Bohr, Niels Henrik David

Born Oct. 7, 1885, in Copenhagen; died there Nov. 18, 1962. Danish physicist. Created the first quantum theory of the atom, and subsequently participated in the development of the fundamentals of quantum mechanics. Also made significant contributions to the development of the theory of the atomic nucleus and nuclear reactions and the processes of interaction of elementary particles with the environment.

Bohr graduated from the University of Copenhagen in 1908. At the university he carried out his first investigations of the surface tension of fluids (1907–10) and the classical electron theory of metals (1911). In 1911–12 he worked in Cambridge with J. J. Thomson and in Manchester with E. Rutherford. From 1914 to 1916 he taught a mathematical physics course in Manchester. In 1916 he obtained a professorship in theoretical physics in Copenhagen. From 1920 until the end of his life he directed the Institute of Theoretical Physics in Copenhagen, which he founded and which now bears his name. In 1943, when it became known that the Hitlerites who occupied Denmark were preparing a reprisal against Bohr, he was rescued by boat by the Resistance and taken to Sweden; from there he was taken to the USA by an English military plane. Here Bohr participated in the production of the atomic bomb. After the war he returned to Denmark. He actively took part in the struggle against the danger of atomic holocaust.

Working in Manchester, Bohr accepted the conception of the planetary atom formulated by Rutherford in 1911. However, at that time it was already clear that such a structure (a nucleus and electrons rotating around it in orbits) violated the laws of classical electrodynamics and mechanics. According to the laws of classical electrodynamics, the electron in an atom must continuously radiate electromagnetic waves, lose its energy during an insignificantly small fraction of a second, and fall into the nucleus. Consequently, according to classical physics, stable motions of electrons in an atom are impossible and the atom cannot exist as a dynamic system. Proceeding from the idea of the quantization of energy, proposed earlier by M. Planck in the theory of radiation, Bohr developed and in 1913 published a theory of the atom in which he showed that the atom’s planetary structure and the properties of its radiation spectrum can be explained if one considers the motion of the electron to be subject to certain additional restrictions, the so-called Bohr postulates. According to these postulates, there are selected, or “allowed” orbits for the electron. Moving in these orbits, the electron, contrary to the laws of classical electrodynamics, does not radiate energy. It can jump to another allowed orbit which is closer to the nucleus and thereby emit a quantum (portion) of electromagnetic energy which is proportional to the frequency of the electromagnetic wave. The theory of the atom constructed on these postulates and subsequently developed by Bohr himself and other physicists for the first time explained the exceptional stability of the atom and the preservation of its structure and the character of its spectrum during comparatively weak collisions.

In 1923, Bohr quantitatively formulated the so-called correspondence principle, which indicated precisely when these quantum restrictions existed and when classical physics sufficed. That same year, Bohr first succeeded in explaining Mendeleev’s periodic system of elements on the basis of his model of the atom. However, the Bohr theory as a whole contained an internal contradiction in its very foundation, since it joined in a mechanical way the classical concepts and laws with the quantum restrictions and this could not be considered a satisfactory situation. In addition, it was incomplete and insufficiently general because it could not be used for the quantitative explanation of all the various phenomena in the atomic world. A theory that could explain these phenomena was quantum mechanics—the theory of the motion of elementary particles created in 1924–26 by L. de Broglie, W. Heisenberg, and E. Schrödinger.

However, the basic ideas of quantum mechanics, in spite of its formal successes, at first remained unclear in a number of points. For the complete understanding of the physical basis of quantum mechanics and its connection with classical physics, a further thorough analysis was necessary of the relation of classical (macroscopic) and quantum (microscopic, at atomic and subatomic levels) material objects, of the process of measuring the characteristics of an elementary particle, and, in general, of the physical content of the theoretical concepts. This analysis required intensive work in which Bohr played a leading role. His institute became the center of these studies. Bohr’s main idea consisted of the fact that the dynamic characteristics of microparticles (for example, the electron)—its coordinates, momentum, energy, and so forth—which were borrowed from classical physics, are not at all intrinsic to the particle by itself. The significance and the specific value of one or another of the characteristics of the electron—for example, its momentum—are revealed only in the interaction with classical objects, for which these quantities have a specific meaning and can all simultaneously have a specific value. (Such a classical object is conventionally called a measuring device.) This idea is not only of major physical but also of philosophical significance. As a result, a systematic, extremely general, and internally consistent theory was developed explaining all the known processes in the microscopic world for the nonrelativistic region (that is, for particle velocities that are small in comparison with the velocity of light) and which, in the limiting case, leads automatically to the classical laws and concepts when the object becomes macroscopic. In addition, the foundations of the relativistic theory were laid.

In 1927, Bohr formulated the major principle of complementarity. This principle asserted the impossibility, in observing the microworld, of combining the instruments of two fundamentally different classes, corresponding to the fact that in the microworld there are no conditions under which an object simultaneously has exact dynamic characteristics that belong to two welldefined, mutually exclusive classes. This, in its turn, is determined by the fact that there exist no such sets of classical objects (measuring devices) in relation to which the microobject simultaneously has exact values of all dynamic quantities.

In 1936, Bohr formulated a picture fundamental to nuclear physics of the course of nuclear reactions (the compound nucleus model). In 1939, together with J. A. Wheeler, he developed a theory of nuclear fission, a process releasing enormous quantities of nuclear energy. In the 1940’s and 1950’s, Bohr worked mainly on the problem of the interaction of elementary particles with the environment.

Bohr created a large school of physicists and greatly contributed to the development of cooperation between physicists all over the world. Bohr’s institute became one of the most important scientific centers in the world. Physicists trained in this institute work in almost all the countries of the world. Bohr also accepted Soviet scientists into his institute, many of whom worked there for considerable lengths of time. Bohr came to the USSR a number of times and in 1929 was elected a foreign member to the Academy of Sciences of the USSR. He was a member of the Danish Royal Scientific Society (from 1917) and of many of the world’s academies and scientific societies. He won a Nobel prize in 1922.