Paul Ehrenfest

Biography

Paul Ehrenfest was born on 18 January 1880 in Vienna to Jewish parents, who were originally from Loštice in Moravia (now part of the Czech Republic). His parents, Sigmund Ehrenfest and Johanna Jellinek, managed a grocery store. Ehrenfest excelled in grade school but did not do well at the Akademisches Gymnasium, his best subject being mathematics. After transferring to the Franz Josef Gymnasium, his marks improved. In 1899, he passed the final exams.

He majored in chemistry at the Vienna Institute of Technology, but took courses at the University of Vienna, in particular from Ludwig Boltzmann on his kinetic theory of thermodynamics. These lectures had a profound influence: they were instrumental in developing Ehrenfest's interest in theoretical physics, defined his main area of research for years to come, and provided an example of inspired teaching. At the time, it was customary in the German-speaking world to study at more than one university, and in 1901, Ehrenfest transferred to University of Göttingen, which until 1933 was an important site for mathematics and theoretical physics. There he met his future wife, Tatyana Afanasyeva, a young mathematician born in Kiev (then the capital of the Kiev Governorate, Russian Empire) and educated in St Petersburg. In 1903, he met Dutch physicist Hendrik Lorentz during a short visit to Leiden, Netherlands. He wrote his dissertation on Die Bewegung starrer Körper in Flüssigkeiten und die Mechanik von Hertz (The Motion of Rigid Bodies in Fluids and the Mechanics of Hertz) and obtained his Ph.D. on 23 June 1904 in Vienna, where he stayed from 1904 to 1905.

On 21 December 1904, he married Afanasyeva, who collaborated with him in his work. They had two daughters and two sons: Tatyana ('Tanja') (1905–1984) also became a mathematician; Galinka ('Galja') (1910–1979) became an author and illustrator of children's books; Paul Jr. ('Pavlik') (1915–1939) became a physicist; and Vassily ('Wassik') (1918–1933).

The Ehrenfests returned to Göttingen in September 1906. They did not see Boltzmann again: on 5 September Boltzmann took his own life in Duino near Trieste. Ehrenfest published an extensive obituary describing Boltzmann's accomplishments. Felix Klein, dean of the Göttingen mathematicians and chief editor of the Enzyklopädie der mathematischen Wissenschaften ("Encyclopedia of Mathematical Sciences"), had counted on Boltzmann for a review about statistical mechanics. Now he asked Ehrenfest to take on this task. Together with his wife, Ehrenfest worked on it for several years; the article was not published until 1911. It is a review of the work of Boltzmann and his school, and has a style all its own: a sharp logical analysis of the fundamental hypotheses, clear delineation of unsolved questions, and an explanation of general principles by cleverly chosen transparent examples.

In 1907, the couple relocated to St. Petersburg. Ehrenfest found good friends there, in particular the physicist A.F. Joffe, but felt scientifically isolated. Moreover, because he was unwilling to declare belief in any religious denomination, he could not apply for a professorship and therefore had no prospect of securing a permanent position. In 1912, Ehrenfest toured German-speaking universities in the hope of a position. He visited Berlin, where he saw Max Planck; Leipzig, where he met his old friend German mathematician Gustav Herglotz; Munich, where he met theoretical physicist Arnold Sommerfeld; Zürich; and Vienna. In Prague he met Albert Einstein for the first time, and they remained close friends thereafter. Einstein recommended that Ehrenfest succeed him in his position in Prague, but the plan failed since Ehrenfest declared himself an atheist. Sommerfeld offered him a position in Munich, but Ehrenfest received a better offer; at the same time there was an unexpected turn of events: H. A. Lorentz resigned his position at the University of Leiden, and on his advice, Ehrenfest was appointed as his successor.

Academic career

In October 1912, Ehrenfest arrived in Leiden, and on 4 December, he gave his inaugural lecture, Zur Krise der Lichtaether-Hypothese (About the crisis of the light-ether hypothesis). He remained in Leiden for the rest of his career. To stimulate interaction and exchange among physics students, Ehrenfest organized a discussion group and a study association called De Leidsche Flesch ("The Leyden jar"). He maintained close contact with prominent physicists in the country and abroad, and invited them to visit Leiden and give presentations in his lecture series. Ehrenfest was an outstanding debater, quick to point out weaknesses and summarize the essentials.

In his lectures, Ehrenfest emphasized simple models and examples to illustrate and clarify basic assumptions. His classes were small, and he made an effort to get to know students who made use of the reading room. Though few of them were accepted as majors in theoretical physics, he had long discussions with them almost daily. According to Einstein:

He was not merely the best teacher in our profession whom I have ever known; he was also passionately preoccupied with the development and destiny of men, especially his students. To understand others, to gain their friendship and trust, to aid anyone embroiled in outer or inner struggles, to encourage youthful talent—all this was his real element, almost more than his immersion in scientific problems.

If Ehrenfest felt there was little more he could teach a student, he would send them to other centers in Europe for more training. He would also encourage students to accept positions abroad. Among his students were Johannes Burgers, Hendrik Kramers, Dirk Coster, George Uhlenbeck and Samuel Goudsmit, who became famous for jointly proposing the concept of electron spin, Jan Tinbergen, Arend Rutgers, Hendrik Casimir, Gerhard Dieke, Dirk Struik, and Gerard Kuiper. His assistants included Yuri Krutkov, Viktor Trkal, Adriaan Fokker, Paul Epstein, and Gregory Breit. Other young foreign scientists who spent extended periods in his laboratory included Gunnar Nordström, Enrico Fermi, Igor Tamm, Oskar Klein, J. Robert Oppenheimer, Walter Elsasser, Ralph Kronig, Werner Heisenberg, Paul Dirac, and David Dennison.

Ehrenfest had ambivalent opinions about science, technological progress, and cultural and social issues.

In 1919, he became a member of the Royal Netherlands Academy of Arts and Sciences.

Final years

Going by his correspondence with his friends, it seems that Ehrenfest suffered from severe depression at latest by May 1931. By August 1932, Einstein was so worried that he wrote to the Board of the University of Leiden, expressing concern and suggesting ways in which Ehrenfest's workload could be reduced.

Having made arrangements for the care of his other children, on 25 September 1933, in Amsterdam, Ehrenfest fatally shot his younger son Wassik, who had Down syndrome, then killed himself.