Editing Niels Bohr (section)

=== Quantum mechanics ===
The introduction of [[Spin (physics)|spin]] by [[George Uhlenbeck]] and [[Samuel Goudsmit]] in November 1925 was a milestone. The next month, Bohr travelled to [[Leiden]] to attend celebrations of the 50th anniversary of Hendrick Lorentz receiving his doctorate. When his train stopped in [[Hamburg]], he was met by Wolfgang Pauli and [[Otto Stern]], who asked for his opinion of the spin theory. Bohr pointed out that he had concerns about the interaction between electrons and magnetic fields. When he arrived in Leiden, [[Paul Ehrenfest]] and Albert Einstein informed Bohr that Einstein had resolved this problem using [[Theory of relativity|relativity]]. Bohr then had Uhlenbeck and Goudsmit incorporate this into their paper. Thus, when he met Werner Heisenberg and [[Pascual Jordan]] in [[Göttingen]] on the way back, he had become, in his own words, "a prophet of the electron magnet gospel".{{sfn|Pais|1991|p=243}}

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Heisenberg first came to Copenhagen in 1924, then returned to Göttingen in June 1925, shortly thereafter developing the mathematical foundations of quantum mechanics. When he showed his results to Max Born in Göttingen, Born realised that they could best be expressed using [[Matrix (mathematics)|matrices]]. This work attracted the attention of the British physicist [[Paul Dirac]],{{sfn|Pais|1991|pp=275–279}} who came to Copenhagen for six months in September 1926. Austrian physicist [[Erwin Schrödinger]] also visited in 1926. His attempt at explaining quantum physics in classical terms using wave mechanics impressed Bohr, who believed it contributed "so much to mathematical clarity and simplicity that it represents a gigantic advance over all previous forms of quantum mechanics".{{sfn|Pais|1991|pp=295–299}}

When Kramers left the institute in 1926 to take up a chair as professor of theoretical physics at the [[Utrecht University]], Bohr arranged for Heisenberg to return and take Kramers's place as a ''[[lektor]]'' at the University of Copenhagen.{{sfn|Pais|1991|p=263}} Heisenberg worked in Copenhagen as a university lecturer and assistant to Bohr from 1926 to 1927.{{sfn|Pais|1991|pp=272–275}}

Bohr became convinced that light behaved like both waves and particles and, in 1927, experiments confirmed the [[de Broglie hypothesis]] that matter (like electrons) also behaved like waves.{{sfn|Pais|1991|p=301}} He conceived the philosophical principle of [[Complementarity (physics)|complementarity]]: that items could have apparently mutually exclusive properties, such as being a wave or a stream of particles, depending on the experimental framework.{{sfn|MacKinnon|1985|pp=112–113}} He felt that it was not fully understood by professional philosophers.{{sfn|MacKinnon|1985|p=101}}

In February 1927, Heisenberg developed the first version of the [[uncertainty principle]], presenting it using a [[thought experiment]] where an electron was observed through a [[gamma-ray microscope]]. Bohr was dissatisfied with Heisenberg's argument, since it required only that a measurement disturb properties that already existed, rather than the more radical idea that the electron's properties could not be discussed at all apart from the context they were measured in. In a paper presented at the [[Como Conference]] in September 1927, Bohr emphasised that Heisenberg's uncertainty relations could be derived from classical considerations about the resolving power of optical instruments.{{sfn|Pais|1991|pp=304–309}} Understanding the true meaning of complementarity would, Bohr believed, require "closer investigation".{{sfn|Bohr|1928|p=582}} Einstein preferred the determinism of classical physics over the probabilistic new quantum physics to which he himself had contributed. Philosophical issues that arose from the novel aspects of quantum mechanics became widely celebrated subjects of discussion. Einstein and Bohr had [[Bohr–Einstein debates|good-natured arguments]] over such issues throughout their lives.{{sfn|Dialogue|1985|pp=121–140}}

In 1914 [[Carl Jacobsen]], the heir to [[Carlsberg Group|Carlsberg breweries]], bequeathed his mansion (the Carlsberg Honorary Residence, currently known as Carlsberg Academy) to be used for life by the Dane who had made the most prominent contribution to science, literature or the arts, as an honorary residence ({{langx|da|Æresbolig|links=no}}). Harald Høffding had been the first occupant, and upon his death in July 1931, the Royal Danish Academy of Sciences and Letters gave Bohr occupancy. He and his family moved there in 1932.{{sfn|Pais|1991|pp=332–333}} He was elected president of the Academy on 17 March 1939.{{sfn|Pais|1991|pp=464–465}}

By 1929 the phenomenon of [[beta decay]] prompted Bohr to again suggest that the [[law of conservation of energy]] be abandoned, but [[Wolfgang Pauli]]'s hypothetical [[neutrino]] and the subsequent 1932 discovery of the [[neutron]] provided another explanation. This prompted Bohr to create a new theory of the [[compound nucleus]] in 1936, which explained how neutrons could be captured by the nucleus. In this model, the nucleus could be deformed like a drop of liquid. He worked on this with a new collaborator, the Danish physicist Fritz Kalckar, who died suddenly in 1938.{{sfn|Pais|1991|pp=337–340, 368–370}}<ref>{{cite journal |title=Transmutations of Atomic Nuclei |last=Bohr |first=Niels |journal=[[Science (journal)|Science]] |date=20 August 1937 |volume=86 |issue=2225 |pages=161–165 |doi=10.1126/science.86.2225.161 |bibcode = 1937Sci....86..161B |pmid=17751630}}</ref>

The discovery of [[nuclear fission]] by [[Otto Hahn]] in December 1938 (and its theoretical explanation by [[Lise Meitner]]) generated intense interest among physicists. Bohr brought the news to the United States where he opened the fifth [[Washington Conference on Theoretical Physics]] with Fermi on 26 January 1939.{{sfn|Stuewer|1985|pp=211–216}} When Bohr told [[George Placzek]] that this resolved all the mysteries of [[transuranic elements]], Placzek told him that one remained: the neutron capture energies of uranium did not match those of its decay. Bohr thought about it for a few minutes and then announced to Placzek, [[Léon Rosenfeld]] and [[John Archibald Wheeler|John Wheeler]] that "I have understood everything."{{sfn|Pais|1991|p=456}} Based on his [[liquid drop model]] of the nucleus, Bohr concluded that it was the [[uranium-235]] isotope and not the more abundant [[uranium-238]] that was primarily responsible for fission with thermal neutrons. In April 1940, [[John R. Dunning]] demonstrated that Bohr was correct.{{sfn|Stuewer|1985|pp=211–216}} In the meantime, Bohr and Wheeler developed a theoretical treatment, which they published in a September 1939 paper on "The Mechanism of Nuclear Fission".<ref>{{cite journal |last1=Bohr |first1=Niels |last2=Wheeler |first2=John Archibald |author-link2=John Archibald Wheeler |title=The Mechanism of Nuclear Fission |journal=[[Physical Review]] |volume=56 |issue=5 |pages=426–450 |date=September 1939 |doi=10.1103/PhysRev.56.426 |url=http://www.pugetsound.edu/files/resources/7579_Bohr%20liquid%20drop.pdf |bibcode=1939PhRv...56..426B |doi-access=free |access-date=22 October 2013 |archive-date=24 September 2015 |archive-url=https://web.archive.org/web/20150924083202/http://www.pugetsound.edu/files/resources/7579_Bohr%20liquid%20drop.pdf |url-status=live }}</ref>