Austrian theoretical physicist who helped clarify the foundations of quantum theory and statistical mechanics.

Ehrenfest studied with Ludwig Boltzmann (Boltzmann, Ludwig Eduard) at the University of Vienna, where he received his doctorate in 1904. Ehrenfest and his wife, Russian mathematician Tatiana A. Afanassjewa, renounced their religions (Judaism and Christianity, respectively) because such interconfessional marriages were not allowed in Austro-Hungary. Having seriously complicated their chances to find regular academic positions, the couple moved to St. Petersburg, Russia, where they subsisted on temporary teaching incomes between 1907 and 1912, before Paul Ehrenfest obtained an appointment as a professor of theoretical physics at Leiden University in The Netherlands.

During the embryonic stage of quantum theory, Ehrenfest clarified that Max Planck (Planck, Max)'s formula for blackbody radiation necessarily implies a fundamental postulate of discontinuous energy—the existence of discrete quantum energy levels—which classical physics proved incapable of explaining. In 1911 Ehrenfest also pointed out that Albert Einstein (Einstein, Albert)'s light quanta differ from classical particles in being statistically indistinguishable, and he explicitly constructed this statistics—now known as Bose-Einstein statistics—in a 1915 paper with Dutch physicist Heike Kamerlingh Onnes (Kamerlingh Onnes, Heike). Instead of corpuscular quanta, Ehrenfest preferred to work with a model of quantized waves that he first proposed in 1906 and that later became essential in quantum field theory. Ehrenfest's adiabatic principle of 1913 allowed physicists to quantize new varieties of systems, linked together by adiabatic processes (adiabatic process). Reputed for his great ability to teach and foster new research talent, Ehrenfest encouraged his students Samuel Abraham Goudsmit (Goudsmit, Samuel Abraham) and George Eugene Uhlenbeck (Uhlenbeck, George Eugene) to propose the concept of electron spin in 1925.

In 1911 Paul and Tatiana Ehrenfest published an influential critical review of the field of statistical mechanics and its conceptual foundations, in particular drawing the attention of subsequent researchers to the crucial problem of the ergodic hypothesis (the assumption that all “microstates” at the same energy level are equally likely).