In other words, he gives us the abstract mathematical scheme that describes how nature behaves, without explaining the earlier history of physical ideas out of which this scheme arose. In those days I wrote a letter to my parents once a week, so I have a contemporary record of the impact of his lectures. I wrote in February 1942:
Dirac reached a climax of difficulty in his last lecture. [Another professor] has promised to invite me next term to a tea-party with Dirac. Dirac is a man I should like to talk to,…but he lectures like a gramophone record and nobody seems to know him at all.
There is no record of the tea party with Dirac, if it ever happened. But nine months later there was another tea party, which I recorded on November 30, 1942:
There were present also two young Diracs, by name Gabriel and Judith. Gabriel is 17 and a first-year undergraduate from St. John’s, Judith is 15 and at a school in Cambridge. They are Hungarians by upbringing, and know a lot about Central Europe; also they used to row on the Danube with von Neumann the great topologist, an almost legendary figure now in America. Gabriel is a very ardent member of the communist party, and thus kept the ball rolling from the start. The young Dirac is reading mathematics but is at present more interested in politics.
Gabriel and Judith were the step-children who moved into Dirac’s life when he married their mother in 1937. As teenagers they were outstandingly bright and lively. When I met them at the tea party, the Battle of Stalingrad was raging, the Soviet Union was our gallant ally bearing the main brunt of the war against Hitler, and for a bright teenager to be a Communist was not unusual. Dirac himself was not a Communist, but he was strongly socialist. He had traveled many times to Russia, where some of his closest friends were living, and he had been welcomed by the Soviet government as a visitor even when his friends were in political trouble.
Six years later, when I arrived at the Institute for Advanced Study in Princeton for the first time, I encountered Dirac with his own daughters, Mary and Monica, then aged eight and six. Here was the scene, dated September 14, 1948:
When I visited the Institute there were more children there than grown-ups, Dirac with his family shortly leaving for England, and various other children playing cowboys and Indians, and Von Neumann looking rather vague in the midst of the confusion.
This vivid scene, with Mary and Monica chasing each other around the table in the middle of the institute common room, was very different from the staid formality of college common rooms in Cambridge to which both Dirac and I were accustomed.
When Dirac approached the age of retirement from his professorship in Cambridge, Manci decided that the time had come to move him to America. Mary and Monica were both married and settled there, and Manci had had enough of England. Manci took Dirac to Tallahassee, where Mary was living, and the Physics Department of Florida State University offered him a job as “visiting eminent professor.” Dirac accepted the offer, and lived the last thirteen years of his life in Tallahassee as an honored sage, becoming more sociable and more talkative as he grew older. Manci was happy to open her house to a stream of American friends who came to escape the northern winters and to talk with her famous husband. Manci and Dirac understood each other’s needs and achieved a certain serenity in their old age.
Farmelo ends his book with two provocative chapters, one entitled “On Dirac’s Brain and Persona” and the other “Legacy.” Each of them raises intriguing questions. The questions are perhaps unanswerable. Farmelo gives them tentative answers based on his own opinions. I shall explain why I give them different answers, equally tentative, based on my opinions. The chapter on Dirac’s brain and persona asks whether he was autistic. Autism was until recently a rare disease, characterized by mental disorders that made the patient incapable of a normal life. The main symptom was a failure to achieve or understand social relationships with other human beings. This was often accompanied by failure to speak or by severe retardation of speech. The typical autistic child was obsessed with repetitive activities, resistant to any change in established routines, and uninterested in communication with family or friends.
Judged by these criteria, Dirac was clearly not autistic. My wife found him a friendly and amusing companion when she went for a walk with him in Princeton. He was intensely and personally involved with his physicist friends such as Kapitza, Heisenberg, and Bohr. He had close friendships with at least three women before he married Manci. And he had normal fatherly relationships with his stepchildren and children. If Dirac was autistic, then the word “autism” must have a different meaning.
During the last twenty years the concept of autism has been broadened so as to include a much wider variety of people. Debates have raged among the experts concerning the criteria for a medical diagnosis. The phrase Autistic Spectrum Disorder has been introduced to give official recognition to the wider meaning of autism. As a result, autism is now no longer rare, and it includes many people who can function normally in society but have the classic symptoms of autism in milder form.
To be autistic in the wider sense, it is enough to be insensitive to other people’s feelings, to be more interested in things than in people, and to be intensely interested in things that normal people find bizarre. The diagnosis of Autistic Spectrum Disorder includes a wide spectrum of disability, from mute and severely retarded people languishing in mental institutions to articulate and gifted university professors living active professional lives. Temple Grandin is a famous example of the high-functional autistic: a professor of engineering, author of several books, and world-class expert on the design of buildings and machines for the humane handling of livestock. Farmelo describes two aspects of Grandin’s autism—her sensitivity to sudden sounds and the visual nature of her thought processes—and makes a case for Dirac’s autism by analogy.
