Isaak Markovich Khalatnikov. 17 October 1919—9 January 2021

Abstract

Isaak Khalatnikov made major contributions to two different branches of theoretical physics: quantum liquids, particularly liquid helium, and general relativity and cosmology. He made outstanding contributions to our theoretical understanding of superfluidity in liquid 4He and in 3He–4He mixtures. He developed the formulation of the two-fluid hydrodynamic equations, including the proper description of very complex dissipative and nonlinear effects. He also developed the idea, originally due to Landau, that the normal fluid can be described in terms of a gas of weakly-interacting excitations. His work on general relativity and cosmology began with the paper written with E. M. Lifshitz (ForMemRS 1982) in 1963. They extended Lifshitz's pioneering paper of 1946, showing how small departures from homogeneity and isotropy in the early Universe would have grown as the Universe expanded. They also tackled the problem of whether or not the equations of general relativity predict that space-time contains singularities, either in the past or in the future. They initially concluded that more general solutions would not contain singularities, but later work with Belinskiĭ not only showed that general solutions could contain singularities, but also indicated how the Universe would behave near a singularity. Later, Khalatnikov showed that nearly all cosmological solutions with a massive scalar field have a period of exponential or inflationary expansion. He became the founding director of the Landau Institute for Theoretical Physics of the USSR Academy of Sciences in 1964. He brought together a powerful group of theoreticians, creating a scientific centre that has played a major role in the development of theoretical physics.