The Danish physicist Niels Henrik David Bohr (1885-1962) formulated the first successful explanation of some major lines of the hydrogen spectrum. The Bohr theory of the atom has become the foundation of modern atomic physics.

Niels Bohr was born on Oct. 7, 1885, in Copenhagen, the son of Christian Bohr and Ellen Adler Bohr. He studied physics and philosophy at the University of Copenhagen. His postgraduate work culminated in 1911 in a doctoral dissertation on the electron theory of metals.

In the same year he went to Cambridge University and worked with J. J. Thompson at the Cavendish Laboratory. By the spring of 1912 he was working with Ernest Rutherford at the University of Manchester. It was there that Bohr made some valuable suggestions about the chemical relevance of radioactive decay which proved to be most instrumental in formulating the concept of isotopes.

Secret of the Atom

Bohr's principal interest lay, however, in the planetary model of the atom, which Rutherford proposed in 1911. While pondering the implications of that model, Bohr became acquainted with Johannes Rydberg's studies of spectral lines and with J. J. Balmer's formula. As Bohr himself recalled in 1934, "As soon as I saw Balmer's formula the whole thing was immediately clear to me." The "thing" was the recognition on Bohr's part that basically different laws govern the atom when it is not in its stationary state but is absorbing or emitting radiation. He was no longer at Rutherford's laboratory when he succeeded in developing this revolutionary notion into a consistent and concise picture of the atom.

Meanwhile, in 1912 Bohr married Margrethe Norlund shortly after his return to Copenhagen, where he was appointed assistant professor at the university.

When Bohr asked Rutherford to recommend his now historic paper "On the Constitution of Atoms and Molecules" for publication, Rutherford admitted that Bohr's ideas as to the mode of origin of the spectra of hydrogen were very ingenious and worked very well, but he was unwilling to agree with Bohr's own evaluation of the paper. It took a special trip by Bohr to Rutherford in Manchester and a series of evenings during which the two carefully went over every paragraph in the paper before Rutherford's objections could be overcome. When the paper was published, in three parts in the Philosophical Magazine, June, September, and November 1913, reactions were divided. Some immediately expressed unreserved admiration, but there were doubters as well. In Einstein's eyes the paper was one of the great discoveries.

Copenhagen School

Bohr spent 2 years with Rutherford before returning to Copenhagen, where he began to think that the most effective cultivation of atomic and nuclear physics demanded a special institute, sheltering not only a well equipped laboratory, but also playing host to a large number of physicists from all over the world. In 1917 he approached the university with his plan, and as soon as the war was over the plan was enthusiastically approved. The institute was financed by public subscription, and the city donated a choice site to the Institute for Theoretical Physics, which soon established itself as the world center of theoretical physics.

Bohr's first major scientific award was the Hughes Medal of the Royal Society in 1921. The Nobel Prize followed the next year, but the finest tribute to Bohr was the steady stream of brilliant young physicists to his institute, which was dedicated on Sept. 15, 1920. Among the first to arrive at Bohr's institute was Wolfgang Pauli, and 2 years later, in 1924, came Werner Heisenberg, and shortly afterward Paul Dirac, to mention only some most important names in modern physics. In fact, there was hardly a major theoretical breakthrough in physics in the 1920s without some connection with the so-called Copenhagen school. Heisenberg's matrix mechanics, Erwin Schrödinger's wave mechanics, the demonstration of their equivalence by Max Born, Dirac, and P. Jordan, Pauli's theory of electron spin, Louis de Broglie's wave theory of matter--all entered the mainstream of physics through the animated discussions at Bohr's institute. Reminiscing on the 1920s, Bohr could rightly say that "in these years a unique cooperation of a whole generation of theoretical physicists from many countries created step by step, a logically consistent generalization of quantum mechanics and electromagnetics, and has sometimes been designated as the heroic age in quantum physics."

Principle of Complementarity

To use Bohr's own words, "a new outlook emerged," which put the comprehension of physical experience into radically new perspectives. Bohr contributed an important part to that new outlook when he outlined his principle of Complementarity in 1927. According to Bohr, waves and particles were two complementary aspects of nature which, as far as human perception and reasoning went, represented mutually irreducible aspects of nature. The wider implications of such an outlook were further articulated by Bohr in subsequent years, as he came to grips with such philosophical questions as indeterminism versus causality, and life versus mechanism.

Bohr's famous extension of the principle of complementarity to the question of life versus mechanism came in 1932 in a lecture entitled "Light and Life." In this lecture he first pointed out that an exhaustive investigation of the basic units of life was impossible because those life units would most likely be destroyed by the high-speed particles needed for their observation. For Bohr, the units of life represented irreducible entities similar to the quantum of energy. According to him, the "essential non analyzability of atomic stability in mechanical terms presents a close analogy to the impossibility of a physical or chemical explanation of the peculiar functions characteristic of life." Scientists who, because of the subsequent startling developments in molecular biology, claimed to have come to the threshold of a mechanistic explanation of life found no ally in Bohr. To the end of his life he held fast to the basic message of his now-classic lecture, as may be seen from his essay "Light and Life Revisited," written in 1962, the year he died.

