Oskar Benjamin Klein (15.09.1894-5.02.1977) was a Swedish theoretical physicist and a member of the Royal Swedish Academy of Sciences (1945). He was born in Stockholm. He graduated from Stockholm University (1922). In 1927-28 he worked at the Institute of Theoretical Physics in Copenhagen, and in 1928—62 at Stockholm University (professor from 1931).
His research concerned quantum mechanics, quantum electrodynamics, the physics of elementary particles, general relativity, cosmology and statistical physics. In 1926 he wrote down the simplest relativistic wave equation for particles of zero spin (the Klein — Fock — Gordon equation). In 1927-28 Klein, and also P. Jordan and E. Wigner, showed that the fundamental equation of non-relativistic quantum mechanics can be formulated in the language of field operators (the Jordan — Klein — Wigner operator field theory). Together with Y. Nishina he developed (1929) the theory of the Compton effect (the Klein — Nishina equation). In relativistic quantum theory he is also known for the “Klein paradox”. Independently of others he put forward the idea of the universality of the Fermi interaction. He was one of the first to consider general relativity in connection with quantum theory. He constructed (1928) a theory of gravitation (the Klein — Kaluza theory). His research was also in the field of meson theory and the systematics of particles.
In 1953 —65 he was a member of the Nobel Committee. The M. Planck Medal (1959).