Niels Bohr

Niels Henrik David Bohr ( 7 October 1885 – 18 November 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr mentored and collaborated with many of the top physicists of the century at his institute in Copenhagen. He was part of a team of physicists working on the Manhattan Project. Bohr married Margrethe Nørlund in 1912, and one of their sons, Aage Bohr, grew up to be an important physicist who in 1975 also received the Nobel Prize. Bohr has been described as one of the most influential scientists of the 20th century.In 1922, Bohr was awarded the Nobel Prize in physics "for his services in the investigation of the structure of atoms and of the radiation emanating from them." The award recognized his early leading work in the emerging field of Quantum Mechanics.

While at Manchester University, Bohr had adapted Rutherford's nuclear structure to Max Planck's quantum theory and so obtained a model of atomic structure which, with later improvements – mainly as a result of Heisenberg's concepts – remains valid to this day. Bohr published his model of atomic structure in 1913. Here he introduced the theory of electrons traveling in orbits around the atom'snucleus, the chemical properties of each element being largely determined by the number of electrons in the outer orbits of its atoms. Bohr also introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a photon (light quantum) of discrete energy. This became a basis for quantum theory.

Among the international community of nuclear physicists, Bohr came to play the role of convener of discussion groups and lectures, as well as being a mentor and an advisor. With the assistance of the Danish government and the Carlsberg Foundation, he succeeded in founding the Institute of Theoretical Physics in 1921, of which he became director. Bohr's institute served as a focal point for researchers into Quantum Mechanics and related subjects in the 1920s and '30s, when most of the world's best known theoretical physicists spent some time in his company. Bohr became widely appreciated as their congenial host and eminent colleague, both at the Institute and at the Foundation's mansion in Carlsberg, where he and his family resided after 1932.

Bohr also conceived the principle of complementarity: that items could be separately analyzed as having several contradictory properties. For example, physicists currently conclude that light behaves either as a wave or a stream of particles depending on the experimental framework – two apparently mutually exclusive properties – on the basis of this principle. Bohr found philosophical applications for this daring principle.^[specify] Albert Einstein much preferred the determinism of classical physics over the probabilistic new quantum physics (to which among many others Einstein himself had 'unwittingly' contributed). Philosophical issues that arose from the novel aspects of Quantum Mechanics became widely celebrated subjects of discussion. Einstein and Bohr had good-natured arguments over such issues throughout their lives. See article Bohr–Einstein debates.

Werner Heisenberg worked as an assistant to Bohr and university lecturer in Copenhagen from 1926 to 1927. It was in Copenhagen, in 1927, that Heisenberg developed his uncertainty principle, while working on the mathematical foundations of quantum mechanics. Heisenberg later became head of the German nuclear energy project. In April 1940, early in World War II, Germany invaded and occupied Denmark. In September 1941, Bohr was visited by Heisenberg in Copenhagen.