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{{Short description|Danish theoretical physicist (1885–1962)}}
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{{Infobox scientist
| name              = Niels Bohr
| image             = Niels Bohr.jpg
| caption           = Bohr in 1922
| alt               = Photograph showing the head and shoulders of a man in a suit and tie
| birth_name        = Niels Henrik David Bohr
| birth_date        = {{Birth date|1885|10|07|df=yes}}
| death_date        = {{Death date and age|1962|11|18|1885|10|07|df=yes}}
| birth_place       = [[Copenhagen]], Denmark
| death_place       = Copenhagen, Denmark
| resting_place     = [[Assistens Cemetery (Copenhagen)|Assistens Cemetery]], Copenhagen
| alma_mater        = [[University of Copenhagen]] ([[Doctor of Philosophy|PhD]])
| known_for         = {{Indented plainlist|
* [[Bohr magneton]]
* [[Bohr model]]
* [[Bohr–Van Leeuwen theorem]]
* [[Bohr–Kramers–Slater theory]]
* [[Complementarity (physics)|Complementarity]]
* [[Copenhagen interpretation]]
* [[Correspondence principle]]}}
| spouse            = {{Marriage|[[Margrethe Nørlund]]|1912}}
| children          = 6, including [[Aage Bohr|Aage]] and [[Ernest Bohr|Ernest]]
| father            = [[Christian Bohr]]
| family            = [[Bohr family|Bohr]]
| awards            = {{Indented plainlist|
* [[Hughes Medal]] (1921)
* [[Nobel Prize in Physics]] (1922)
* [[Matteucci Medal]] (1923)
* [[Franklin Medal]] (1926)
* [[Faraday Lectureship Prize]] (1930)
* [[Max Planck Medal]] (1930)
* [[Copley Medal]] (1938)
* [[Atoms for Peace Award]] (1957)
* [[Sonning Prize]] (1961)}}
| fields            = {{Plain list|
* [[Atomic physics]]
* [[Quantum physics]]}}
| work_institutions = University of Copenhagen
| thesis_title      = Studier over metallernes elektrontheori
| thesis_url        = https://openlibrary.org/books/OL14372449M/Studier_over_metallernes_elektrontheori.
| thesis_year       = 1911
| doctoral_advisor  = [[Christian Christiansen (physicist)|Christian Christiansen]]
| academic_advisors = {{Indented plainlist|
* [[Harald Høffding]]
* [[Thorvald Thiele]]}}
| notable_students  = {{Collapsible list|title={{Nobold|''See list''}}<ref>{{Cite web|title=Niels Bohr - Physics Tree|url=https://academictree.org/physics/peopleinfo.php?pid=1943|website=academictree.org|access-date=13 June 2025}}</ref>
 | [[Homi J. Bhabha]]
 | [[Aage Bohr]]
 | [[Max Delbrück]]
 | [[David M. Dennison]]
 | [[Hugo Fricke]]
 | [[Werner Heisenberg]]
 | [[Walter Heitler]]
 | [[Frank Hoyt]]
 | [[Arthur Kerman]]
 | [[Hans Kopfermann]]
 | [[Hans Kramers]]
 | [[Hans Kuhn (chemist)|Hans Kuhn]]
 | [[Lev Landau]]
 | [[Thomas Lauritsen]]
 | [[Robert Bruce Lindsay]]
 | [[Ben Mottelson]]
 | [[Wolfgang Pauli]]
 | [[Linus Pauling]]
 | [[Svein Rosseland]]
 | [[John C. Slater]]
 | [[Bengt Strömgren]]
 | [[Llewellyn Thomas]]
 | [[Harold Urey]]
 | [[Victor Weisskopf]]
 | [[John Archibald Wheeler]]}}
| signature         = Niels Bohr Signature.svg
}}

'''Niels Henrik David Bohr''' ({{IPAc-en|n|i:|l|z|_|b|o:r}};<ref>{{cite AV media |title=Study Physics at the Niels Bohr Institute |url=https://www.youtube.com/watch?v=jpRQ-Y2U-Z4 |publisher=[[Niels Bohr Institute]] - [[University of Copenhagen]], Faculty of Science |access-date=20 December 2025 |time=16 seconds and 32 seconds |language=en |date=11 April 2016 |via=YouTube |type=video}}</ref> {{IPA|da|ˈne̝ls ˈpoɐ̯ˀ|lang}}; 7 October 1885 – 18 November 1962)<ref name=NYTOBit/> was a Danish [[theoretical physicist]] who made foundational contributions to understanding [[atomic structure]] and [[old quantum theory|quantum theory]], for which he received the [[Nobel Prize in Physics]] in 1922.<ref name="Nobel Prize">{{Cite web|title=Nobel Prize in Physics 1922|url=https://www.nobelprize.org/prizes/physics/1922/summary/|url-status=live|publisher=[[Nobel Foundation]]|archive-url=https://web.archive.org/web/20081017213314/http://nobelprize.org/nobel_prizes/physics/laureates/1922/index.html |archive-date=2008-10-17|access-date=2008-10-09}}</ref> He was also a [[philosopher]] and a promoter of scientific research.

Bohr developed the [[Bohr model]] of the [[atom]], in which he proposed that energy levels of [[electron]]s are discrete and that the electrons revolve in stable orbits around the [[atomic nucleus]] but can jump from one energy level (or orbit) to another. Although the Bohr model has been supplanted by other models, its underlying principles remain valid. He conceived the principle of [[Complementarity (physics)|complementarity]]: that items could be separately analysed in terms of contradictory properties, like behaving as a [[Wave–particle duality|wave or a stream of particles]]. The notion of complementarity dominated Bohr's thinking in both science and philosophy.

Bohr founded the Institute of Theoretical Physics at the [[University of Copenhagen]], now known as the [[Niels Bohr Institute]], which opened in 1920. Bohr mentored and collaborated with physicists including [[Hans Kramers]], [[Oskar Klein]], [[George de Hevesy]], and [[Werner Heisenberg]]. He predicted the properties of a new [[zirconium]]-like element, which was named [[hafnium]], after the Latin name for Copenhagen, where it was discovered. Later, the synthetic element [[bohrium]] was named after him because of his groundbreaking work on the structure of atoms.

During the 1930s, Bohr helped refugees from [[Nazism]]. After [[German invasion of Denmark (1940)|Denmark was occupied by the Germans]], he met with Heisenberg, who had become the head of the [[German nuclear weapon project]]. In September 1943 word reached Bohr that he was about to be arrested by the Germans, so he fled to Sweden. From there, he was flown to Britain, where he joined the British [[Tube Alloys]] nuclear weapons project, and was part of the British mission to the [[Manhattan Project]]. After the war, Bohr called for international cooperation on nuclear energy. He was involved with the establishment of [[CERN]] and the [[Risø DTU National Laboratory for Sustainable Energy|Research Establishment Risø of the Danish Atomic Energy Commission]] and became the first chairman of the [[Nordic Institute for Theoretical Physics]] in 1957.<ref name=NYTOBit>{{cite web| title=Niels Bohr | date=November 20, 1962| work=[[The New York Times]]| url=https://www.nytimes.com/1962/11/20/archives/niels-bohr.html}}</ref> In 1999, he was named the fourth greatest physicist of all time.<ref>{{cite news| title=Physics: past, present, future| work=[[Physics World]]| date=December 6, 1999| url=https://physicsworld.com/a/physics-past-present-future/}}</ref>

== Early life and education ==
Niels Henrik David Bohr was born on 7 October 1885 in [[Copenhagen]], Denmark, the second of three children of [[Christian Bohr]],<ref name=cphpolice3308989>{{cite book |author= <!--Staff writer(s); no by-line.--> |title= Politiets Registerblade |trans-title= Register cards of the Police |location= Copenhagen |publisher= Københavns Stadsarkiv |url= http://www.politietsregisterblade.dk/en/component/sfup/?controller=politregisterblade&task=viewRegisterblad&id=3308989 |at= Station Dødeblade (indeholder afdøde i perioden). Filmrulle 0002. Registerblad 3341 |date= 7 June 1892 |id= ID 3308989 |language= da |archive-url= https://web.archive.org/web/20141129033630/http://www.politietsregisterblade.dk/en/component/sfup/?controller=politregisterblade&task=viewRegisterblad&id=3308989 |archive-date= 29 November 2014}}</ref>{{sfn|Pais|1991|pp=44–45, 538–539}} Professor of Physiology at the [[University of Copenhagen]], and Ellen Adler, the daughter of [[Danish Jewish]] banker [[David Baruch Adler]].{{sfn|Pais|1991|pp=35–39}} He had an elder sister, Jenny, and a younger brother [[Harald Bohr|Harald]].<ref name=cphpolice3308989 /> Jenny became a teacher,{{sfn|Pais|1991|pp=44–45, 538–539}} while Harald became a [[mathematician]] and [[association football|footballer]] who played for the [[Denmark national football team|Danish national team]] at the [[1908 Summer Olympics]] in London. Niels was a passionate footballer as well, and the two brothers played several matches for the Copenhagen-based [[Akademisk Boldklub]] (Academic Football Club), with Niels as [[Goalkeeper (association football)|goalkeeper]].<ref>There is no truth in the oft-repeated claim that Bohr emulated his brother, Harald, by playing for the Danish national team. {{cite news |last=Dart |first=James |date=27 July 2005 |url=https://www.theguardian.com/football/2005/jul/27/theknowledge.panathinaikos |title=Bohr's footballing career |work=The Guardian |location=London |access-date=26 June 2011 |archive-date=27 May 2023 |archive-url=https://web.archive.org/web/20230527043508/https://www.theguardian.com/football/2005/jul/27/theknowledge.panathinaikos |url-status=live }}</ref>

Bohr was educated at Gammelholm Latin School, starting when he was seven.<ref>{{cite web |url=http://www.nbi.ku.dk/english/www/niels/bohr/skole/ |title=Niels Bohr's school years |publisher=Niels Bohr Institute |access-date=14 February 2013 |date=18 May 2012 |archive-date=4 October 2013 |archive-url=https://web.archive.org/web/20131004220548/http://www.nbi.ku.dk/english/www/niels/bohr/skole/ |url-status=live }}</ref> In 1903, Bohr enrolled as an undergraduate at the University of Copenhagen. His major was physics, which he studied under [[Christian Christiansen (physicist)|Christian Christiansen]], the university's only professor of physics at that time. He also studied astronomy and mathematics under [[Thorvald Thiele]], and philosophy under [[Harald Høffding]], a friend of his father.{{sfn|Pais|1991|pp=98–99}}<ref name="university">{{cite web |url=http://www.nbi.ku.dk/english/www/niels/bohr/universitetet/ |title=Life as a Student |publisher=Niels Bohr Institute |access-date=14 February 2013 |date=16 July 2012 |archive-date=4 October 2013 |archive-url=https://web.archive.org/web/20131004220131/http://www.nbi.ku.dk/english/www/niels/bohr/universitetet/ |url-status=live }}</ref>

[[File:Niels Bohr - LOC - ggbain - 35303.jpg|thumb|left|upright|Bohr as a young man, {{circa|1910}}.|alt=Head and shoulders of young man in a suit and tie]]

In 1905, a gold medal competition was sponsored by the [[Royal Danish Academy of Sciences and Letters]] to investigate a method for measuring the [[surface tension]] of liquids that had been proposed by [[Lord Rayleigh]] in 1879. This involved measuring the frequency of oscillation of the radius of a water jet. Bohr conducted a series of experiments using his father's laboratory in the university; the university itself had no physics laboratory. To complete his experiments, he had to [[glass blowing|make his own glassware]], creating test tubes with the required [[ellipse|elliptical]] cross-sections. He went beyond the original task, incorporating improvements into both Rayleigh's theory and his method, by taking into account the [[viscosity]] of the water, and by working with finite amplitudes instead of just infinitesimal ones. His essay, which he submitted at the last minute, won the prize. He later submitted an improved version of the paper to the [[Royal Society]] in London for publication in the ''[[Philosophical Transactions of the Royal Society]]''.{{sfn|Rhodes|1986|pp=62–63}}{{sfn|Pais|1991|pp=101–102}}<ref name="university" />{{sfn|Aaserud|Heilbron|2013|p=155}}

