German-born American physicist notable for a variety of contributions to science.

Elsasser received the Ph.D. from the University of Göttingen in 1927, then accepted teaching appointments at Frankfurt, Paris, and the California Institute of Technology. He became a U.S. citizen in 1940 and engaged in research for the U.S. Army Signal Corps and the Radio Corporation of America before returning to academic life, holding successive positions at the universities of Pennsylvania, Utah, and California; the Scripps Institution of Oceanography; and Princeton, Maryland, and Johns Hopkins universities.

While Elsasser was a graduate student he correctly predicted that a beam of electrons would be diffracted by a crystalline material; after the neutron was discovered, he predicted the same behaviour for neutrons. Independently of Nobel Prize winner Hans Bethe, Elsasser carried out important work on the likelihood of certain interactions between neutrons and atomic nuclei.

Elsasser formulated what is called the dynamo (dynamo theory) model of the Earth's structure to account for the origin and properties of its magnetic field. He suggested that convection within the core, driven by the energy of radioactive decay, generates electric currents that interact with concentric spherical shells of the Earth's mantle, which are rotating at different speeds, and that the Coriolis effect produces eddies at the boundaries of these shells.

He also investigated the applicability of contemporary theoretical concepts of physics, especially quantum mechanics, to the biological sciences, presenting his views in The Physical Foundation of Biology (1958), Atom and Organism (1966), and The Chief Abstractions of Biology (1975).