Epstein, Paul Sophus

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(b. Warsaw, Poland [then Russia], 20 March 1883; d Pasadena, California, 8 February 1966)

theoretical physics.

Epstein’s research career spanned the development of quantum theory based on classical mechanics, to which he made fundamental contributions, into quantum mechanics. He was born into a patrician Polish-Russian family of successful businessmen, rabbinical scholars, and philanthropists. His father, Siegmund Simon Epstein, was a road-building contractor and insurance broker. His maternal grandfather, Chaim Lur’ia, was a prominent civic leader in the Jewish community in the city of Minsk, Russia, Epstein’s mother, Sarah Sof’ia Lur’ia, who was considerably more ambitious and intellectual than her husband, studied at the medical school for women at St. Petersburg University, corresponded with many literary figures, including Fedor Dostoeevskii, and nurtured her son’s scientific aspirations. Epstein’s parents divorced when he was a child and he grew up in Minsk, graduating in 1901 from the local high school with a gold medal in mathematics. He spoke Russian and German fluently as a boy and by the age of ten had decided to study physics.

In 1901 Epstein entered the School of Physics and Mathematics at the Imperial University of Moscow. He intended to study mathematical physics, but since there were no professors in this field at Moscow, he worked under Petr N. Lebedev, an experimental physicist best known for measuring the pressure of light. The lectures were available in printed form, so Epstein rarely attended them, aside from the basic two-year lecture demonstration course in physics. The problem courses interested him intensely, however, and the experience of solving problems at the blackboard before the whole class, he later said, gave him a feeling for the subject. An excellent physics library, coupled with the research colloquium that undergraduates could attend but not participate in, directed him to books and journal articles. He earned a bachelor’s degree in science in 1906 (the revolution of 1905 having delayed his senior examinations by one year) and then enrolled as a graduate student.

Epstein was a laboratory instructor in physics at the Moscow Institute of Agriculture (1906–1907), and then at the Imperial University of Moscow (1907–1909), while conducting experimental research on the dielectric constant of gases. In 1909 he received a master’s degree in physics and became the equivalent of an assistant professor. That December, at a scientific congress in Moscow, he met the theoretical physicist Paul Ehrenfest, who directed his attention to the West.

Having decided that he was not cut out to be an experimentalist (“My hands were not clever enough” ), Epstein left Moscow in early 1910 for Munich. There he attended Arnold Sommerfeld’s lectures on relativity, then studied the theory of electromagnetic waves, particularly the theory of diffraction, and finally spent, by his own account, four years at Sommerfeld’s Institute for The oretical Physics.

Epstein’s first contact with psychoanalysis dates from this period. Plagued by depression and stomach troubles, he spent several months in 1911, and again in 1912, in a sanitarium in Switzerland, undergoing analysis. An enthusiastic and ardent student of Freud’s ideas from then on, Epstein went on to cofound the Los Angeles Psychoanalytic Study Group in the late 1920’s, the precursor of the Los Angeles Psychoanalytic Society and Institute.

In 1914 Epstein received the Ph.D. in physics with minors in mathematics and crystallography from the University of Munich. During World War I, Epstein was classified as an enemy alien by the Germans, and he was interned briefly. After his release, Epstein continued his research, but was not allowed to leave Germany until the war’s end. His interest in problems of quantum theory coincided with the publication of Sommerfeld’s paper on the fine structure of atomic hydrogen (1916).

Epstein wrote a series of important papers on quantum theory and its applications. In his classic paper on the theory of the Stark effect, the splitting of the spectral lines in a hydrogen atom by a strong electric field (1916), he worked out the quantization rules in an invariant form and then used them to calculate the splitting of the hydrogen lines. The splitting effect, first observed by Johannes Stark in 1913, could not be explained along classical lines. Showing that Niels Bohr’s Quantum description of the hydrogen atom could solve the problem made Epstein’s reputation as a theoretical physicist. The match between his theoretical predictions and Stark’s data furnished striking support for the RutherfordBohr atomic theory.