Harald became the first of the two Bohr brothers to earn a [[master's degree]], which he earned for mathematics in April 1909. Niels took another 9 months to earn his for the electron theory of metals, a topic assigned by his supervisor, Christiansen. Bohr subsequently elaborated his master's thesis into his much-larger [[Ph.D.]] thesis. He surveyed the literature on the subject, settling on [[Drude model|a model]] developed by [[Paul Drude]] and elaborated by [[Hendrik Lorentz]], in which the electrons in a metal are considered to behave like a gas. Bohr extended Lorentz's model, but was still unable to account for phenomena like the [[Hall effect]], and concluded that electron theory could not fully explain the magnetic properties of metals. The thesis was accepted in April 1911,<ref>{{Cite encyclopedia|url=https://www.britannica.com/biography/Niels-Bohr|title=Niels Bohr {{!}} Danish physicist|encyclopedia=Encyclopedia Britannica|access-date=25 August 2017|archive-date=8 August 2023|archive-url=https://web.archive.org/web/20230808044009/https://www.britannica.com/biography/Niels-Bohr|url-status=live}}</ref> and Bohr conducted his formal defence on 13 May. Harald had received his doctorate the previous year.{{sfn|Pais|1991|pp=107–109}} Bohr's thesis was groundbreaking, but attracted little interest outside Scandinavia because it was written in Danish, a Copenhagen University requirement at the time. In 1921, the Dutch physicist [[Hendrika Johanna van Leeuwen]] would independently derive a theorem in Bohr's thesis that is today known as the [[Bohr–Van Leeuwen theorem]].{{sfn|Kragh|2012|pp=43–45}}

== Physics ==
=== Bohr model ===
{{Main|Bohr model}}

In September 1911, Bohr, supported by a fellowship from the [[Carlsberg Foundation]], travelled to England, where most of the theoretical work on the structure of atoms and molecules was being done.{{sfn|Kragh|2012|p=122}} He met [[J. J. Thomson]] of the [[Cavendish Laboratory]] and [[Trinity College, Cambridge]]. He attended lectures on [[electromagnetism]] given by [[James Jeans]] and [[Joseph Larmor]], and did some research on [[cathode ray]]s, but failed to impress Thomson.{{sfn|Kennedy|1985|p=6}}{{sfn|Pais|1991|pp=117–121}} He had more success with younger physicists like the Australian [[William Lawrence Bragg]],{{sfn|Kragh|2012|p=46}} and New Zealand's [[Ernest Rutherford]], whose 1911 small central nucleus [[Rutherford model]] of the [[atom]] had challenged Thomson's 1904 [[plum pudding model]].{{sfn|Pais|1991|pp=121–125}} Bohr received an invitation from Rutherford to conduct post-doctoral work at [[Victoria University of Manchester]],{{sfn|Kennedy|1985|p=7}} where Bohr met [[George de Hevesy]] and [[Charles Galton Darwin]] (whom Bohr referred to as "the grandson of the [[Charles Darwin|real Darwin]]").{{sfn|Pais|1991|pp=125–129}}

Bohr returned to Denmark in July 1912 for his wedding, and travelled around England and Scotland on his honeymoon. On his return, he became a ''[[Privatdocent]]'' at the University of Copenhagen, giving lectures on [[thermodynamics]]. [[Martin Knudsen]] put Bohr's name forward for a ''[[docent]]'', which was approved in July 1913, and Bohr then began teaching medical students.{{sfn|Pais|1991|pp=134–135}} His three papers, which later became famous as "the trilogy",{{sfn|Kennedy|1985|p=7}} were published in ''[[Philosophical Magazine]]'' in July, September and November of that year.<ref>{{cite journal | first=Niels | last=Bohr | title=On the Constitution of Atoms and Molecules, Part I | journal=[[Philosophical Magazine]] | year=1913 | volume=26 | pages=1–24 | doi=10.1080/14786441308634955 | url=http://web.ihep.su/dbserv/compas/src/bohr13/eng.pdf | issue=151 | bibcode=1913PMag...26....1B | access-date=4 June 2009 | archive-date=2 September 2011 | archive-url=https://web.archive.org/web/20110902020206/http://web.ihep.su/dbserv/compas/src/bohr13/eng.pdf | url-status=live }}</ref><ref name="Bohr 1913 476">{{cite journal | first=Niels | last=Bohr | title=On the Constitution of Atoms and Molecules, Part II Systems Containing Only a Single Nucleus | journal=[[Philosophical Magazine]] | year=1913 | volume=26 | pages=476–502 | url=http://web.ihep.su/dbserv/compas/src/bohr13b/eng.pdf | doi=10.1080/14786441308634993 | issue=153 | bibcode=1913PMag...26..476B | access-date=21 October 2013 | archive-date=9 December 2008 | archive-url=https://web.archive.org/web/20081209111729/http://web.ihep.su/dbserv/compas/src/bohr13b/eng.pdf | url-status=live }}</ref><ref>{{cite journal | first=Niels | last=Bohr | title=On the Constitution of Atoms and Molecules, Part III Systems containing several nuclei | journal=[[Philosophical Magazine]] | year=1913 | volume=26 | pages=857–875 | issue=155 | doi=10.1080/14786441308635031 | url=https://zenodo.org/record/1430922 | bibcode=1913PMag...26..857B | access-date=1 July 2019 | archive-date=22 June 2021 | archive-url=https://web.archive.org/web/20210622091927/https://zenodo.org/record/1430922 | url-status=live }}</ref>{{sfn|Pais|1991|p=149}} He adapted Rutherford's nuclear structure to [[Max Planck]]'s quantum theory and so created his [[Bohr model]] of the atom.<ref name="Bohr 1913 476" />

Planetary models of atoms were not new, but Bohr's treatment was.{{sfn|Kragh|2012|p=22}} Taking the 1912 paper by Darwin on the role of electrons in the interaction of alpha particles with a nucleus as his starting point,<ref name="Darwin1912">{{cite journal|last1=Darwin|first1=Charles Galton|title=A theory of the absorption and scattering of the alpha rays|journal=[[Philosophical Magazine]]|volume=23|issue=138|year=1912|pages=901–920|issn=1941-5982|doi=10.1080/14786440608637291|url=https://zenodo.org/record/1430804|access-date=1 July 2019|archive-date=7 April 2020|archive-url=https://web.archive.org/web/20200407004543/https://zenodo.org/record/1430804|url-status=live}}</ref><ref name="Arabatzis2006">{{cite book|last=Arabatzis|first=Theodore |title=Representing Electrons: A Biographical Approach to Theoretical Entities|url=https://books.google.com/books?id=CdKZYot85OcC&pg=PA118|year=2006|publisher=University of Chicago Press|isbn=978-0-226-02420-2|page=118}}</ref> he advanced the theory of electrons travelling in [[orbit]]s of quantised "stationary states" around the atom's nucleus in order to stabilise the atom, but it wasn't until his 1921 paper that he showed that the chemical properties of each element were largely determined by the number of electrons in the outer orbits of its atoms.<ref>Kragh, Helge. "Niels Bohr's Second Atomic Theory". Historical Studies in the Physical Sciences, vol. 10, University of California Press, 1979, pp. 123–86, https://doi.org/10.2307/27757389 {{Webarchive|url=https://web.archive.org/web/20221017193849/https://online.ucpress.edu/hsns/article-abstract/doi/10.2307/27757389/47571/Niels-Bohr-s-Second-Atomic-Theory?redirectedFrom=fulltext |date=17 October 2022 }}.</ref><ref>N. Bohr, "Atomic Structure", Nature, 107. Letter dated 14 February 1921.</ref><ref>See [[Bohr model]] and [[Periodic Table]] for full development of electron structure of atoms.</ref>{{sfn|Kragh|1985|pp=50–67}} He introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting a [[quantum]] of discrete energy. This became a basis for what is now known as the [[old quantum theory]].{{sfn|Heilbron|1985|pp=39–47}}

[[File:Bohr-atom-PAR.svg|thumb|right|The [[Bohr model]] of the [[hydrogen atom]]. A negatively charged electron, confined to an [[atomic orbital]], orbits a small, positively charged nucleus; a quantum jump between orbits is accompanied by an emitted or absorbed amount of [[electromagnetic radiation]].|alt=Diagram showing electrons with circular orbits around the nucleus labelled n=1, 2 and 3. An electron drops from 3 to 2, producing radiation delta E = hv]]
[[File:Evolution of atomic models infographic.svg|thumb|right|The evolution of [[atomic model]]s in the 20th century: [[Plum pudding model|Thomson]], [[Rutherford model|Rutherford]], [[Bohr model|Bohr]], [[Atomic orbital|Heisenberg/Schrödinger]]]]
In 1885, [[Johann Balmer]] had come up with his [[Balmer series]] to describe the visible [[spectral line]]s of a [[hydrogen]] atom:
:<math>\frac{1}{\lambda} = R_\mathrm{H}\left(\frac{1}{2^2} - \frac{1}{n^2}\right) \quad \text{for} \ n=3,4,5,...</math>
where λ is the wavelength of the absorbed or emitted light and ''R''<sub>H</sub> is the [[Rydberg constant]].{{sfn|Heilbron|1985|p=43}} Balmer's formula was corroborated by the discovery of additional spectral lines, but for thirty years, no one could explain why it worked. In the first paper of his trilogy, Bohr was able to derive it from his model:
:<math> R_Z = { 2\pi^2 m_e Z^2 e^4 \over h^3 } </math>
where ''m''<sub>e</sub> is the electron's mass, ''e'' is its charge, ''h'' is the [[Planck constant]] and ''Z'' is the atom's [[atomic number]] (1 for hydrogen).{{sfn|Pais|1991|pp=146–149}}

The model's first hurdle was the [[Pickering series]], lines that did not fit Balmer's formula. When challenged on this by [[Alfred Fowler]], Bohr replied that they were caused by [[ionised]] [[helium]], helium atoms with only one electron. The Bohr model was found to work for such ions.{{sfn|Pais|1991|pp=146–149}} Many older physicists, like Thomson, Rayleigh and [[Hendrik Lorentz]], did not like the trilogy, but the younger generation, including Rutherford, [[David Hilbert]], [[Albert Einstein]], [[Enrico Fermi]], [[Max Born]] and [[Arnold Sommerfeld]] saw it as a breakthrough.{{sfn|Pais|1991|pp=152–155}}{{sfn|Kragh|2012|pp=109–111}} Einstein called Bohr's model "the highest form of musicality in the sphere of thought."<ref>{{cite book |last=Pais |first=Abraham |author-link=Abraham Pais |title=[[Subtle is the Lord: The Science and the Life of Albert Einstein]] |year=1982 |page=416}}</ref> The trilogy's acceptance was entirely due to its ability to explain phenomena that stymied other models, and to predict results that were subsequently verified by experiments.{{sfn|Kragh|2012|pp=90–91}}<ref>{{cite web|url=https://blogs.cranfield.ac.uk/leadership-management/cbp/forecasting-prediction-is-very-difficult-especially-if-its-about-the-future|title=Forecasting – Prediction is very difficult, especially if it's about the future!|website=cranfield.ac.cuk|date=10 July 2017|quote=Prediction is very difficult, especially if it's about the future|access-date=14 July 2021|archive-date=14 July 2021|archive-url=https://web.archive.org/web/20210714133210/https://blogs.cranfield.ac.uk/leadership-management/cbp/forecasting-prediction-is-very-difficult-especially-if-its-about-the-future|url-status=live}}</ref> Today, the Bohr model of the atom has been superseded, but is still the best known model of the atom, as it often appears in high school physics and chemistry texts.{{sfn|Kragh|2012|p=39}}

Bohr did not enjoy teaching medical students. He later admitted that he was not a good lecturer, because he needed a balance between clarity and truth, between "Klarheit und Wahrheit".<ref>{{cite book |last=Weisskopf |first=Victor |title="Niels Bohr, the Quantum, and the World" Social Research 51, no. 3 |date=1984 |pages=593}}</ref> He decided to return to Manchester, where Rutherford had offered him a job as a [[reader (academic rank)|reader]] in place of Darwin, whose tenure had expired. Bohr accepted. He took a leave of absence from the University of Copenhagen, which he started by taking a holiday in [[South Tyrol|Tyrol]] with his brother Harald and aunt [[Hanna Adler]]. There, he visited the [[University of Göttingen]] and the [[Ludwig Maximilian University of Munich]], where he met Sommerfeld and conducted seminars on the trilogy. The First World War broke out while they were in Tyrol, greatly complicating the trip back to Denmark and Bohr's subsequent voyage with Margrethe to England, where he arrived in October 1914. They stayed until July 1916, by which time he had been appointed to the Chair of Theoretical Physics at the University of Copenhagen, a position created especially for him. His docentship was abolished at the same time, so he still had to teach physics to medical students. New professors were formally introduced to King [[Christian X]], who expressed his delight at meeting such a famous football player.{{sfn|Pais|1991|pp=164–167}}

