to cause plate tectonics. The toughness of the sublayers of the Earth's crust is much more stronger than assumed by Epstein. In 1930 he was elected a member of the National Academy of Sciences. In 1930, Epstein married Alice Emelie Ryckman, and the couple had a daughter Sari (Mittelbach). After World War II, concerned that some young American intellectuals were becoming attracted to communism, he joined the American Committee for Cultural Freedom and in 1951 he served as one of the three US delegates to the seminar the Congress held in Strasbourg. As well as quantum theory, Epstein also published papers in other fields. Examples include ""Zur Theorie des Radiometers" (1929), "Reflection of Waves in an Inhomogeneous Absorbing Medium" (1930) and "On the Air Resistance of Projectiles" (1931). Other subjects that he worked on were the settling of gases in the atmosphere, the theory of vibrations of shells and plates and the absorption of sound in fogs and suspensions. He retired as Emeritus Professor at Caltech in 1953 and continued to serve as a consultant for a number of industrial companies. One of the significant reports he wrote during this period was "Theory of Wave Propagation in a Gyromagnetic Medium" (1956). Epstein had other interests as well. He was very interested in Freudian psychology and was one of the founding members of a Psychoanalytic Study Group (together with Thomas Libbin) that later merged into the Los Angeles Institute for Psychoanalysis. In the 1930s he published two articles in the monthly literary and scientific magazine Reflex - "The Frontiers of Science" and "Uses and Abuses of Nationalism". Although he was not an active Zionist, Epstein served as a member of the American National Society of Friends of the Hebrew University and was very friendly with the prominent Israeli mathematician Abraham Fraenkel. The Epstein Memorial Fund was established through donations from more than fifty of his former students and a bronze bust of Epstein stands in the Physics and Mathematics section of the Millikan library at Caltech. See also Stark effect Notes External links Jesse W.M. DuMond, Paul Sophus Epstein, Biographical Memoirs V.45 (1974), National Academy of Sciences (NAS) Caltech Archives Oral Histories Online Interview with Paul S. Epstein Oral history interview transcript with Paul Sophus Epstein on 25 May 1962, American Institute of Physics, Niels Bohr Library & Archives - Session I Oral history interview transcript with Paul Sophus Epstein on