Walter Elsasser (born 20.03.1904) was a physicist and geophysicist. He was born in Mannheim (Germany). He studied at the universities of Heidelberg, Munich and Göttingen. In 1928 — 30 he worked at the Technical University in Berlin, in 1930 — 33 in Frankfurt, in 1933 — 36 at the A. Poincaré Institute at the University of Paris, in 1936 — 41 at the California Institute of Technology, in 1947 — 50 at the University of Pennsylvania, in 1950—56 he was professor at the university in Ithaca, in 1956—62 he was at the University of California in San Diego, and in 1962 — 68 at Princeton University. In 1968 — 74 he was professor at the University of Maryland.
His work concerned nuclear physics and the physics of the Earth. In 1933—34 he came to the conclusion that nuclei with numbers of protons or neutrons equal to 2, 8, 20, 50, 82 and 126, later named “magic numbers”, should have a particular stability. In trying to give a theoretical basis to the empirical regularities, he suggested that each nucleon of the nucleus moves in the averaged field of the remaining nucleons, considered the quantum-mechanical problem of the motion of a nucleon in a rectangular potential well, and found the order in which the levels are filled (the shell structure of the nucleus). In 1925 he proposed using crystals to observe the diffraction of electrons and to prove their wave nature, and in 1936 he predicted the interference of neutrons.
He was a member of the National Academy of Sciences of the USA (1957).