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A pioneer in the field of quantum mechanics and nuclear fission, Niels Bohr is considered one of the most important theoretical physicists of the twentieth century. He was born in Copenhagen, Denmark, in 1885, into a distinguished scientific family. After receiving his Ph.D. in 1911, Bohr went to England to work with Joseph John Thomson, discoverer of the electron, and later with Ernest Rutherford, who, in 1911, had shown that the atom consists of a small central nucleus surrounded by relatively distant electrons. At this time, the process involving electron orbits and energy transfer was not well understood due to the limitations of classical electrodynamics. In 1913, Bohr elucidated the process by proposing an explanation based on earlier research of Max Planck, who had argued that radiation is emitted or absorbed by atoms in discrete units of quanta of energy. Bohr proposed that electrons exchange energy in quanta. By applying quantum theory to the atom, he derived a theoretical formula for the spectral lines in hydrogen, long observed but never explained. Bohr's formula matched the empirical formula, thus verifying the theory. For this accomplishment, Bohr received the Nobel Prize in physics in 1922. Among Bohr's many contributions to the early development of quantum theory was his formulation of the correspondence principle in 1916 and the complementarity principle in 1927. The former principle requires that the quantum theoretical description of the atom correspond to classical physics at large magnitudes. The second principle states that it is impossible to distinguish between the actual behavior of atomic objects and their interaction with the measuring instrument. In 1910, the government of Denmark created the Institute for Theoretical Physics to facilitate Bohr's research. Bohr served as its director until his death. The institute under Bohr's leadership became a world center for the exchange of ideas and information on nuclear physics. Bohr was president of the Royal Danish Academy of Sciences from 1939 until his death. When the Germans occupied Denmark in 1940, Bohr became active in the resistance movement. In 1943, he and his family escaped to Sweden in a fishing boat and then to the United States to assist in developing the atomic bomb at Los Alamos, New Mexico, on the Manhattan Project. After the war, he became a passionate advocate of nuclear disarmament. In 1952 Bohr helped create the European Centre for Nuclear Research (CERN) in Geneva, Switzerland. In 1955, he organized the first Atoms for Peace Conference in Geneva. Bohr died in 1962.