=== Institute of Theoretical Physics ===
{{Main|Niels Bohr Institute}}

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}}

The '''Bohr Festival''' ({{langx|de|Bohrfestspiele}}) was a series of seven lectures given by Bohr from 12 to 22 June 1922<ref name="Lindner L-L">{{cite web |last1=Lindner |first1=Konrad |title=Friedrich Hund über die Bohrfestspiele von 1922 |url=https://www.leipzig-lese.de/persoenlichkeiten/h/hund-friedrich/friedrich-hund-ueber-die-bohrfestspiele-von-1922/ |website=Leipzig-Lese |access-date=17 January 2023 |language=de}}</ref> at the Institute of Theoretical Physics in [[Göttingen]]. These were the Wolfskehl Lectures, funded by the Wolfskehl Foundation. Taking place in the fortnight leading up to the [[Göttingen International Handel Festival]], it became known as the Bohr Festival.<ref name="SH G&QM">{{cite web |last1=Schönhammer |first1=K. |title=Göttingen and Quantum Mechanics |url=https://www.theorie.physik.uni-goettingen.de/~schoenh/QMGemit.pdf |publisher=University of Göttingen |access-date=17 January 2023}}</ref> In 1991, [[Friedrich Hund]] suggested that [[James Franck]] was responsible for the comparison.<ref name="Lindner L-L"/> In the lectures, Bohr outlined the current development of the [[Bohr–Sommerfeld model|Bohr-Sommerfeld theory]], remarking "how incomplete and uncertain everything still is".<ref name="SH G&QM"/>

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}}

=== 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}}

{{multiple image
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| footer            = 1927 [[Solvay Conference]] in Brussels, October 1927. Bohr is on the right in the middle row, next to [[Max Born]].
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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 ''[[Lecturer|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>

== Philosophy ==
Heisenberg said of Bohr that he was "primarily a philosopher, not a physicist".{{sfn|Honner|1982|p=1}} Bohr read the 19th-century Danish [[Christian existentialist]] philosopher [[Søren Kierkegaard]]. [[Richard Rhodes]] argued in ''[[The Making of the Atomic Bomb]]'' that Bohr was influenced by Kierkegaard through Høffding.{{sfn|Rhodes|1986|p=60}} 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 home; but I do not believe that it would be very easy to find anything better&nbsp;... 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 [[Philosophy of Søren Kierkegaard|Kierkegaard's philosophy]].{{sfn|Faye|1991|p=37}} Some of Bohr's biographers suggested that this disagreement stemmed from Kierkegaard's advocacy of Christianity, while Bohr was an [[atheist]].{{sfn|Stewart|2010|p=416}}<ref name="Aaserud-Heilbron-2013-a">{{harvnb|Aaserud|Heilbron|2013|pp=159–160}}: "A statement about religion in the loose notes on Kierkegaard may throw light on the notion of wildness that appears in many of Bohr's letters. 'I, who do not feel in any way united with, and even less, bound to a God, and therefore am also much poorer [than Kierkegaard], would say that the good [is] the overall lofty goal, as only by being good [can one] judge according to worth and right.{{'"}}</ref><ref name="Aaserud-Heilbron-2013-b">{{harvnb|Aaserud|Heilbron|2013|p=110}}: "Bohr's sort of humor, use of parables and stories, tolerance, dependence on family, feelings of indebtedness, obligation, and guilt, and his sense of responsibility for science, community, and, ultimately, humankind in general, are common traits of the Jewish intellectual. So too is a well-fortified atheism. Bohr ended with no religious belief and a dislike of all religions that claimed to base their teachings on revelations."</ref>

There has been some dispute over the extent to which Kierkegaard influenced Bohr's philosophy and science. [[David Favrholdt]] argued that Kierkegaard had minimal influence over Bohr's work, taking Bohr's statement about disagreeing with Kierkegaard at face value,{{sfn|Favrholdt|1992|pp=42–63}} while Jan Faye argued that one can disagree with the content of a theory while accepting its general premises and structure.{{sfn|Richardson|Wildman|1996|p=289}}{{sfn|Faye|1991|p=37}}

Bohr sat on the Board of Editors of the book series ''[[World Perspectives]]'' which published a variety of books on philosophy.<ref>{{cite journal |last1=Egerod |first1=Soren |title="Voices of Man. The Meaning and Function of Language", by Mario Pei (Book Review) |journal=Romance Philology |date=November 1963 |volume=17 |issue=2 |pages=458–61}}</ref>

=== Quantum physics ===
[[File:Niels Bohr Albert Einstein4 by Ehrenfest cr.jpg|thumb|Bohr ''(left)'' and [[Albert Einstein]], pictured on 11 December 1925, had [[Bohr–Einstein debates|a long-running debate]] about the metaphysical implication of quantum physics.]]

There has been much subsequent debate and discussion about Bohr's views and philosophy of quantum mechanics.{{sfn|Camilleri|Schlosshauer|2015}} Regarding his ontological interpretation of the quantum world, Bohr has been seen as an [[anti-realist]], an [[Instrumentalism|instrumentalist]], a phenomenological realist or some other kind of realist. Furthermore, though some have seen Bohr as being a [[subjectivist]] or a [[positivist]], most philosophers agree that this is a misunderstanding of Bohr as he never argued for [[verificationism]] or for the idea that the subject had a direct impact on the outcome of a measurement.<ref name=":0" />

Bohr has often been quoted saying that there is "no quantum world" but only an "abstract quantum physical description". This was not publicly said by Bohr, but rather a private statement attributed to Bohr by Aage Petersen in a reminiscence after his death. [[N. David Mermin]] recalled [[Victor Weisskopf]] declaring that Bohr wouldn't have said anything of the sort and exclaiming, "Shame on Aage Petersen for putting those ridiculous words in Bohr's mouth!"{{sfn|Mermin|2004}}<ref>{{cite journal |last1=Petersen |first1=Aage |title=The Philosophy of Niels Bohr |journal=Bulletin of the Atomic Scientists |date=1963 |volume=19 |issue=7 |pages=8–14|doi=10.1080/00963402.1963.11454520 |bibcode=1963BuAtS..19g...8P }}</ref>

Numerous scholars have argued that the philosophy of [[Immanuel Kant]] had a strong influence on Bohr. Like Kant, Bohr thought distinguishing between the subject's experience and the object was an important condition for attaining knowledge. This can only be done through the use of causal and spatial-temporal concepts to describe the subject's experience.<ref name=":0">{{Cite encyclopedia |last=Faye |first=Jan |title=Copenhagen Interpretation of Quantum Mechanics |url=https://plato.stanford.edu/entries/qm-copenhagen/ |url-status=live |archive-url=https://web.archive.org/web/20221128011247/https://plato.stanford.edu/archives/win2019/entries/qm-copenhagen/ |archive-date=28 November 2022 |editor-last=Zalta |editor-first=Edward N. |encyclopedia=The Stanford Encyclopedia of Philosophy |edition=Winter 2019 |access-date=27 December 2023}}</ref> Thus, according to Jan Faye, Bohr thought that it is because of "classical" concepts like "space", "position", "time", "causation", and "momentum" that one can talk about objects and their objective existence. Bohr held that basic concepts like "time" are built in to our ordinary language and that the concepts of classical physics are merely a refinement of them.<ref name=":0" /> Therefore, for Bohr, classical concepts need to be used to describe experiments that deal with the quantum world. Bohr writes:

<blockquote>[T]he account of all evidence must be expressed in classical terms. The argument is simply that by the word 'experiment' we refer to a situation where we can tell to others what we have done and what we have learned and that, therefore, the account of the experimental arrangement and of the results of the observations must be expressed in unambiguous language with suitable application of the terminology of classical physics (''APHK'', p. 39).<ref name=":0" /></blockquote>

According to Faye, there are various explanations for why Bohr believed that classical concepts were necessary for describing quantum phenomena. Faye groups explanations into five frameworks: empiricism (i.e. [[logical positivism]]); [[Kantianism]] (or [[Neo-Kantian]] models of [[epistemology]]); [[Pragmatism]] (which focus on how human beings experientially interact with atomic systems according to their needs and interests); Darwinianism (i.e. we are adapted to use classical type concepts, which [[Léon Rosenfeld]] said that we evolved to use); and Experimentalism (which focuses strictly on the function and outcome of experiments that thus must be described classically).<ref name=":0" /> These explanations are not mutually exclusive, and at times Bohr seems to emphasise some of these aspects while at other times he focuses on other elements.<ref name=":0" />

According to Faye "Bohr thought of the atom as real. Atoms are neither heuristic nor logical constructions." However, according to Faye, he did not believe "that the quantum mechanical formalism was true in the sense that it gave us a literal ('pictorial') rather than a symbolic representation of the quantum world."<ref name=":0" /> Therefore, Bohr's theory of [[Complementarity (physics)|complementarity]] "is first and foremost a semantic and epistemological reading of quantum mechanics that carries certain ontological implications".<ref name=":0" /> As Faye explains, Bohr's ''indefinability thesis'' is that

<blockquote>[T]he truth conditions of sentences ascribing a certain kinematic or dynamic value to an atomic object are dependent on the apparatus involved, in such a way that these truth conditions have to include reference to the experimental setup as well as the actual outcome of the experiment.<ref name=":0" /></blockquote>

Faye notes that Bohr's interpretation makes no reference to a "collapse of the wave function during measurements" (and indeed, he never mentioned this idea). Instead, Bohr "accepted the Born statistical interpretation because he believed that the [[Wave function|''ψ''-function]] has only a symbolic meaning and does not represent anything real". Since for Bohr, the ''ψ''-function is not a literal pictorial representation of reality, there can be no real collapse of the wavefunction.<ref name=":0" />

A much debated point in recent literature is what Bohr believed about atoms and their reality and whether they are something else than what they seem to be. Some like Henry Folse argue that Bohr saw a distinction between observed phenomena and a [[Noumenon|transcendental reality]].<!-- Source? --> Jan Faye disagrees with this position and holds that for Bohr, the quantum formalism and complementarity was the only thing we could say about the quantum world and that "there is no further evidence in Bohr's writings indicating that Bohr would attribute intrinsic and measurement-independent state properties to atomic objects [...] in addition to the classical ones being manifested in measurement."<ref name=":0" />

== World War II ==
=== Assistance to refugee scholars===
The rise of [[Nazism]] in Germany prompted many scholars to flee their countries, either because they were Jewish or because they were political opponents of the Nazi regime. In 1933, the [[Rockefeller Foundation]] created a fund to help support refugee academics, and Bohr discussed this programme with the President of the Rockefeller Foundation, [[Max Mason]], in May 1933 during a visit to the United States. Bohr offered the refugees temporary jobs at the institute, provided them with financial support, arranged for them to be awarded fellowships from the Rockefeller Foundation, and ultimately found them places at institutions around the world. Those that he helped included [[Guido Beck]], [[Felix Bloch]], [[James Franck]], George de Hevesy, [[Otto Frisch]], [[Hilde Levi]], [[Lise Meitner]], George Placzek, [[Eugene Rabinowitch]], [[Stefan Rozental]], Erich Ernst Schneider, [[Edward Teller]], [[Arthur R. von Hippel|Arthur von Hippel]] and [[Victor Weisskopf]].{{sfn|Pais|1991|pp=382–386}}

In April 1940, early in the Second World War, [[Nazi Germany]] [[Occupation of Denmark|invaded and occupied Denmark]].{{sfn|Pais|1991|p=476}} To prevent the Germans from discovering [[Max von Laue]]'s and James Franck's [[Nobel Prize medal|gold Nobel medal]]s, Bohr had de Hevesy dissolve them in [[aqua regia]]. In this form, they were stored on a shelf at the Institute until after the war, when the gold was precipitated and the medals re-struck by the Nobel Foundation. Bohr's own medal had been donated to an auction to the [[Finnish Relief Fund]], and was auctioned off in March 1940, along with the medal of [[August Krogh]]. The buyer later donated the two medals to the Danish Historical Museum in [[Frederiksborg Castle]], where they are still kept,<ref>{{cite web |title=A unique gold medal |website=www.nobelprize.org |url=https://www.nobelprize.org/prizes/about/the-nobel-medals-and-the-medal-for-the-prize-in-economic-sciences/ |access-date=6 October 2019 |archive-date=11 April 2017 |archive-url= https://web.archive.org/web/20170411152140/https://historical.ha.com/itm/miscellaneous/georg-wittig-nobel-prize-medal-in-chemistry-received-in-1979-together-with-four-additional-medals/a/6165-49227.s |url-status=live }}</ref> although Bohr's medal temporarily went to space with [[Andreas Mogensen]] on [[International Space Station|ISS]] [[Expedition 70]] in 2023–2024.<ref>{{cite web |last1=Howell |first1=Elizabeth |title=Astronaut shows off vintage Nobel Prize in space — and talks 'quantum dots' ISS experiment (video) |url=https://www.space.com/international-space-station-nobel-prize-experiment |website=[[Space.com]] |date=12 December 2023}}</ref><ref>{{cite web |title=Andreas Mogensen leverer Nobelprismedalje retur til Frederiksborg |url=https://dnm.dk/pressepakke/andreas-mogensen-leverer-nobelpris-medalje-retur-til-frederiksborg/ |website=Frederiksborg |language=da-DK |date=14 June 2024}}</ref>

Bohr kept the Institute running, but all the foreign scholars departed.{{sfn|Pais|1991|pp=480–481}}

=== Meeting with Heisenberg ===
[[File:Heisenbergbohr.jpg|thumb|right|[[Werner Heisenberg]] ''(left)'' with Bohr at the Copenhagen Conference, 1934.|alt=A young man in a white shirt and tie and an older man in suit and tie sit at a table, on which there is a tea pot, plates, cups and saucers and beer bottles.]]

