The shy gazelle of anti-matter

Paul Dirac
March 10, 2000

The English physicist Paul Adrien Maurice Dirac was not only one of the founders of quantum theory but also the discoverer of anti-matter. According to Peter Goddard in his preface to this collection of four lectures celebrating Dirac's life and work, Dirac should be mentioned alongside Newton, Maxwell, Einstein and Rutherford as one of the greatest physicists of all time.

It is therefore all the more puzzling that the name of Dirac is not better known to the general public. It is true that Dirac determinedly avoided the public gaze: he was persuaded to accept the 1933 Nobel prize only after Rutherford told him that "a refusal will get you much more publicity". Of course, winning the Nobel prize inevitably leads to some publicity, but faced with Dirac's self-effacing personality, the London papers were reduced to characterising him as "shy as a gazelle and modest as a Victorian maid".

A memorial plaque to Dirac has now been erected in Westminster Abbey, and Stephen Hawking's memorial address is reprinted here. Hawking tries to explain why it took so long for the nation to recognise that Dirac "was probably the greatest theoretical physicist since Newton". One of Hawking's most telling remarks is that he never once met Dirac in the seven years they were both in the same department in Cambridge. As Hawking says, Dirac did not believe in "new-fangled departments" and chose to work in isolation in his rooms in St John's College.

The lectures are arranged in order of accessibility. First comes an account of Dirac's life and work by that consummate professional, Abraham Pais, who not only had a successful career as a theoretical physicist but also a second one as the definitive biographer of Einstein and Bohr. Pais knew Einstein, Bohr and Dirac personally, in Copenhagen before the second world war, and later at Princeton, when they were in what may uncharitably be described as the twilight of their research careers. As ever, Pais skilfully blends historical fact with an appreciation of Dirac's scientific contributions, leavening the mix with a judicious sprinkling of anecdotes.

After first studying electrical engineering at Bristol, Dirac enrolled at Cambridge as a research student in 1923. Nine years later, he was offered the Lucasian chair of mathematics at Cambridge, once held by Newton. In the intervening years Dirac had made major contributions to the foundations of quantum mechanics, rising from obscure graduate student to the scientific equal of Heisenberg and Schrodinger. Yet for all the originality of his pioneering work in quantum theory, Dirac is undoubtedly best remembered for his equation describing the motion of an electron moving with relativistic velocity.

Dirac's equation was born in 1928, into a world in which there were only two known elementary particles: the proton and the electron. Pais tells how Dirac was eventually bold enough to suggest that the puzzling negative energy states of his equation actually signalled the existence of an anti-electron, the positron. Maurice Jacob then takes up the story and describes the amazing things physicists are now able to do with antimatter. Dirac's once-debatable positrons are now used for tumour detection in more than 100 positron emission tomography centres worldwide.

The final two lectures are more mathematical in nature. In the first, David Olive describes another of Dirac's creations, the magnetic monopole. While no such single magnetic charges have yet been observed, Olive describes the modern role of monopoles in the context of gauge-field theories.

In the last lecture, Michael Atiyah describes the profound implications of the Dirac equation for geometry and topology. These two lectures will be hard going for non-theoretical physicists, but taken as a whole, this collection succeeds admirably in capturing the unique scope of Dirac's physics and in giving some insight into Dirac the man.

Finally, many famous Dirac stories - illustrating his passion for precision - are recounted in this collection. My favourite concerns Dirac and Heisenberg taking a boat trip to Japan in 1929. In the evenings Heisenberg liked to dance, and Dirac, who did not, would sit in a chair and observe the dancers. Heisenberg's story goes as follows: "Once I came back from a dance and took the chair beside him, and he asked me:'Heisenberg, why do you dance?' I said:'Well, when there are nice girls it is a pleasure to dance.' He thought for a long time about it, and after about five minutes he said: 'Heisenberg, how do you know beforehand that the girls are nice?'"

A. J. G. Hey is professor of computation and dean of engineering, University of Southampton.

Paul Dirac: The Man and His Work

Editor - Peter Goddard
ISBN - 0 521 58382 9
Publisher - Cambridge University Press
Price - £12.95
Pages - 124

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