The phrase “Asperger syndrome” is an alternative name for the high-functional end of the autistic spectrum. Asperger was an Austrian psychologist who studied children who were socially inept but intellectually sharp. Several parents of my acquaintance are proud to claim that their gifted children “have a touch of Asperger” when the children develop a passion for painting or for mathematics. Asperger syndrome has become a distinction rather than a disease. If every child who is silent and withdrawn and has an unusual talent has a touch of Asperger, then Dirac certainly had a touch of Asperger. If Asperger syndrome is included in the autistic spectrum, then Farmelo is justified in concluding that Dirac was autistic.
The definition of autism is today based on symptoms that are poorly defined and on medical judgments that are largely subjective. That is why the posthumous diagnosis of Dirac as autistic or nonautistic is a matter of opinion. But in the future, an objective diagnosis may become possible. We have strong evidence that autism is associated with anatomical abnormalities in the brain, and that autism is heritable. When this evidence from neurology and genetics has been consolidated, it is possible that the diagnosis of autism based on symptoms will be replaced by a diagnosis based on the objective observation of brains and genomes. At some time in the future, when the intricacies of brains and genomes are understood in detail, a reliable posthumous diagnosis of Dirac’s personality may become possible, provided that some fragment of tissue carrying his DNA has been preserved.
Since the era of diagnosis based on DNA has not yet arrived, I base my tentative diagnosis on anecdotal evidence. Two anecdotes in Farmelo’s book strike me as strong evidence that Dirac’s peculiarities had nothing to do with autism. Both episodes occurred before he married Manci and became domesticated. In 1935, when Kapitza and his wife were detained in Russia, their two sons were left behind in England, and the Kapitzas appointed Dirac as legal guardian of the boys. Dirac took care of the boys until they joined their parents in Russia. While the boys were with him, Guy Fawkes Day occurred, the English equivalent of July 4, traditionally celebrated with bonfires and fireworks. Dirac organized a fireworks show for the boys.
The second anecdote concerns another famous Russian physicist, George Gamow, who was also a close friend of Dirac. Gamow was notorious as a practical joker. He had emigrated from Russia and settled in Washington with his wife. Dirac was traveling in Florida and saw some alligators for sale. He decided to give Gamow a taste of his own medicine, bought a baby alligator, and mailed it anonymously in a parcel to Washington. The joke succeeded even better than Dirac intended. Gamow’s wife opened the parcel, was seriously bitten by the alligator, and accused her husband of perpetrating the joke. Dirac had hit two birds with one stone. He let a month pass before confessing that he was the guilty party. These two stories show us Dirac as he was in his thirties, a young man with a fondness for children and a robust sense of humor, very far from the pathological self-absorption that is the basic symptom of autism.
The last chapter of Farmelo’s book concerns Dirac’s legacy to later generations. The legacy consists of three parts, first the laws of nature that Dirac discovered in his wonder years from 1925 to 1933, second the doctrine of mathematical beauty that he preached for the remaining fifty years of his life, and third his distaste for philosophical interpretation of his discoveries. For practicing scientists, the chief legacy of Dirac is the starburst of discoveries that he made as a young man. His legacy to nonscientists is not so clear. Farmelo emphasizes the doctrine that he preached later, proclaiming that mathematical beauty is the key to scientific truth. In order to discover the true laws of nature, the searcher after truth should pay more attention to abstract beauty than to practical details. The beauty and simplicity of the laws of nature would be revealed in abstract mathematics. The second legacy is summarized in a statement that Dirac wrote at the end of his life: “If you are receptive and humble, mathematics will lead you by the hand.”
The doctrine of mathematical beauty is itself beautiful, and there is no doubt that Dirac believed it to be true. But it does not agree well with the historical facts. During the wonder years when he was making his great discoveries, his thinking was more concerned with practical details and less with abstract beauty. And during the long second half of Dirac’s life, when he was preaching the doctrine of mathematical beauty, it did not lead him to important new discoveries.
During Dirac’s middle years, the grand edifice of modern particle physics was growing up around him, with discoveries of new particles and new symmetries bursting out in rapid succession. Nature was screaming at him to pay attention to her revelations. But his love for abstract beauty told him to stay aloof. He ignored the new discoveries of particles and symmetries, because he judged them to be too complicated, not beautiful enough to be true. Instead of listening to Nature, he was telling Nature how to behave. As a result, the second half of his life was comparatively sterile.
In addition to the two legacies of discoveries and aesthetic principles, Dirac also left a third legacy, which I consider precious but Farmelo does not. This legacy is Dirac’s refusal to engage in philosophical arguments about the interpretation of quantum mechanics. These philosophical debates raged during his lifetime and have raged even more fiercely after his death. Dirac took no part in these debates and considered them to be meaningless. He said, as Galileo said three hundred years earlier, that mathematics is the language that nature speaks. When expressed in mathematical equations, the laws of quantum mechanics are clear and unambiguous. Confusion arises from misguided attempts to translate the laws from mathematics to human language.
Human language describes the world of everyday life, and lacks the concepts that could describe quantum processes accurately. Dirac said we should stop arguing about words, stay with mathematics, and allow the philosophical fog to blow away. I consider Dirac’s disengagement from verbal disputes about the meaning of quantum mechanics to be an essential part of his legacy. But I am, as usual, in the minority.