Bohr was aware of the possibility of using uranium-235 to construct an [[atomic bomb]], referring to it in lectures in Britain and Denmark shortly before and after the war started, but he did not believe that it was technically feasible to extract a sufficient quantity of uranium-235.{{sfn|Gowing|1985|pp=267–268}} In September 1941, Heisenberg, who had become head of the [[German nuclear energy project]], visited Bohr in Copenhagen. During this meeting the two men took a private moment outside, the content of which has caused much speculation, as both gave differing accounts.
According to Heisenberg, he began to address nuclear energy, morality and the war, to which Bohr seems to have reacted by terminating the conversation abruptly while not giving Heisenberg hints about his own opinions.{{sfn|Heisenberg|1984|p=77}} [[Ivan Supek]], one of Heisenberg's students and friends, claimed that the main subject of the meeting was [[Carl Friedrich von Weizsäcker]], who had proposed trying to persuade Bohr to mediate peace between Britain and Germany.<ref>{{cite web |author=Portal Jutarnji.hr |date=19 March 2006 |url=http://jutarnji.hr/clanak/art-2006,3,19,supek_intervju,17440.jl?artpg=1 |title=Moj život s nobelovcima 20. stoljeća |trans-title=My Life with the 20th century Nobel Prizewinners |work=[[Jutarnji list]] |language=hr |access-date=13 August 2007 |quote={{lang|hr |Istinu sam saznao od Margrethe, Bohrove supruge.&nbsp;... Ni Heisenberg ni Bohr nisu bili glavni junaci toga susreta nego Carl Friedrich von Weizsaecker.&nbsp;... Von Weizsaeckerova ideja, za koju mislim da je bila zamisao njegova oca koji je bio Ribbentropov zamjenik, bila je nagovoriti Nielsa Bohra da posreduje za mir između Velike Britanije i Njemačke.}} [I learned the truth from Margrethe, Bohr's wife.&nbsp;... Neither Bohr nor Heisenberg were the main characters of this encounter, but Carl Friedrich von Weizsaecker. Von Weizsaecker's idea, which I think was the brainchild of [[Ernst von Weizsäcker|his father]] who was [[Ribbentrop]]'s deputy, was to persuade Niels Bohr to mediate for peace between Great Britain and Germany.] |archive-date=28 June 2009 |archive-url=https://web.archive.org/web/20090628102407/http://jutarnji.hr/clanak/art-2006,3,19,supek_intervju,17440.jl?artpg=1  }} An interview with Ivan Supek relating to the 1941 Bohr&nbsp;– Heisenberg meeting.</ref>

In 1957, Heisenberg wrote to [[Robert Jungk]], who was then working on the book ''[[Brighter than a Thousand Suns: A Personal History of the Atomic Scientists]]''. Heisenberg explained that he had visited Copenhagen to communicate to Bohr the views of several German scientists, that production of a nuclear weapon was possible with great efforts, and this raised enormous responsibilities on the world's scientists on both sides.<ref>{{cite web |url=http://www.childrenofthemanhattanproject.org/MP_Misc/Bohr_Heisenberg/bohr_2.htm |title=Letter From Werner Heisenberg to Author Robert Jungk |access-date=21 December 2006 |last=Heisenberg |first=Werner |author-link=Werner Heisenberg |publisher=The Manhattan Project Heritage Preservation Association, Inc. |url-status=usurped |archive-url = https://web.archive.org/web/20061017232033/http://childrenofthemanhattanproject.org/MP_Misc/Bohr_Heisenberg/bohr_2.htm <!-- Bot retrieved archive --> |archive-date = 17 October 2006}}</ref> When Bohr saw Jungk's depiction in the Danish translation of the book, he drafted (but never sent) a letter to Heisenberg, stating that he deeply disagreed with Heisenberg's account of the meeting,<ref>"I am greatly amazed to see how much your memory has deceived you in your letter to the author of the book"</ref> that he recalled Heisenberg's visit as being to encourage cooperation with the inevitably victorious Nazis<ref>"...you and Weizsäcker expressed your definite conviction that Germany would win and that it was therefore quite foolish for us to maintain the hope of a different outcome of the war and to be reticent as regards all German offers of cooperation'</ref> and that he was shocked that Germany was pursuing nuclear weapons under Heisenberg's leadership.<ref>"...you spoke in a manner that could only give me the firm impression that, under your leadership, everything was being done in Germany to develop atomic weapons... [...] If anything in my behaviour could be interpreted as shock, it did not derive from such reports but rather from the news, as I had to understand it, that Germany was participating vigorously in a race to be the first with atomic weapons."</ref><ref>{{cite web |url=http://www.nbarchive.dk/collections/bohr-heisenberg/ |title=Release of documents relating to 1941 Bohr-Heisenberg meeting |access-date=4 June 2007 |archive-url = https://web.archive.org/web/20170217070953/http://www.nbarchive.dk/collections/bohr-heisenberg/ |archive-date =17 February 2017|last=Aaserud |first=Finn |date=6 February 2002 |publisher=Niels Bohr Archive}}</ref>

[[Michael Frayn]]'s 1998 play ''[[Copenhagen (play)|Copenhagen]]'' explores what might have happened at the 1941 meeting between Heisenberg and Bohr.<ref>{{cite web |url=http://www.complete-review.com/reviews/fraynm/cophagen.htm |title=Copenhagen – Michael Frayn |publisher=The Complete Review |access-date=27 February 2013 |archive-date=29 April 2013 |archive-url=https://web.archive.org/web/20130429014906/http://www.complete-review.com/reviews/fraynm/cophagen.htm |url-status=live }}</ref> [[Copenhagen (2002 film)|A television film version]] of the play by the [[BBC]] was first screened on 26 September 2002, with [[Stephen Rea]] as Bohr. With the subsequent release of Bohr's letters, the play has been criticised by historians as being a "grotesque oversimplification and perversion of the actual moral balance" due to adopting a pro-Heisenberg perspective.<ref>{{cite journal|title='Copenhagen': An Exchange|first1=Gerald |last1=Holton |author-link1=Gerald Holton |first2=Jonothan |last2=Logan |first3=Thomas |last3=Powers |author-link3=Thomas Powers |first4=Michael |last4=Frayn |journal=The New York Review of Books |author-link4=Michael Frayn |publisher=The New York Review|date=11 April 2002|volume=49 |issue=6 |url=https://www.nybooks.com/articles/2002/04/11/copenhagen-an-exchange/ |access-date=18 May 2024}}</ref>

The same meeting had previously been dramatised by the BBC's ''[[Horizon (BBC TV series)|Horizon]]'' science documentary series in 1992, with [[Anthony Bate]] as Bohr, and Philip Anthony as Heisenberg.<ref>''Horizon: Hitler's Bomb'', [[BBC Two]], 24 February 1992</ref> The meeting is also dramatised in the Norwegian/Danish/British miniseries ''[[The Heavy Water War]]''.<ref>{{cite web |url=http://www.channel4.com/programmes/the-saboteurs/episode-guide/ |title=The Saboteurs – Episode Guide |publisher=Channel 4 |access-date=3 March 2017 |archive-date=3 March 2017 |archive-url=https://web.archive.org/web/20170303123755/http://www.channel4.com/programmes/the-saboteurs/episode-guide/ |url-status=live }}</ref>

=== Manhattan Project ===
In September 1943, word reached Bohr and his brother Harald that the Nazis [[Nuremberg Laws|considered their family to be Jewish]], since their mother was Jewish, and that they were therefore in danger of being arrested. The Danish resistance helped Bohr and his wife escape by sea to Sweden on 29 September.{{sfn|Rozental|1967|p=168}}{{sfn|Rhodes|1986|pp=483–484}} The next day, Bohr persuaded King [[Gustaf V of Sweden]] to make public Sweden's willingness to provide asylum to Jewish refugees. On 2 October 1943, Swedish radio broadcast that Sweden was ready to offer asylum, and the mass [[rescue of the Danish Jews]] by their countrymen followed swiftly thereafter. Some historians claim that Bohr's actions led directly to the mass rescue, while others say that, though Bohr did all that he could for his countrymen, his actions were not a decisive influence on the wider events.{{sfn|Rhodes|1986|pp=483–484}}{{sfn|Hilberg|1961|p=596}}{{sfn|Kieler|2007|pp=91–93}}{{sfn|Stadtler|Morrison|Martin|1995|p=136}} Eventually, over 7,000 Danish Jews escaped to Sweden.{{sfn|Pais|1991|p=479}}

[[File:Portrait of Albert Einstein, Niels Bohr, James Franck and Rabi.jpg|thumb|Bohr ''(left)'' with [[James Franck]], [[Albert Einstein]] and [[Isidor Isaac Rabi]], 1954.]]

When the news of Bohr's escape reached Britain, [[Lord Cherwell]] sent a telegram to Bohr asking him to come to Britain. Bohr arrived in Scotland on 6 October in a [[de Havilland Mosquito]] operated by the [[British Overseas Airways Corporation]] (BOAC).{{sfn|Jones|1985|pp=280–281}}{{sfn|Powers|1993|p=237}} The Mosquitos were unarmed high-speed bomber aircraft that had been converted to carry small, valuable cargoes or important passengers. By flying at high speed and high altitude, they could cross German-occupied Norway, and yet avoid German fighters. Bohr, equipped with parachute, flying suit and oxygen mask, spent the three-hour flight lying on a mattress in the aircraft's [[bomb bay]].{{sfn|Thirsk|2006|p=374}} During the flight, Bohr did not wear his flying helmet as it was too small, and consequently did not hear the pilot's intercom instruction to turn on his oxygen supply when the aircraft climbed to high altitude to overfly Norway. He passed out from oxygen starvation and only revived when the aircraft descended to lower altitude over the North Sea.{{sfn|Rife|1999|p=242}}{{sfn|Medawar|Pyke|2001|p=65}}{{sfn|Jones|1978|pp=474–475}} Bohr's son Aage followed his father to Britain on another flight a week later, and became his personal assistant.{{sfn|Jones|1985|pp=280–282}}

Bohr was warmly received by [[James Chadwick]] and Sir [[John Anderson, 1st Viscount Waverley|John Anderson]], but for security reasons Bohr was kept out of sight. He was given an apartment at [[St James's Palace]] and an office with the British [[Tube Alloys]] nuclear weapons development team. Bohr was astonished at the amount of progress that had been made.{{sfn|Jones|1985|pp=280–282}}{{sfn|Pais|1991|pp=491}} Chadwick arranged for Bohr to visit the United States as a Tube Alloys consultant, with Aage as his assistant.{{sfn|Cockcroft|1963|p=46}} On 8 December 1943, Bohr arrived in [[Washington, D.C.]], where he met with the director of the [[Manhattan Project]], Brigadier General [[Leslie R. Groves Jr.]] He visited Einstein and Pauli at the [[Institute for Advanced Study]] in [[Princeton, New Jersey]], and went to [[Los Alamos National Laboratory|Los Alamos]] in [[New Mexico]], where the nuclear weapons were being designed.{{sfn|Pais|1991|pp=498–499}} For security reasons, he went under the name of "Nicholas Baker" in the United States, while Aage became "James Baker".{{sfn|Gowing|1985|p=269}} In May 1944 the Danish resistance newspaper ''[[De frie Danske]]'' reported that they had learned that 'the famous son of Denmark Professor Niels Bohr' in October the previous year had fled his country via Sweden to London and from there travelled to [[Moscow]] from where he could be assumed to support the war effort.<ref>{{cite news |title= Professor Bohr ankommet til Moskva |trans-title= Professor Bohr arrived in Moscow |url= http://www.illegalpresse.dk/papers#/paper?paper=72&page=826 |newspaper= [[De frie Danske]] |date= May 1944 |page= 7 |access-date= 18 November 2014 |language= da |archive-date= 16 November 2018 |archive-url= https://web.archive.org/web/20181116000610/http://www.illegalpresse.dk/papers#/paper?paper=72&page=826 |url-status= live }}</ref>

Bohr did not remain at Los Alamos, but paid a series of extended visits over the course of the next two years. [[Robert Oppenheimer]] credited Bohr with acting "as a scientific father figure to the younger men", most notably [[Richard Feynman]].{{sfn|Pais|1991|p=497}} Bohr is quoted as saying, "They didn't need my help in making the atom bomb."{{sfn|Pais|1991|p=496}} Oppenheimer gave Bohr credit for an important contribution to the work on [[modulated neutron initiator]]s. "This device remained a stubborn puzzle", Oppenheimer noted, "but in early February 1945 Niels Bohr clarified what had to be done".{{sfn|Pais|1991|p=497}}

