Niels Henrik David Bohr Biography

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Niels Henrik David Bohr was born in Copenhagen, Denmark, on October 7, 1885, the second of three children born to Christian and Ellen Adler Bohr. Bohr's mother was a member of a wealthy Jewish banking family, and his father was a talented professor of physiology at the University of Copenhagen who conducted quantitative research on the physical processes underlying physiological functions. Bohr received his primary and secondary education at the Gammelholm School in Copenhagen.

In 1903, Bohr enrolled at the University of Copenhagen, where he majored in physics. His first research project was a precise measurement of the surface tension of water. He earned his bachelor of science degree in 1907, his master of science degree in 1909, and his doctorate in 1911, all at the University of Copenhagen. Bohr's thesis, which dealt with the electron theory of metals, convinced him that classical electromagnetic theory was inadequate for describing the quantitative properties of metals.

After completing his degree, Bohr joined J. J. Thomson, who had discovered the electron, at the Cavendish Laboratory at Cambridge University. Finding that Thomson was uninterested in his ideas, Bohr moved on in 1912 to spend three months at the University of Manchester working with Ernest Rutherford. Rutherford had the previous year published his nuclear model of the atom in which electrons orbit the atomic nucleus much like planets orbiting the sun. The chief shortcoming of Rutherford's model was that it could not explain why electrons in orbit about the nucleus did not lose energy and spiral into the nucleus. In July of 1912, Bohr returned to the University of Copenhagen to take a position as assistant professor of physics.

In 1913, Bohr published a series of papers describing his idea that electrons orbit the nucleus at fixed distances from the center of the atom, and that it requires the gain of loss of a discrete amount of energy for an electron to change its orbit. In Bohr's model, the electron's orbital momentum was quantized, i.e., it assumed only discrete values. Bohr explained atomic radiation as the emission of quantum amounts of radiation as electrons underwent transitions from one orbit to another. Unlike Rutherford's model, which predicted that the hydrogen spectrum should be continuous, Bohr's model accounted for the set of spectral lines that were actually observed. Although Bohr's work seemed to have a commonality with earlier work on quantized systems carried out by Max Planck and Albert Einstein, there really was no theoretical justification at the time for his ideas. But as a descriptive model of observed phenomena, Bohr's model seemed to work.

Bohr's ideas were slow to catch on with his contemporaries, many of whom were reluctant to accept the need for a break with classical physics that Bohr's theory seemed to imply. But as new spectral data were obtained that confirmed the predictions of the theory, the model, which was later shown to be a special solution of the Schrödinger equation, gained acceptance. The model was eventually found capable of explaining variations in properties of the elements and the chief features of X-ray and optical spectra exhibited by all of the elements.

In 1914, Bohr was appointed Reader in Theoretical Physics at Manchester University, where he remained for the next two years. In 1916, he returned to Copenhagen, and two years later he became the first director of the Institute for Theoretical Physics at the University of Copenhagen. This institute was to become one of the leading intellectual centers in Europe for young theoretical physicists. Bohr later characterized his early years at the Institute as a time of "unique co-operation of the whole generation of theoretical physicists from many countries." In 1922, he was awarded the Nobel Prize for physics for his work on the quantum mechanical model of the atom, which by that time had been accepted universally among physicists.

In 1927, in a lecture presented at the International physical Congress, Bohr introduced his notion of complementarity, which expressed the idea that it is impossible to make a sharp distinction between the behavior of physical objects and the objects' interaction with the instruments used to measure that behavior. This principle was brought to bear on the problem of wave-particle duality in which particles could be observed to behave both as waves and particles.

Also at this time, Bohr introduced his correspondence principle, which stated that, while quantum mechanical principles are required to described atomic phenomena, these principles must remain consistent with observations of the real world made at the macroscopic, or everyday, level.

In the 1930's, Bohr's interest turned to nuclear physics and the composition of the atomic nucleus. His chief contribution to nuclear structure theory was to point out that the atomic nucleus consisted of many protons and neutrons clustered together like molecules in a drop of water. This idea was to heavily influence nuclear reaction theory.

Bohr also studied the physics of uranium fission, and the role of fast and slow neutrons. He showed, for example, that uranium-235 usually decayed by emitting slow neutrons, but uranium-238 usually absorbs neutrons without undergoing fission. Bohr's observation led to the awareness that one would have to use uranium-235 if one wanted to produce a fast chain reaction.

In 1943, when he learned that he was at risk of imprisonment in German-occupied Copenhagen for being a patriot, Bohr escaped with his family in fishing boats to Sweden. In October of that year, at the intervention of the British authorities, Bohr and his younger son, Aage, were flown to England. (Aage Bohr was himself later to win the Nobel Prize for physics in 1975 for his work on the structure of the atomic nucleus.)

Co-operative work in atomic energy had resumed between the United States and the United Kingdom by September of 1943, and arrangements were made to bring Bohr to the United States, where he spent time in Washington and at Los Alamos. At Los Alamos, Bohr contributed to the development of the atomic bomb, and also served as an elder statesman in the community of prominent scientists.

Bohr was particularly concerned with the implications that the atomic bomb had for future world history, and with keeping control of it out of the hands of the military and under civilian supervision. In 1944, he wrote to the United Kingdom's minister in charge of atomic energy research expressing his concern about the future control of these weapons.

Upon returning to Denmark after the war, Bohr continued to address these problems. In 1950, he sent a letter to the United Nations in which he made 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 1950's, Bohr assumed a prominent role in the establishment of the European Centre for Nuclear Research, and served as a Council member of the center for the rest of his life. Although academic policy mandated his retirement at the age of 70 (in 1955) from the University of Copenhagen, Bohr retained his position as director of the Institute of Theoretical Physics until his death.

In his later years, Bohr attempted to apply the principle of complementary relationships to psychical phenomena, i.e., the relationship between human emotions as evidenced by behavior and consciousness of them; and to human society, i.e., the relationship between hereditary factors and tradition. He had thus come to view the principle of complementarity as a means for including in rational analysis any line of thought that provided useful results, and of maximizing the number of potential avenues of investigation.

Bohr was the first chairman of the Danish Atomic Energy Commission. He also served as president of the Royal Danish Academy of Sciences from 1939 until his death. He chaired a meeting of Academy just two days before his death in Copenhagen on November 18, 1962. His greatest contributions to physics were the formulation of the quantum theory of the hydrogen atom, and the explanation of the origin of the atomic spectra of hydrogen and helium.