Paul Dirac, still in his 20s and already recognized as one of the most brilliant theoretical physicists in the world, has just finished giving a lecture to colleagues. Someone in the audience raises his hand and says: "I don't understand the equation on the top-right-hand corner of the blackboard." Dirac says nothing. The audience shuffles nervously. After a long interval of uneasy silence, the moderator asks Dirac if he wants to answer the question. Dirac laconically replies: "That was not a question, it was a comment."
This is just one of the anecdotes that led the great Danish physicist Niels Bohr to call Dirac "the strangest man," a remark that gave Graham Farmelo the title of this superb biography.
Dirac's contributions to the birth and development of quantum physics in the period between the two world wars were many and fundamental. He was the first to bring together quantum physics and relativity. The famous Dirac equation - "achingly beautiful," according to one physicist - describes the behaviour of every electron, proton and neutron that ever existed. The equation predicted the existence of antimatter, which was subsequently observed by experimentalists.
Dirac also predicted the existence of magnetic monopoles, exotic particles widely accepted by theorists but still waiting to be discovered. Physicist Freeman Dyson said of Dirac: "His great discoveries were like exquisitely carved marble statues falling out of the sky, one after another. He seemed to be able to conjure laws of nature from pure thought."
Dirac shared the 1933 Nobel Prize for physics with Erwin Schrödinger. Always reticent, he accepted the prize - the youngest theoretician ever to get it - only when his mentor, Ernest Rutherford, assured him that "a refusal will get you more publicity."
"Strange" is the word that came often to the lips of those who knew him. He seldom spoke unless asked a direct and sensible question, and then his replies were terse, even monosyllabic. During his most creative years, he lived to work, six days a week, disdaining ordinary social contact, reserving Sundays for long walks in the countryside. He cared little for material possessions (he wore the same overcoat for half a century). Even the people who knew him best had little idea what was going on inside his head. Albert Einstein said to a friend: "I have trouble with Dirac. This balancing on the dizzying path between genius and madness is awful."
It is tempting to call Dirac an eccentric, and many did. This may be too easy. Some years ago, neuropsychologist David Weeks did a study of eccentrics when he realized there was nothing on the subject in the scientific literature. He discovered that eccentrics are non-conforming, intelligent, creative, strongly motivated by curiosity, idealistic, gleefully obsessed, non-competitive, possessed of a mischievous sense of humour - and, of course, completely loony. But not, according to Weeks, mentally ill or in need of a cure. In general, the hundreds of eccentrics Weeks interviewed and tested in Britain and the United States were happier than the rest of us.
This almost, but not quite, describes Dirac. Certainly he was not gleeful or manifestly happier than the rest of us, and his work was very much in the mainstream of physics. Dirac laid the blame for his bizarre social behaviour on his Swiss-born father, Charles, a control-freak husband and parent with whom Dirac had a fraught relationship. The family situation as Paul was growing up was dysfunctional. His brother, Felix, was an early suicide.
Farmelo strongly suggests that Dirac was autistic, and that perhaps his father was too. The relationship between Paul and Charles was, in Farmelo's guess, doomed by nature rather than nurture: "The young Dirac was born to be a child of few words and was pitiably unable to empathize with others, including his closest family."
Whatever the cause of Dirac's oddness, he was able to make a few close friends, including Russian physicist Peter Kapitza, and, though showing no particular interest in women, married and fathered two children.
Those who are interested in the psychology of genius will find Dirac's story, as told by Farmelo, compelling. The book is also a wonderful romp through the golden age of quantum physics. The cast of characters will be familiar by name to many readers: Einstein, Rutherford, Bohr, Schrödinger, Heisenberg, Fermi, Oppenheimer and Feynman, to name just a few. And the story is set against a background of epochal world events: the regimes of Hitler and Stalin, the Second World War and the atomic bomb.
One of the more interesting themes to run through this book is the contrast of two ways of doing physics, which might be called "bottom-up" and "top-down." In the former, one gathers data by experiment, then looks for a mathematical theory that describes the data. In the latter, one contrives beautiful mathematical theories, then looks to see if nature conforms. Dirac was very much in the "top-down" camp. His confidence in his theories derived exclusively from their mathematical elegance.
Farmelo writes: "[W]at is most remarkable about the story of antimatter is that human beings first understood and perceived it not through sight, smell, taste and touch, but through purely theoretical reasoning inside Dirac's head."
The same might be said about Einstein's theory of general relativity, which was conceived in his head before any experiment indicated its necessity, and today's string theory, which is awaiting empirical verification. In the end, of course, observation is the ultimate test of any theory.
Dirac outlived almost all of his contemporaries, spending the last 13 years of his life at Florida State University, still strange, still revered as an intellectual giant of his times, but no longer at the cutting edge of his field. He died in 1984 at the age of 82, lamenting that his life was a failure. This excellent biography is worthy of its remarkable subject.
Chet Raymo is the author of a dozen books on science and nature. He resides on the Web at www.sciencemusings.com.