Professor Paul Ehrenfest, c mid-20th century

Paul Ehrenfest, born 18 January 1880, died 25 January 1933. He was an Austrian theoretical physicist who contributed to statistical mechanics, quantum mechanics, and created the Ehrenfest theorem. He was born in Vienna, Austria-Hungary. Ehrenfest studied at Vienna Institute of Technology in chemistry, later transferring to the University of Göttingen, where he met his wife Tatyana Afanasyeva. He obtained his PhD in 1904 in Vienna. He married Tatyana in 1904 and they had two daughters and two sons. He and his wife, a fellow mathematician, worked together on projects. In 1907, the family moved to St. Petersburg. However, although from a Jewish background, Ehrenfest was unwilling to declare a religious denomination which meant he was not able to apply for a professorship. He then looked towards German-speaking universities in 1912. He was friends with Albert Einstein after they met in Prague in 1912. In 1912, he finally found a position at the University of Leiden, where he remained for the rest of his career. He was a devoted and respected teacher, who Einstein called "not merely the best teacher in our profession whom I have ever known". In 1919, he became a member of the Royal Netherlands Academy of Arts and Sciences. Ehrenfest seemed to suffer from depression from at least 1931, and by 1932, Einstein was concerned about his wellbeing to the point of writing to the University of Leiden to ask them to support him. Unfortunately, on 25 September 1933, Ehrenfest fatally shot his youngest son Wassik, who had Downs syndrome, and then committed suicide.