’28 Mill would have approved. 

During Dirac’s assault on religion, Pauli had been uncharacteristically silent. When asked what he thought, he replied, ‘Well our friend Dirac, too, has a religion, and its guiding principle is “There is no God and Dirac is his prophet”.’ It was an old joke, but everyone laughed, including Dirac.*? The opinions he expressed here, with uncharacteristic forwardness, were entirely in keeping with Kapitza’s views and would not have drawn comment from any of the intelleccuals who were flirting with Bolshevism. Although Dirac never put any of his political views on paper, it was clear from his actions in the coming decade where his sympathies lay. 

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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. 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.

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 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.*? 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.

Since the mid-1920s, Gamow and Landau had been two leaders of the informal group of young Soviet theorists nicknamed the ‘Jazz Band’.>! 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. 

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‘If physics laboratories have to be run dictatorially [. . .] I would rather be my own dictator.’ Blackett carved out a niche in the Cavendish, working with an Italian visitor, Giuseppe Occhialini, a light-hearted Bohemian commonly known by his nickname ‘Beppo’. Ten years younger than Blackett, Occhialini was an exper experimenter who tended to rely on his intuition, rarely pausing to write down an equation, preferring to spell out the steps in his reasoning with an impressive range of accompanying gesticulations, When Occhialini arrived in Cambridge the year before, in July 1931, he had already been involved in experiments to detect cosmic rays and brought to the Cavendish years of experience working with Geiger counters, only recently introduced to Cambridge. These counters were delicate and unreliable, Blackett later remembered: ‘In order to make it work you had to spit on the wire on some Friday evening in Lent.’ For Occhialini, Blackett was a jack of all trades in the laboratory: 

I remember his hands, skilfully designing the cloud chamber, drawing each piece in the smallest detail, without an error, lovingly shaping some delicate parts on his schoolboy’s lathe. They were the sensitive yet powerful hands of an artisan, of an artist, and what he built had beauty. Some of my efforts produced what he called ‘very ugly bits’.

Occhialini often visited Blackett at home in the evening. The two of them would relax in the front room and review their day’s work ovef glasses of lemonade and a plate of biscuits, while Blackett fondled the ears of his sheepdog. During their conversations at home and iS the Cavendish, they came up with a clever way of getting cosmic rays £0 take photographs of themselves: the trick was to place one Geiger counter above their cloud chamber and another counter below it,

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Dirac begins by considering belief. Some of the things a person pelieves 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 srovide a lo: ical 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.’ 

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.... Einstein’s general theory of relativity provided the sturdy theoretical foundations of modern cosmology, but the subject was handicapped by a dearth of reliable data. As a result, theoretical cosmologists had more room for manoeuvre than was good for them and had to rely heavily on intuition. 

Without question, the most successful observational astronomer was the former lawyer Edwin Hubble, an Anglophile American in his mid-forties, given to declaiming on conference platforms in a strangely affected English accent, akin to Oppenheimer’s. Hubble had created a public sensation in 1929 when he suggested that galaxies (aggregates of stars and other matter) do not stay still with respect to one another but are always rushing apart. In what became known as Hubble’s law, he used the data in his charts and tables to propose that the further a galaxy is away from the Earth, the faster the galaxy is moving away from it. This picture of galaxies dashing away from each other was consistent with Lemaitre’s ‘primeval atom’ theory of the origin of the universe, a precursor of the modern theory of the Big Bang.