Niels Henrik David Bohr (7.10.1885 - 18.11.1962) was an outstanding Danish theoretical physicist, one of the creators of modern physics and a member of the Royal Danish Academy of Sciences and Letters (1917), of which he was president from 1939. He was born in Copenhagen. He graduated from the University of Copenhagen (1908). In 1911 — 12 he worked in Cambridge under J. J. Thomson, and in 1912—13 in Manchester under E. Rutherford. From 1916 he was professor at the University of Copenhagen, and from 1920 director of the Institute of Theoretical Physics, which he had founded and which became an international centre for theoretical physicists and played a large part in international contact between scientists (the Niels Bohr Institute).
Bohr took shape as a scientist in a period of great tension for physics, when it had come right up against the study of the world of atomic processes and of the fields associated with them. The work of M. Planck and A. Einstein and the analysis of the emission spectra of atoms had already shown how unusual the regularities of the microworld were. An enormous body of experimental material had been accumulated, highly contradictory in the light of the laws known previously. A fundamentally new approach was needed to create a physical picture of atomic processes. Bohr's important achievement was that he found such an approach. He directed physicists towards the study of the contradictory aspects of the physical reality of the microworld, formulated the idea that the energy states of atoms are discrete, constructed an atomic model in the light of the new ideas, discovering the conditions for the stability of atoms, and explained a wide range of phenomena.
In 1913, proceeding from M. Planck's idea of the quantisation of energy, Bohr constructed on the basis of Rutherford's model of the atom his own theory of the hydrogen-like atom, founded on two postulates that directly contradicted classical ideas and laws. This was the first quantum model of the atom, and it began a new era in atomic theory. According to this theory the planetary structure of the atom and the properties of its emission spectrum are easily explained if one assumes that the motion of an electron in the atom “obeys” certain restrictions, which Bohr formulated as two postulates. Bohr established that the atom has stationary allowed orbits, moving along which an electron, contrary to the laws of electrodynamics, does not radiate energy, but can jump to another allowed orbit closer to the nucleus, emitting in the process a quantum of energy equal to the difference between the energies of the atom in the stationary states; and he worked out some rules of quantisation. Bohr's theory made it possible to explain a whole series of difficult questions of the structure of the atom, and of facts, that classical physics was unable to explain. In particular, Bohr found the fundamental laws of the spectral lines and the electron shells of atoms, explained (1923) the features of the periodic system of the chemical elements, proposing a version of his own for representing the periodic system of the elements, and in the same year arrived at the idea of the shell structure of the atom, based on the classification of electron orbits by the principal and azimuthal quantum numbers. In 1922 he was awarded the Nobel Prize for constructing the quantum theory of the planetary atom.
In 1918 he formulated the correspondence principle, important for the new atomic theory, which shows exactly when quantum restrictions matter and when classical physics is sufficient. Bohr did much for the establishment and the interpretation of quantum mechanics, which arose in the 1920s. Through the efforts of Bohr and of his colleagues and pupils a coherent system of the physical ideas of quantum mechanics was built up. In 1927 he formulated the principle of complementarity, important for understanding it, and this led to his well-known discussions with A. Einstein about determinism. Einstein's criticism, as Bohr himself acknowledged, helped him towards a deeper understanding of quantum mechanics.
In interpreting the new ideas and facts, and in the theory of knowledge, Bohr sometimes advanced mistaken philosophical propositions, and some of his formulations gave grounds for his statements to be construed in a positivist spirit. However, as V. A. Fock writes, “the general impression made by all of Bohr's work, from the very earliest, is of its deep dialectical quality. Bohr is not troubled by the contradictions that arise when essentially new phenomena of nature are approached from the point of view of old concepts and old views; he looks for the resolution of the contradictions in new ideas”. Soviet physicists and philosophers did much to give a materialist interpretation of Bohr's positive contribution to physics. They pursued a line intended to rule out any possibility whatever of the positivists making use of that contribution.
Bohr also did much for the development of nuclear physics. He was the author of the theory of the compound nucleus (1936), one of the creators of the liquid-drop model of the nucleus (1936) and of the theory of the fission of the atomic nucleus (1939). Together with J. Wheeler (independently of Ya. I. Frenkel) he gave a quantitative interpretation of nuclear fission, introducing the so-called fission parameter, and predicted the spontaneous fission of uranium.
In 1912 he formulated an important theorem of statistical mechanics, rediscovered in 1919 by J. van Leeuwen (the Bohr — van Leeuwen theorem).
He founded a large international school of physicists (F. Bloch, A. Bohr, V. Weisskopf, H. Casimir, O. Klein, H. Kramers, L. D. Landau, C. Møller, Y. Nishina, A. Pais, L. Rosenfeld, S. Rosseland, J. Wheeler and others)
An honorary member of more than 20 academies of sciences of the world, and in particular a foreign member of the USSR Academy of Sciences (1929).