Niels Henrik David Bohr
|Born||October 7, 1885
|Died||November 18, 1962
|Institution||University of Copenhagen|
|Doctoral Advisor||Christian Christiansen|
|Known for||Copenhagen interpretation
|Notable Prizes||Nobel Prize (1922)|
Niels (Henrik David) Bohr [nels ˈb̥oɐ̯ˀ] (October 7, 1885 – November 18, 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics. Bohr is widely considered one of the greatest physicists of the twentieth century.
He received the Nobel Prize for Physics for this work in 1922.
Niels Bohr was born in Copenhagen, Denmark in 1885. His father, Christian Bohr, was professor of physiology at the University of Copenhagen, while his mother, Ellen Adler Bohr, came from a wealthy Sephardic Jewish family prominent in Danish banking and parliamentary circles. His brother was Harald Bohr, a mathematician and Olympic soccer player who played in the Danish national team; Niels Bohr was a passionate soccer player as well, and the two brothers played a number of matches for Akademisk Boldklub.
Bohr received his doctorate from Copenhagen University in 1911 under Christian Christiansen. He then studied under Ernest Rutherford in the Victoria University of Manchester in England. On the basis of Rutherford's theories, Bohr published his model of atomic structure in 1913, introducing the theory of electrons traveling in orbits around the atom's nucleus, the chemical properties of the element being largely determined by the number of electrons in the outer orbits. Bohr also introduced the idea that an electron could drop from a higher-energy orbit to a lower one, emitting a photon (light quantum) of discrete energy. This became a basis for quantum theory.
In 1916, Niels Bohr became a professor at the University of Copenhagen, and director of the newly constructed "Institute of Theoretical Physics" in 1920. 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". Bohr's institute served as a focal point for theoretical physicists in the 1920s and '30s, and most of the world's best known theoretical physicists of that period spent some time there.
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 is both a wave and a stream of particles — two apparently mutually exclusive properties — on the basis of this principle. Bohr also found philosophical applications for this daringly original principle. Albert Einstein much preferred the determinism of classical physics over the probabilistic new physics of Bohr (to which Max Planck and Einstein himself had contributed). He and Bohr had good-natured arguments over the truth of this principle throughout their lives (see Bohr Einstein debate). One of Bohr's most famous students was Werner Heisenberg, a crucial figure in the development of quantum mechanics, who was also head of the German atomic bomb project.
Niels Bohr and his wife Margrethe Nørlund had six children. Two died young, and most of the others went on to lead successful lives. One, Aage Niels Bohr, also became a very successful physicist; like his father, he won a Nobel Prize.
In 1941, during the German occupation of Denmark in World War II, Bohr was visited by Heisenberg in Copenhagen (see next section). In 1943, shortly before he was to be arrested by the German police, Bohr escaped to Sweden, and then traveled to London.
He worked at the secret Los Alamos laboratory in New Mexico, USA, on the Manhattan Project, where, according to Richard Feynman, he was known by the assumed name of Nicholas Baker for security reasons. His role in the project was important. He was seen as a knowledgeable consultant or "father confessor" on the project. He was concerned about a 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 Bohr visit President Franklin Roosevelt to convince him that the Manhattan Project should be shared with the Russians in the hope of speeding up its results. Roosevelt suggested Bohr return to England to try to win British approval. Churchill opposed the idea.
After the war he returned to Copenhagen, advocating the peaceful use of nuclear energy. He died in Copenhagen in 1962. He is buried in the Assistens Kirkegård in the Nørrebro section of Copenhagen.
The element bohrium is named in his honor. He is pictured on the 500 kr. Danish bank note. In 1965, three years after Bohr's death, the institute of physics at the university of Copenhagen changed its name to the Niels Bohr Institute.
It is generally accepted that Bohr read the 19th century Danish philosopher Søren Kierkegaard. In 1909, Bohr sent his brother Kierkegaard's Stages on Life's Way as a birthday gift. In the enclosed letter, Bohr wrote, "It is the only thing I have to send; but I do not believe that it would be very easy to find anything better.... I even think it is one of the most delightful things I have ever read." Bohr enjoyed Kierkegaard's language and literary style, but mentioned that he had some "disagreement with [Kierkegaard's ideas]". 
Given this, there has been some dispute over whether Kierkegaard influenced Bohr's philosophy and science. David Favrholdt argues that Kierkegaard had minimal influence over Bohr's work; taking Bohr's statement about disagreeing with Kierkegaard at face value, while Jan Faye argues the opposing point of view; by arguing that one can disagree with the content of a theory while accepting its general premise and structure. 
Bohr and Heisenberg enjoyed a strong mentor/mentee relationship up to the onset of World War II. At that point, the relationship became somewhat strained because Bohr, with his Jewish heritage, remained in occupied Denmark, while Heisenberg remained in Germany. Heisenberg made a now-famous visit to Bohr in September 1941, and during a private moment, began to discuss nuclear weapons and the war efforts. Michael Frayn's play Copenhagen, which ran on Broadway for a time, explores what might have happened at the 1941 meeting between Heisenberg and Bohr. The truth of the historical event is still a matter of scholarly debate, as neither Bohr nor Heisenberg spoke about it in any detail, and they were alone in the woods. While some suggest that the relationship became somewhat strained at this meeting, other evidence suggests that the fracture occurred much later. In correspondence to his wife, Heisenberg described the final visit of the trip: "Today I was once more, with Weizsaecker, at Bohr's. In many ways this was especially nice, the conversation revolved for a large part of the evening around purely human concerns, Bohr was reading aloud, I played a Mozart Sonata (A-Major)." 
In 1957, while the author Robert Jungk was working on the book Brighter Than a Thousand Suns, Heisenberg wrote to Jungk explaining that he had visited Copenhagen to communicate to Bohr his view that scientists on neither side should help develop the atomic bomb, that the German attempts were entirely focused on energy production, and that Heisenberg's circle of colleagues tried to keep it that way.  However, Heisenberg acknowledged that his cryptic approach of the subject had so-alarmed Bohr that the discussion failed. Heisenberg nuanced his claims, though, and avoided implication that he and his colleagues had purposely sabotaged the bomb effort. However, this nuance was lost in Jungk's original publication of the book, which strongly implied that the German atomic bomb project was rendered purposely stillborn by Heisenberg.
When Bohr saw this erroneous depiction in the Danish translation of Jungk's book, he disagreed wholeheartedly. He said that while Heisenberg had indeed discussed the subject of nuclear weapons in Copenhagen that Heisenberg never alluded to the fact that Heisenberg might be resisting efforts to build such weapons. He dismissed the idea of any pact as an after-the-fact construction. He drafted several letters to inform Heisenberg about this but never sent any of them.