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Paul Adrien Maurice Dirac was a legend in his own lifetime, and his status in the history of recent physics has not diminished since his death. He died on October 20, 1984, in Florida and was buried at Roselawn Cemetery in Tallahassee. Shortly after his death, the Institute of Physics, the organization for physicists in the United Kingdom of which Dirac had been an Honorary Fellow since 1971, established the Paul Dirac Medal, to be awarded annually to outstanding contributors to physics. The first recipient was the famous astrophysicist and expert in general relativity Stephen Hawking, who at the time occupied the Lucasian Chair formerly held by Dirac. Another Dirac Medal was established by the Abdus Salam International Centre for Theoretical Physics located in Trieste, Italy. The publishing branch of the Institute of Physics is located in the Dirac House in his native Bristol, where one can also find the small and unimpressive Dirac Road. In 1989, The Paul A. M. Dirac Science Library was inaugurated at Florida State University, containing, among other items, Dirac’s papers and archival materials related to his life and career.

Dirac did not invent quantum mechanics, but together with Heisenberg, Born, Jordan and Pauli, he belonged to the small group of highly talented physicists who originally established the theory. Much of Dirac’s early work in the area was independent of the work done by his German colleagues and friendly competitors, yet his breakthrough in the fall of 1925 relied crucially on Heisenberg’s original insight. Would Dirac have made inroads in quantum mechanics without Heisenberg’s paper? It is impossible to say, but probably not. In any case, Dirac always acknowledged Heisenberg as the founder of quantum mechanics and never claimed priority either to this field or other theoretical discoveries. Priority claims were just not his style.

Dirac’s name is first of all associated with the Dirac equation, the relativistic wave equation for the electron, which he had ready in January 1928 and which had a galvanizing effect on theoretical physics—leading to the idea of antimatter, and much more. Connected to the wave equation are names such as Dirac operators, Dirac matrices, and Dirac measure, which are familiar not only to physicists but, perhaps even more so, to mathematicians. Even before the emergence of the Dirac wave equation, the name of the young British physicist was turned into an eponymy in the Dirac delta function and in the Fermi-Dirac quantum statistics characterizing the electron and many other elementary particles. While Dirac operators and Fermi-Dirac statistics are accepted and semi-official names, the “dirac” unit is mostly anecdotal and based on the physicist’s succinct manner. It is a measure of taciturnity or slowness of speech, the smallest number of words used in a conversation. Its exact value is unknown but, according to some experts, one dirac corresponds to one word per hour.

The “Dirac-Kapitza effect” may be little known, but it is not without either scientific or historical interest. An arch-theorist with a predilection for mathematical theories and fundamental physics, Dirac at times descended from his theoretical heaven to more mundane experimental problems. Collaborating with Kapitza, an experimental physicist, in 1933, he discussed the possibility of obtaining electron diffraction from a grating of standing light waves. Given the wave-particle dualism of quantum mechanics, such an effect—the opposite of the ordinary diffraction of light in a material grating—should be at least theoretically possible. Dirac and Kapitza calculated the probability and obtained a ridiculously small number, which suggested that the effect was outside the reach of an experiment. Not very much more happened until 1986, when both physicists had passed away. In that year, physicists at the Massachusetts Institute of Technology succeeded in detecting the phenomenon predicted by Dirac and Kapitza 53 years earlier, albeit using sodium atoms instead of electrons. Two years later, the diffraction of electrons by standing light waves was demonstrated. Today the Dirac-Kapitza effect (sometimes referred to as the Kapitza-Dirac effect) is well known and a thriving field of research in the community of condensed matter physics.

While Dirac had a tremendous influence on scientific thought, his impact on British science and science education was much more limited. Considering that Dirac was at Cambridge University for 47 years, of which he spent 37 years as a Lucasian professor, his local influence may be said to have been very small. His taciturn and reticent nature was part of the reason, and so was his dislike of public appearances. Dirac tried hard and largely successfully to stay out of the public limelight, not to be part of committees, and not to take on administrative duties. He was totally uninterested in exerting power or influence of any kind. The idea of building up a school of physics, something equivalent to what Rutherford and Bohr had done, probably never crossed his mind. Although he was pleased to be appointed as a Lucasian professor, the position itself did not seem to have interested him very much. More than once he was compared to Newton, but the comparison did not impress him. Nor did he pay any attention to the history of the Lucasian chair or to his illustrious predecessors, including not only Newton, but also Gabriel Stokes, Joseph Larmor, and other important physicists. In his letters to colleagues from 1932 to 1933, he did not even mention the new position or commented about being Lucasian professor in his later writings and correspondence.

