Vol 459|21 May 2009 BOOKS & ARTS Paul Dirac: a physicist of few words 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.

The Strangest Man: The Hidden Life of Paul Dirac by Graham Farmelo Faber & Faber: 2009. 560 pp. £22.50 BETTMANN/CORBIS Among scientists, Paul Dirac is widely regarded as being in the same league as Albert Einstein. In London’s Westminster Abbey, Dirac’s epony- mous equation describing the quantum behav- iour of electrons is set in stone. But in his home town of Bristol, UK, his reputation is overshad- owed 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 state- ments are legendary. Six years elapsed before even close colleagues learned any of P. A. M. Paul Dirac predicted the anti-electron’s existence, but did little to encourage others to hunt for it. Dirac’s forenames. When he came up with the equations of quantum mechanics, his weekly a sensation once people began to understand at the University of Cambridge, UK, showing postcard home merely said, “Not much to its unusual structure. However, it contained Anderson’s images of particle tracks from report here.” After solving a decades-old prob- puzzling solutions in which the electron had cosmic rays — including some that looked like lem by creating Fermi–Dirac statistics later negative energy. Dirac proposed that a vacuum those of electrons but which curved the wrong that year, the news once more was: “Not much is filled with a sea of negative-energy electrons. way in a magnetic field. No one associated these to report.” When he arrived at the relativistic Any hole in this vacuum would appear as a tracks with Dirac’s holes. quantum equation that describes the elec- positively charged, positive-energy particle. By 1932, the holes had become a joke. At a tron, he didn’t even send a postcard. Even his At first, he thought that this particle was the meeting in Copenhagen, when Bohr lost his colleagues were unaware of it. proton, until J. Robert Oppenheimer pointed patience and confronted Dirac with: “Do you In this elegantly written biography, Far- out that if this were so, the electron and proton believe all that stuff?”, he simply replied, “I melo’s meticulous research sheds considerable could destroy each other and matter would be don’t think anyone has put a conclusive argu- light on Dirac’s personality and the circum- unstable. Wolfgang Pauli was equally scepti- ment against it.” Dirac no longer seemed to stances behind it. Several members of Dirac’s cal, remarking that anyone making a theory be strongly committed to the anti-electron; extended family developed acute depression, of matter should first apply it to the atoms of the absence of the particle was, Farmelo says, six committing suicide within a century, their own body. Pauli went on to prove that the “sapping his morale”. He even told Werner including his brother. His father was cold and positive particle must have the same mass as an Heisenberg that he had ceased to believe in it. authoritarian, his mother overweening — the electron, which was worrying because experi- On 2 August 1932, Anderson found his first description of her excruciating behaviour at menters had not found any such particle. With clear single particle trail, now hailed in text- the Nobel prize ceremony, haranguing journa- the debate unresolved, many began to wonder books as the ‘discovery of the positron’. This lists and officials on behalf of her idolized son, if Dirac’s equation might be wrong. realization was far from clear cut, however. In a is pure entertainment. Dirac immersed him- In 1931, Dirac referred for the first time to the series of missed opportunities, no one seemed self in mathematics. ‘anti-electron’, remarking that it could not occur able to put two and two together to link Dirac’s The received wisdom is that in producing his in nature owing to its immediate destruction by holes and Anderson’s ‘positron’. equation for the electron, Dirac ‘discovered’ the ubiquitous electrons. Although he commented Anderson published his positron paper in concept of antimatter in 1928, and four years that it could be made transiently in experiments, September 1932 in the journal Science. But later, Carl Anderson’s discovery of the positron he was surprisingly circumspect, more con- remarkably, no one in Cambridge seemed to in cosmic rays validated Dirac’s idea. But in cerned with the difficulties of detection than the have read it. By that autumn, British physicist Farmelo’s account the reality is rather different. inevitability of its existence. He made no sugges- Patrick Blackett had his own images of posi- Dirac’s electron equation — declared “ach- tion as to how experimentalists might make it, trons, and had even shown them in a talk with ingly beautiful” by physicist Frank Wilczek — or recognize it. He was away in the United States Dirac and Soviet physicist Peter Kapitsa in the described the spin of the electron, and caused later that year when Robert Millikan gave a talk audience. Dirac stayed silent. Kapitsa exclaimed

