Brillouin, Léon

Born Aug. 7, 1889, in Sèvres, near Paris. French physicist.

Brillouin studied at universities in Munich and Paris. From 1928 to 1932 he was a professor at the University of Paris and from 1932 to 1939 a professor at the Collège de France. In 1941 he moved to the USA, where he taught at various universities. He was a professor at Columbia University. The scope of Brillouin’s scientific interests is extremely broad and includes classical electrodynamics, quantum mechanics, solid-state physics, radiophysics, statistical physics, and information theory.

At the same time as German physicists G. Wentzel and H. Kramers (1926), Brillouin worked out the method of semi-classical approximation in quantum mechanics. He discovered the dependence of the magnetization of a paramagnet on the intensity of the magnetic field and temperature (Brillouin’s formula). He discovered the existence of zones of “permitted” values of the energy of electrons in a solid body (the so-called Brillouin zones) and was the first to explain many physical properties of metals and semiconductors. Brillouin developed a new concept of the mechanism of propagation of electromagnetic waves in wave guides.

Beginning in the 1940’s, Brillouin devoted special attention to information theory. He studied the common character of and the interconnection between the concept of entropy in thermodynamics and information theory. He showed that obtaining information about the state of a physical system is equivalent to a corresponding decrease of its entropy in accordance with the second principle of thermodynamics and is unavoidably accompanied by a compensatory increase of entropy in some other system.