Paul Sophus Epstein (20.03.1883-9.02.1966) was a theoretical physicist. He was born in Warsaw. He graduated from Moscow University (1906) and worked there in 1907-14, and in 1919-21 at the University of Zurich. In 1921—53 he was professor at the California Institute of Technology.
His work was in the field of the theory of diffraction, quantum theory and quantum mechanics. He constructed the theory of diffraction by a parabolic cylinder. In 1916, independently of K. Schwarzschild, he formulated the general quantum theory of multiply periodic systems, which he applied successfully to the explanation of the Stark effect.
He was a member of the National Academy of Sciences of the USA (1930).