Bohr developed theBohr model of theatom, in which he proposed that energy levels ofelectrons are discrete and that the electrons revolve in stable orbits around theatomic 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 ofcomplementarity: that items could be separately analysed in terms of contradictory properties, like behaving as awave 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 theUniversity of Copenhagen, now known as theNiels Bohr Institute, which opened in 1920. Bohr mentored and collaborated with physicists includingHans Kramers,Oskar Klein,George de Hevesy, andWerner Heisenberg. He predicted the properties of a newzirconium-like element, which was namedhafnium, after the Latin name for Copenhagen, where it was discovered. Later, the synthetic elementbohrium was named after him because of his groundbreaking work on the structure of atoms.
Bohr was educated at Gammelholm Latin School, starting when he was seven.[6] In 1903, Bohr enrolled as an undergraduate at the University of Copenhagen. His major was physics, which he studied underChristian Christiansen, the university's only professor of physics at that time. He also studied astronomy and mathematics underThorvald Thiele, and philosophy underHarald Høffding, a friend of his father.[7][8]
Bohr as a young man
In 1905, a gold medal competition was sponsored by theRoyal Danish Academy of Sciences and Letters to investigate a method for measuring thesurface tension of liquids that had been proposed byLord 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 tomake his own glassware, creating test tubes with the requiredelliptical cross-sections. He went beyond the original task, incorporating improvements into both Rayleigh's theory and his method, by taking into account theviscosity 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 theRoyal Society in London for publication in thePhilosophical Transactions of the Royal Society.[9][10][8][11]
Harald became the first of the two Bohr brothers to earn amaster'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-largerPh.D. thesis. He surveyed the literature on the subject, settling ona model developed byPaul Drude and elaborated byHendrik 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 theHall effect, and concluded that electron theory could not fully explain the magnetic properties of metals. The thesis was accepted in April 1911,[12] and Bohr conducted his formal defence on 13 May. Harald had received his doctorate the previous year.[13] 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 physicistHendrika Johanna van Leeuwen would independently derive a theorem in Bohr's thesis that is today known as theBohr–Van Leeuwen theorem.[14]
Bohr returned to Denmark in July 1912 for his wedding, and travelled around England and Scotland on his honeymoon. On his return, he became aPrivatdocent at the University of Copenhagen, giving lectures onthermodynamics.Martin Knudsen put Bohr's name forward for adocent, which was approved in July 1913, and Bohr then began teaching medical students.[22] His three papers, which later became famous as "the trilogy",[20] were published inPhilosophical Magazine in July, September and November of that year.[23][24][25][26] He adapted Rutherford's nuclear structure toMax Planck's quantum theory and so created hisBohr model of the atom.[24]
Planetary models of atoms were not new, but Bohr's treatment was.[27] Taking the 1912 paper by Darwin on the role of electrons in the interaction of alpha particles with a nucleus as his starting point,[28][29] he advanced the theory of electrons travelling inorbits 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.[30][31][32][33] He introduced the idea that an electron could drop from a higher-energy orbit to a lower one, in the process emitting aquantum of discrete energy. This became a basis for what is now known as theold quantum theory.[34]
where λ is the wavelength of the absorbed or emitted light andRH is theRydberg constant.[35] 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:
The model's first hurdle was thePickering series, lines that did not fit Balmer's formula. When challenged on this byAlfred Fowler, Bohr replied that they were caused byionisedhelium, helium atoms with only one electron. The Bohr model was found to work for such ions.[36] Many older physicists, like Thomson, Rayleigh andHendrik Lorentz, did not like the trilogy, but the younger generation, including Rutherford,David Hilbert,Albert Einstein,Enrico Fermi,Max Born andArnold Sommerfeld saw it as a breakthrough.[37][38] Einstein called Bohr's model "the highest form of musicality in the sphere of thought."[39] 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.[40][41] 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.[42]
