Editing Niels Bohr (section)

=== Institute of Physics ===
In April 1917, Bohr began a campaign to establish an Institute of Theoretical Physics. He gained the support of the Danish government and the Carlsberg Foundation, and sizeable contributions were also made by industry and private donors, many of them Jewish. Legislation establishing the institute was passed in November 1918. Now known as the [[Niels Bohr Institute]], it opened on 3 March 1921, with Bohr as its director. His family moved into an apartment on the first floor.<ref>{{cite web | url=http://www.nbi.ku.dk/english/www/institute/History/history/ | title=History of the institute: The establishment of an institute | publisher=Niels Bohr Institute |last=Aaserud |first=Finn |archive-url=https://web.archive.org/web/20080405160424/http://www.nbi.ku.dk/english/about/history/ |archive-date=5 April 2008 |access-date=11 May 2008| date=January 1921 }}</ref>{{sfn|Pais|1991|pp=169–171}} 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. Early arrivals included [[Hans Kramers]] from the Netherlands, [[Oskar Klein]] from Sweden, George de Hevesy from Hungary, [[Wojciech Rubinowicz]] from Poland, and [[Svein Rosseland]] from Norway. Bohr became widely appreciated as their congenial host and eminent colleague.{{sfn|Kennedy|1985|pp=9, 12, 13, 15}}{{sfn|Hund|1985|pp=71–73}} Klein and Rosseland produced the institute's first publication even before it opened.{{sfn|Pais|1991|pp=169–171}}

[[File:Niels Bohr Institute 1.jpg|thumb|The [[Niels Bohr Institute]], part of the [[University of Copenhagen]]|alt=A block-shaped beige building with a sloped, red tiled roof]]
The Bohr model worked well for hydrogen and ionized single-electron helium, which impressed Einstein<ref>From Bohr's Atom to Electron Waves https://galileo.phys.virginia.edu/classes/252/Bohr_to_Waves/Bohr_to_Waves.html {{Webarchive|url=https://web.archive.org/web/20210810030204/http://galileo.phys.virginia.edu/classes/252/Bohr_to_Waves/Bohr_to_Waves.html |date=10 August 2021 }}</ref><ref>The Age of Entanglement, Louisa Gilder, p.799, 2008.</ref> but could not explain more complex elements. By 1919, Bohr was moving away from the idea that electrons orbited the nucleus and developed [[heuristic]]s to describe them. The [[rare-earth element]]s posed a particular classification problem for chemists because they were so chemically similar. An important development came in 1924 with [[Wolfgang Pauli]]'s discovery of the [[Pauli exclusion principle]], which put Bohr's models on a firm theoretical footing. Bohr was then able to declare that the as-yet-undiscovered element 72 was not a rare-earth element but an element with chemical properties similar to those of [[zirconium]]. (Elements had been predicted and discovered since 1871 by chemical properties<ref>See [[Periodic Table]] and [[History of the periodic table]] showing elements predicted by chemical properties since [[Mendeleev]].</ref>), and Bohr was immediately challenged by the French chemist [[Georges Urbain]], who claimed to have discovered a rare-earth element 72, which he called "celtium". At the Institute in Copenhagen, [[Dirk Coster]] and George de Hevesy took up the challenge of proving Bohr right and Urbain wrong. Starting with a clear idea of the chemical properties of the unknown element greatly simplified the search process. They went through samples from Copenhagen's Museum of Mineralogy looking for a zirconium-like element and soon found it. The element, which they named [[hafnium]] (''hafnia'' being the Latin name for Copenhagen), turned out to be more common than gold.{{sfn|Kragh|1985|pp=61–64}}{{sfn|Pais|1991|pp=202–210}}

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".{{sfn|Pais|1991|p=215}} The award thus recognised both the trilogy and his early leading work in the emerging field of quantum mechanics. For his Nobel lecture, Bohr gave his audience a comprehensive survey of what was then known about the structure of the atom, including the [[correspondence principle]], which he had formulated. This states that the behaviour of systems described by quantum theory reproduces [[classical physics]] in the limit of large [[quantum number]]s.{{sfn|Bohr|1985|pp=91–97}}

The discovery of [[Compton scattering]] by [[Arthur Holly Compton]] in 1923 convinced most physicists that light was composed of [[photon]]s and that energy and momentum were conserved in collisions between electrons and photons. In 1924, Bohr, Kramers, and [[John C. Slater]], an American physicist working at the Institute in Copenhagen, proposed the [[Bohr–Kramers–Slater theory]] (BKS). It was more of a program than a full physical theory, as the ideas it developed were not worked out quantitatively. The BKS theory became the final attempt at understanding the interaction of matter and electromagnetic radiation on the basis of the old quantum theory, in which quantum phenomena were treated by imposing quantum restrictions on a classical wave description of the electromagnetic field.<ref>{{cite journal|last1=Bohr |first1=N. |first2=H. A. |last2=Kramers |author-link2=Hans Kramers |last3=Slater |first3=J. C. |author-link3=John C. Slater |journal=Philosophical Magazine |doi=10.1080/14786442408565262 |url=http://www.cond-mat.physik.uni-mainz.de/~oettel/ws10/bks_PhilMag_47_785_1924.pdf |title=The Quantum Theory of Radiation |series=6 |volume=76 |issue=287 |year=1924 |access-date=18 February 2013 |pages=785–802 |archive-url=https://web.archive.org/web/20130522110143/http://www.cond-mat.physik.uni-mainz.de/~oettel/ws10/bks_PhilMag_47_785_1924.pdf |archive-date=22 May 2013 }}</ref>{{sfn|Pais|1991|pp=232–239}}

Modelling atomic behaviour under incident electromagnetic radiation using "virtual oscillators" at the absorption and emission frequencies, rather than the (different) apparent frequencies of the Bohr orbits, led Max Born, [[Werner Heisenberg]] and Kramers to explore different mathematical models. They led to the development of [[matrix mechanics]], the first form of modern [[quantum mechanics]]. The BKS theory also generated discussion of, and renewed attention to, difficulties in the foundations of the old quantum theory.{{sfn|Jammer|1989|p=188}} The most provocative element of BKS – that momentum and energy would not necessarily be conserved in each interaction, but only statistically – was soon shown to be in conflict with experiments conducted by [[Walther Bothe]] and [[Hans Geiger]].{{sfn|Pais|1991|p=237}} In light of these results, Bohr informed Darwin that "there is nothing else to do than to give our revolutionary efforts as honourable a funeral as possible".{{sfn|Pais|1991|p=238}}