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At the turn of the millennium in the year 2000, a senior Italian physicist reflected on which individual had had the greatest influence on 20th-century physics. “Paul Dirac had a much bigger impact on modern science in the 20th century than Albert Einstein,” he concluded. Paul Dirac? Who was he? While Einstein is the undisputed icon of modern theoretical physics, Dirac is little known outside the physics community, certainly less so than Niels Bohr, Werner Heisenberg, Erwin Schrödinger, and Richard Feynman. And yet, Dirac unquestionably was a creative physicist of the same elevated rank, and his relative anonymity is unwarranted. For one, Einstein recognized his genius early on. While trying, in 1926, to understand a paper by the then 24-year-old British physicist who, at the time, was unknown to him, Einstein wrote in a letter, almost in despair: “I have trouble with Dirac. This balancing on the dizzying path between genius and madness is awful.”

Paul Adrien Maurice Dirac—for that was his full name—was a late prodigy. Born on August 8, 1902, in Bristol, the second child of Charles Dirac and Florence née Holten, Paul had two siblings: an older brother Reginald (who committed suicide at 24) and a younger sister, Beatrice. His father was a French-speaking Swiss immigrant who had settled in Bristol around 1890 and, for this reason, Paul was registered as Swiss by birth. Only in 1919, when he was 17 years old, did he acquire British nationality. The family lived in a small house on 15 Monk Road in suburban Bristol, a location unlikely to attract tourists. Today, a plaque informs passers-by of the building’s erstwhile occupant: “Nobel Prize Laureate in 1933 who revealed the secrets of quantum physics and antimatter.” In 1913, the Dirac family moved to a bigger and more expensive home at 6 Julius Road in another part of Bristol.

Paul had a rather unhappy childhood and youth, primarily the result of his authoritarian father’s distaste for social contacts (a trait that the elder Dirac passed on to his son). A highly regarded teacher, Charles recognized Paul’s unusual abilities and encouraged his education. On the other hand, he also dominated his family and kept his children in a virtual prison as far as emotional and social life was concerned. They were brought up in a crippling atmosphere of cold silence and isolation. “I had no social life at all as a child,” Paul once recalled. As a young man, he never had a girlfriend—or any other friend for that matter. When he finally broke out of that “prison” and left Bristol, he started to hate his father and avoided him if at all possible. Most likely, Paul’s reserved and solitary nature was a product of his childhood.

In 1918, Paul entered Bristol University as a student of electrical engineering; he graduated three years later with top honors. Unable to find a job, he was lucky to get accepted as a student of mathematics at the same university. He was more interested in mathematics than engineering anyhow. Finally, in the fall of 1923, a new chapter in his life started as he left Bristol to enroll as a research student at Cambridge’s St. John’s College.

Relativity and quantum theory were not subjects that Dirac was taught at Bristol University. “We were studying engineering, and all of our work was based on Newton,” he said in 1979 at a conference celebrating the centenary of Einstein’s birth. “We had absolute faith in Newton, and now we learned that Newton was wrong in some mysterious way.” When Dirac arrived in Cambridge, he was already thoroughly acquainted with Einstein’s theory of relativity.

In 1919, British astronomers Arthur Eddington and Frank Dyson had confirmed from observations of a solar eclipse that starlight would bend around the Sun in agreement with the general theory of relativity. This meant that Einstein was right and Newton wrong. After this confirmation of Einstein’s theory, relativity was on everybody’s lips. Dirac was no exception. “We discussed it very much,” he recalled. “Relativity was a subject that everybody felt himself competent to write about in a general philosophical way. The philosophers just put forward the view that everything had to be considered relatively to something else, and they rather claimed that they had known about relativity all along.” Dirac took a course on relativity theory and scientific thought given by the Bristol philosopher Charlie Broad and later followed up on it by self-studies of the mathematical content of Einstein’s theory. Thirty years later, Broad remembered that Dirac had attended his lectures. He described him as “one whose shoe-laces I was not worthy to unloose.” When Dirac came to Cambridge, he had mastered the essentials of both the special and general theory of relativity, including much of the abstract mathematical apparatus underlying the theories. Relativity was Dirac’s first love, but it was quantum theory that made his fortune.

As a research student in Cambridge, Dirac lived a quiet life, confining himself entirely to scientific studies and taking almost no part in social activities. One of his fellow research students was the American John Slater, who would soon emerge as a leader of quantum physics and its applications in chemistry. Characteristically, Dirac and Slater never talked together and only realized years later that they had followed some of the same courses. Still, as Dirac met more people, he gradually became a little less shy and introverted, as shown by his membership in 1924 in two academic clubs for mathematicians and physicists—the ∇²V (del-squared V) Club and the Kapitza Club. The latter was an informal discussion group founded by and named after the Russian physicist Peter Kapitza.

