SUPPLEMENT TO “REMINISCENCES OF PROFESSOR P. EHRENFEST” BY G. E. UHLENBECK*)

A. F. Ioffe

Pavel Sigizmundovich Ehrenfest exerted a great influence not only on the physicists of Holland, where after Lorentz he held the chair of theoretical physics, but also on the development of Soviet physics. The circumstance that he had taken the chair of Lorentz himself later constituted the tragedy of his life. He considered himself unworthy of such a high position and thought that he had usurped the place of someone more worthy whenever one of his pupils came to the fore. He persistently sought a way out, trying, for example, to obtain a chair in Siberia or in the Urals. For Leningrad, Moscow, and Kiev he saw physicists whom he regarded as far superior to himself, in particular L. D. Landau. Ehrenfest died in the year when Hitler seized power in Germany.

Meanwhile, it was precisely in Ehrenfest’s time that Leiden became a world center of theoretical physics, where all the best physicists came to study or to talk with Ehrenfest. On the second floor of his house there was a room on whose white wall those who lived in it left their signatures. Here were Einstein and Bohr, and Germans, and Americans, and Englishmen, and, of course, Russians, who were especially close to the heart of Pavel Sigizmundovich.

For several years Pavel Sigizmundovich lived in Leningrad, passed, as one of the first after a 25-year interval, the master’s examination, but was not admitted to lecture at the university. Only the Polytechnic Institute—then the most progressive—ventured to offer him an optional course on the differential equations of physics. And what a course it was! In no treatment of this classical subject were physics and mathematics combined into such a harmonious whole.

I remember that, during my master’s examinations, when I told Academician Steklov some of Ehrenfest’s generalizations (referring to him), this made a strong impression and, I think, for a long time determined Steklov’s opinion of my mathematical erudition.

Still more was given by the seminars held under Ehrenfest’s direction, or even merely with his participation, and by conversations with him. Uhlenbeck has shown this convincingly.

I remember a seminar in Leiden where the same Uhlenbeck, then still a beginning physicist, presented the hypothesis of the rotating electron and of “spin” as a fourth coordinate in Pauli’s scheme. One may say with certainty that for the speakers themselves (Goudsmit and Uhlenbeck) the physical meaning

*) Academician Abram Fedorovich Ioffe, who at one time was closely associated with P. S. Ehrenfest, kindly sent his supplement to G. E. Uhlenbeck’s “Reminiscences of Professor P. Ehrenfest,” which is published here. (Editors.)

their hypotheses became fully clear only in the course of discussion, as a result of Ehrenfest’s questions and answers.

For me personally, contact with Ehrenfest was of enormous importance. During all the years of his stay in St. Petersburg, he set out his thoughts daily in letters, and twice a week we discussed them together with other physical questions at his apartment. I still have these letters.

Those were pre-revolutionary years, and much in the system of scientific work aroused our indignation. In particular, the arrangement of master’s examinations, which for several years tore people away from active scientific work and yet proved insurmountable for such physicists as V. K. Lebedinsky, A. A. Gershun, and V. F. Mitkevich. I remember Ehrenfest’s speech at the congress of natural scientists in Moscow, when, after telling how the master’s examinations were organized in Petersburg, he described how D. S. Rozhdestvensky, having left his scientific research and declined to take part in the congress in order to cram himself with useless knowledge, could not endure it and burst into tears.

Ehrenfest lived by the interests of Russian science, loved and respected it, as he later proved when, after the revolution and the blockade, he became the link connecting young Soviet physics with the West.

I recall with gratitude how much energy Ehrenfest expended in arranging scientific trips abroad for the present academicians V. A. Fock and D. V. Skobeltsyn.

In a brief note it is difficult to tell of the significance and the distinctive character of Ehrenfest’s contribution to the development of physics.

We owe to him the physical understanding of Boltzmann’s \(H\)-theorem (Boltzmann was Ehrenfest’s teacher), of Planck’s quantum hypothesis, of Wien’s displacement law and its generalization in adiabatic invariants, of the mechanism of thermoelectromotive forces, and of many other fundamental foundations of modern physics. To this day the best exposition of statistical physics is the article in the Encyclopedia of Mathematical Sciences, written by P. S. Ehrenfest together with his wife, Tatyana Alekseevna Afanasyeva-Ehrenfest.

In the person of Pavel Sigizmundovich, physics lost one of the brightest minds; he was able, in every physical question, to reveal its essence; he was able to see in physics not isolated facts, but an integral, interconnected picture of captivating beauty and harmony. For us he was a faithful friend of Soviet science, a friend and teacher of many Soviet physicists.

Scattered throughout the world are scientists indebted to Ehrenfest for his interest in the problems of physics. I shall end with what I began: Ehrenfest’s activity left its mark on the development of physics not only through his published works, but also through his personal influence.