# Dirac, P(aul) A(drien) M(aurice)

### electron energy mechanics quantum

(1902–84) British theoretical physicist: major contributor to quantum mechanics; predicted existence of the positron and other antiparticles.

Dirac, the son of a Swiss father and English mother, studied electrical engineering at Bristol and mathematics at Cambridge. After teaching in America and visiting Japan and Siberia, Dirac was appointed in 1932 to the Lucasian professorship in Page 98 mathematics at Cambridge, where he remained until his retirement in 1969. He was then a visiting lecturer at four US universities before becoming professor of physics at Florida State University in 1971.

A uniquely gifted theoretician, Dirac contributed creatively to the rapid development of quantum mechanics. In 1926, just after , he developed a general theoretical structure for quantum mechanics. In 1928 he produced the relativistic form of the theory, describing the properties of the electron and correcting the failure of theory to explain electron spin, discovered by in 1925.

From the relativistic theory he proposed in 1930 that the theoretically possible negative energy solutions for the electron exist as states but these states are filled with particles of negative energy so that other electrons cannot enter them. He predicted that a sufficiently energetic photon could create an electron–positron pair apparently from nowhere by knocking an electron out of one of these negative energy states. The positively charged hole left is the antiparticle to an electron, called a positron. Also, an electron meeting a positron can give mutual annihilation, releasing energy as a photon (light). All these predictions were observed experimentally by in 1932. Dirac’s argument applies to all particles, not just electrons, so that all particles possess corresponding antiparticles.

In 1930 Dirac published *The Principles of Quantum Mechanics* , which is a classic work which confirmed his stature as a 20th-c in the minds of many physicists. The Nobel Prize for physics for 1933 was shared by Dirac and Schrödinger.

## User Comments