The greatest British physicist since Newton

The Strangest Man: The hidden life of Paul Dirac, quantum genius

By Graham Farmelo

Published by Faber and Faber, 2009, £9.99

Reviewed by

Geoff Jones

THE MODERN technological world around us – television, computers, the internet, GPS, DNA profiling, lasers, medical scanners – all stems from the work of a small number of brilliant physicists in the first half of the twentieth century. Of these, four stand out above the rest: Einstein, Heisenberg, Schrödinger and Dirac. Everyone has heard of Einstein. Heisenberg (of the uncertainty principle) and Schrödinger (of the cat) are also familiar names. But who was Dirac? Farmelo’s book aims to answer the question.

Paul Dirac (born 1902) was brought up in Bristol in an unhappy family, ruled by a tyrannical father, a schoolteacher of Swiss background. At 16, he studied engineering at Bristol University where his outstanding mathematical talent soon became evident. Still in his teens, he was reading Einstein’s theory of relativity which only one or two physicists in Britain claimed to understand. At 18, he won a scholarship to Cambridge, but his father denied him funding to take it up. He therefore took a mathematics degree at Bristol in two years, then gained a Cambridge scholarship to do postgraduate work in theoretical physics. In Cambridge, Dirac cut a lonely figure. Utterly taciturn, only answering precisely framed questions with short exact answers in his strong Bristol accent, he was nevertheless recognised at once as a brilliant scientist.

In the 1920s, the world of physics had been turned upside down. First, Einstein’s theory of relativity had brought into question the whole nature of time. Second, experiments had shown classical theory to be completely unable to explain the structure and properties of atoms. The idea that energy must be ‘quantised’ – only absorbed or emitted in minute but definite amounts – appeared unavoidable. But how to frame a theory to describe this, the way Newton’s theory described the motion of bodies like stars and planets?

The German physicist, Werner Heisenberg, and Austrian Jewish, Erwin Schrödinger, put forward possible but apparently contradictory approaches. Dirac was able to demonstrate that the two pictures were just two equivalent ways of looking at the same theory and to formulate that theory in a clear and concise fashion – quantum mechanics. In 1930, he published a textbook, Principles of Quantum Mechanics, which is still in print. Dirac’s analysis led him to predict the existence of particles identical to the electron but positively charged – positrons – which were observed experimentally a few years later. Most important, he managed to marry quantum mechanics with Einstein’s special theory of relativity to produce an equation which bears his name. He was awarded the Nobel Prize in 1933.

But the scientific arguments and debates of the 1920s and 1930s were not isolated from the political battles taking place outside the laboratory, even for scientists as unsociable as Dirac. In fact, they were intimately connected not just in the lives of the physicists themselves but in the whole direction physics was to take. In Britain, many academics, Dirac included, were desperately concerned about mass unemployment and the rise of fascism. In Cambridge – where students had mobilised to break the 1926 general strike – the late 1920s and early 1930s saw the growth of socialist ideas.

Many scientists looked towards the Soviet Union as a model of a better society – a planned economy seemed a positive alternative to the depression gripping the west and among Dirac’s few friends at Cambridge was Peter Kapitza, a Russian physicist. Dirac himself used his Nobel speech in 1933 to call for redistribution to provide goods for all, despite the fact that speeches are expected to be non-controversial. He visited the USSR several times in the 1930s with Kapitza, made friends with other leading Russian physicists, and was elected to the Soviet Academy of Sciences. But when, in 1937, Stalin cracked down on Kapitza, refusing to allow him to return to Cambridge and putting him under virtual house arrest, Dirac organised a petition of leading physicists and lobbied the Russians, leading to an odd compromise where Kapitza’s whole laboratory was shipped from Cambridge to a new institute in Moscow purpose-built for him!

As the crisis of the 1930s slipped into war, physicists scattered. Einstein fled to the USA. Schrödinger left Austria and finally settled in neutral Dublin. Heisenberg remained in Germany and continued working but not openly supporting Hitler, unlike some leading German scientists who became avid Nazis. Back in Cambridge, Dirac organised support for Jewish scientists fleeing from Germany. Horrified even before the war by the treatment of Jews in Germany, he resolved never to speak German again, except to refugees. He took part in the programme to build a nuclear bomb, devising a method of isotope separation and carrying out calculations on chain reactions.

But such changes were only part of a major change in the whole structure of physics. Until the 1930s, physics was a small international ‘cottage industry’, isolated from world events, with individuals across the world from Denmark to Japan corresponding, meeting, sharing results and arguing. But as well as the rise of Hitler, 1933 saw the invention of the cyclotron – ancestor of today’s Large Hadron Collider (LHC). This marked the start of ‘big physics’, its absorption in national and international economic and political power struggles, and the beginning of an extreme specialisation among physicists. Rather than following their independent research interests, they were now seen as part of the nation’s economic capital and their research areas directed by government.

Dirac, as a pure theoretician, was mainly immune to these trends – he resisted a wartime invitation to join the code breakers at Bletchley. His quantum theory was extended by a younger generation of physicists who had cut their teeth on the atom bomb project. Richard Feynman, the flamboyant US theoretical physicist who shared the 1965 Nobel Prize, often said with uncharacteristic modesty: "I’m no Dirac". Dirac himself believed that much of his theoretical work had reached a dead end. Indeed, near the end of his life he was quoted as saying: "My life has been a failure". Nevertheless, he stood head and shoulders above other physicists of his generation. Suggestions that he threw off almost in passing pointed to quantum optics and even to the idea that electrons might be one-dimensional ‘strings’.

Although practically unknown to the general public, Dirac was honoured by the physics community. He was Lucasian Professor of Mathematics at Cambridge, a post held more recently by Stephen Hawking. He was offered a knighthood, which he refused, but later accepted the Order of Merit presented by Queen Elizabeth. When asked about his impressions of her, his characteristically brief comment was merely: "Very small".

Since Dirac’s death in 1984, physics has changed again. Experimentation, especially in fundamental research, now requires machines so expensive that no individual state can afford to fund them. The LHC cost an estimated $6 billion and is the result of collaboration between over 20 countries. The internet enables physicists around the world to share results and arguments more quickly and easily than in the past. At the same time, the question of how quantum theory describes reality is being raised once again. The view generally accepted today had been bitterly opposed by Einstein in the 1930s. Dirac was never happy with the consensus, as is well summed up by a wry comment: "I listened to their arguments but I did not join in them, essentially because I was not very interested… I was more interested in getting the correct equations… It seems clear that quantum mechanics is not in its final form… I think it very likely, or at least quite possible, that in the long run Einstein will turn out to be correct, even though for the time being physicists will have to accept the Bohr probability interpretation, especially if they have examinations in front of them".

Farmelo’s book is not without its faults: it dwells in detail on Dirac’s family, marriage and health, for example, while saying little on his political thinking, and Farmelo is clearly not sympathetic to socialist ideas. It is quite likely that Kapitza introduced Dirac to Marxist theory. A committed atheist, Dirac was very impatient with references to God, regarding the idea as "a product of the human imagination", and taught only "to keep the lower classes quiet". On balance, however, this is a thorough and highly readable biography of the man in his period and can be recommended to scientists and non-scientists alike. The book has been awarded the Costa Biography Prize so should be readily available in libraries.