Niels Henrik David Bohr (in Danish; 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 also part of the team of physicists working on the Manhattan Project. Bohr married Margrethe Nørlund in 1912, and one of their sons, Aage Niels Bohr, grew up to be an important physicist who, like his father, received the Nobel prize, in 1975. Bohr has been described as one of the most influential physicists of the 20th century.
Bohr studied as an undergraduate, graduate and, under Christian Christiansen, as a doctoral student at Copenhagen University, receiving his doctorate in 1911. As a post-doctoral student, Bohr first conducted experiments under J. J. Thomson at Trinity College, Cambridge. He then went on to study under Ernest Rutherford at the 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.
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 1975.
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 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.
In 1941, during the German occupation of Denmark in World War II, Bohr was visited by Heisenberg in Copenhagen (see section below). In 1943, shortly before he was to be arrested by the German police, Bohr escaped to Sweden, and then traveled to London.
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 D. 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 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.
After the war Bohr returned to Copenhagen, advocating 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 Latin motto contraria sunt complementa: opposites are complementary. He died in Copenhagen in 1962. He is buried in the Assistens Kirkegård in the Nørrebro section of Copenhagen.
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 endorses the opposing point of view by arguing that one can disagree with the content of a theory while accepting its general premises and structure.
By the time of World War II, the relationship became strained; this was in part because Bohr, with his partially-Jewish heritage, remained in occupied Denmark, while Heisenberg remained in Germany and became head of the German nuclear efforts. Heisenberg made a now-famous visit to Bohr in September/October 1941, and during a private moment, it seems that he began to address nuclear energy and morality as well as the war effort. Neither Bohr nor Heisenberg spoke about it in any detail or left written records of this part of the meeting, and they were alone and outside. Bohr seems to have reacted by terminating that conversation abruptly while not giving Heisenberg hints in any direction.
While some suggest that the relationship became strained at this meeting, other evidence shows that the level of contact had been reduced considerably for some time already. Heisenberg himself suggested that the fracture occurred 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). Ivan Supek, one of Heisenberg's students and friends, claimed that the main figure of the meeting was actually Weizsäcker who tried to persuade Bohr to mediate for peace between Great Britain and Germany.
After leaving Denmark in the dramatic day and night (October 1943) when most Jews were able to escape to Sweden due to a series of very exceptional circumstances (see Rescue of the Danish Jews), Bohr was quickly asked, again, to join British efforts, and he was flown to the UK for that purpose. He was evacuated from Stockholm in 1943 in an unarmed De Havilland Mosquito bomber (carried in an improvised cabin in the bomb bay) sent by the RAF. The flight almost ended in tragedy as Bohr did not don his oxygen equipment as instructed, and passed out. He would have died had not the pilot, surmising from Bohr's lack of response to intercom communication that he had lost consciousness, descended to a lower altitude for the remainder of the flight. Bohr's comment was that he had slept like a baby for the entire flight.
As part of the UK team on "Tube Alloys" Bohr was also included at Los Alamos. Oppenheimer credited Bohr warmly for his guiding help during certain discussions among scientists there. Discreetly, he met President Franklin D. Roosevelt and later Winston Churchill to warn against the perilous perspectives that would follow from separate development of nuclear weapons by several powers rather than some form of controlled sharing of the basic scientific knowledge, which would spread quickly in any case. Only in the 1950s, after the immense surprise that the Soviets could and did in fact develop the weapons independently, was it possible to create the International Atomic Energy Agency along the lines of Bohr's old suggestion.
When Bohr saw Jungk's erroneous depiction in the Danish translation of the book, he disagreed wholeheartedly. He drafted (but never sent) a letter to Heisenberg, stating that while Heisenberg had indeed discussed the subject of nuclear weapons in Copenhagen, Heisenberg had never alluded to the fact that he might be resisting efforts to build such weapons. Bohr dismissed the idea of any pact as an after-the-fact construction.
Michael Frayn's play Copenhagen, which was performed in London (for five years), Copenhagen, Gothenburg, Rome, Athens (Greece), Geneva and on Broadway in New York, explores what might have happened at the 1941 meeting between Heisenberg and Bohr. Frayn points in particular to the onus of being one of the few, or the first one, to understand what it would mean in practice to create a nuclear weapon.