When in 1912 H A Lorentz resigned his chair of theoretical physics at the University of Leyden to take up a research position that had been specially created for him at Haarlem, he recommended Ehrenfest as his successor. Thus Paul Ehrenfest, who had been born in Vienna on January 18, 1880, had studied in Vienna and Göttingen, had obtained his doctor's degree under Boltzmann in 1904 and who since 1907 had been living at St Petersburg with his Russian-born wife and collaborator, Tatiana Afanassjewa (whom he had married in Vienna in 1904), became a full professor at Leyden. He stayed at Leyden until his death on September 25, 1933.
It is characteristic of Lorentz's width of vision that he should esteem and encourage a young man so entirely different from himself and so utterly at variance with then prevailing ideas about what a professor should be like. Ehrenfest from his side showed towards Lorentz a devoted admiration beautifully expressed in the obituary reprinted on page 559 of the present volume. It is true that both were interested in a broad range of problems and that both were accomplished lecturers to physicists as well as to general audiences, but there all resemblance stops. Lorentz's lectures were masterpieces exposing the subject in a systematic and lucid way in well-balanced and beautifully correct language whether he spoke his native Dutch, or French, English or German. Ehrenfest's lectures were brilliant too, but in an unconventional way: he emphasized salient points rather than continuity of argumentation; the essential formulae appeared on the blackboard almost as aesthetical entities and not only as links in a chain of deductions. He shunned calculations of any length and numerical constants were often considered irrelevant. "4" - mind the quotation marks - could mean almost anything. His language was vivid, full of surprising metaphor, and quite ungrammatical, at least when he spoke Dutch - and more so when he spoke English - but even his German was occasionally spiced with Viennese vernacular. One had very little inclination to go to sleep during Ehrenfest's lectures, but if one ever showed any tendency in that direction one was immediately and ruthlessly called to order.
The difference between Lorentz and Ehrenfest was even more pronounced when it came to seminars and colloquia. Lorentz did not like to speak about a problem before he had arrived at a solution, and he reproved - always in an extremely polite and mild way - those who made remarks without due cogitation. To Ehrenfest, on the other hand, discussions and arguments were an essential part of his scientific activity and the best way to clarify an obscure point. He was never afraid to ask a stupid question and he encouraged others to do the same. How often during drowsy meetings when an audience is placidly dozing away the hours while incomprehensible speakers are muttering un-understandable words, projecting irrelevant slides or writing illegible formulae, have I longed for some one with both Ehrenfest's courage to cut a speaker short and Ehrenfest's ability to expose sham and bombast, and also to bring forward the quintessence and to explain the beautiful and important. For Ehrenfest was not only a merciless critic of the stupid and the unclear but perhaps even more a fervid admirer and interpreter of the beautiful and profound, and in this he was entirely unselfish. Few people have shown, greater loyalty to their friends and their pupils.
Ehrenfest's own contributions are mainly in the field of statistical mechanics and its relations with quantum mechanics. He has elucidated many basic points in Boltzmann's theories, and the article in the Encyklopädie der mathematischen Wissenschaften - written jointly with Mrs Ehrenfest - is still invaluable for all those who are interested in the fundamentals of statistical theory. His work on quantum statistics led to the formulation of his theorem of adiabatic invariance which played an important role throughout the further development of quantum mechanics.
Now that atoms have become almost tangible realities and the theory of atomic structure can be built up from first principles, it is instructive to realize to what extent the development of quantum theory was initially due to statistical and thermodynamical considerations. But perhaps Ehrenfest's papers are even more fascinating because of the striking peculiarities of his work. Every single page bears witness to the intensity of his preoccupation with physical science. Most of his papers are concerned with fundamentals - there are hardly any dealing with the application of well-established theory to experimental facts - and many deal with single points rather than with the systematic exposition of a body of theory or the elaboration of mathematical details. He has a great preference for the use of simple models that show the essential traits of a problem - and is a master at inventing them; this is a common feature of his lectures and his writings. Otherwise his written prose is much more restrained. In his papers, one will look in vain for the graphic imagery of his lectures and discussions and only here and there do we gather between the lines of published speeches a glimpse of his exuberant wit. Ehrenfest himself realized that his oral style would not go down well in writing; without his gestures and intonation, without the ambiance that he created around him it would lose its significance. We, Ehrenfest's pupils, shall value these collected papers as a work of reference, as a historical document and as a worthy tribute to the memory of a great physicist, but reading them we shall also wistfully remember a great and inspiring teacher who was for us the central figure in a happy era of physics that will not come again.
H B G Casimir