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“Now, Dirac, put that into your theory! Positive electrons, eh!” Farmelo comments that Kapitsa The end of the invasion? “had spent hours talking with Dirac but had evi- dently not even heard of the anti-electron” and Invasion Biology foothold only by finding a vacant niche or by that Dirac simply replied “Positive electrons by Mark A. Davis throwing out another species. have been in the theory for a very long time”. Yet Oxford University Press: 2009. 288 pp. $55 Niche theory gives rise to the diversity- there is no sense that Dirac was claiming any- invasibility hypothesis, which posits that the thing, apparently convinced that the positive Ascension Island in the South Atlantic Ocean more species there are in an ecosystem, the trails in the pictures were “a mirage”. Farmelo is a good example of the changes that invasive more niches will be filled and the harder it will sees Dirac as exhibiting “reticence taken to species can wreak. Its volcanic mountain tops be for a new species to become established. the point of perversity”. His colleagues so mis- once hosted a monotonous carpet of ferns. But But the evidence does not bear this out. trusted his abstract theory that they could not in 1843, botanist Joseph Hooker recommended Many studies have failed to find any strong accept that it predicted new particles. that the bleak island be wooded by importing relationship between how diverse a place is The first link between hole theory and the many new plants — what modern ecologists and how easy it is to invade. Davis concludes positron came from Blackett, who showed would see as a massive, human-mediated bio- that, despite its appeal and its “implicit affir- sensational images of electron–positron pair logical invasion. Surprisingly, this resulted not mation of the value of diversity”, the hypothesis creation at a meeting at the Royal Society in in ecological meltdown, but in the creation of a is not true. In fact, the opposite may hold. In London, saying that they “fit extraordinarily lush cloud forest. The forest traps mists, cycles any ecosystem, each individual plant or animal well with Dirac’s hole theory”. Immediately nutrients and survives, generation after genera- has to get a foothold, irrespective of its origin. afterwards, journalists rushed to interview tion, without its species having evolved together. A seed does not care whether it is exotic or native him. Meanwhile Dirac, who was lecturing in A study of this anomalous system is cited in when it lands on the ground, and neither do the another room in the same building, was “una- Mark Davis’s new book Invasion Biology. Why? surrounding species. The key insight is that vailable for comment”. Maybe because it is not so anomalous. there is nothing fundamentally different about According to Farmelo, Dirac later realized Invasion Biology starts out as a graduate-level exotics other than where they came from. that he held responsibility for not having advo- text on how organisms brought cated that experimentalists should hunt for far from their homes by humans positrons, nor advising on how to detect them. can flourish, often at the expense Had he done so, the positron could have been of native species in the places they discovered “in a single afternoon”, as Ander- ‘invade’. But on turning the pages, son put it. When asked later why he did not the book reveals itself to be an K. SCHAFER/ALAMY speak out and predict the positron, Dirac said, iconoclastic argument that much “pure cowardice”. of the field’s conventional wisdom Nonetheless, Dirac on other occasions is wrong, that biologists are more believed that he had predicted it, although not swayed by their emotions about every one agreed. Blackett said: “Dirac nearly invasive species than they care to but not quite predicted the positron.” So much admit, and that invasion biology as for history; today, Dirac’s role in foreseeing the a field should be disbanded. Davis positron, and the mirror world of antimatter, writes, “This may be the first time was, as Farmelo describes it, “one of the greatest that an author has concluded a Ascension Island: not all imported species are destructive. achievements in science”. book, the title of which is the same Farmelo concludes The Strangest Man by as the discipline being reviewed, by recom- Davis challenges other received wisdom, analysing Dirac’s singular character and genius. mending that participants consider abolishing such as the idea that newcomers are more likely He makes a sound case that Dirac was autistic, their discipline.” to compete with or predate on natives than help and argues that his behavioural traits were Davis is not on the fringe. His arguments them flourish, and that introduced populations crucial to his success as a theoretical physicist. crystallize a rumbling of dissent recently heard are unlikely to be genetically diverse. He refuses Cambridge in the 1920s was the ideal environ- among those who study invasive species. As he to exaggerate the differences between natives ment for him: tolerant of eccentricity; college puts it, “There is little about biological invasions and exotics, or to see exotics as the enemy. life providing for his every need; the rules of that make them so unique that a specialized sub- Elton’s 1958 book was an expansion of a dining at High Table enabling a rigidly pre- discipline need be sustained to study them.” series of radio broadcasts aimed at the public. dictable form of social contact. These unusual Invasion biology began in earnest in 1958 Davis speculates that this audience was the circumstances enabled Dirac’s special genius when ecologist Charles Elton published his reason behind Elton’s colourful, militaristic to flower. As to autism, this is thought to be pioneering book, The Ecology of Invasions by comparisons of “ecological explosions” with caused by disrupted brain development, which Animals and Plants (see Nature 452, 34; 2008). bombs. This may have sown the seeds of the can show up as irregularities in brain tissue. Elton saw species ‘invasions’ in the context current ‘good-versus-evil’ rhetoric of species These can be visualized using positron emis- of niches. In an intact, co-evolved ecosystem, invasion, with its talk of biological pollution, sion tomography scans — the medical applica- every species will have a slightly different role, killer weeds and battling garlic mustard. tion of Dirac’s antimatter. Irony indeed. ■ or niche, and often every niche will be filled. Davis is not a fan of such heated rhetoric. Frank Close is professor of physics and a Fellow For example, predators eat herbivores; herbiv- He feels that the dichotomous approach is not at Exeter College, University of Oxford, Oxford, ores eat plants; some plants grow on wet soil ecologically enlightening. Life is much messier, UK. He is the author of Antimatter. and some grow on dry. When new species are more dynamic and more complex, he says. He e-mail: [email protected] introduced, the theory goes, they can get a stuffs the book with examples of exotic species

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