Niels Bohr
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Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a
Danish Danish may refer to: * Something of, from, or related to the country of Denmark People * A national or citizen of Denmark, also called a "Dane," see Demographics of Denmark * Culture of Denmark * Danish people or Danes, people with a Danish a ...
physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...
who made foundational contributions to understanding
atomic structure Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...
and
quantum theory Quantum theory may refer to: Science *Quantum mechanics, a major field of physics *Old quantum theory, predating modern quantum mechanics * Quantum field theory, an area of quantum mechanics that includes: ** Quantum electrodynamics ** Quantum ...
, for which he received the
Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
in 1922. Bohr was also a
philosopher A philosopher is a person who practices or investigates philosophy. The term ''philosopher'' comes from the grc, φιλόσοφος, , translit=philosophos, meaning 'lover of wisdom'. The coining of the term has been attributed to the Greek th ...
and a promoter of scientific research. Bohr developed the
Bohr model In atomic physics, the Bohr model or Rutherford–Bohr model, presented by Niels Bohr and Ernest Rutherford in 1913, is a system consisting of a small, dense nucleus surrounded by orbiting electrons—similar to the structure of the Solar Syst ...
of the
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, ...
, in which he proposed that energy levels of
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...
s are discrete and that the electrons revolve in stable orbits around the
atomic nucleus The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron i ...
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: that items could be separately analysed in terms of contradictory properties, like behaving as a 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 The University of Copenhagen ( da, Københavns Universitet, KU) is a prestigious public university, public research university in Copenhagen, Copenhagen, Denmark. Founded in 1479, the University of Copenhagen is the second-oldest university in ...
, now known as the
Niels Bohr Institute The Niels Bohr Institute (Danish: ''Niels Bohr Institutet'') is a research institute of the University of Copenhagen. The research of the institute spans astronomy, geophysics, nanotechnology, particle physics, quantum mechanics and biophysics ...
, which opened in 1920. Bohr mentored and collaborated with physicists including
Hans Kramers Hendrik Anthony "Hans" Kramers (17 December 1894 – 24 April 1952) was a Dutch physicist who worked with Niels Bohr to understand how electromagnetic waves interact with matter and made important contributions to quantum mechanics and statistical ...
, Oskar Klein,
George de Hevesy George Charles de Hevesy (born György Bischitz; hu, Hevesy György Károly; german: Georg Karl von Hevesy; 1 August 1885 – 5 July 1966) was a Hungarian radiochemist and Nobel Prize in Chemistry laureate, recognized in 1943 for his key rol ...
, and
Werner Heisenberg Werner Karl Heisenberg () (5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the main pioneers of the theory of quantum mechanics. He published his work in 1925 in a breakthrough paper. In the subsequent serie ...
. He predicted the existence of a new
zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...
-like element, which was named
hafnium Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri M ...
, after the Latin name for Copenhagen, where it was discovered. Later, the element bohrium was named after him. During the 1930s, Bohr helped refugees from
Nazism Nazism ( ; german: Nazismus), the common name in English for National Socialism (german: Nationalsozialismus, ), is the far-right totalitarian political ideology and practices associated with Adolf Hitler and the Nazi Party (NSDAP) i ...
. After Denmark was occupied by the Germans, he had a famous meeting with Heisenberg, who had become the head of the
German nuclear weapon project The Uranverein ( en, "Uranium Club") or Uranprojekt ( en, "Uranium Project") was the name given to the project in Germany to research nuclear technology, including nuclear weapons and nuclear reactors, during World War II. It went through seve ...
. In September 1943 word reached Bohr that he was about to be arrested by the Germans, and he fled to Sweden. From there, he was flown to Britain, where he joined the British
Tube Alloys Tube Alloys was the research and development programme authorised by the United Kingdom, with participation from Canada, to develop nuclear weapons during the Second World War. Starting before the Manhattan Project in the United States, the ...
nuclear weapons project, and was part of the British mission to the
Manhattan Project The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project w ...