An even more fundamental aspect of the principle of complementarity was the recognition that the observer and the observed represented a continuous interaction in which the two influenced and altered one another, however slightly. This meant that the rigid line of separation between the subjective and the objective needed some modification. This also meant a radical modification of the physicist's concept of the external world. The impact of the new insight into the correlation of the objective and the subjective was enormous also on the philosophical temper of the age. It seems indeed that the enunciation of the principle of complementarity by Bohr produced an insurmountable stumbling block for a mechanistic or reductionist explanation of the realm of reality as it is conceived and experienced by man.

Compound Nucleus and the Fission Process

With the discovery of the neutron in 1932, attention rapidly turned from electrons, which form the outer part of the atom, to the nucleus. To understand the various phenomena produced when nuclei of atoms were exposed to bombardment by neutrons, physicists first turned to Bohr's atom model. There the electrons moved largely independently of one another and were subject mainly to a field of force that was the average effect of the motion and position of all of them. The great number of nuclear resonances seemed, however, to point toward a rather different situation. The recognition of this came from Bohr himself, who proposed in 1936 that the protons and neutrons in the nucleus should be considered as a strongly coupled system of particles, in a close analogy to molecules making up a drop of water. In such a system there had to be a very large number of resonance levels of energy, and it also followed that a fairly long time could elapse before the available energy would concentrate on a single neutron resulting in its emission.

This picture of the "compound nucleus" formed the basis of Bohr's other crucial contribution to nuclear physics, the analysis of the fission process. In a paper written jointly with John A. Wheeler in 1939, he showed in quantitative detail the behavior of the compound nucleus for the cases of radiation, neutron emission, and fission. On this last point their all-important contribution consisted in arguing that in the fission of uranium it was mainly the isotope U²³⁵ that produced the effect under the impact of slow neutrons. It then became immediately clear that to obtain either a large-scale or a sustained, low-rate energy process by fissioning uranium, one had to achieve a separation of U²³⁵ in sufficient quantities from uranium ore in which the nonfissionable U²³⁸ was predominant.

A Towering Figure

After 1939, Bohr's life was largely devoted to humanitarian efforts, such as intervening for the Danish Jews; he had to save human lives, including his own and those of his family. Moreover, he felt duty-bound to prevent science from turning into a tool of wholesale destruction. Following his escape to Sweden in September 1943, he was quickly flown to England and from there to the United States. There he lent his talents to the Manhattan Project, and during his stay at Los Alamos he did work on the initiator phase of the activation of the atomic bomb. He also began to stress the need for international control of atomic weapons and energy. His view and arguments helped shape the Acheson-Lilienthal plan and the Baruch proposals to the United Nations on behalf of the American government. In 1950 he submitted in a letter to the United Nations a plea for an "open world where each nation can assert itself solely by the extent to which it can contribute to the common culture, and is able to help others with experience and resources." In the 1950s Bohr's principal contribution to science consisted in taking a leading part in the development of the European Center for Nuclear Research (CERN). It was at his institute that the decision was made to build the 28-Bev (billion-electron-volt) accelerator near Geneva.

From 1938 until his death he was the president of the Royal Danish Academy of Sciences, acted as chairman of the Danish Atomic Energy Commission, and supervised the first phase of the Commission's program for the peaceful uses of atomic energy. Bohr's last major appearance was to deliver the Rutherford Memorial Lecture in 1961, which gave a fascinating portrayal not only of the great master but also of his equally famous disciple.

Bohr's death came rather suddenly but quietly on Nov. 18, 1962, at his home. Einstein and he were possibly the most towering and influential figures of 20th-century physics.

Further Reading

• The best biography of Bohr is Ruth Moore, Niels Bohr: The Man, His Science and the World They Changed (1966). Stefan Rozental, ed., Niels Bohr: His Life and Work as Seen by His Friends and Colleagues (trans. 1967), is a most valuable collection of essays contributed by Bohr's closest friends and associates. On Bohr's role in 20th-century physics one should consult the papers written in his honor on his seventieth birthday, W. Pauli, ed., Niels Bohr and the Development of Physics (1955). See also Niels Hugh de Vaudrey Heathcote, Nobel Prize Winners in Physics, 1901-1950 (1953); Arthur March and Ira Freeman, New World of Physics (1962); and Henry A. Boorse and Lloyd Motz, ed., The World of the Atom (2 vols., 1966).