Bohr recognised early that nuclear weapons would change international relations. In April 1944, he received a letter from [[Peter Kapitza]], written some months before when Bohr was in Sweden, inviting him to come to the [[Soviet Union]]. The letter convinced Bohr that the Soviets were aware of the Anglo-American project, and would strive to catch up. He sent Kapitza a non-committal response, which he showed to the authorities in Britain before posting.{{sfn|Gowing|1985|p=270}} Bohr met Churchill on 16 May 1944, but found that "we did not speak the same language".{{sfn|Gowing|1985|p=271}} 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."{{sfn|Aaserud|2006|p=708}}

Oppenheimer suggested that Bohr visit President [[Franklin D. Roosevelt]] to convince him that the Manhattan Project should be shared with the Soviets in the hope of speeding up its results. Bohr's friend, Supreme Court Justice [[Felix Frankfurter]], informed President Roosevelt about Bohr's opinions, and a meeting between them took place on 26 August 1944. Roosevelt suggested that Bohr return to the United Kingdom to try to win British approval.{{sfn|Rhodes|1986|pp=528–538}}{{sfn|Aaserud|2006|pp=707–708}} When Churchill and Roosevelt met at Hyde Park on 19 September 1944, they rejected the idea of informing the world about the project, and the [[Quebec Agreement#Hyde Park Aide-Mémoire|aide-mémoire]] of their conversation contained a rider that "enquiries should be made regarding the activities of Professor Bohr and steps taken to ensure that he is responsible for no leakage of information, particularly to the Russians".{{sfn|U.S. Government|1972|pp=492–493}}

In June 1950, Bohr addressed an "Open Letter" to the [[United Nations]] calling for international cooperation on nuclear energy.{{sfn|Aaserud|2006|pp=708–709}}<ref>{{cite journal |last=Bohr |first=Niels |date=9 June 1950 |url=http://www.fredsakademiet.dk/library/dkpeace/dkpeace15.htm |title=To the United Nations (open letter) |journal=Impact of Science on Society |volume=I |issue=2 |page=68 |access-date=12 June 2012 |archive-date=8 March 2013 |archive-url=https://web.archive.org/web/20130308122903/http://www.fredsakademiet.dk/library/dkpeace/dkpeace15.htm |url-status=live }}<br />• {{cite journal |last=Bohr |first=Niels |date=July 1950 |pages=213–219 |url=https://books.google.com/books?id=4g0AAAAAMBAJ&q=%22atomic+energy+project%22+1944&pg=PA214 |title=For An Open World |journal=Bulletin of the Atomic Scientists |volume=6 |issue=7 |access-date=26 June 2011 |doi=10.1080/00963402.1950.11461268 |bibcode=1950BuAtS...6g.213B |archive-date=30 October 2023 |archive-url=https://web.archive.org/web/20231030202300/https://books.google.com/books?id=4g0AAAAAMBAJ&q=%22atomic+energy+project%22+1944&pg=PA214#v=snippet&q=%22atomic%20energy%20project%22%201944&f=false |url-status=live }}</ref>{{sfn|Pais|1991|pp=513–518}} In the 1950s, after the [[RDS-1|Soviet Union's first nuclear weapon test]] in 1949, the [[International Atomic Energy Agency]] was created along the lines of Bohr's suggestion.{{sfn|Gowing|1985|p=276}} In 1957, he received the first ever [[Atoms for Peace Award]].<ref>{{cite web |url=http://libraries.mit.edu/archives/research/collections/collections-mc/pdf/mc10.pdf |title=Guide to Atoms for Peace Awards Records |first=Elizabeth |last=Craig-McCormack |publisher=[[Massachusetts Institute of Technology]] |access-date=28 February 2013 |archive-url=https://web.archive.org/web/20100311073706/http://libraries.mit.edu/archives/research/collections/collections-mc/pdf/mc10.pdf |archive-date=11 March 2010 }}</ref>

== Later life ==
[[File:Coat of Arms of Niels Bohr.svg|thumb|right|240px|upright|Bohr's coat of arms, 1947. [[Argent]], a ''[[taijitu]]'' (yin-yang symbol) [[Gules]] and [[Sable (heraldry)|Sable]]. Motto: ''Contraria sunt complementa'' ("opposites are complementary")<ref>{{cite web |url=http://www.numericana.com/arms/#bohr |publisher=Numericana |title=Escutcheons of Science |first=Gérard P. |last=Michon |access-date=13 March 2017 |archive-date=22 February 2012 |archive-url=https://web.archive.org/web/20120222211800/http://www.numericana.com/arms/#bohr |url-status=live }}</ref>]]
Following the ending of the war, Bohr returned to Copenhagen on 25 August 1945, and was re-elected President of the Royal Danish Academy of Arts and Sciences on 21 September.{{sfn|Pais|1991|p=504}} At a memorial meeting of the Academy on 17 October 1947 for King [[Christian X]], who had died in April, the new king, [[Frederik IX]], announced that he was conferring the [[Order of the Elephant]] on Bohr. This award was normally awarded only to royalty and heads of state, but the king said that it honoured not just Bohr personally, but Danish science.{{sfn|Pais|1991|pp=166, 466–467}}{{sfn|Wheeler|1985|p=224}} Bohr designed his own [[coat of arms]], which featured a [[taijitu]] (symbol of yin and yang) and a motto in {{langx|la|contraria sunt complementa|links=no}}, "opposites are complementary".<ref>{{cite web |title=Bohr crest | publisher=University of Copenhagen | date=17 October 1947 | url=http://www.nbi.dk/hehi/logo/bohr_crest.png | access-date=9 September 2019 |archive-url=https://web.archive.org/web/20190502082514/https://www.nbi.dk/hehi/logo/bohr_crest.png |archive-date=2 May 2019}}</ref>{{sfn|Wheeler|1985|p=224}}<ref>{{Cite web |title=A Complementary Relationship: Niels Bohr and China* |url=https://www.nbarchive.dk/doc/Aaserud-confucius.pdf |website=Niels Bohr Archive |access-date=15 July 2023 |archive-date=9 October 2021 |archive-url=https://web.archive.org/web/20211009004016/https://www.nbarchive.dk/doc/Aaserud-confucius.pdf |url-status=live }}</ref>

The Second World War demonstrated that science, and physics in particular, now required considerable financial and material resources. To avoid a [[brain drain]] to the United States, twelve European countries banded together to create [[CERN]], a research organisation along the lines of the national laboratories in the United States, designed to undertake [[Big Science]] projects beyond the resources of any one of them alone. Questions soon arose regarding the best location for the facilities. Bohr and Kramers felt that the Institute in Copenhagen would be the ideal site. [[Pierre Victor Auger|Pierre Auger]], who organised the preliminary discussions, disagreed; he felt that both Bohr and his Institute were past their prime, and that Bohr's presence would overshadow others. After a long debate, Bohr pledged his support to CERN in February 1952, and [[Geneva]] was chosen as the site in October. The CERN Theory Group was based in Copenhagen until their new accommodation in Geneva was ready in 1957.{{sfn|Pais|1991|pp=519–522}} Victor Weisskopf, who later became the [[List of Directors General of CERN|Director General of CERN]], summed up Bohr's role, saying that "there were other personalities who started and conceived the idea of [[CERN]]. The enthusiasm and ideas of the other people would not have been enough, however, if a man of his stature had not supported it."{{sfn|Pais|1991|p=521}}<ref>{{cite journal|last1=Weisskopf|first1=Victor|title=Tribute to Niels Bohr|journal=CERN Courier|date=July 1963|volume=2|issue=11|page=89|url=https://cds.cern.ch/record/1728615|access-date=26 March 2015|archive-date=17 August 2018|archive-url=https://web.archive.org/web/20180817091438/https://cds.cern.ch/record/1728615|url-status=live}}</ref>

Meanwhile, Scandinavian countries formed the [[Nordic Institute for Theoretical Physics]] in 1957, with Bohr as its chairman. He was also involved with the founding of the [[Risø DTU National Laboratory for Sustainable Energy|Research Establishment Risø of the Danish Atomic Energy Commission]], and served as its first chairman from February 1956.{{sfn|Pais|1991|pp=523–525}}

Bohr died of heart failure on 18 November 1962 at his home in [[Carlsberg (district)|Carlsberg]], [[Copenhagen]].<ref>{{cite journal|title=Niels Bohr|journal=CERN Courier|date=November 1962|volume=2|issue=11|page=10|url=https://cds.cern.ch/record/1728506|access-date=24 March 2015|archive-date=17 August 2018|archive-url=https://web.archive.org/web/20180817091106/https://cds.cern.ch/record/1728506|url-status=live}}</ref> He was cremated, and his ashes were buried in the family plot in the [[Assistens Cemetery (Copenhagen)|Assistens Cemetery]] in the [[Nørrebro]] section of Copenhagen, along with those of his parents, his brother Harald, and his son Christian. Years later, his wife's ashes were also interred there.{{sfn|Pais|1991|p=529}} On 7 October 1965, on what would have been his 80th birthday, the Institute for Theoretical Physics at the University of Copenhagen was officially renamed to what it had been called unofficially for many years: the [[Niels Bohr Institute]].<ref>{{cite web |url=http://www.nbi.dk/nbi-history.html |title=History of the Niels Bohr Institute from 1921 to 1965 |publisher=Niels Bohr Institute |access-date=28 February 2013 |archive-url=https://web.archive.org/web/20030608060003/http://www.nbi.dk/nbi-history.html |archive-date=8 June 2003 }}</ref><ref>{{cite journal|last1=Reinhard|first1=Stock|title=Niels Bohr and the 20th century|journal=CERN Courier|date=October 1998|volume=38|issue=7|page=19|url=https://cds.cern.ch/record/1732841|access-date=26 March 2015|archive-date=24 October 2017|archive-url=https://web.archive.org/web/20171024200749/https://cds.cern.ch/record/1732841|url-status=live}}</ref>

== Family ==
{{Main|Bohr family}}
[[File:Niels Bohr and Margrethe engaged 1910.jpg|thumb|right|upright|Bohr and [[Margrethe Nørlund]] on their engagement in 1910.|alt=A young man in a suit and tie and a young woman in a light coloured dress sit on a stoop, holding hands]]

In 1910, Bohr met [[Margrethe Nørlund]], the sister of mathematician [[Niels Erik Nørlund]].{{sfn|Pais|1991|p=112}} Bohr resigned his membership in the [[Church of Denmark]] on 16 April 1912, and he and Margrethe were married in a civil ceremony at the town hall in [[Slagelse]] on 1 August. Years later, his brother, [[Harald Bohr|Harald]], similarly left the church before getting married.{{sfn|Pais|1991|pp=133–134}} Bohr and Margrethe had six sons.{{sfn|Pais|1991|pp=226, 249}} The eldest, Christian, died in a boating accident in 1934.{{sfn|Stuewer|1985|p=204}} Another son, Harald, was severely mentally disabled, and was placed in an institution away from his family's home at the age of 4 and died of childhood [[meningitis]] six years later.<ref>{{Cite web |date=11 April 2022 |title=Udstilling om Brejnings historie hitter i Vejle |url=https://ugeavisen.dk/ugeavisenvejle/artikel/udstilling-om-brejnings-historie-hitter-i-vejle |access-date=17 July 2022 |website=ugeavisen.dk |language=da |archive-date=14 July 2022 |archive-url=https://web.archive.org/web/20220714232348/https://ugeavisen.dk/ugeavisenvejle/artikel/udstilling-om-brejnings-historie-hitter-i-vejle |url-status=live }}</ref>{{sfn|Pais|1991|pp=226, 249}} [[Aage Bohr]] became a successful physicist, and in 1975 was awarded the [[Nobel Prize in Physics]], like his father. A son of Aage, Vilhelm A. Bohr, is a scientist affiliated with the University of Copenhagen<ref>{{Cite web |last=Schou |first=Mette Kjær |date=22 August 2019 |title=Bohr Group |url=https://icmm.ku.dk/english/research-groups/bohr-group/ |access-date=19 October 2022 |website=icmm.ku.dk |language=en |archive-date=19 October 2022 |archive-url=https://web.archive.org/web/20221019200544/https://icmm.ku.dk/english/research-groups/bohr-group/ |url-status=live }}</ref> and the [[National Institute on Aging]] in the U.S.<ref>{{Cite web |title=Neuroscience@NIH > Faculty > Profile |url=https://dir.ninds.nih.gov/Faculty/Profile/vilhelm-bohr.html |access-date=19 October 2022 |website=dir.ninds.nih.gov |archive-date=19 October 2022 |archive-url=https://web.archive.org/web/20221019200544/https://dir.ninds.nih.gov/Faculty/Profile/vilhelm-bohr.html |url-status=dead }}</ref> {{Interlanguage link|Hans Bohr|da|lt=Hans}} became a physician; {{Interlanguage link|Erik Bohr|da|lt=Erik}}, a chemical engineer; and [[Ernest Bohr|Ernest]], a lawyer.<ref name="nobelprize.org">{{cite web|title=Niels Bohr&nbsp;– Biography|url=https://www.nobelprize.org/nobel_prizes/physics/laureates/1922/bohr-bio.html|publisher=[[Nobelprize.org]]|access-date=10 November 2011|archive-date=11 November 2011|archive-url=https://web.archive.org/web/20111111014801/http://www.nobelprize.org/nobel_prizes/physics/laureates/1922/bohr-bio.html|url-status=live}}</ref> Like his uncle Harald, Ernest Bohr became an Olympic athlete, playing [[field hockey]] for Denmark at the [[1948 Summer Olympics]] in London.<ref name="hockey">{{cite web |url=https://www.sports-reference.com/olympics/athletes/bo/ernest-bohr-1.html |archive-url=https://web.archive.org/web/20200418093355/https://www.sports-reference.com/olympics/athletes/bo/ernest-bohr-1.html  |archive-date=18 April 2020 |title=Ernest Bohr Biography and Olympic Results – Olympics |publisher=Sports-Reference.com |access-date=12 February 2013}}</ref>

