Nikolai Nikolayevich Bogoliubov (born 21.08.1909) is a Soviet theoretical physicist and mathematician, an academician of the USSR Academy of Sciences (1953; corresponding member 1946) and an academician of the Academy of Sciences of the Ukrainian SSR (1948). He was born in Nizhny Novgorod (now Gorky). In 1928—73 he worked at the Academy of Sciences of the Ukrainian SSR (from 1965 as director of its Institute of Theoretical Physics) and in 1936—59 he was professor at Kiev University. At the same time he has been, from 1948, at the V. A. Steklov Mathematical Institute of the USSR Academy of Sciences, from 1956 at the Joint Institute for Nuclear Research (from 1965 as director) and from 1950 professor at Moscow University.
His research in physics was in the fields of statistical physics, quantum field theory, the theory of elementary particles and mathematical physics. In 1932—43, together with N. M. Krylov, he created the theory of nonlinear oscillations. He developed methods of the asymptotic integration of the nonlinear equations that describe various oscillatory processes, and gave them a mathematical foundation. He extended the methods he had developed to statistical mechanics. He was the first to put forward (1945) the idea of a hierarchy of relaxation times, which is of great importance in the statistical theory of irreversible processes, and he created (1946) an effective method of chains of equations for the distribution functions of groups of particles.
Bogoliubov obtained important results in quantum statistics. He extended (1947) the method of kinetic distribution functions to quantum systems, developing a general procedure for constructing kinetic equations for such systems, and he applied the method of constructing hydrodynamic equations (1963) to building the hydrodynamics of a superfluid liquid. He developed (1946) the method of approximate second quantisation for determining the energy spectrum of the weakly excited states of quantum systems, and calculated (1947—48) the spectrum of the elementary excitations of a weakly non-ideal degenerate Bose gas, showing that its spectrum has the same properties as the spectrum of helium II, as a result of which a theoretical model was created to explain the phenomenon of the superfluidity of helium II. For his research in statistical physics Bogoliubov was awarded USSR State Prizes in 1947 and in 1953. He constructed (1958) the microscopic theory of superconductivity, showed that superconductivity can be regarded as the superfluidity of the electron gas, and developed the microscopic theory of superfluidity. The further development of the idea of superconductivity as the superfluidity of Fermi systems led Bogoliubov (1958) to the discovery of the superfluidity of nuclear matter. In 1958, for the development of a new method in quantum field theory and statistical physics which led, in particular, to a foundation for the theory of superfluidity and superconductivity, Bogoliubov was awarded the Lenin Prize. For the study of superconducting and superfluid systems Bogoliubov developed (1968) a generalised Hartree — Fock method in which the existence of correlated pairs of particles is taken into account. To him belongs also the idea of “quasi-averages”, which is important in the theory of phase transitions.
The fundamental results obtained by Bogoliubov in statistical physics were extended into quantum field theory, where a number of results of the first importance belong to him, in particular a new formulation of quantum field theory (the S-matrix in the interaction representation). At the basis of this formulation Bogoliubov placed not the traditional Hamiltonian formalism but the Heisenberg scattering S-matrix, which satisfies the requirements of covariance, unitarity and causality. The causality condition for the S-matrix, written in the language of variational derivatives, is known as Bogoliubov's microcausality condition (1955). Together with D. V. Shirkov he developed the renormalisation group method (1955). He gave the first proof of the dispersion relations in quantum field theory. The system of basic propositions used to derive the dispersion relations was later developed by him into an independent general approach to the construction of a relativistic quantum theory of the interaction of particles. In the field of the symmetry of strong interactions he worked on quark models. Independently of others he proposed (1965) a three-triplet quark model and introduced a new quantum number, colour.
He was the founder of a large school of theoretical physics (A. A. Logunov, D. V. Shirkov, O. S. Parasyuk, V. A. Moskalenko, A. N. Tavkhelidze, I. Todorov, V. P. Shelest, N. N. Bogoliubov junior, P. N. Bogoliubov, V. L. Bonch-Bruevich, V. G. Kadyshevsky, V. A. Meshcheryakov, M. K. Polivanov, V. G. Solovyev, L. D. Solovyev, B. V. Struminsky, S. V. Tyablikov and others). An honorary member of many academies of sciences and scientific societies. Twice Hero of Socialist Labour (1969, 1979). The M. V. Lomonosov Prize, the M. Planck (1973) and B. Franklin (1974) medals and others.