Dirac was fortunate to be assigned Ralph Fowler as his supervisor—not because Fowler was very active in supervising him, but because he was one of the very few British physicists with expert knowledge of atomic and quantum theory. The quantum theory of atomic structure was effectively founded by Niels Bohr in 1913 when he applied Max Planck’s idea of discrete energy quanta to explain the structure of atoms. Apart from Bohr and his school in Copenhagen, the theory was dominated by German physicists from the universities of Göttingen and Munich. On the other hand, British physicists generally resisted or ignored the new theory.

In the early 1920s, the semi-classical quantum theory based on Bohr’s planetary model of the atom encountered an increasing number of difficulties. It pictured the atom as a system of electrons revolving in definite orbits around the nucleus, not unlike the planets in our solar system. Although the theory was in many respects successful, physicists working with it realized that there was something seriously wrong. They thought that perhaps the very notion of electron orbits, a remnant of classical theory, ought to be abandoned. Unfortunately, neither Bohr nor his colleagues in Germany had a better alternative. It was in these circumstances that Dirac was first exposed to quantum theory. Guided by Fowler, he soon mastered the intricacies of quantum atomic theory, subsequently writing a couple of papers on this topic. In the summer of 1925, less than two years after he had arrived in Cambridge, he had published six papers on theoretical physics. Although an impressive record, none of the papers were strikingly original.

In 1927, only a year after he had completed his Ph.D. at Cambridge University, Dirac was invited to participate in the prestigious Solvay conference in Brussels, which focused on the physical meaning of the new quantum mechanics and featured a soon-to-be-famous dialogue between Einstein and Bohr. The following year, Dirac was offered the professorship in applied mathematics at Manchester University but turned down the offer. Then, in 1930, he was elected a Fellow of the Royal Society, one of the youngest Fellows ever in the history of the distinguished academy founded in 1662. And in 1932, he was appointed Lucasian professor of mathematics at Cambridge, the chair once occupied by Isaac Newton. The name of the chair derives from Henry Lucas, a Cambridge member of the British parliament who founded it in 1663.

The culmination of Dirac’s scientific career came in November 1933, when he had been awarded the Nobel Prize in Physics, sharing it with Schrödinger. One would imagine that young Dirac was overjoyed to become a Nobel laureate, but this was not the case. In fact, at first, he seriously contemplated rejecting the prize because of the publicity it would inevitably bring with it. But as the British physicist Ernest Rutherford pointed out to him, a refusal would surely create much more publicity, so Dirac reluctantly accepted it. At the time, a London newspaper, the Sunday Dispatch, described the 31-year-old Cambridge professor to be “as shy as a gazelle and as modest as a Victorian maid.” It added that he “fears all women.”

By the way, when Dirac arrived in Stockholm to receive the prize and deliver the traditional Nobel lecture in front of the Swedish king, he brought his mother with him. According to the rules of the Nobel Foundation, he was permitted to invite one guest, and he deliberately chose not to ask his father, which is not surprising, considering the ill-feeling Dirac had harbored towards Charles.

Nobel Prize winners have the right to nominate scientists within their field for future prizes. Most laureates take advantage of this right, realizing that the award is an important instrument of international science policy. Dirac was not interested in science policy, but he did use his right in 1946 and again in 1950 to nominate his friend Peter Kapitza. Dirac met Kapitza when both worked in Cambridge, from about 1921 to 1934, and also knew him from his many travels in Russia. Although Dirac’s two nominations were unsuccessful, in 1978 Kapitza finally received the Nobel Prize for his significant contributions to low-temperature physics. He was 84 years old.

There is a concept in elementary particle physics called “strangeness,” a kind of quantum number that characterizes a rapidly decaying particle. While the proton and the electron are not strange particles, more exotic elementary particles are ascribed a strangeness number different from zero. For example, the omega-minus particle consisting of three quarks has strangeness – 3, which makes it a very strange particle indeed. Had Dirac been an elementary particle, he definitely would be classified as strange, perhaps even stranger than omega-minus. A major biography of Dirac published in 2009 carries the title The Strangest Man. This is undoubtedly an exaggeration, but Dirac’s personality was in some ways peculiar and eccentric, exhibiting character traits that went back to his childhood and can well be described as strange. One of these traits, as illustrated in the case of the Nobel Prize, was his dislike of honors and publicity of any kind. In 1953, for example, he turned down a knighthood, mainly because he did not want to be addressed as “Sir Paul.” Of the few honors that he did accept, the most prestigious was the Order of Merit, which he received in 1973 (and which did not require any change of name). Dirac also received numerous offers of honorary doctorates, but invariably declined them all. In a letter to his friend, the American physicist and later Nobel laureate John Van Vleck, he wrote, “I like conferences and lectures less and less as I get older, and I never did like celebrations.”