Working independently, the German astronomer Karl Schwarzschild publicly announced the solution to the same problem one day after Epstein’s paper appeared. (As Epstein relates it, Schwarzchild initially had the wrong formula. He corrected it after seeing an announcement of the development of Epstein’s result.) Epstein’s other contributions to the development of the quantum theory of atomic structure between 1916 and 1921 ranged from his extension of Bohr’s theory to nonperiodic motions, including beta decay and the photoelectric effect, to the interference of spectral lines.

After two years as Privatdozent at Zurich (1919–1921) and a short time as assistant to Hendrik A. Lorentz at Leiden, Epstein joined the faculty of the new California Institute of Technology in 1921 as professor of theoretical physics. Except for two years that he spent as an exchange professor (1927, 1929) at the Aachen Institute of Technology in Germany, he served for thirty-two years on the Caltech faculty, becoming professor emeritus in 1953.

Epstein played a significant role in the foundation of Caltech’s physics division, introducing and teaching virtually all the theoretical physics courses in the early years. Out of this experience came Textbook of Thermodynamics (1937) and a deep interest in Willard Gibbs’s statistical mechanics. With Robert A. Millikan he organized and ran the weekly physics research seminar, which became a Thursday afternoon tradition; he also built up the physics library. In 1930 he was elected to the National Academy of Sciences.

At Pasadena, Epstein worked on perturbation theory and the application of Schrödinger’s wave mechanics to the Stark effect. In later years his research work mainly fell at the border of physics, acoustics, and hydrodynamics. During World War II he was a consultant for the U.S. Navy Sound Laboratory in San Diego and for the Army Air Corps’s Meteorology Project at the Institute.

Outside of physics Epstein’s interests ranged over the fields of philosophy, art, history, and psychoanalysis. He married twice. His first wife, Mina, was a concert pianist. They were married in 1909, separated in 1911, and divorced in 1919. Epstein called the marriage “the greatest stupidity that I ever committed in my life” and kept Mina’s surname a secret. In 1930 he married Alice Emelie Ryckman; they had one daughter.


I. Original Works. Epstein’s papers referred to in the text are “Zur Theorie des Starkeffektes,” in Annalen der Physik, 4th ser., 50 (1916), 489–520; “Versuch einer Anwendung der Quantenlehre auf die Theorie des lichtelektrischen Effekts und der β-Strahlung radioaktiver Substanzen,” ibid., 815–840; “Über die Interferenzfähigkeit von Spektrallinien vom Standpunkt der Quantentheorie,” in Sitzungsberichte der Bayerischen Akademie der Wissenschaften zu München, Math.-phys. K1. (11 January 1919), 73–90; “Problems of Quantum Theory in the Light of the Theory of Perturbation,” in Physical Review19 (1922), 578–608; “The Stark Effect from the Point of View of Schrödinger’s Quantum Theory,” ibid., 28 (1926), 695–710; “Application of Gibbs’ Methods to Modern Problems of Thermodynamics,” in Commentary on the Scientific Writings of J. Willard Gibbs. I (New Haven, 1936), 59–112; and “Critical Appreciation of Gibbs’statistical Mechanics,” ibid., 521–584. One scientific article omitted from the bibliography (see below) is “Ferrite Post in a Rectangular Wave Guide,” in Journal of Applied Physics, 27 (1956), 1328–1335, written with A. D. Berk.

Epstein’s notebooks, autobiographical notes, oral history, correspondence, and manuscripts are deposited in the archives of the California Institute of Technology. For an appreciation of his intellectual development, see “Paul S. Epstein,” transcript of an oral interview conducted by John L. Heilbron, Archive for the History of Quantum Physics, Office for History of Science and Technology, University of California, Berkeley, 25 and 26 May and 2 June 1962. Additional source material is listed in Thomas S. Kuhn et al., Sources for History of Quantum Physics (Philadelphia, 1967).

II. Secondary Literature. See Jesse W. M. DuMond, “Paul Sophus Epstein,” in Biographical Memoirs. National Academy of Sciences, 45 (1974), 127–152, with portrait and bibliography; and the obituary notice in Naturwissenschaftliche Rundschau, 19 (1966), 170, Technical details of Epstein’s work on the Stark effect are covered in G. Birtwistle, The Quantum Theory of the Atom (Cambridge, 1926), 97–111.

Judith R. Goodstein