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".[43] He decided to return to Manchester, where Rutherford had offered him a job as areader 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 inTyrol with his brother Harald and auntHanna Adler. There, he visited theUniversity of Göttingen and theLudwig 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 KingChristian X, who expressed his delight at meeting such a famous football player.[44]
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 theNiels Bohr Institute, it opened on 3 March 1921, with Bohr as its director. His family moved into an apartment on the first floor.[45][46] Bohr's institute served as a focal point for researchers intoquantum 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 includedHans Kramers from the Netherlands,Oskar Klein from Sweden, George de Hevesy from Hungary,Wojciech Rubinowicz from Poland, andSvein Rosseland from Norway. Bohr became widely appreciated as their congenial host and eminent colleague.[47][48] Klein and Rosseland produced the institute's first publication even before it opened.[46]
The Bohr model worked well for hydrogen and ionized single-electron helium, which impressed Einstein[49][50] but could not explain more complex elements. By 1919, Bohr was moving away from the idea that electrons orbited the nucleus and developedheuristics to describe them. Therare-earth elements posed a particular classification problem for chemists because they were so chemically similar. An important development came in 1924 withWolfgang Pauli's discovery of thePauli 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 ofzirconium. (Elements had been predicted and discovered since 1871 by chemical properties[51]), and Bohr was immediately challenged by the French chemistGeorges 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 namedhafnium (hafnia being the Latin name for Copenhagen), turned out to be more common than gold.[52][53]
In 1922, Bohr was awarded theNobel Prize in Physics "for his services in the investigation of the structure of atoms and of the radiation emanating from them".[54] 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 thecorrespondence principle, which he had formulated. This states that the behaviour of systems described by quantum theory reproducesclassical physics in the limit of largequantum numbers.[55]
The discovery ofCompton scattering byArthur Holly Compton in 1923 convinced most physicists that light was composed ofphotons and that energy and momentum were conserved in collisions between electrons and photons. In 1924, Bohr, Kramers, andJohn C. Slater, an American physicist working at the Institute in Copenhagen, proposed theBohr–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.[56][57]
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 ofmatrix mechanics, the first form of modernquantum mechanics. The BKS theory also generated discussion of, and renewed attention to, difficulties in the foundations of the old quantum theory.[58] 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 byWalther Bothe andHans Geiger.[59] 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".[60]
Quantum mechanics
The introduction ofspin byGeorge Uhlenbeck andSamuel Goudsmit in November 1925 was a milestone. The next month, Bohr travelled toLeiden to attend celebrations of the 50th anniversary of Hendrick Lorentz receiving his doctorate. When his train stopped inHamburg, he was met by Wolfgang Pauli andOtto 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 usingrelativity. Bohr then had Uhlenbeck and Goudsmit incorporate this into their paper. Thus, when he met Werner Heisenberg andPascual Jordan inGöttingen on the way back, he had become, in his own words, "a prophet of the electron magnet gospel".[61]
1927Solvay Conference in Brussels, October 1927. Bohr is on the right in the middle row, next toMax Born.
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 usingmatrices. This work attracted the attention of the British physicistPaul Dirac,[62] who came to Copenhagen for six months in September 1926. Austrian physicistErwin 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".[63]
When Kramers left the institute in 1926 to take up a chair as professor of theoretical physics at theUtrecht University, Bohr arranged for Heisenberg to return and take Kramers's place as alektor at the University of Copenhagen.[64] Heisenberg worked in Copenhagen as a university lecturer and assistant to Bohr from 1926 to 1927.[65]
Bohr became convinced that light behaved like both waves and particles and, in 1927, experiments confirmed thede Broglie hypothesis that matter (like electrons) also behaved like waves.[66] He conceived the philosophical principle ofcomplementarity: that items could have apparently mutually exclusive properties, such as being a wave or a stream of particles, depending on the experimental framework.[67] He felt that it was not fully understood by professional philosophers.[68]
