Paul Adrien Maurice Dirac (1902-1984) was born on August 8th, 1902, in Bristol, England, to a Swiss father and English mother. He was handicapped by a harsh childhood which left him, in his own words, "without any social contact." His father, a stern high school French teacher, insisted that while he was home Paul speak only in French, a language he was unable to confidently use. "Since I found I couldn't express myself in French, it was better for me to stay silent than to talk in English," as he recalled. This led to a terseness in his language and manner that persisted throughout his life.
At first he took up electrical engineering at Bristol University, obtaining his Bachelor's degree in 1921. Since he couldn't find a good job even with that degree, Bristol offered him funding to take graduate studies in mathematics there for two more years, and finally, he went to St. John's College, Cambridge where he eventually received a Ph.D in physics. He became a fellow of St. John's College in 1929 and in 1932 was elected to the Lucasian Professorship of Mathematics at Cambridge, the prestigious post once held by Isaac Newton (and now held by Stephen Hawking), smack dab in the middle of the greatest revolution in physics since Newton's day.
Dirac is considered one of the pioneers of quantum mechanics, along with Niels Bohr, Werner Heisenberg, Max Born, and Pascual Jordan. after having read Bohr and Heisenberg's original papers on the subject in 1925, he was immediately entranced. "I remember what a surprise it was to me when I first learned about the Bohr theory," Dirac recalled in a 1975 lecture. "I still remember very well how strongly I was impressed... It is really the most surprising thing that such a radical departure from the laws of Newton should be so successful." He later developed more general theories that could explain the Pauli exclusion principle in quantum-mechanical terms. Dirac then began investigating the statistical behavior of particles that obey that principle, such as electrons, work that had been begun a little earlier by Enrico Fermi, so the theory is called Fermi-Dirac statistics in their honor.
By the mid-1920's the main theories of quantum mechanics were beginning to take shape, with Erwin Schrödinger developing his wave mechanics. Turns out that Schrödinger at first attempted to derive a form of his wave equation that incorporated Albert Einstein's Special Theory of Relativity in it, but while calculating his results he found answers that did not agree very well with the experimental measurements of his day. So he published the nonrelativistic version instead. As it turned out, the experiments were inaccurate. As Dirac comments: "Schrödinger lacked the courage to publish an equation that gave results that disagreed with experiment." Other relativistic wave equations, such as that developed by Oskar Klein in Copenhagen (the Klein-Gordon equation) already existed, but Dirac was dissatisfied by what he perceived as their impurity. "There was a real difficulty in making quantum mechanics agree with relativity," Dirac recalled. "That difficulty bothered me very much at the time, but it did not seem to bother other physicists, for some reason which I am not very clear about." However, by 1928 he felt he overcame many of these difficulties, and produced what would be known as the Dirac Equation in his honor. Much to his amazement, the equation could be used to predict electron spin (which just been discovered from the Stern-Gerlach experiments made only a few years before) and the electron's magnetic moment with great accuracy.
Dirac's equation did, however, possess a troublesome feature: the electrons described by the equation could possess either positive or negative energy. As everything naturally seeks its lowest possible state of energy, what was to stop all of the electrons in the universe from falling into states of negative energy? Dirac rescued his theory by making use of the Pauli exclusion principle, saying that the vacuum of space is actually filled with negative energy electrons, so all the electrons we can see can no longer occupy these negative energy states. "Let us assume... that all states of negative energy are occupied except perhaps in a few of small velocity," he explained in a 1930 paper outlining these ideas. "We shall have an infinite number of electrons in negative energy states... but if their distribution is exactly uniform we should expect them to be completely unobservable. Only the small departures from exact uniformity, brought about by some of the negative-energy states being unoccupied, can we hope to observe." That would mean that an unoccupied negative-energy state would act like a positively-charged particle, like a hole in a semiconductor lattice. Dirac here was on the verge of predicting the existence of antimatter, but in this case he lacked courage just as Schrödinger did. At the time Dirac, along with every other physicist of the time, only knew of one particle with a positive charge, the proton, and conjectured that these holes were protons, despite their large difference in mass. "When I first thought of the idea," Dirac remembered, "it occurred to me that the mass would have to be the same as that of the electron because of the symmetry. But I did not dare to put forward that idea, because it occurred to me that if this new kind of particle (having the same mass as an electron but of opposite charge) existed, it would certainly have been discovered by experimenters."
Soon after, the mathematician Hermann Weyl noted this flaw in Dirac's arguments and explained mathematically that the two particles would need to be equal in mass. J. Robert Oppenheimer (yes, the father of the atomic bomb) published a paper in 1930 that blew the whole proton idea to shreds. By then, Dirac was prepared to take the consequences of the mathematics he produced, and he wrote another paper in 1931 that explicitly stated that the holes would be new particles, positively charged, with precisely the same mass as the electron. As luck would have it, only a year later, Carl Anderson at Caltech observed in bubble chambers tracks that were unmistakably those of an electron, but moved in the opposite way to that of an electron in the face of a magnetic field, indicating it had a positive charge. Another year after, Paul Dirac shared the 1933 Nobel Prize in Physics with Erwin Schrödinger for all this work.
He is also remembered for his invention of a formalism in wide use for quantum mechanics that bears his name, which he introduced in his groundbreaking (but alas, not very lucid) book The Principles of Quantum Mechanics. The book is tough reading, even for seasoned veterans of texts on mathematics and physics, and reflects the terseness in style that many who have attended Dirac's lectures know all too well, however it covers much of the old quantum mechanics recast in that formalism, as well as (in later editions), the theory of the electron he developed.
In an attempt to explain the quantization of charge, in 1931 Dirac posited the existence of magnetic monopoles, but could find no observational evidence for such particles (and none have as yet been found as of this writing). He said of this much later in 1981: "From the theoretical point of view one would think that monopoles should exist, because of the prettiness of the mathematics. Many attempts to find them have been made, all of them unsuccessful. One should conclude that pretty mathematics by itself is not an adequate reason for nature to have made use of a theory."
He married Margit Witner of Budapest in 1937, and remained in Cambridge until 1971, when he moved to Florida State University. He died on October 22, 1984.