Paul Ehrenfest (poul ā´rənfĕst), 1880–1933, Austrian physicist. In 1904, Ehrenfest received his doctorate in theoretical physics in Vienna and married the Russian mathematician Tatyana Alexeyevna Afanassyewa. Together they wrote what has become a classical exposition of statistical mechanics. He was one of the first to take German physicist Max Planck's quantum theory seriously and to try to define its relation to the older physics. In 1912 he succeeded Dutch physicist H. A. Lorentz in the chair of theoretical physics at the Univ. of Leiden. He proved an energetic, lucid, and inspiring teacher. His acute criticisms and his formulation of the adiabatic principle—which Niels Bohr placed among the foundations of quantum theory—were important contributions to advancing modern physics.
See M. J. Klein, Paul Ehrenfest: The Making of a Theoretical Physicist (Vol. 1, 2d ed. 1985).