In February 1927, Heisenberg developed the first version of theuncertainty principle, presenting it using athought experiment where an electron was observed through agamma-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 theComo Conference in September 1927, Bohr emphasised that Heisenberg's uncertainty relations could be derived from classical considerations about the resolving power of optical instruments.[69] Understanding the true meaning of complementarity would, Bohr believed, require "closer investigation".[70] 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 hadgood-natured arguments over such issues throughout their lives.[71]
In 1914,Carl Jacobsen, the heir toCarlsberg 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 (Danish:Æresbolig). 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.[72] He was elected president of the Academy on 17 March 1939.[73]
By 1929, the phenomenon ofbeta decay prompted Bohr to again suggest that thelaw of conservation of energy be abandoned, butWolfgang Pauli's hypotheticalneutrino and the subsequent 1932 discovery of theneutron provided another explanation. This prompted Bohr to create a new theory of thecompound 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.[74][75]
Thediscovery of nuclear fission byOtto Hahn in December 1938 (and its theoretical explanation byLise Meitner) generated intense interest among physicists. Bohr brought the news to the United States where he opened the fifthWashington Conference on Theoretical Physics with Fermi on 26 January 1939.[76] When Bohr toldGeorge Placzek that this resolved all the mysteries oftransuranic 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 andJohn Wheeler that "I have understood everything."[77] Based on hisliquid drop model of the nucleus, Bohr concluded that it was theuranium-235 isotope and not the more abundanturanium-238 that was primarily responsible for fission with thermal neutrons. In April 1940,John R. Dunning demonstrated that Bohr was correct.[76] In the meantime, Bohr and Wheeler developed a theoretical treatment, which they published in a September 1939 paper on "The Mechanism of Nuclear Fission".[78]
Philosophy
Heisenberg said of Bohr that he was "primarily a philosopher, not a physicist".[79] Bohr read the 19th-century DanishChristian existentialist philosopherSøren Kierkegaard.Richard Rhodes argued inThe Making of the Atomic Bomb that Bohr was influenced by Kierkegaard through Høffding.[80] In 1909, Bohr sent his brother Kierkegaard'sStages 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 ... 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 withKierkegaard's philosophy.[81] Some of Bohr's biographers suggested that this disagreement stemmed from Kierkegaard's advocacy of Christianity, while Bohr was anatheist.[82][83][84]
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,[85] while Jan Faye argued that one can disagree with the content of a theory while accepting its general premises and structure.[86][81]
Bohr sat on the Board of Editors of the book seriesWorld Perspectives which published a variety of books on philosophy.[87]
Quantum physics
Bohr (left) andAlbert Einstein (right), pictured on 11 December 1925, hada 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.[88] Regarding his ontological interpretation of the quantum world, Bohr has been seen as ananti-realist, aninstrumentalist, a phenomenological realist or some other kind of realist. Furthermore, though some have seen Bohr as being asubjectivist or apositivist, most philosophers agree that this is a misunderstanding of Bohr as he never argued forverificationism or for the idea that the subject had a direct impact on the outcome of a measurement.[89]
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 recalledVictor 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!"[90][91]
Numerous scholars have argued that the philosophy ofImmanuel 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.[89] 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.[89] Therefore, for Bohr, classical concepts need to be used to describe experiments that deal with the quantum world. Bohr writes:
[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).[89]
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 (orNeo-Kantian models ofepistemology);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, whichLéon 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).[89] 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.[89]
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."[89] Therefore, Bohr's theory ofcomplementarity "is first and foremost a semantic and epistemological reading of quantum mechanics that carries certain ontological implications".[89] As Faye explains, Bohr'sindefinability thesis is that
[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.[89]
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 theψ-function has only a symbolic meaning and does not represent anything real". Since for Bohr, theψ-function is not a literal pictorial representation of reality, there can be no real collapse of the wavefunction.[89]