. After the war, Bohr called for international cooperation on nuclear energy. He was involved with the establishment of
CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
and the Research Establishment Risø of the Danish Atomic Energy Commission and became the first chairman of the
Nordic Institute for Theoretical Physics The Nordic Institute for Theoretical Physics, or NORDITA, or Nordita ( da, Nordisk Institut for Teoretisk Fysik), is an international organisation for research in theoretical physics. It was established as Nordisk Institut for Teoretisk Atomfys ...
in 1957.


Early years

Niels Henrik David Bohr was born in
Copenhagen Copenhagen ( or .; da, København ) is the capital and most populous city of Denmark, with a proper population of around 815.000 in the last quarter of 2022; and some 1.370,000 in the urban area; and the wider Copenhagen metropolitan ar ...
, Denmark, on 7 October 1885, the second of three children of
Christian Bohr Christian Harald Lauritz Peter Emil Bohr (1855–1911) was a Danish physician, father of the physicist and Nobel laureate Niels Bohr, as well as the mathematician and football player Harald Bohr and grandfather of another physicist and Nobel lau ...
, a professor of
physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...
at the University of Copenhagen, and Ellen Bohr from the wealthy banking family. He had an elder sister, Jenny, and a younger brother
Harald Harald or Haraldr is the Old Norse form of the given name Harold. It may refer to: Medieval Kings of Denmark * Harald Bluetooth (935–985/986) Kings of Norway * Harald Fairhair (c. 850–c. 933) * Harald Greycloak (died 970) * Harald Hardra ...
. Jenny became a teacher, while Harald became a
mathematician A mathematician is someone who uses an extensive knowledge of mathematics in their work, typically to solve mathematical problems. Mathematicians are concerned with numbers, data, quantity, structure, space, models, and change. History On ...
and
footballer A football player or footballer is a sportsperson who plays one of the different types of football. The main types of football are association football, American football, Canadian football, Australian rules football, Gaelic football, rugby ...
who played for the 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 Akademisk Boldklub Gladsaxe (or AB) is a Danish professional football club from Gladsaxe north of Copenhagen, currently playing at the 3rd highest level of Danish domestic football in the Danish 2nd Division group 1. The club was established on ...
(Academic Football Club), with Niels as
goalkeeper In many team sports which involve scoring goals, the goalkeeper (sometimes termed goaltender, netminder, GK, goalie or keeper) is a designated player charged with directly preventing the opposing team from scoring by blocking or intercepting o ...
. Bohr was educated at Gammelholm Latin School, starting when he was seven. In 1903, Bohr enrolled as an undergraduate at
Copenhagen University The University of Copenhagen ( da, Københavns Universitet, KU) is a prestigious public research university in Copenhagen, Denmark. Founded in 1479, the University of Copenhagen is the second-oldest university in Scandinavia after Uppsala Unive ...
. His major was physics, which he studied under Professor Christian Christiansen, the university's only professor of physics at that time. He also studied astronomy and mathematics under Professor Thorvald Thiele, and philosophy under Professor
Harald Høffding Harald Høffding (11 March 1843 – 2 July 1931) was a Danish philosopher and theologian. Life Born and educated in Copenhagen, he became a schoolmaster, and ultimately in 1883 a professor at the University of Copenhagen. He was strongly influe ...
, a friend of his father. In 1905 a gold medal competition was sponsored by the
Royal Danish Academy of Sciences and Letters {{Infobox organization , name = The Royal Danish Academy of Sciences and Letters , full_name = , native_name = Det Kongelige Danske Videnskabernes Selskab , native_name_lang = , logo = Royal ...
to investigate a method for measuring the surface tension of liquids that had been proposed by
Lord Rayleigh John William Strutt, 3rd Baron Rayleigh, (; 12 November 1842 – 30 June 1919) was an English mathematician and physicist who made extensive contributions to science. He spent all of his academic career at the University of Cambridge. Am ...
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 make his own glassware, creating test tubes with the required
elliptical Elliptical may mean: * having the shape of an ellipse, or more broadly, any oval shape ** in botany, having an elliptic leaf shape ** of aircraft wings, having an elliptical planform * characterised by ellipsis (the omission of words), or by conc ...
cross-sections. He went beyond the original task, incorporating improvements into both Rayleigh's theory and his method, by taking into account the
viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...
