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First Published Friday, 22 May 2009


Paul Dirac

Paul Dirac: a physicist of few words


Frank Close


A detailed biography argues that the Nobel prizewinner’s notorious reticence delayed experimentalists from discovering the antimatter that would confirm his elegant theory, explains Frank Close.


BOOK REVIEWEDThe Strangest Man: The Hidden Life of Paul Dirac


by Graham Farmelo


Faber & Faber: 2009. 560 pp. £22.50


Paul Dirac: a physicist of few words

BETTMANN/CORBIS


Paul Dirac predicted the anti-electron’s existence, but did little to encourage others to hunt for it.


Among scientists, Paul Dirac is widely regarded as being in the same league as Albert Einstein. In London’s Westminster Abbey, Dirac’s eponymous equation describing the quantum behaviour of electrons is set in stone. But in his home town of Bristol, UK, his reputation is overshadowed by that of his fellow student at the Bishop Road School, Archie Leach — better known as the film star Cary Grant. On asking the Bristol Record Office for material about Dirac for his new book, author Graham Farmelo received the response: “Who?”


Danish physicist Niels Bohr described Dirac as “the strangest man”. His extreme reticence, monosyllabic responses and repetitious statements are legendary. Six years elapsed before even close colleagues learned any of P. A. M. Dirac’s forenames. When he came up with the equations of quantum mechanics, his weekly postcard home merely said, “Not much to report here.” After solving a decades-old problem by creating Fermi–Dirac statistics later that year, the news once more was: “Not much to report.” When he arrived at the relativistic quantum equation that describes the electron, he didn’t even send a postcard. Even his colleagues were unaware of it.


In this elegantly written biography, Farmelo’s meticulous research sheds considerable light on Dirac’s personality and the circumstances behind it. Several members of Dirac’s extended family developed acute depression, six committing suicide within a century, including his brother. His father was cold and authoritarian, his mother overweening — the description of her excruciating behaviour at the Nobel prize ceremony, haranguing journalists and officials on behalf of her idolized son, is pure entertainment. Dirac immersed himself in mathematics.


The received wisdom is that in producing his equation for the electron, Dirac ‘discovered’ the concept of antimatter in 1928, and four years later, Carl Anderson’s discovery of the positron in cosmic rays validated Dirac’s idea. But in Farmelo’s account the reality is rather different.


Dirac’s electron equation — declared “achingly beautiful” by physicist Frank Wilczek — described the spin of the electron, and caused a sensation once people began to understand its unusual structure. However, it contained puzzling solutions in which the electron had negative energy. Dirac proposed that a vacuum is filled with a sea of negative-energy electrons. Any hole in this vacuum would appear as a positively charged, positive-energy particle. At first, he thought that this particle was the proton, until J. Robert Oppenheimer pointed out that if this were so, the electron and proton could destroy each other and matter would be unstable. Wolfgang Pauli was equally sceptical, remarking that anyone making a theory of matter should first apply it to the atoms of their own body. Pauli went on to prove that the positive particle must have the same mass as an electron, which was worrying because experimenters had not found any such particle. With the debate unresolved, many began to wonder if Dirac’s equation might be wrong.


In 1931, Dirac referred for the first time to the ‘anti-electron’, remarking that it could not occur in nature owing to its immediate destruction by ubiquitous electrons. Although he commented that it could be made transiently in experiments, he was surprisingly circumspect, more concerned with the difficulties of detection than the inevitability of its existence. He made no suggestion as to how experimentalists might make it, or recognize it. He was away in the United States later that year when Robert Millikan gave a talk at the University of Cambridge, UK, showing Anderson’s images of particle tracks from cosmic rays — including some that looked like those of electrons but which curved the wrong way in a magnetic field. No one associated these tracks with Dirac’s holes.


By 1932, the holes had become a joke. At a meeting in Copenhagen, when Bohr lost his patience and confronted Dirac with: “Do you believe all that stuff?”, he simply replied, “I don’t think anyone has put a conclusive argument against it.” Dirac no longer seemed to be strongly committed to the anti-electron; the absence of the particle was, Farmelo says, “sapping his morale”. He even told Werner Heisenberg that he had ceased to believe in it.


On 2 August 1932, Anderson found his first clear single particle trail, now hailed in textbooks as the ‘discovery of the positron’. This realization was far from clear cut, however. In a series of missed opportunities, no one seemed able to put two and two together to link Dirac’s holes and Anderson’s ‘positron’.


Anderson published his positron paper in September 1932 in the journal Science. But remarkably, no one in Cambridge seemed to have read it. By that autumn, British physicist Patrick Blackett had his own images of positrons, and had even shown them in a talk with Dirac and Soviet physicist Peter Kapitsa in the audience. Dirac stayed silent. Kapitsa exclaimed “Now, Dirac, put that into your theory! Positive electrons, eh!” Farmelo comments that Kapitsa “had spent hours talking with Dirac but had evidently not even heard of the anti-electron” and that Dirac simply replied “Positive electrons have been in the theory for a very long time”. Yet there is no sense that Dirac was claiming anything, apparently convinced that the positive trails in the pictures were “a mirage”. Farmelo sees Dirac as exhibiting “reticence taken to the point of perversity”. His colleagues so mistrusted his abstract theory that they could not accept that it predicted new particles.


The first link between hole theory and the positron came from Blackett, who showed sensational images of electron-positron pair creation at a meeting at the Royal Society in London, saying that they “fit extraordinarily well with Dirac’s hole theory”. Immediately afterwards, journalists rushed to interview him. Meanwhile Dirac, who was lecturing in another room in the same building, was “unavailable for comment”.


According to Farmelo, Dirac later realized that he held responsibility for not having advocated that experimentalists should hunt for positrons, nor advising on how to detect them. Had he done so, the positron could have been discovered “in a single afternoon”, as Anderson put it. When asked later why he did not speak out and predict the positron, Dirac said, “pure cowardice”.


Nonetheless, Dirac on other occasions believed that he had predicted it, although not everyone agreed. Blackett said: “Dirac nearly but not quite predicted the positron.” So much for history; today, Dirac’s role in foreseeing the positron, and the mirror world of antimatter, was, as Farmelo describes it, “one of the greatest achievements in science”.


Farmelo concludes The Strangest Man by analysing Dirac’s singular character and genius. He makes a sound case that Dirac was autistic, and argues that his behavioural traits were crucial to his success as a theoretical physicist. Cambridge in the 1920s was the ideal environment for him: tolerant of eccentricity; college life providing for his every need; the rules of dining at High Table enabling a rigidly predictable form of social contact. These unusual circumstances enabled Dirac’s special genius to flower. As to autism, this is thought to be caused by disrupted brain development, which can show up as irregularities in brain tissue. These can be visualized using positron emission tomography scans — the medical application of Dirac’s antimatter. Irony indeed.




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