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 atranscendental reality. 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."[89]
Second World War
Assistance to refugee scholars
The rise ofNazism 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, theRockefeller 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 includedGuido 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 von Hippel andVictor Weisskopf.[92]
In April 1940, early in the Second World War,Nazi Germanyinvaded and occupied Denmark.[93] To prevent the Germans from discoveringMax von Laue's and James Franck'sgold Nobel medals, Bohr had de Hevesy dissolve them inaqua 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 theFinnish Relief Fund, and was auctioned off in March 1940, along with the medal ofAugust Krogh. The buyer later donated the two medals to the Danish Historical Museum inFrederiksborg Castle, where they are still kept,[94] although Bohr's medal temporarily went to space withAndreas Mogensen onISSExpedition 70 in 2023–2024.[95][96]
Bohr kept the Institute running, but all the foreign scholars departed.[97]
Meeting with Heisenberg
Werner Heisenberg (left) with Bohr at the Copenhagen Conference in 1934
Bohr was aware of the possibility of using uranium-235 to construct anatomic 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.[98] In September 1941, Heisenberg, who had become head of theGerman 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.[99]Ivan Supek, one of Heisenberg's students and friends, claimed that the main subject of the meeting wasCarl Friedrich von Weizsäcker, who had proposed trying to persuade Bohr to mediate peace between Britain and Germany.[100]
In 1957, Heisenberg wrote toRobert Jungk, who was then working on the bookBrighter 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.[101] 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,[102] that he recalled Heisenberg's visit as being to encourage cooperation with the inevitably victorious Nazis[103] and that he was shocked that Germany was pursuing nuclear weapons under Heisenberg's leadership.[104][105]
Michael Frayn's 1998 playCopenhagen explores what might have happened at the 1941 meeting between Heisenberg and Bohr.[106]A television film version of the play by theBBC was first screened on 26 September 2002, withStephen 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.[107]
The same meeting had previously been dramatised by the BBC'sHorizon science documentary series in 1992, withAnthony Bate as Bohr, and Philip Anthony as Heisenberg.[108] The meeting is also dramatised in the Norwegian/Danish/British miniseriesThe Heavy Water War.[109]
Manhattan Project
In September 1943, word reached Bohr and his brother Harald that the Nazisconsidered 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.[110][111] The next day, Bohr persuaded KingGustaf 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 massrescue 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.[111][112][113][114] Eventually, over 7,000 Danish Jews escaped to Sweden.[115]
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 ade Havilland Mosquito operated by theBritish Overseas Airways Corporation (BOAC).[116][117] 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'sbomb bay.[118] 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.[119][120][121] Bohr's son Aage followed his father to Britain on another flight a week later, and became his personal assistant.[122]
Bohr was warmly received byJames Chadwick and SirJohn Anderson, but for security reasons Bohr was kept out of sight. He was given an apartment atSt James's Palace and an office with the BritishTube Alloys nuclear weapons development team. Bohr was astonished at the amount of progress that had been made.[122][123] Chadwick arranged for Bohr to visit the United States as a Tube Alloys consultant, with Aage as his assistant.[124] On 8 December 1943, Bohr arrived inWashington, D.C., where he met with the director of theManhattan Project, Brigadier GeneralLeslie R. Groves Jr. He visited Einstein and Pauli at theInstitute for Advanced Study inPrinceton, New Jersey, and went toLos Alamos inNew Mexico, where the nuclear weapons were being designed.[125] For security reasons, he went under the name of "Nicholas Baker" in the United States, while Aage became "James Baker".[126] In May 1944 the Danish resistance newspaperDe 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 toMoscow from where he could be assumed to support the war effort.[127]
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 notablyRichard Feynman.[128] Bohr is quoted as saying, "They didn't need my help in making the atom bomb."[129] Oppenheimer gave Bohr credit for an important contribution to the work onmodulated neutron initiators. "This device remained a stubborn puzzle", Oppenheimer noted, "but in early February 1945 Niels Bohr clarified what had to be done".[128]