== Recognition ==
=== Awards ===
{| class="wikitable"
! Year
! Organization
! Award
! Citation
! {{Reference column heading}}
|-
| 1921
| {{Flagicon|UKGBI}} [[Royal Society]]
| [[Hughes Medal]]
| "For his research in theoretical physics."
| <ref>{{Cite web|title=Hughes Medal|url=https://royalsociety.org/medals-and-prizes/hughes-medal/|website=royalsociety.org|access-date=2025-10-31}}</ref>
|-
| 1922
| {{Flagicon|Sweden}} [[Royal Swedish Academy of Sciences]]
| [[Nobel Prize in Physics]]
| "For his services in the investigation of the structure of atoms and of the radiation emanating from them."
| <ref name="Nobel Prize"/>
|-
| 1923
| {{Flagicon|Kingdom of Italy}} [[Accademia dei XL]]
| [[Matteucci Medal]]
|
| <ref>{{Cite web|title=Medaglie|url=https://www.accademiaxl.it/attivita/medaglie/|website=www.accademiaxl.it|language=it|access-date=2025-11-04}}</ref>
|-
| 1926
| {{Flagicon|US|1912}} [[Franklin Institute]]
| [[Franklin Medal]]
| "For contributions to the mechanics of atomic structures and theory of the origin of spectrum lines."
| <ref>{{Cite web|title=Niels Bohr|url=https://fi.edu/en/awards/laureates/niels-bohr|url-status=live|publisher=[[Franklin Institute]]|archive-url=https://web.archive.org/web/20140814044251/https://www.fi.edu/laureates/niels-bohr|archive-date=2014-08-14|access-date=2013-10-21}}</ref>
|-
| 1930
| {{Flagicon|UK}} [[Royal Society of Chemistry]]
| [[Faraday Lectureship Prize]]
|
| <ref>{{Cite web|title=Faraday Lectureship Prize - previous winners|url=http://www.rsc.org/ScienceAndTechnology/Awards/FaradayLectureshipPrize/PreviousWinners.asp|url-status=dead|publisher=[[Royal Society of Chemistry]]|archive-url=https://web.archive.org/web/20241108220123/https://www.rsc.org/prizes-funding/prizes/find-a-prize/faraday-division-open-award-faraday-lectureship-prize/previous-winners/|archive-date=2024-11-08|access-date=2010-03-05}}</ref>
|-
| 1930
| {{Flagicon|Weimar Republic}} [[German Physical Society]]
| [[Max Planck Medal]]
|
| <ref>{{Cite web|title=Preisträgerinnen und Preisträger|url=https://www.dpg-physik.de/auszeichnungen/dpg-preise/max-planck-medaille/preistraeger|url-status=live|website=www.dpg-physik.de|language=de|archive-url=https://web.archive.org/web/20251009202127/https://www.dpg-physik.de/auszeichnungen/dpg-preise/max-planck-medaille/preistraeger|archive-date=2025-10-09|access-date=2021-03-23}}</ref>
|-
| 1938
| {{Flagicon|UK}} [[Royal Society]]
| [[Copley Medal]]
| "In recognition of his distinguished work in the development of the quantum theory of atomic structure."
| <ref>{{Cite web|title=Copley Medal|url=https://royalsociety.org/medals-and-prizes/copley-medal/|url-status=live|website=royalsociety.org |archive-url=https://web.archive.org/web/20150906190948/https://royalsociety.org/grants-schemes-awards/awards/copley-medal/|archive-date=2015-09-06|access-date=2016-10-19}}</ref>
|-
| 1957
|
| [[Atoms for Peace Award]]
|
| <ref>{{Cite journal|title=Bohr Wins Atoms for Peace Prize|journal=[[Physics Today]]|volume=10|issue=5|page=16|date=1957-05-01|doi=10.1063/1.3060363}}</ref>
|-
| 1961
| {{Flagicon|Denmark}} [[University of Copenhagen]]
| [[Sonning Prize]]
|
| <ref>{{Cite web|title=Sonning Prize recipients|url=https://event.ku.dk/sonning_prize/recipients/|url-status=live|website=event.ku.dk|archive-url=https://web.archive.org/web/20250518065138/https://event.ku.dk/sonning_prize/recipients/|archive-date=2025-05-18|access-date=2025-11-30}}</ref>
|}

=== Memberships ===
{| class="wikitable"
! Year
! Organization
! Type
! {{Reference column heading}}
|-
| 1923
| {{Flagicon|Netherlands}} [[Royal Netherlands Academy of Arts and Sciences]]
| Foreign Member
| <ref>{{Cite web|title=N.H.D. Bohr (1885 - 1962)|url=https://dwc.knaw.nl/biografie/biografisch-apparaat/pmknaw/?pagetype=authorDetail&aId=PE00003380|url-status=live|website=dwc.knaw.nl|archive-url=https://web.archive.org/web/20150923232750/http://www.dwc.knaw.nl/biografie/pmknaw/?pagetype=authorDetail&aId=PE00003380|archive-date=2015-09-23|access-date=2015-07-21}}</ref>
|-
| 1925
| {{Flagicon|US|1912}} [[National Academy of Sciences]]
| International Member
| <ref>{{Cite web|title=Niels Bohr|url=http://www.nasonline.org/member-directory/deceased-members/20001177.html|url-status=dead|publisher=[[National Academy of Sciences]]|archive-url=https://web.archive.org/web/20230504142351/http://www.nasonline.org/member-directory/deceased-members/20001177.html|archive-date=2023-05-04|access-date=2023-05-04}}</ref>
|-
| 1926
| {{Flagicon|UK}} [[Royal Society]]
| Foreign Member
| <ref>{{Cite web|title=Search Results|url=https://catalogues.royalsociety.org/CalmView/Record.aspx?src=CalmView.Persons&id=NA3135&pos=1|website=catalogues.royalsociety.org|access-date=2025-11-30}}</ref>
|-
| 1940
| {{Flagicon|US|1912}} [[American Philosophical Society]]
| International Member
| <ref>{{Cite web|title=APS Member History|url=https://search.amphilsoc.org/memhist/search?creator=Niels+Bohr&title=&subject=&subdiv=&mem=&year=&year-max=&dead=&keyword=&smode=advanced|url-status=live|website=search.amphilsoc.org|archive-url=https://web.archive.org/web/20230504142402/https://search.amphilsoc.org/memhist/search?creator=Niels+Bohr&title=&subject=&subdiv=&mem=&year=&year-max=&dead=&keyword=&smode=advanced|archive-date=2023-05-04|access-date=2023-05-04}}</ref>
|-
| 1945
| {{Flagicon|US|1912}} [[American Academy of Arts and Sciences]]
| International Honorary Member
| <ref>{{Cite web|title=Niels Henrik David Bohr|url=https://www.amacad.org/person/niels-henrik-david-bohr|url-status=live|website=www.amacad.org|archive-url=https://web.archive.org/web/20230504142400/https://www.amacad.org/person/niels-henrik-david-bohr|archive-date=2023-05-04|access-date=2023-05-04}}</ref>
|-
| 1950
| {{Flagicon|Occupation of Japan}} [[Japan Academy]]
| Honorary Member
| <ref>{{Cite web|title=Deceased Honorary Members|url=https://www.japan-acad.go.jp/en/members/bukko/bukko_kyakuin.html|url-status=live|website=www.japan-acad.go.jp|archive-url=https://web.archive.org/web/20251221202229/https://www.japan-acad.go.jp/en/members/bukko/bukko_kyakuin.html|archive-date=2025-12-21|access-date=2026-02-02}}</ref>
|}

== Commemoration == 
The Bohr model's semicentennial was commemorated in Denmark on 21 November 1963 with a [[Commemorative stamp|postage stamp]] depicting Bohr, the hydrogen atom and the formula for the difference of any two hydrogen energy levels: <math>h\nu = \epsilon_2 - \epsilon_1</math>. Several other countries have also issued postage stamps depicting Bohr.{{sfn|Kennedy|1985|pp=10–11}} In 1997, the [[Danish National Bank]] began circulating the [[Banknotes of Denmark, 1997 series|500-krone banknote]] with the portrait of Bohr smoking a pipe.{{sfn|Danmarks Nationalbank|2005|pp=20–21}}<ref>{{cite web|url=http://www.nationalbanken.dk/DNUK/NotesAndCoins.nsf/side/Denmarks_banknote_series!OpenDocument |title=500-krone banknote, 1997 series |publisher=Danmarks Nationalbank |access-date=7 September 2010 |archive-url=https://web.archive.org/web/20100825003955/http://www.nationalbanken.dk/DNUK/NotesAndCoins.nsf/side/Denmarks_banknote_series%21OpenDocument |archive-date=25 August 2010 }}</ref> On 7 October 2012, Bohr's birthday was celebrated in a [[Google Doodle]] depicting the Bohr model of the hydrogen atom.<ref>{{cite web|title=Niels Bohr's 127th Birthday|url=https://doodles.google/doodle/niels-bohrs-127th-birthday/|website=www.google.com/doodles#archive|access-date=7 October 2021| publisher=[[Google]]| archive-date=6 October 2021|archive-url=https://web.archive.org/web/20211006233829/http://www.google.com/doodles/niels-bohrs-127th-birthday|url-status=live}}</ref> An asteroid, [[3948 Bohr]], was named after him,<ref>{{cite journal |url=http://www.minorplanet.info/MPB/MPB_40-1.pdf |access-date=28 February 2013 |title=Lightcurve Analysis of 3948 Bohr and 4874 Burke: An International Collaboration |journal=Minor Planet Bulletin |volume=40 |issue=1 |date=January–March 2013 |page=15 |last1=Klinglesmith |first1=Daniel A. III |last2=Risley |first2=Ethan |last3=Turk |first3=Janek |last4=Vargas |first4=Angelica |bibcode=2013MPBu...40...15K |last5=Warren |first5=Curtis |last6=Ferrero |first6=Andera |archive-url=https://web.archive.org/web/20130603072504/http://www.minorplanet.info/MPB/MPB_40-1.pdf |archive-date=3 June 2013 }}</ref> as was the [[Bohr (crater)|Bohr lunar crater]], and [[bohrium]], the chemical element with atomic number 107, in acknowledgement of his work on the structure of atoms.<ref name=IUPAC97>{{cite journal|doi=10.1351/pac199769122471|title=Names and symbols of transfermium elements (IUPAC Recommendations 1997)|year=1997|journal=Pure and Applied Chemistry|volume=69|page=2472|issue=12|doi-access=free}}</ref><ref>{{Cite web |title=Bohrium |url=https://www.utoledo.edu/nsm/ic/elements/bohrium.html#:~:text=The%20namesake%20of%20Bohrium,%20Niels,separate%20orbits%20in%20an%20atom. |access-date=3 January 2025 |publisher=University of Toledo}}</ref>

