Leonid Isaakovich Mandelstam (4.05.1879 —27.06.1944) was a Soviet physicist and an academician (1929; a corresponding member in 1928). He was born in Mogilev. He studied at Novorossiysk University in Odessa, from which he was expelled for taking part in student unrest. He graduated from the University of Strasbourg (1902), where he worked (professor from 1913). In 1914 he returned to Russia.
In 1918-22 he was head of the department of physics at the Odessa Polytechnic Institute. From 1925 he was head of the department of theoretical physics at Moscow University and a member of the university's Research Institute of Physics, and from 1934 he also worked at the Physical Institute of the USSR Academy of Sciences.
His work was in the fields of optics, radiophysics, the theory of non-linear oscillations, quantum mechanics, and the history and methodology of physics. As early as 1907, in the paper “On optically homogeneous and turbid media”, which dealt with the scattering of light, he proved that the theories of Planck and Rayleigh then prevailing were mistaken, showing that scattering is caused by the inhomogeneity of the medium and that homogeneous media do not scatter light. Later research by M. Smoluchowski, A. Einstein and Mandelstam himself proved that these inhomogeneities are caused by statistical fluctuations of the density of the medium. In 1926, independently of the French physicist L. Brillouin, he predicted (the idea and clear arguments had been put forward as early as 1918 — 21) that the scattering of light by an elastic medium should show a splitting of the line of the scattered light (the Brillouin — Mandelstam effect). Attempts to detect this effect experimentally led him and G. S. Landsberg in 1928 to the discovery of the Raman scattering of light (independently of the Indian physicists C. Raman and K. S. Krishnan). Together with G. S. Landsberg he discovered the selective scattering of light, and with M. A. Leontovich he developed a theory of the scattering of light in solids. He gave a rigorous mathematical theory of optical images. In quantum theory he proposed many fruitful ideas. Together with M. A. Leontovich he developed (1928) a theory of the passage of a particle through a potential barrier. The scattering matrix was predicted in his ideas. Mandelstam was one of the first to understand the physical significance of relativity and did much to explain and promote it. Together with I. E. Tamm he gave a more general interpretation of the uncertainty relation in terms of “energy — time”. For his work in physics he was awarded the V. I. Lenin Prize in 1931.
Mandelstam's research on the theory of oscillations, most of it carried out together with N. D. Papalexi, led to the creation of a new line of work, the theory of non-linear oscillations. In extending this theory Mandelstam covered oscillatory processes in radio engineering, acoustics, automatic control and aerodynamics. His name is connected with the discovery of new kinds of resonance, in particular resonance of the nth kind, and with the creation of its theory, with the generalisation and deepening of the concept of resonance, with the construction of a theory of steady oscillations at resonance of the nth kind, with the detection of asynchronous excitation and with the development of a new method of generating electromagnetic energy by means of parametric generators.
Together with N. D. Papalexi he made a substantial contribution to radiophysics and radio engineering. He developed new methods of radiotelegraph and radiotelephone modulation and radio-interference methods of studying the propagation of radio waves and of measuring distance, and he created a new field of radio engineering, radiogeodesy, and others. For the development of the radio-interference technique and the practical results obtained he was awarded a USSR State Prize in 1942.
He founded a school of physicists (A. A. Andronov, A. A. Vitt, G. S. Landsberg, M. A. Leontovich, I. E. Tamm, S. P. Shubin, S. M. Rytov, G. S. Gorelik, V. V. Migulin, S. P. Strelkov, P. A. Ryazin, S. E. Khaikin and others).