Bohr recognised early that nuclear weapons would change international relations. In April 1944, he received a letter fromPeter Kapitza, written some months before when Bohr was in Sweden, inviting him to come to theSoviet 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.[130] Bohr met Churchill on 16 May 1944, but found that "we did not speak the same language".[131] 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."[132]
Oppenheimer suggested that Bohr visit PresidentFranklin 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 JusticeFelix 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.[133][134] When Churchill and Roosevelt met at Hyde Park on 19 September 1944, they rejected the idea of informing the world about the project, and theaide-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".[135]
Bohr's coat of arms, 1947.Argent, ataijitu (yin-yang symbol)Gules andSable. Motto:Contraria sunt complementa ("opposites are complementary")[141]
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.[142] At a memorial meeting of the Academy on 17 October 1947 for KingChristian X, who had died in April, the new king,Frederik IX, announced that he was conferring theOrder 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.[143][144] Bohr designed his owncoat of arms, which featured ataijitu (symbol of yin and yang) and a motto in Latin:contraria sunt complementa, "opposites are complementary".[145][144][146]
The Second World War demonstrated that science, and physics in particular, now required considerable financial and material resources. To avoid abrain drain to the United States, twelve European countries banded together to createCERN, a research organisation along the lines of the national laboratories in the United States, designed to undertakeBig 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 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, andGeneva 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.[147] Victor Weisskopf, who later became theDirector General of CERN, summed up Bohr's role, saying that "there were other personalities who started and conceived the idea ofCERN. The enthusiasm and ideas of the other people would not have been enough, however, if a man of his stature had not supported it."[148][149]
Bohr died of heart failure on 18 November 1962 at his home inCarlsberg,Copenhagen.[151] He was cremated, and his ashes were buried in the family plot in theAssistens Cemetery in theNørrebro 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.[152] 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: theNiels Bohr Institute.[153][154]
In 1910, Bohr metMargrethe Nørlund, the sister of mathematicianNiels Erik Nørlund.[155] Bohr resigned his membership in theChurch of Denmark on 16 April 1912, and he and Margrethe were married in a civil ceremony at the town hall inSlagelse on 1 August. Years later, his brother,Harald, similarly left the church before getting married.[156] Bohr and Margrethe had six sons.[157] The eldest, Christian, died in a boating accident in 1934.[158] 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 childhoodmeningitis six years later.[159][157]Aage Bohr became a successful physicist, and in 1975 was awarded theNobel Prize in Physics, like his father. A son of Aage, Vilhelm A. Bohr, is a scientist affiliated with the University of Copenhagen[160] and theNational Institute on Aging in the U.S.[161]Hans [da] became a physician;Erik [da], a chemical engineer; andErnest, a lawyer.[162] Like his uncle Harald, Ernest Bohr became an Olympic athlete, playingfield hockey for Denmark at the1948 Summer Olympics in London.[163]
—— (2008). Nielsen, J. Rud (ed.).Volume 1: Early Work (1905–1911). Niels Bohr Collected Works. Amsterdam: Elsevier.ISBN978-0-444-53286-2.OCLC272382249.
—— (2008). Hoyer, Ulrich (ed.).Volume 2: Work on Atomic Physics (1912–1917). Niels Bohr Collected Works. Amsterdam: Elsevier.ISBN978-0-444-53286-2.OCLC272382249.
—— (2008). Nielsen, J. Rud (ed.).Volume 3: The Correspondence Principle (1918–1923). Niels Bohr Collected Works. Amsterdam: Elsevier.ISBN978-0-444-53286-2.OCLC272382249.
—— (2008). Nielsen, J. Rud (ed.).Volume 4: The Periodic System (1920–1923). Niels Bohr Collected Works. Amsterdam: Elsevier.ISBN978-0-444-53286-2.OCLC272382249.
—— (2008). Stolzenburg, Klaus (ed.).Volume 5: The Emergence of Quantum Mechanics (mainly 1924–1926). Niels Bohr Collected Works. Amsterdam: Elsevier.ISBN978-0-444-53286-2.OCLC272382249.
—— (2008). Kalckar, Jørgen (ed.).Volume 6: Foundations of Quantum Physics I (1926–1932). Niels Bohr Collected Works. Amsterdam: Elsevier.ISBN978-0-444-53286-2.OCLC272382249.
—— (2008). Kalckar, Jørgen (ed.).Volume 7: Foundations of Quantum Physics I (1933–1958). Niels Bohr Collected Works. Amsterdam: Elsevier.ISBN978-0-444-53286-2.OCLC272382249.
