Paul Ehrenfest (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.
Paul Ehrenfest's father, Sigmund Ehrenfest, came from a poor Jewish family. He was working in a weaving mill in the Jewish village of Loschwitz in Moravia when he married Johanna Jellinek. After the marriage they moved to Vienna where they set up a grocery business which fared rather well. They had five children who survived birth, and Paul was the youngest, having four older brothers Arthur, Emil, Hugo, and Otto. Johanna Ehrenfest worked long hours in their shop and Paul was looked after at home by a nursemaid.
As a child Paul's health was poor. He was sickly, had dizzy spells, and suffered frequent nosebleeds. He suffered from anti-Semitic comments from other children in the neighbourhood but his brothers supported him strongly and played an important role in his childhood. His oldest brother was twenty-two years old by the time Paul was five and it was through his brothers that Paul became interested in education. They gave him puzzles which he enjoyed solving.
By the time he was six years old Paul could read, write and count. Mostly he had taught himself these things, helped a little by his mother, and encouraged by his brothers. At this age, in 1886, he began to attend primary school, moving to a different primary school in 1888. He was introduced to science and mathematics by his brothers, rather than from his school, and their attitude was one which would have quite an influence on him.
Paul Ehrenfest was a student at the Technische Hochschule in Vienna. There he formed a close friendship with three other students of mathematics, Heinrich Tietze, Hans Hahn and Herglotz. They called themselves the 'inseparable four'. Ehrenfest attended Boltzmann's lectures on the mechanical theory of heat during 1899-1900.
In 1901 he studied at Gottingen under Klein and Hilbert. There he took Max Abraham's course on the electromagnetic theory of light. He also attended courses by Stark, Walther Nernst, Schwarzschild and Zermelo.
Ehrenfest obtained his doctorate from Vienna in 1904, under Boltzmann's supervision, on a topic in classical mechanics. He returned to Gottingen in 1906 and there was influenced by Klein, Hilbert, Minkowski and Caratheodory.
In 1907 Ehrenfest went to St Petersburg. He was appointed to a chair to succeed Lorentz at the University of Leiden in 1912. Einstein was one of the main influences in Ehrenfest's appointment. They first met in Prague in 1912 and their deep friendship lasted until Ehrenfest's death. Ehrenfest was also a close friend of Niels Bohr.
Ehrenfest worked on quantum theory applying it to rotating bodies. He recognised that Ampere's molecular currents are incompatible with classical statistical mechanics. He proposed a model of diffusion in the early 1900s in order to illuminate the statistical interpretation of the second law of thermodynamics, that the entropy of a closed system can only increase. The modern theory of nonequilibrium thermodynamics brings together the molecular, collisional ideas of Boltzmann with the statistical ideas of Ehrenfest's to give a nonlinear, statistical theory.
All through his life Ehrenfest had suffered from low self esteem, which was in marked contrast to the high esteem in which he was held by his fellow scientists. He was also greatly saddened by his son Wassik being a mongol and having severe problems both physically and mentally.
Ehrenfest comitted suicide in 1933. His last letter (which was never sent) is a sad document :
My dear friends: Bohr, Einstein, Franck, Herglotz, Joffe, Kohnstamm, and Tolman!
I absolutely do not know any more how to carry further during the next few months the burden of my life which has become unbearable. I cannot stand it any longer to let my professorship in Leiden go down the drain. I must vacate my position here. Perhaps it may happen that I can use up the rest of my strength in Russia. .. If, however, it will not become clear rather soon that I can do that, then it is as good as certain that I shall kill myself. And if that will happen some time then I should like to know that I have written, calmly and without rush, to you whose friendship has played such a great role in my life. ...
In recent years it has become ever more difficult for me to follow the developments in physics with understanding. After trying, ever more enervated and torn, I have finally given up in desperation. This made me completely weary of life .. I did feel condemned to live on mainly because of the economic cares for the children. I tried other things but that helps only briefly. Therefore I concentrate more and more on the precise details of suicide. I have no other practical possibility than suicide, and that after having first killed Wassik. Forgive me ...
May you and those dear to you stay well.
This letter, and a similar letter which he wrote to his students, was never sent. Ehrenfest shot Wassik in the waiting room of the Professor Watering Institute in Amsterdam where Wassik was being treated. Then he shot himself. The Dutch papers only reported his sudden death and gave lengthy accounts of his achievements.
Einstein said of Ehrenfest that he was "the best teacher in our profession I have ever known".