Paul Ehrenfest (18.01.1880-25.09.1933) was a theoretical physicist. He was born in Vienna. He graduated from the University of Vienna (1904). In 1907 he moved to St Petersburg, where he taught at the Polytechnic Institute and ran a theoretical seminar organised at his home. From 1912 he was professor at the University of Leiden.
His main work concerned thermodynamics, statistical mechanics, nuclear physics, relativity and quantum theory. In 1911, together with T. A. Afanasyeva, he carried out a logical analysis of statistical mechanics and proposed the quasi-ergodic hypothesis. In 1933 he introduced the concept of second-order phase transitions.
In the field of quantum statistics he formulated the so-called adiabatic hypothesis. In 1931, with R. Oppenheimer, he showed that nuclei with an odd atomic number obey Fermi — Dirac statistics and those with an even one obey Bose — Einstein statistics (the Ehrenfest — Oppenheimer theorem), and noted in this connection that the proton-electron hypothesis of the structure of nuclei accepted at that time led, as applied to the nitrogen-14 nucleus, to a number of contradictions with the known properties of nitrogen.
He created a school of physicists (S. Goudsmit, H. Kramers, D. Koster, G. Uhlenbeck, J. Burgers, V. G. Bursian, G. G. Veikhard, Yu. A. Krutkov, V. K. Frederiks and others). He was a foreign member of the USSR Academy of Sciences (1924) and a member of the Royal Netherlands Academy of Arts and Sciences.