—— (2008). Thorsen, Jens (ed.).Volume 8: The Penetration of Charged Particles Through Matter (1912–1954). Niels Bohr Collected Works. Amsterdam: Elsevier.ISBN978-0-444-53286-2.OCLC272382249.
^abPolitiets Registerblade [Register cards of the Police] (in Danish). Copenhagen: Københavns Stadsarkiv. 7 June 1892. Station Dødeblade (indeholder afdøde i perioden). Filmrulle 0002. Registerblad 3341. ID 3308989. Archived fromthe original on 29 November 2014.
^There is no truth in the oft-repeated claim that Bohr emulated his brother, Harald, by playing for the Danish national team.Dart, James (27 July 2005)."Bohr's footballing career".The Guardian. London.Archived from the original on 27 May 2023. Retrieved26 June 2011.
^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/10.2307/27757389Archived 17 October 2022 at theWayback Machine.
^N. Bohr, "Atomic Structure", Nature, 107. Letter dated 14 February 1921.
^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.'"
^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."
^Portal Jutarnji.hr (19 March 2006)."Moj život s nobelovcima 20. stoljeća" [My Life with the 20th century Nobel Prizewinners].Jutarnji list (in Croatian). Archived fromthe original on 28 June 2009. Retrieved13 August 2007.Istinu sam saznao od Margrethe, Bohrove supruge. ... Ni Heisenberg ni Bohr nisu bili glavni junaci toga susreta nego Carl Friedrich von Weizsaecker. ... 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. ... 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 ofhis father who wasRibbentrop's deputy, was to persuade Niels Bohr to mediate for peace between Great Britain and Germany.] An interview with Ivan Supek relating to the 1941 Bohr – Heisenberg meeting.
^"I am greatly amazed to see how much your memory has deceived you in your letter to the author of the book"
^"...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'
^"...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."
^"Bohrium". University of Toledo. Retrieved3 January 2025.
References
Aaserud, Finn (2006). Kokowski, M. (ed.).Niels Bohr's Mission for an 'Open World'(PDF). Proceedings of the 2nd ICESHS. Cracow. pp. 706–709. Archived fromthe original(PDF) on 2 September 2011. Retrieved26 June 2011.
Aaserud, Finn; Heilbron, J. L. (2013).Love, Literature and the Quantum Atom: Niels Bohr's 1913 Trilogy Revisited. Oxford: Oxford University Press.ISBN978-0-19-968028-3.
Kieler, Jørgen (2007).Resistance Fighter: A Personal History of the Danish Resistance. Translated from the Danish by Eric Dickens. Jerusalem: Gefen Publishing House.ISBN978-965-229-397-8.
Kragh, Helge (2012).Niels Bohr and the quantum atom: the Bohr model of atomic structure, 1913–1925. Oxford: Oxford University Press.ISBN978-0-19-965498-7.OCLC769989390.
Medawar, Jean; Pyke, David (2001).Hitler's Gift: The True Story of the Scientists Expelled by the Nazi Regime. New York: Arcade Publishing.ISBN978-1-55970-564-6.
Rozental, Stefan (1967).Niels Bohr: His Life and Work as Seen by his Friends and Colleagues. Amsterdam: North-Holland.ISBN978-0-444-86977-7. (Previously published by John Wiley & Sons in 1964)
Stadtler, Bea; Morrison, David Beal; Martin, David Stone (1995).The Holocaust: A History of Courage and Resistance. West Orange, New Jersey: Behrman House.ISBN978-0-87441-578-0.
Stewart, Melville Y. (2010).Science and Religion in Dialogue, Two Volume Set. Maiden, Massachusetts: John Wiley & Sons.ISBN978-1-4051-8921-7.
Blaedel, Niels (1988).Harmony and Unity: The Life of Niels Bohr. Madison, Wisconsin: Science Tech.ISBN978-0-910239-14-1.OCLC17411890.
Feilden, Tom (3 February 2010)."The Gunfighter's Dilemma".news.bbc.co.uk.Archived from the original on 21 July 2012. Retrieved2 March 2013. Bohr's researches on reaction times.
%2c_1922_%25E2%2580%2593_BEIC_10990185.jpg)
