Why should anyone who is not a physicist be interested in Paul Dirac? Dirac is interesting for the same reasons that Einstein is interesting. Both made profound discoveries that changed our way of thinking. And both were unique human beings with strong opinions and strong passions. Besides these two major similarities, many details of their lives were curiously alike. Both won the Nobel Prize for physics, Einstein in 1921 and Dirac in 1933. Both had two children of their own and two stepchildren from a wife’s previous marriage. Both were intensely involved in the community of professional scientists in Europe when they were young. Both of them emigrated to the United States and became isolated from the American scientific community when they were old. The main difference between them is the fact that Einstein was one of the most famous people in the world while Dirac remained obscure.
There are many reasons why Einstein became inordinately famous. The main reason is that he enjoyed being famous and entertained the public with provocative statements that made good newspaper headlines. Dirac had neither the desire nor the gift for publicity. He discouraged inquisitive journalists by remaining silent. Einstein has had dozens of books written about him, while Dirac has only two, Dirac: A Scientific Biography by Helge Kragh, published in 1990, and this new biography by Graham Farmelo. The Kragh biography is full of equations and is addressed to experts only. The enormous fame of Einstein and the obscurity of Dirac have given the public a false picture of the two revolutions that they led. The public is aware of one revolution and correctly gives credit for it to Einstein. That was the revolution that changed the way we think about space and time. The new way of thinking was called relativity.
The second revolution that came ten years later was more profound, and changed the way we think about almost everything, not only in physics but in chemistry and biology and philosophy. It changed the way we think about the nature of science, about cause and effect, about past and future, about facts and probabilities. This new way of thinking was called quantum mechanics. The second revolution was led by a group of half a dozen people including Einstein. It does not belong to a single leader. But the purest and boldest thinker of the second revolution was Dirac. If we wish to give the second revolution a human face, the most appropriate face is Dirac. Farmelo writes that “in one of his greatest achievements,” Dirac arranged
what had seemed an unlikely marriage—between quantum mechanics and Einstein’s theory of relativity—in the form of an exquisitely beautiful equation to describe the electron. Soon afterwards, with no experimental clues to prompt him, he used his equation to predict the existence of antimatter, previously unknown particles with the same mass as the corresponding particles of matter but with the opposite charge. The success of this prediction is, by wide agreement, one of [the] most outstanding triumphs of theoretical physics.
In Farmelo’s book we see Dirac as a character in a human drama, carrying his full share of tragedy as well as triumph. He is as strange a figure as Einstein. He is less famous because he preferred to fight his battles alone.
The title, The Strangest Man: The Hidden Life of Paul Dirac, Mystic of the Atom, is not well chosen. The English edition published by Faber and Faber has a better title, with “Mystic of the Atom” replaced by “Quantum Genius.” Mystic and genius are not synonymous. The phrase “The Strangest Man” is a quote from Niels Bohr, the great Danish physicist who invited Dirac to visit his Institute for Theoretical Physics in Copenhagen in 1926 when Dirac was twenty-four years old. Bohr said many years later that Dirac was the strangest man who had ever come to his institute.
It was true that Dirac’s inner life was well hidden. He did not like to reveal what he was thinking, either about science or about himself. But Bohr did not say that Dirac was a mystic, and it is not true. Dirac was the opposite of a mystic. He worked in a straightforward way, trying out mathematical schemes to describe the way nature behaves. What was strange about Dirac was not mysticism but formidable concentration of attention upon a single problem. He was silent and aloof because he liked to think about one thing at a time. In his choice of problems to think about, he was guided by his ability to set aside irrelevancies, to see clearly what was important and what was not. For him, most of the subjects that people talk about in everyday conversation were irrelevant, and so his conversation was mostly silence.
Although Dirac spoke little about himself, he preserved almost all the letters and papers that he received from his family and friends, all the way back to his childhood. These papers are now accessible in the Dirac archive at Florida State University. They provide a solid documentary basis for Farmelo’s biography. In spite of Dirac’s legendary silence, we know more about his early life than we know about his more talkative contemporaries. Farmelo also interviewed everyone still alive who had known Dirac, and obtained detailed accounts of rare conversations in which Dirac as an old man talked at length about his youth. The most dramatic of these conversations was reported by Kurt Hofer, a biologist colleague of Dirac at Florida State University. Farmelo places Hofer’s story at the beginning of his biography, emphasizing its importance for the understanding of Dirac’s struggles. The truth of the story is confirmed by other witnesses and documents.