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 The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...
in London for publication in the ''
Philosophical Transactions of the Royal Society ''Philosophical Transactions of the Royal Society'' is a scientific journal published by the Royal Society. In its earliest days, it was a private venture of the Royal Society's secretary. It was established in 1665, making it the first journa ...
''. Harald became the first of the two Bohr brothers to earn a
master's degree A master's degree (from Latin ) is an academic degree awarded by universities or colleges upon completion of a course of study demonstrating mastery or a high-order overview of a specific field of study or area of professional practice.
, which he earned for mathematics in April 1909. Niels took another nine months to earn his on the electron theory of metals, a topic assigned by his supervisor, Christiansen. Bohr subsequently elaborated his master's thesis into his much-larger
Doctor of Philosophy A Doctor of Philosophy (PhD, Ph.D., or DPhil; Latin: or ') is the most common Academic degree, degree at the highest academic level awarded following a course of study. PhDs are awarded for programs across the whole breadth of academic fields ...
(dr. phil.) thesis. He surveyed the literature on the subject, settling on a model postulated by
Paul Drude Paul Karl Ludwig Drude (; 12 July 1863 – 5 July 1906) was a German physicist specializing in optics. He wrote a fundamental textbook integrating optics with Maxwell's theories of electromagnetism. Education Born into an ethnic German family, D ...
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 The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor that is transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current. It was dis ...
, and concluded that electron theory could not fully explain the magnetic properties of metals. The thesis was accepted in April 1911, and Bohr conducted his formal defence on 13 May. Harald had received his doctorate the previous year. 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. In 1910, Bohr met Margrethe Nørlund, the sister of the mathematician
Niels Erik Nørlund Niels Erik Nørlund (26 October 1885, in Slagelse – 4 July 1981, in Copenhagen) was a Danish mathematician. His book ''Vorlesungen über Differenzenrechnung'' (1924, reprinted 1954) was the first book on complex function solutions of dif ...
. Bohr resigned his membership in the
Church of Denmark The Evangelical-Lutheran Church in Denmark or National Church, sometimes called the Church of Denmark ( da, Folkekirken, literally: "The People's Church" or unofficially da, Den danske folkekirke, literally: "The Danish People's Church"; kl, ...
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 similarly left the church before getting married. Bohr and Margrethe had six sons. The oldest, Christian, died in a boating accident in 1934, and another, Harald, was severely mentally disabled. He was placed in an instution away from his family's home at the age of four and died from childhood meningitis six years later.
Aage Bohr Aage Niels Bohr (; 19 June 1922 – 8 September 2009) was a Danish nuclear physicist who shared the Nobel Prize in Physics in 1975 with Ben Roy Mottelson and James Rainwater "for the discovery of the connection between collective motion and pa ...
became a successful physicist, and in 1975 was awarded the Nobel Prize in physics, like his father. A son of Aage, Vilhem A. Bohr, is a scientist affiliated with the University of Copenhagen and the National Institute on Aging of the USA. became a physician; , a chemical engineer; and
Ernest Ernest is a given name derived from Germanic languages, Germanic word ''ernst'', meaning "serious". Notable people and fictional characters with the name include: People *Archduke Ernest of Austria (1553–1595), son of Maximilian II, Holy Roman ...
, a lawyer. Like his uncle Harald, Ernest Bohr became an Olympic athlete, playing
field hockey Field hockey is a team sport structured in standard hockey format, in which each team plays with ten outfield players and a goalkeeper. Teams must drive a round hockey ball by hitting it with a hockey stick towards the rival team's shooting ci ...
for Denmark at the 1948 Summer Olympics in London.


Physics


Bohr model

In September 1911, Bohr, supported by a fellowship from the
Carlsberg Foundation Carlsberg Foundation ( da, Carlsbergfondet) was founded by J. C. Jacobsen in 1876, by allocating some of his shares in the Carlsberg Brewery to fund and operate the Carlsberg Laboratory and the Museum of National History at Frederiksborg Palace. ...
