Niels Henrik David Bohr ( 7 October 1885 – 18 November 1962) was a Danish physicist who made foundational contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. He developed the model of the atom with the nucleus at the center and electrons in orbit around it, which he compared to the planets orbiting the sun. He worked on the idea in quantum mechanics that electrons move from one energy level to another in discrete steps, not continuously. Bohr mentored and collaborated with many of the top physicists of the century at his institute in Copenhagen. He was part of the British team of physicists working on the Manhattan Project. Bohr married Margrethe Nørlund in 1912, and one of their sons, Aage Bohr, was also a physicist and in 1975 also received the Nobel Prize. Bohr has been described as one of the most influential scientists of the 20th century.
Bohr was born in Copenhagen, Denmark, in 1885. His father, Christian Bohr, was professor of physiology at the University of Copenhagen (it is his name which is given to the Bohr shift or Bohr effect), while his mother, Ellen Adler Bohr, came from a wealthy Jewish family prominent in Danish banking and parliamentary circles (in 1891, Bohr was baptized a Lutheran, his father’s religion). Despite having a religious background, he later resigned his membership from the Lutheran Church and became an atheist. His brother was Harald Bohr, a mathematician and Olympic footballer who played on the Danish national team. Niels Bohr was a passionate footballer as well, and the two brothers played a number of matches for the Copenhagen-based Akademisk Boldklub, with Niels in goal.
In 1903, Bohr enrolled as an undergraduate at Copenhagen University, initially studying philosophy and mathematics. In 1905, prompted by a gold medal competition sponsored by the Royal Danish Academy of Sciences and Letters, he conducted a series of experiments to examine the properties of surface tension, using his father’s laboratory in the university, familiar to him from assisting there since childhood. His essay won the prize, and it was this success that prompted Bohr’s decision to abandon philosophy and adopt physics. He continued as a graduate student at the University of Copenhagen, under the physicist Christian Christiansen, receiving his doctorate in 1911.
As a post-doctoral student, Bohr first conducted experiments under J. J. Thomson, of Trinity College, Cambridge and Cavendish Laboratory. In 1912 he met and later joined Ernest Rutherford at Victoria University of Manchester, where on and off he spent four fruitful years in association with the older physics professor and became part of the ‘Nuclear Family’ which included scientists such as William Lawrence Bragg, James Chadwick and Hans Geiger. In 1916, Bohr returned permanently to the University of Copenhagen, where he was appointed to the Chair of Theoretical Physics, a position created especially for him. In 1918 he began efforts to establish the University Institute of Theoretical Physics, which he later directed.
Earlier in 1910 Bohr had met Margrethe Nørlund, sister of the mathematician Niels Erik Nørlund. They were married in Copenhagen in 1912. Of their six sons, the oldest died in a boating accident and another died from childhood meningitis. The others went on to lead successful lives, including Aage Bohr, who became a very successful physicist and, like his father, was awarded the Nobel Prize in physics in 1975. His other sons were Hans Henrik, a physician, Erik, a chemical engineer, and Ernest, a lawyer.
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’s nucleus, 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 1930s, 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. 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.
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.
In September 1943, reliable word reached Bohr about his imminent arrest by the German police; the Danish resistance quickly managed to help Bohr and his wife escape by sea to Sweden. Soon after, Bohr was flown in a military aircraft to Britain. There he was introduced to the then-secret atomic bomb project (see Political Activities section below for greater detail). Eventually he was directed to the project’s principal location in the United States of America.
Bohr worked on the Manhattan Project at the top-secret Los Alamos laboratory in New Mexico, where he was known by the name of Nicholas Baker for security reasons. His role on the project was as the knowledgeable consultant or “father confessor”. He often expressed social concern about such weaponry and an eventual nuclear arms race, and is quoted as saying, “That is why I went to America. They didn’t need my help in making the atom bomb.”
Bohr believed that atomic secrets should be shared by the international scientific community. After meeting with Bohr, J. Robert Oppenheimer suggested that Bohr visit President Franklin D. Roosevelt to convince him that the Manhattan Project should be shared with the Soviets in the hope of speeding up its results. Roosevelt suggested that Bohr return to the United Kingdom to try to win British approval. Winston Churchill disagreed with the idea of openness towards the Russians to the point that he wrote in a letter: “It seems to me Bohr ought to be confined or at any rate made to see that he is very near the edge of mortal crimes.”
Following the war Bohr returned to Copenhagen. He continued to advocate the peaceful use of nuclear energy. When awarded the Order of the Elephant by the Danish government, he designed his own coat of arms which featured a taijitu (symbol of yin and yang) and the motto in Latin:contraria sunt complementa: “opposites are complementary”. He died in Copenhagen in 1962 of heart failure. He is buried in the Assistens Kirkegård in the Nørrebro section of Copenhagen.