The broader impact Dirac did have was primarily through his lectures on quantum mechanics and his textbook on which the lectures were always based. Physicists following his course have hailed Dirac’s “exceptionally clear” presentation, noting that the audience “was carried along in the unfolding of an argument which seemed as majestic and inevitable as the development of a Bach fugue.” Dirac’s lectures were indeed clear and well structured, but for three decades they largely followed the same pattern as outlined in his textbook, which he considered to be the way of presenting quantum mechanics. He delivered his lectures tersely and with little interaction with his audience. If the students did not understand him, they could not expect much help. One can safely assume that not all students appreciated Dirac’s lucid and majestic arguments. Some might have wished to listen to a Liszt rhapsody or even a folk song rather than a Bach fugue. Dirac had only a few Ph.D. students and was, in general, uninterested in them; in fact, he expected the students to work largely on their own. The German-American physicist Victor Weisskopf called him “absolutely unusable for any student.” It is probably correct to say that, although Dirac’s physics was a model of inspiration, Dirac himself was not a role model for any of his students. But then, he had no intention of being a role model.

Dirac’s life can be divided into four phases, the first being his youth and early training in Bristol. Only when he arrived in Cambridge in 1923 did his genius unfold, and two years later he entered the avant-garde of international theoretical physics. During the next decade or so, the second phase, he produced a phenomenal record of innovative ideas and theories that forever changed the physical world picture. All of these ideas and theories were concerned with quantum mechanics. This second phase of his life was far the most significant from the perspective of science, the theory of antiparticles perhaps being the deepest, most original, and most astounding of all his contributions. Not only was the substance of his work unique, but the way he presented his discoveries to the world of science was also in a style rarely, if ever, seen before. The British-American mathematical physicist Freeman Dyson, one of the chief architects of the new quantum electrodynamics, wrote about Dirac’s explosion of creativity: “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—it was this purity that made him unique.”

Had Dirac died in 1936, perhaps fallen from one of the summits he climbed so eagerly, it would not have mattered much to his legacy in physics. During the third and longest phase of his life, Dirac, now married, continued doing important work in quantum theory but not of the same quality and innovative nature as previously. There were gems here and there, but fewer and less precious. Dissatisfied with the new trends in theoretical physics in general and with renormalization quantum electrodynamics, in particular, after World War II Dirac increasingly distanced himself from mainstream physics. His long and lonely fight against the infinite quantities appearing in quantum theory was the fight of a loser. Dirac more or less knew it, but he just fought on. For a period, he switched to problems of general relativity, a line of work he enjoyed and which was appreciated by contemporary physicists. Yet, the foundation of quantum mechanics was constantly on his mind.

At the age of 67, Dirac was forced to retire from his chair in Cambridge, much to his regret. He was upset that the university did not want to give him even the space of the small room he occupied at Cambridge. Although he felt marginalized and was dissatisfied with the modernization of the university’s organization, he wanted to go on as usual. He spent the fourth and last phase of his life in Florida. In 1968, he had been invited to visit the Center for Theoretical Studies at the University of Miami, and three years later he joined the physics department at Florida State University, Tallahassee. He remained scientifically active and continued his project of reformulating quantum mechanics into a logically more satisfactory theory. However, Dirac’s main occupation during the Florida period was cosmology rather than quantum theory. The project to establish a new cosmological theory based on the hypothesis of a varying gravitational constant was a resurrection of the idea he had introduced as early as 1937. It was even less mainstream than his work in quantum mechanics. Although attracting some interest among physicists, geologists, and astronomers, observations consistently disagreed with the predictions of Dirac’s gravitation hypothesis.

When Dirac is commemorated as one of the greatest physicists ever, it is mainly because of his epoch-making contributions to quantum physics made in the second phase of his life. Given the outstanding quality of these contributions, it is only natural that he was unable to reach the same level of successful creativity later in life. After all, and despite his reputation, Dirac was only a human. Even Einstein’s career was divided into a highly creative early phase and much less productive later years. At the end of his life, Dirac felt that his earlier successes did not quite make up for his later failures. In 1983, he was addressed by Pierre Ramond, then a 40-year-old Florida theoretical physicist known for his important work in string theory. Ramond sought to engage the famous physicist in a conversation, but Dirac evaded: “I have nothing to talk about. My life has been a failure.” One understands Ramond’s surprise and even shock: “I could hardly believe that such a great man could look back on his life as a failure,” he noted. “What did that say about the rest of us?”