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Bohr's contributions to physics
- Bohr's model of atomic structure.
- The electron's orbital angular momentum is quantized; L=nħ.
- The theory that electrons travel in discrete orbits around the atom's nucleus, with the chemical properties of the element being largely determined by the number of electrons in the outer orbits.
- 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 the basis for quantum theory).
- The Copenhagen interpretation of quantum mechanics.
- The principle of complementarity: that items could be separately analyzed as having several contradictory properties.
He received the Nobel Prize for Physics for this work in 1922.
Bohr was born in Copenhagen in 1885. His father, Christian Bohr, was a professor of physiology at the University of Copenhagen, while his mother, Ellen Adler Bohr , came from a wealthy Jewish family prominent in Danish banking and parliamentary circles.
Bohr received his doctorate from Copenhagen University in 1911. He then studied under Ernest Rutherford in the Victoria University of Manchester in England. Based on Rutherford's theories, Bohr published his model of atomic structure in 1913, introducing the theory of electrons travelling 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 the 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, he 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 was largely affected by the philosophy of Søren Kierkegaard of sharp sudden "quality" changes and rejection of the continuous changing. These concepts were manifested in Bohr's quantum theory and his development of the Copenhagen interpretation of quantum mechanics.
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 - based on 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 and Max Planck. He and Bohr had good-natured arguments over the verity of this principle throughout their lives. 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 had six children, one of whom, Aage Niels Bohr, also became a very successful physicist: like his father, he won a Nobel prize. Two of his children died, and most of the others were or are successful in life.
Later years, death, and legacy
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 travelled 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. However his role in the project was minor. He is quoted as saying "That is why I went to America. They didn't need my help in making the atom bomb." He was seen as a knowledgeable consultant or "father confessor" on the project . After the war he returned to Copenhagen, advocating for a peaceful use of nuclear energy. He died in Copenhagen in 1962. He is buried in the Assistens Cemetery, in Copenhagen, Denmark.
The element bohrium is named in his honor.
Relationship with Heisenberg
Werner Heisenberg claimed in an interview after the war, when the author Robert Jungk was working on the book Brighter Than a Thousand Suns, that he had tried to establish a pact with Bohr such that scientists on neither side should help develop the atomic bomb. He also said that the German attempts were entirely focused on energy production, and that his circle of colleagues tried to keep it that way. 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 depiction in the Danish translation of Jungk's book, he disagreed wholeheartedly. He said that Heisenberg had indeed let him know in Copenhagen that he was working on an atomic bomb project, and that he thought that Germany would win the war. 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.
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.
"Anyone who is not shocked by quantum theory has not understood a single word."
Books about Bohr
- Niels Bohr: The Man, His Science, and the World They Changed, by Ruth Moore ISBN 0262631016
- Niels Bohr's Times, In Physics, Philosophy and Polity, by Abraham Pais, ISBN 0-19-852049-2
- Suspended In Language: Niels Bohr's Life, Discoveries, And The Century He Shaped by Jim Ottaviani (graphic novel) ISBN 0966010655
- Asteroid 3948 Bohr, named after the physicist.
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