== Bibliography ==
[[File:Bohr, Niels – The theory of spectra and atomic constitution (Drei Aufsätze über Spektren und Atombau), 1922 – BEIC 10990185.jpg|thumb|upright|''The Theory of Spectra and Atomic Constitution (Drei Aufsätze über Spektren und Atombau)'', 1922]]
{{refbegin|30em}}
* {{cite book | last=Bohr |first=Niels |title=The Theory of Spectra and Atomic Constitution; three essays|publisher=Cambridge University Press |location =Cambridge|year=1922|url=https://gutenberg.org/ebooks/47464 |ref=none}}
* {{cite book| last=Bohr |first=Niels |editor-last= Nielsen |editor-first=J. Rud |series=Niels Bohr Collected Works |location=Amsterdam |publisher=Elsevier |year=2008 |isbn=978-0-444-53286-2 |oclc=272382249 |title=Volume 1: Early Work (1905–1911) | author-mask=2 |ref=none }}
* {{cite book| last=Bohr |first=Niels |editor-last=Hoyer |editor-first=Ulrich |series=Niels Bohr Collected Works |location=Amsterdam |publisher=Elsevier |year=2008 |isbn=978-0-444-53286-2 |oclc=272382249 | author-mask=2 |title=Volume 2: Work on Atomic Physics (1912–1917) |ref=none }}
* {{cite book| last=Bohr |first=Niels |editor-last= Nielsen |editor-first=J. Rud |series=Niels Bohr Collected Works |location=Amsterdam |publisher=Elsevier |year=2008 |isbn=978-0-444-53286-2 |oclc=272382249 | author-mask=2 |title=Volume 3: The Correspondence Principle (1918–1923) |ref=none }}
* {{cite book| last=Bohr |first=Niels |editor-last= Nielsen |editor-first=J. Rud |series=Niels Bohr Collected Works |location=Amsterdam |publisher=Elsevier |year=2008 |isbn=978-0-444-53286-2 |oclc=272382249 | author-mask=2 |title=Volume 4: The Periodic System (1920–1923) |ref=none }}
* {{cite book| last=Bohr |first=Niels |editor-last= Stolzenburg |editor-first=Klaus |series=Niels Bohr Collected Works |location=Amsterdam |publisher=Elsevier |year=2008 |isbn=978-0-444-53286-2 |oclc=272382249 | author-mask=2 |title=Volume 5: The Emergence of Quantum Mechanics (mainly 1924–1926) |ref=none }}
* {{cite book| last=Bohr |first=Niels |editor-last= Kalckar |editor-first=Jørgen |series=Niels Bohr Collected Works |location=Amsterdam |publisher=Elsevier |year=2008 |isbn=978-0-444-53286-2 |oclc=272382249 | author-mask=2 |title=Volume 6: Foundations of Quantum Physics I (1926–1932) |ref=none }}
* {{cite book| last=Bohr |first=Niels |editor-last= Kalckar |editor-first=Jørgen |series=Niels Bohr Collected Works |location=Amsterdam |publisher=Elsevier |year=2008 |isbn=978-0-444-53286-2 |oclc=272382249 | author-mask=2 |title=Volume 7: Foundations of Quantum Physics I (1933–1958) |ref=none }}
* {{cite book| last=Bohr |first=Niels |editor-last= Thorsen |editor-first=Jens|series=Niels Bohr Collected Works |location=Amsterdam |publisher=Elsevier |year=2008 |isbn=978-0-444-53286-2 |oclc=272382249 | author-mask=2 |title=Volume 8: The Penetration of Charged Particles Through Matter (1912–1954) |ref=none }}
* {{cite book| last=Bohr |first=Niels |editor-last= Peierls |editor-first=Rudolf |editor-link=Rudolf Peierls |series=Niels Bohr Collected Works |location=Amsterdam |publisher=Elsevier |year=2008 |isbn=978-0-444-53286-2 |oclc=272382249 | author-mask=2 |title=Volume 9: Nuclear Physics (1929–1952)  |ref=none }}
* {{cite book| last=Bohr |first=Niels |editor-last= Favrholdt |editor-first=David  |series=Niels Bohr Collected Works |location=Amsterdam |publisher=Elsevier |year=2008 |isbn=978-0-444-53286-2 |oclc=272382249 | author-mask=2 |title=Volume 10: Complementarity Beyond Physics (1928–1962) |ref=none }}
* {{cite book| last=Bohr |first=Niels |editor-last= Aaserud |editor-first=Finn |series=Niels Bohr Collected Works |location=Amsterdam |publisher=Elsevier |year=2008 |isbn=978-0-444-53286-2 |oclc=272382249 | author-mask=2 |title=Volume 11: The Political Arena (1934–1961) |ref=none }}
* {{cite book| last=Bohr |first=Niels |editor-last= Aaserud |editor-first=Finn |series=Niels Bohr Collected Works |location=Amsterdam |publisher=Elsevier |year=2008 |isbn=978-0-444-53286-2 |oclc=272382249 | author-mask=2 |title=Volume 12: Popularization and People (1911–1962) |ref=none}}
* {{cite book| last=Bohr |first=Niels |editor-last= Aaserud |editor-first=Finn |series=Niels Bohr Collected Works |location=Amsterdam |publisher=Elsevier |year=2008 |isbn=978-0-444-53286-2 |oclc=272382249 | author-mask=2 |title=Volume 13: Cumulative Subject Index |ref=none}}
{{refend}}

== See also ==
* {{anl|Einstein–Podolsky–Rosen paradox}}

== Notes ==
{{reflist|22em}}

== References ==
{{refbegin|30em}}
* {{cite conference |last=Aaserud |first=Finn |year=2006 |pages=706–709 |url=http://www.2iceshs.cyfronet.pl/2ICESHS_Proceedings/Chapter_25/R-17_Aaserud.pdf |title=Niels Bohr's Mission for an 'Open World' |editor-last=Kokowski |editor-first=M. |conference=Proceedings of the 2nd ICESHS |location=Kraków |access-date=26 June 2011 |archive-date=2 September 2011 |archive-url=https://web.archive.org/web/20110902221649/http://www.2iceshs.cyfronet.pl/2ICESHS_Proceedings/Chapter_25/R-17_Aaserud.pdf |url-status=dead }}
* {{cite book
 |last1=Aaserud |first1=Finn |last2=Heilbron |first2=J. L. |year=2013
 |title=Love, Literature and the Quantum Atom: Niels Bohr's 1913 Trilogy Revisited
 |publisher=Oxford University Press |location=Oxford |isbn=978-0-19-968028-3
}}
* {{cite journal|last=Bohr |first=Niels |year=1928 |title=The Quantum Postulate and the Recent Development of Atomic Theory |journal=Nature |volume=121 |issue=3050 |pages=580–590 |doi=10.1038/121580a0|bibcode=1928Natur.121..580B |s2cid=4097746 |doi-access=free }}
* {{cite book |last=Bohr |first=Niels |year=1985 |orig-date=1922 |chapter=Nobel Prize Lecture: The Structure of the Atom (excerpts) |pages=[https://archive.org/details/nielsbohrcentena00bohr/page/91 91–97] |editor1-last=French |editor1-first=A. P. |editor-link=Anthony French |editor2-last=Kennedy |editor2-first=P. J. |title=Niels Bohr: A Centenary Volume |location=Cambridge, Massachusetts |publisher=Harvard University Press |isbn=978-0-674-62415-3 |chapter-url=https://archive.org/details/nielsbohrcentena00bohr/page/91 }}
* {{cite book |last=Bohr |first=Niels |year=1985 |orig-date=1949 |chapter=The Bohr-Einstein Dialogue |pages=[https://archive.org/details/nielsbohrcentena00bohr/page/121 121–140] |editor1-last=French |editor1-first=A. P. |editor-link=Anthony French |editor2-last=Kennedy |editor2-first=P. J. |title=Niels Bohr: A Centenary Volume |location=Cambridge, Massachusetts |publisher=Harvard University Press |isbn=978-0-674-62415-3 |ref={{harvid|Dialogue|1985}} |chapter-url=https://archive.org/details/nielsbohrcentena00bohr/page/121 }}
** Excerpted from: {{cite book |last=Bohr |first=Niels |editor-first=Paul Arthur |editor-last=Schilpp |title=Albert Einstein: Philosopher-Scientist |chapter-url=https://books.google.com/books?id=MCk0QwAACAAJ |year=1949 |publisher=Library of Living Philosophers |location=Evanston, Illinois |pages=208–241 |chapter=Discussions with Einstein on Epistemological Problems in Atomic Physics |ref=none }}
* {{cite journal|first1=K. |last1=Camilleri |first2=M. |last2=Schlosshauer |title=Niels Bohr as Philosopher of Experiment: Does Decoherence Theory Challenge Bohr's Doctrine of Classical Concepts? |arxiv=1502.06547 |journal=Studies in History and Philosophy of Modern Physics |volume=49 |pages=73–83 |year=2015 |doi=10.1016/j.shpsb.2015.01.005|bibcode=2015SHPMP..49...73C |s2cid=27697360 }}
* {{cite journal |last=Cockcroft |first=John D. |date=1 November 1963 |author-link1=John Cockcroft |title=Niels Henrik David Bohr. 1885–1962 |journal=Biographical Memoirs of Fellows of the Royal Society |volume=9 |issue=10 |pages=36–53 |url=http://rsbm.royalsocietypublishing.org/content/9/36 |doi=10.1098/rsbm.1963.0002  |doi-access=free|s2cid=73320447 |access-date=20 October 2013 |archive-date=12 January 2015 |archive-url=https://web.archive.org/web/20150112053121/http://rsbm.royalsocietypublishing.org/content/9/36 |url-status=live |url-access=subscription }}
* {{cite book
 |last=Favrholdt |first=David |year=1992
 |title=Niels Bohr's Philosophical Background
 |location=Copenhagen |publisher=Munksgaard |isbn=978-87-7304-228-1
}}
* {{cite book
 |last=Faye |first=Jan |author-link=Jan Faye |year=1991
 |title=Niels Bohr: His Heritage and Legacy
 |location=Dordrecht |publisher=Kluwer Academic Publishers |isbn=978-0-7923-1294-9
}}
* {{cite book |last=Gowing |first=Margaret |author-link=Margaret Gowing |year=1985 |chapter=Niels Bohr and Nuclear Weapons |pages=[https://archive.org/details/nielsbohrcentena00bohr/page/266 266–277] |editor1-last=French |editor1-first=A. P. |editor-link=Anthony French |editor2-last=Kennedy |editor2-first=P. J. |title=Niels Bohr: A Centenary Volume |location=Cambridge, Massachusetts |publisher=Harvard University Press |isbn=978-0-674-62415-3 |chapter-url=https://archive.org/details/nielsbohrcentena00bohr/page/266 }}
* {{cite book |last=Heilbron |first=John L. |year=1985 |chapter=Bohr's First Theories of the Atom |pages=[https://archive.org/details/nielsbohrcentena00bohr/page/33 33–49] |editor1-last=French |editor1-first=A. P. |editor-link=Anthony French |editor2-last=Kennedy |editor2-first=P. J. |title=Niels Bohr: A Centenary Volume |location=Cambridge, Massachusetts |publisher=Harvard University Press |isbn=978-0-674-62415-3 |chapter-url=https://archive.org/details/nielsbohrcentena00bohr/page/33 }}
* {{cite book
 |last=Heisenberg | first=Elisabeth |year=1984
 |title=Inner Exile: Recollections of a Life With Werner Heisenberg
 |location=Boston |publisher=Birkhäuser | isbn=978-0-8176-3146-8
}}
* {{cite book
 |last=Hilberg |first=Raul |year=1961
 |title=The Destruction of the European Jews
 |volume=2 |location=New Haven, Connecticut |publisher=Yale University Press
}}
* {{cite journal |last=Honner |first=John |title=The Transcendental Philosophy of Niels Bohr |journal=Studies in History and Philosophy of Science Part A |issn=0039-3681 |volume=13 |issue=1 |date=March 1982 |pages=1–29 |doi=10.1016/0039-3681(82)90002-4|bibcode=1982SHPSA..13....1H }}
* {{cite book |last=Hund |first=Friedrich |year=1985 |pages=[https://archive.org/details/nielsbohrcentena00bohr/page/71 71–75] |chapter=Bohr, Göttingen, and Quantum Mechanics |editor1-last=French |editor1-first=A. P. |editor-link=Anthony French |editor2-last=Kennedy |editor2-first=P. J. |title=Niels Bohr: A Centenary Volume |location=Cambridge, Massachusetts |publisher=Harvard University Press |isbn=978-0-674-62415-3 |chapter-url=https://archive.org/details/nielsbohrcentena00bohr/page/71 }}
* {{cite book
 |last=Jammer |first=Max |year=1989
 |title=The Conceptual Development of Quantum Mechanics
 |location=Los Angeles |publisher=Tomash Publishers |isbn=978-0-88318-617-6 |oclc=19517065
}}
* {{cite book |last=Jones |first=R . V. |author-link=R. V. Jones |year=1978 |title=Most Secret War |publisher=Hamilton |location=London |oclc=3717534 |isbn=978-0-241-89746-1 |url=https://archive.org/details/mostsecretwar0000jone }}
* {{cite book |last=Jones |first=R. V. |author-link=R. V. Jones |year=1985 |chapter=Meetings in Wartime and After |pages=[https://archive.org/details/nielsbohrcentena00bohr/page/278 278–287] |editor1-last=French |editor1-first=A. P. |editor-link=Anthony French |editor2-last=Kennedy |editor2-first=P. J. |title=Niels Bohr: A Centenary Volume |location=Cambridge, Massachusetts |publisher=Harvard University Press |isbn=978-0-674-62415-3 |chapter-url=https://archive.org/details/nielsbohrcentena00bohr/page/278 }}
* {{cite book |last=Kennedy |first=P. J. |year=1985 |chapter=A Short Biography |pages=[https://archive.org/details/nielsbohrcentena00bohr/page/3 3–15] |editor1-last=French |editor1-first=A. P. |editor-link=Anthony French |editor2-last=Kennedy |editor2-first=P. J. |title=Niels Bohr: A Centenary Volume |location=Cambridge, Massachusetts |publisher=Harvard University Press |isbn=978-0-674-62415-3 |chapter-url=https://archive.org/details/nielsbohrcentena00bohr/page/3 }}
* {{cite book
 |last=Kieler |first=Jørgen |others=Translated from the Danish by Eric Dickens |year=2007
 |title=Resistance Fighter: A Personal History of the Danish Resistance |location=Jerusalem
 |publisher=Gefen Publishing House |isbn=978-965-229-397-8
}}
* {{cite book |last=Kragh |first=Helge |year=1985 |chapter=The Theory of the Periodic System |pages=[https://archive.org/details/nielsbohrcentena00bohr/page/50 50–67] |editor1-last=French |editor1-first=A. P. |editor-link=Anthony French |editor2-last=Kennedy |editor2-first=P. J. |title=Niels Bohr: A Centenary Volume |location=Cambridge, Massachusetts |publisher=Harvard University Press |isbn=978-0-674-62415-3 |chapter-url=https://archive.org/details/nielsbohrcentena00bohr/page/50 }}
* {{cite book
 |last=Kragh |first= Helge |year=2012
 |title=Niels Bohr and the quantum atom: the Bohr model of atomic structure, 1913–1925
 |location=Oxford |publisher=Oxford University Press |isbn=978-0-19-965498-7 |oclc=769989390 }}
* {{cite book |last=MacKinnon |first=Edward |year=1985 |chapter=Bohr on the Foundations of Quantum Theory |pages=[https://archive.org/details/nielsbohrcentena00bohr/page/101 101–120] |editor1-last=French |editor1-first=A. P. |editor-link=Anthony French |editor2-last=Kennedy |editor2-first=P. J. |title=Niels Bohr: A Centenary Volume |location=Cambridge, Massachusetts |publisher=Harvard University Press |isbn=978-0-674-62415-3 |chapter-url=https://archive.org/details/nielsbohrcentena00bohr/page/101 }}
* {{cite book
 |last1=Medawar |first1=Jean |last2=Pyke |first2=David |year=2001
 |title=Hitler's Gift: The True Story of the Scientists Expelled by the Nazi Regime
 |publisher=Arcade Publishing |location=New York |isbn=978-1-55970-564-6
}}
* {{cite journal |first=N. David |last=Mermin |author-link=N. David Mermin |doi=10.1063/1.1688051 |title=What's Wrong With This Quantum World? |journal=Physics Today |volume=52 |number=2 |year=2004 |page=10|bibcode=2004PhT....57b..10M }}
* {{cite book |last=Pais |first=Abraham |author-link=Abraham Pais |year=1991 |title=Niels Bohr's Times, In Physics, Philosophy and Polity |location=Oxford |publisher=Clarendon Press |isbn=978-0-19-852049-8 |url=https://archive.org/details/nielsbohrstimesi00pais_0 }}
* {{cite book|last=Powers|first=Thomas|year=1993|title=Heisenberg's War: The Secret History of the German Bomb|url=https://archive.org/details/heisenbergswarse00powe_0|url-access=registration|location=New York|publisher=Knopf|isbn=978-0-316-71623-9}}
* {{cite book |last=Rhodes |first=Richard |author-link=Richard Rhodes |year=1986 |title=The Making of the Atomic Bomb |url=https://archive.org/details/makingofatomicbo00rhod |url-access=registration |location=New York |publisher=Simon and Schuster |isbn=978-0-671-44133-3 }}
* {{cite book
 |editor1-last=Richardson |editor1-first=W. Mark |editor2-last=Wildman |editor2-first=Wesley J. |year=1996
 |title=Religion and Science: History, Method, Dialogue
 |location=London, New York |publisher=[[Routledge]] |isbn=978-0-415-91667-7
}}
* {{cite book |last=Rife |first=Patricia |year=1999 |title=Lise Meitner and the Dawn of the Nuclear Age |url=https://archive.org/details/lisemeitnerdawno0000rife |url-access=registration |publisher=Birkhäuser |location=Boston |isbn=978-0-8176-3732-3 }}
* {{cite book
 |last=Rozental |first=Stefan |author-link=Stefan Rozental |year=1967
 |title=Niels Bohr: His Life and Work as Seen by his Friends and Colleagues
 |location=Amsterdam |publisher=North-Holland |isbn=978-0-444-86977-7
}} (Previously published by John Wiley & Sons in 1964)
* {{cite book
 |last1=Stadtler |first1=Bea |last2=Morrison |first2=David Beal |last3=Martin |first3=David Stone |year=1995
 |title=The Holocaust: A History of Courage and Resistance
 |location=West Orange, New Jersey |publisher=Behrman House |isbn=978-0-87441-578-0
}}
* {{cite book
 |last=Stewart |first=Melville Y. |year=2010
 |title=Science and Religion in Dialogue, Two Volume Set
 |publisher=John Wiley & Sons |location=Maiden, Massachusetts |isbn=978-1-4051-8921-7
}}
* {{cite book |last=Stuewer |first=Roger H. |year=1985 |chapter=Niels Bohr and Nuclear Physics |pages=[https://archive.org/details/nielsbohrcentena00bohr/page/197 197–220] |editor1-last=French |editor1-first=A. P. |editor-link=Anthony French |editor2-last=Kennedy |editor2-first=P. J. |title=Niels Bohr: A Centenary Volume |location=Cambridge, Massachusetts |publisher=Harvard University Press |isbn=978-0-674-62415-3 |chapter-url=https://archive.org/details/nielsbohrcentena00bohr/page/197 }}
* {{cite book
 |last=Thirsk |first=Ian |year=2006
 |title=De Havilland Mosquito: An Illustrated History, Volume 2
 |publisher=MBI Publishing Company |location=Manchester |isbn=978-0-85979-115-1
}}
* {{cite book
 |publisher=U.S. Government Printing Office |year=1972
 |series=Foreign Relations of the United States |title=The Conferences at Quebec 1944
 |location=Washington, D.C. |oclc=631921397 |ref={{sfnRef|U.S. Government|1972}}
}}
* {{cite book |last=Wheeler |first=John A. |author-link=John Archibald Wheeler |year=1985 |chapter=Physics in Copenhagen in 1934 and 1935 |pages=[https://archive.org/details/nielsbohrcentena00bohr/page/221 221–226] |editor1-last=French |editor1-first=A. P. |editor-link=Anthony French |editor2-last=Kennedy |editor2-first=P. J. |title=Niels Bohr: A Centenary Volume |location=Cambridge, Massachusetts |publisher=Harvard University Press |isbn=978-0-674-62415-3 |chapter-url=https://archive.org/details/nielsbohrcentena00bohr/page/221 }}
* {{cite book
 |title=The Coins and Banknotes of Denmark
 |publisher=Danmarks Nationalbank
 |year=2005
 |isbn=978-87-87251-55-6
 |ref={{sfnRef|Danmarks Nationalbank|2005}}
 |url=http://www.nationalbanken.dk/C1256BE900406EF3/sysOakFil/Danmarks_penge_2005_ENG/$File/Coins_Banknotes.pdf
 |archive-url=https://web.archive.org/web/20110523100635/http://www.nationalbanken.dk/C1256BE900406EF3/sysOakFil/Danmarks_penge_2005_ENG/%24File/Coins_Banknotes.pdf
 |access-date=7 September 2010
|archive-date=23 May 2011
}}
{{refend}}