Dirac’s reticence and taciturn nature were legendary. He was antisocial and rarely spoke spontaneously, preferring to use as few words as possible. A Cambridge physicist who had known Dirac for many years once told of how Dirac typically responded when asked a scientific question: “He looks for five minutes at the ceiling, five minutes at the windows, and then says ‘Yes’ or ‘No.’ And he is always right.” Other sources claimed that Dirac’s vocabulary was larger, consisting of five and not just two words, for he frequently responded with “I don’t know.” Another of Dirac’s colleagues described him as a “total rationalist,” citing Dirac’s idiosyncratic and exaggerated insistence on logic, as well as intellectual and verbal economy. He would answer a direct question but not address a comment or statement that, from a logical point of view, did not demand a response. According to one anecdote, after Dirac had delivered a lecture, a student in the audience said that he did not understand a formula. Nothing happened, no sign of reaction from the lecturer. After some time of embarrassing silence, Dirac was requested to answer the question. “It is not a question, it is a statement,” he tersely responded.

While other great scientists have sometimes been interested in and inspired by philosophy, literature, music, and political ideas, these aspects of intellectual life were largely absent in Dirac’s universe. In his younger and intensely creative days, he focused one-sidedly on theoretical physics, which completely absorbed his mind. He was close to being a monomaniac. To a Swedish newspaper, interviewing him on the occasion of his Nobel Prize, he said: “I am not interested in literature, I do not go to the theatre, and I do not listen to music. I am occupied with atomic theories.”

When Dirac became aware of the American physicist J. Robert Oppenheimer’s interest in poetry and Buddhist philosophy, he was deeply puzzled. “How can you do physics and poetry at the same time?” he asked. “The aim of science is to make difficult things understandable in a simpler way; the aim of poetry is to state simple things in an incomprehensible way. The two are incompatible.” Once, Kapitza gave Dirac an English translation of Dostoyevsky’s Crime and Punishment and asked him to read it. Later, when Kapitza asked if he had enjoyed the book, Dirac’s only comment was: “It is nice, but in one of the chapters the author made a mistake. He describes the Sun as rising twice on the same day.”

In 1926, Dirac spent a term at Bohr’s Institute in Copenhagen, where he became closely acquainted with the Danish atomic physicist and quantum sage. “We had long talks together,” he recalled, “long talks on which Bohr did practically all the talking.” Bohr was intrigued by the personality of the young Englishman, whom he characterized as a strange yet “complete logical genius.”

Bohr was in most respects very different from Dirac—socially, culturally, and regarding his scientific style. Yet, Dirac admired his colleague and characterized him as the deepest thinker he had ever met. In a 1933 letter to Bohr, Dirac wrote: “I feel that all my deepest ideas have been very greatly and favourably influenced by the talks I have had with you, more than with anyone else.” He later said that Bohr occupied the same position regarding atomic theory as Newton did concerning classical mechanics.

While in Copenhagen, Dirac became for the first time part of a vibrant scientific environment based on collaboration and group discussion. On Bohr’s invitation, he even spent Christmas with Bohr and his family. The stay in Copenhagen was a change in Dirac’s life, but not even the friendly atmosphere at Bohr’s institute could break his solitary habits and deep-rooted preference for isolation.

While Bohr’s creativity was boosted by long conversations with assistants and colleagues, and his publications went through a tortuous process with numerous draft versions, Dirac’s way of working was completely different. His mother had taught him to think first and then write—advice Dirac took to heart. When writing a manuscript, he would first draw up the whole work in his mind. Only then would he write it down in his meticulous handwriting, and this first draft would need few, if any, corrections. One of the students at Bohr’s institute recalled that Dirac appeared “almost mysterious” as he sat alone in the innermost room of the library. He “was so absorbed in his thoughts that we hardly dared to creep into the room, afraid as we were to disturb him. He could spend a whole day in the same position, writing an entire article, slowly and without ever crossing anything out.”