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*.

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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).