Dirac, Paul Adrien Maurice

Dirac, Paul Adrien Maurice (1902-84), British theoretical physicist and Nobel laureate, renowned for his prediction of the existence of the positron, or antielectron, and for his research in quantum theory.

Dirac was born in Bristol, England, and educated at the universities of Bristol and Cambridge. His quantum theory of electron motion led him in 1928 to postulate the existence of a particle identical to the electron in every aspect but charge, the electron having a negative charge and this hypothetical particle a positive one. Dirac's theory was confirmed in 1932 when the American physicist Carl Anderson discovered the positron. In 1933 Dirac shared the Nobel Prize in physics with the Austrian physicist Erwin Schrödinger, and in 1939 he was made a fellow of the Royal Society. He was a professor of mathematics at Cambridge from 1932 to 1968, a professor of physics at Florida State University from 1971 until his death, and a member of the Institute for Advanced Study periodically between 1934 and 1959. Dirac's writings include Principles of Quantum Mechanics (1930).

See also Electron; Physics: Modern Physics.

Dirac, P.A.M.

born Aug. 8, 1902, Bristol, Gloucestershire, Eng.

died Oct. 20, 1984, Tallahassee, Fla., U.S.

in full Paul Adrien Maurice Dirac English theoretical physicist known for his work in quantum mechanics and for his theory of the spinning electron. In 1933 he shared the Nobel Prize for Physics with the Austrian physicist Erwin Schrödinger.

Dirac's mathematical ability showed itself at an early age. At the school he attended in Bristol he was given rather advanced books on mathematics to study independently. His father, a Swiss by birth who was the French master at the same school, encouraged his son to develop his mathematical ability. He wished him also to become fluent in French, to the extent that, according to the son's report, the elder Dirac refused to speak to him unless he was addressed in the French language. This may have fostered Dirac's pronounced tendency to speak seldom and choose his words with utmost care. He avoided company, preferring to work alone. His chief pastime was solitary walks.

Toward the practical end of earning a living, Dirac studied engineering at the University of Bristol. The use of approximations that he learned in this study had a strong influence on his later work; it strengthened his confidence in the intuitive approach to problem solving. He came to believe that a theory expressing fundamental laws of nature could be constructed solely on the basis of approximations, guided by intuition rather than exact knowledge of the actualities. He declared that the actual phenomena were too complex ever to be pinned down in a precise way; a physicist must be satisfied to work only with approximate knowledge of reality.

Dirac's study of theoretical physics began only after he had received a degree in electrical engineering, had failed to find work in this field, and, aided by a grant, had entered St. John's College, Cambridge. From R.H. Fowler, his faculty supervisor, who had collaborated with Niels Bohr in his pioneering work in atomic physics, Dirac learned the current state of that science.

In 1926, while still a graduate student, he made his first major contribution to physics by devising a form of quantum mechanics, the laws of motion that govern atomic particles. Other physicists (Max Born, Pascual Jordan) working in Germany anticipated Dirac in this achievement by only a few months. Dirac's version of quantum mechanics was distinguished, however, by its generality and logical simplicity.

With the object of formulating atomic laws in the most elegant mathematical language, Dirac applied to quantum mechanics the ideas of Einstein's special theory of relativity. He had the revolutionary idea that the electron could be described by four wave functions, satisfying four simultaneous differential equations. It followed from these equations that the electron must rotate on its axis, an idea that had been developed by other physicists, and also that there must be states of negative energy. The latter conclusion did not seem to correspond to physical reality. In a later paper, however, Dirac suggested that a deficiency of an electron in one of these states would be equivalent to a short-lived positively charged particle. This idea was confirmed whenCarl David Anderson obtained cloud chamber photographs showing the existence of positrons—i.e., particles equal to the electron in mass but positively charged. In the experimental confirmation of this phenomenon, an apparent difficulty of Dirac's theory was turned into a triumph.

Because they incorporated relativistic effects, Dirac's wave equations had accurately predicted the electron's motion, spin, and magnetic and other properties. Moreover, these equations laid the foundations for the theory of quantum electrodynamics, which incorporates both quantum and relativity theory in its descriptions of the interactions of charged particles with the electromagnetic field.

In his book The Principles of Quantum Mechanics (1930), Dirac developed the so-called transformation theory of quantum mechanics that furnished a machinery for calculating the statistical distribution of certain variables when others are specified. He also stated his philosophical position with respect to theoretical physics. The fundamental laws of nature, he wrote, “control a substratum of which we cannot form a mental picture without introducing irrelevancies.” In his own work Dirac avoided using any pictorial model or mental picture of the phenomena described by his mathematical symbols.

In addition to refining mathematical descriptions of matter on the atomic scale, Dirac introduced a quantum theory of radiation. He was coinventor of the Fermi-Dirac statistics. In 1933 he was awarded the Nobel Prize for Physics and in 1939 the medal of the Royal Society. Dirac taught at Cambridge after receiving his doctorate there, and in 1932 he was appointed Lucasian Professor of Mathematics, the chair once held by Isaac Newton. He served in that capacity until 1968, shortly after which he moved to the United States. In 1971 he was made professor emeritus at Florida State University, Tallahassee, Fla.

Dirac's other published works include Lectures on Quantum Mechanics (1966), The Development of Quantum Theory (1971), Spinors in Hilbert Space (1974), and GeneralTheory of Relativity (1975).

Barbara Lovett Cline

Additional reading

Helge Kragh, Dirac: A Scientific Biography (1990); Barbara Lovett Cline, The Questioners: Physicists and the Quantum Theory (1965).