Paul Dirac’s father was Charles Dirac, a Swiss schoolteacher who taught modern languages in the English city of Bristol. He was a capable but harsh teacher. Paul’s mother, Florence, was twelve years younger and dominated by her husband. Paul had an older brother, Felix, and a younger sister, Betty. According to Hofer, Charles made Paul’s life miserable by insisting that he speak only French at home and punishing him when he made grammatical mistakes. Since talking brought punishment, Paul acquired the habit of silence. Charles was habitually unfaithful to Florence, and the pair were barely on speaking terms. Paul was close to his mother, Betty to her father, and Felix to neither. Felix suffered acutely from comparisons with his brilliant brother. When Paul was twenty-three and Felix twenty-five, Felix killed himself. By that time Paul had escaped from the hate-filled home and enrolled with a scholarship at St. John’s College, Cambridge. Paul’s hatred of his father endured until he talked with Hofer more than fifty years later. Paul said, “I never knew love or affection when I was a child,” and speaking of his father, “I owe him absolutely nothing.”
In spite of these inner torments, Dirac had a remarkable talent for friendship. His closest friend at Cambridge was Peter Kapitza, the charismatic Russian experimental physicist who later won a Nobel Prize for discovering the superfluidity of liquid helium. Kapitza then lived and worked in England but spent his summers in Russia. Dirac several times went to Russia for long holidays with Kapitza and other Russian friends, climbing mountains and enjoying the comforts of Kapitza’s country club in the Crimea. Dirac also tried his hand at experimental work in Kapitza’s laboratory.
In 1934 Stalin decided to keep Kapitza in Russia and forbade him to return to England. Dirac then traveled to Russia in an unsuccessful campaign to get Kapitza out. Kapitza was depressed, and Dirac stayed for several weeks at his dacha to restore his spirits. In the end, a deal was arranged so that Kapitza stayed in Russia and the Soviet government paid for all of his experimental equipment to be shipped from Cambridge to Moscow. He could then continue his experimental work in Russia as director of his own institute. Dirac and Kapitza remained friends at a distance, and enjoyed a happy reunion in Cambridge thirty years later. Kapitza was a great talker and Dirac was a great listener, so they were well matched.
Dirac was also a faithful friend to several of the other pioneers of quantum mechanics, particularly Niels Bohr and Werner Heisenberg. Heisenberg stayed in Germany through World War II. Although he disliked Hitler, he was a patriotic German and considered it his duty to serve his country and share its fate. After the war, he suffered grievously from the hostility of former friends who never forgave him for leading the German Uranium Project, an abortive effort that never came close to developing a nuclear bomb. Dirac went out of his way to be friendly to Heisenberg, saying that Heisenberg had behaved reasonably in an extremely difficult situation. Dirac had seen his friends in Russia forced to make difficult choices under a capricious government, and understood the pressures under which they lived. He said, “It is easy to be a hero in a democracy.”
Dirac’s talent for human relationships was shown most spectacularly in his marriage to Manci Balasz, which lasted for forty-seven years until his death. Manci was a temperamental Hungarian widow, accustomed to an aristocratic lifestyle. Dirac was a quiet fellow who enjoyed the company of outgoing people. With their opposite qualities they were well matched, just as Dirac and Kapitza had been. Manci took care of Dirac and organized his social life. He enjoyed his stepchildren and his children, letting them run free and be themselves, avoiding the harsh treatment that had alienated him from his own father. He enjoyed working long hours in his garden, growing flowers in peacetime and vegetables in wartime. He enjoyed listening quietly while Manci and her friends talked.
Dirac and Manci lived together in Cambridge for thirty-four years, most of the time amicably. Their most serious disagreements resulted from the fact that Dirac loved the quiet routine of Cambridge while Manci found it boring. According to legend, there was a time when Manci became infuriated as she was serving dinner and said to Dirac, “What would you do if I left you?” After an interval of silence, Dirac replied calmly, “I’d say ‘Goodbye dear.’”
I enjoyed a firsthand glimpse of Dirac when I came to listen to his lectures as a seventeen-year-old undergraduate at Cambridge. Like Niels Bohr in Copenhagen fifteen years earlier, I found him strange. That was in 1941, for England the third year of World War II. Because of the war the number of students was small, but Dirac lectured faithfully to his little band of listeners every Monday, Wednesday, and Friday morning. His lectures were mostly a verbatim recitation of his book The Principles of Quantum Mechanics. In the introduction to that book he says:
This has necessitated a complete break from the historical line of development, but this break is an advantage through enabling the approach to the new ideas to be made as direct as possible.