== Further reading ==
{{refbegin|30em}}
* {{cite web |url=http://nba.nbi.dk/papers/introduction.htm |title=Release of documents relating to 1941 Bohr-Heisenberg meeting |publisher=Niels Bohr Archive |first=Finn |last=Aaserud |date=February 2002 |access-date=2 March 2013 |archive-url=https://web.archive.org/web/20121021120546/http://www.nba.nbi.dk/papers/introduction.htm |archive-date=21 October 2012 |ref=none }}
* {{cite book |last=Blaedel |first=Niels |title=Harmony and Unity: The Life of Niels Bohr |location=Madison, Wisconsin |publisher=Science Tech |year=1988 |oclc=17411890 |isbn=978-0-910239-14-1 |ref=none}}
* {{cite news |url=http://news.bbc.co.uk/today/hi/today/newsid_8493000/8493203.stm |title=The Gunfighter's Dilemma |work=news.bbc.co.uk |first=Tom |last=Feilden |date=3 February 2010 |access-date=2 March 2013 |ref=none |archive-date=21 July 2012 |archive-url=https://web.archive.org/web/20120721015432/http://news.bbc.co.uk/today/hi/today/newsid_8493000/8493203.stm |url-status=live }} Bohr's researches on reaction times.
* {{cite book |last=Moore |first=Ruth |title=Niels Bohr: The Man, His Science, and the World They Changed |url=https://archive.org/details/nielsbohrmanhis00moor |url-access=registration |isbn=978-0-262-63101-3 |oclc=712016 |location=New York |publisher=Knopf |year=1966 |ref=none }}
* {{cite book |last1=Ottaviani |first1=Jim |author-link=Jim Ottaviani |last2=Purvis |first2=Leland |author-link2=Leland Purvis |title=Suspended in Language: Niels Bohr's Life, Discoveries, and the Century He Shaped |location=Ann Arbor, Michigan |publisher=G.T. Labs |year=2004 |isbn=978-0-9660106-5-7 |oclc=55739245 |ref=none }}
* {{cite book |title=Copenhagen |last=Frayn |first=Michael |author-link=Michael Frayn |isbn=978-0-413-72490-8 |location=New York |publisher=Anchor Books |year=2000 |oclc=44467534 |title-link=Copenhagen (play) |ref=none }}
* {{cite book |editor-last=Rozental |editor-first=S. |title=Niels Bohr: His Life and Work as Seen by His Friends and Colleagues |location=New York |publisher=Wiley |year=1967 |oclc=644043790|ref=none}}
* {{cite book |last=Segrè |first=Gino |author-link=Gino Claudio Segre |title=Faust in Copenhagen: A Struggle for the Soul of Physics |isbn=978-0-670-03858-9 |location=New York |publisher=Viking |year=2007 |oclc=76416691 |url=https://archive.org/details/faustincopenhage00segr |ref=none }}
* {{cite journal |last1=Vilhjálmsson |first1=Vilhjálmur Örn |last2=Blüdnikow |first2=Bent |year=2006 |title=Rescue, Expulsion, and Collaboration: Denmark's Difficulties with its World War II Past |url=http://www.jcpa.org/phas/phas-vilhjalmsson-f06.htm |journal=Jewish Political Studies Review |volume=18 |pages=3–4 |issn=0792-335X |access-date=29 June 2011 |ref=none |archive-date=8 April 2013 |archive-url=https://web.archive.org/web/20130408004115/http://www.jcpa.org/phas/phas-vilhjalmsson-f06.htm |url-status=live }}
{{refend}}

== External links ==
{{Sister project links |wikt=no |commons=Niels Bohr |n=no |q= Niels_Bohr|s=no |b=no |voy=no |v=no}}
* [http://www.nbarchive.dk/ Niels Bohr Archive]
* [https://zbmath.org/authors/?q=ai:bohr.niels Author profile] in the database [[zbMATH]]
* {{Gutenberg author|id=44167}}
* {{IMDb name|1106823}}
* {{PM20|FID=pe/002096}}
* {{Nobelprize}} including the Nobel Lecture, 11 December 1922 ''The Structure of the Atom''
* [https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4517-1 Oral history interview transcript for Niels Bohr on 31 October 1962, American Institute of Physics, Niels Bohr Library & Archives] – interviews conducted by [[Thomas S. Kuhn]], [[Leon Rosenfeld]], Erik Rudinger, and Aage Petersen
** [https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4517-2 1 November 1962]
** [https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4517-3 7 November 1962]
** [https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4517-4 14 November 1962]
** [https://www.aip.org/history-programs/niels-bohr-library/oral-histories/4517-5 17 November 1962]
* {{cite web |url=http://www.aip.org/history/heisenberg/bohr-heisenberg-meeting.htm |title=The Bohr-Heisenberg meeting in September 1941 |publisher=[[American Institute of Physics]] |access-date=2 March 2013 |archive-date=4 July 2011 |archive-url=https://web.archive.org/web/20110704121809/http://www.aip.org/history/heisenberg/bohr-heisenberg-meeting.htm  }}
* {{cite web |url=http://web.mit.edu/redingtn/www/netadv/FCintro.html |title=Resources for Frayn's ''Copenhagen'': Niels Bohr |publisher=[[Massachusetts Institute of Technology]] |access-date=9 October 2013}}
* {{cite web |url=http://www.mediatheque.lindau-nobel.org/videos/31564/atomic-physics-and-human-knowledge-1962/laureate-bohr |title=Video – Niels Bohr (1962): Atomic Physics and Human Knowledge |publisher=[[Lindau Nobel Laureate Meetings]] |access-date=9 July 2014 |archive-date=21 December 2014 |archive-url=https://web.archive.org/web/20141221202443/http://www.mediatheque.lindau-nobel.org/videos/31564/atomic-physics-and-human-knowledge-1962/laureate-bohr |url-status=dead }}

{{Copley Medallists 1901–1950}}
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