, travelled to England, where most of the theoretical work on the structure of atoms and molecules was being done. He met
J. J. Thomson Sir Joseph John Thomson (18 December 1856 – 30 August 1940) was a British physicist and Nobel Laureate in Physics, credited with the discovery of the electron, the first subatomic particle to be discovered. In 1897, Thomson showed that ...
of the Cavendish Laboratory and
Trinity College, Cambridge Trinity College is a constituent college of the University of Cambridge. Founded in 1546 by King Henry VIII, Trinity is one of the largest Cambridge colleges, with the largest financial endowment of any college at either Cambridge or Oxford. ...
. He attended lectures on
electromagnetism In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of ...
given by
James Jeans Sir James Hopwood Jeans (11 September 187716 September 1946) was an English physicist, astronomer and mathematician. Early life Born in Ormskirk, Lancashire, the son of William Tulloch Jeans, a parliamentary correspondent and author. Jeans was ...
and
Joseph Larmor Sir Joseph Larmor (11 July 1857 – 19 May 1942) was an Irish and British physicist and mathematician who made breakthroughs in the understanding of electricity, dynamics, thermodynamics, and the electron theory of matter. His most influent ...
, and did some research on
cathode ray Cathode rays or electron beam (e-beam) are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to el ...
s, but failed to impress Thomson. He had more success with younger physicists like the Australian
William Lawrence Bragg Sir William Lawrence Bragg, (31 March 1890 – 1 July 1971) was an Australian-born British physicist and X-ray crystallographer, discoverer (1912) of Bragg's law of X-ray diffraction, which is basic for the determination of crystal structu ...
, and New Zealand's
Ernest Rutherford Ernest Rutherford, 1st Baron Rutherford of Nelson, (30 August 1871 – 19 October 1937) was a New Zealand physicist who came to be known as the father of nuclear physics. ''Encyclopædia Britannica'' considers him to be the greatest ...
, whose 1911 small central nucleus
Rutherford model The Rutherford model was devised by the New Zealand-born physicist Ernest Rutherford to describe an atom. Rutherford directed the Geiger–Marsden experiment in 1909, which suggested, upon Rutherford's 1911 analysis, that J. J. Thomson's plum ...
of the
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, ...
had challenged Thomson's 1904
plum pudding model The plum pudding model is one of several historical scientific models of the atom. First proposed by J. J. Thomson in 1904 soon after the discovery of the electron, but before the discovery of the atomic nucleus, the model tried to explain two pr ...
. Bohr received an invitation from Rutherford to conduct post-doctoral work at
Victoria University of Manchester The Victoria University of Manchester, usually referred to as simply the University of Manchester, was a university in Manchester, England. It was founded in 1851 as Owens College. In 1880, the college joined the federal Victoria University. Afte ...
, where Bohr met
George de Hevesy George Charles de Hevesy (born György Bischitz; hu, Hevesy György Károly; german: Georg Karl von Hevesy; 1 August 1885 – 5 July 1966) was a Hungarian radiochemist and Nobel Prize in Chemistry laureate, recognized in 1943 for his key rol ...
and
Charles Galton Darwin Sir Charles Galton Darwin (19 December 1887 – 31 December 1962) was an English physicist who served as director of the National Physical Laboratory (NPL) during the Second World War. He was a son of the mathematician George Howard Darwin an ...
(whom Bohr referred to as "the grandson of the real Darwin"). 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 ''Privatdozent'' (for men) or ''Privatdozentin'' (for women), abbreviated PD, P.D. or Priv.-Doz., is an academic title conferred at some European universities, especially in German-speaking countries, to someone who holds certain formal qualific ...
'' at the University of Copenhagen, giving lectures on
thermodynamics Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...
.
Martin Knudsen Martin Hans Christian Knudsen (February 15, 1871 in Hasmark on Funen – May 27, 1949 in Copenhagen) was a Danish physicist who taught and conducted research at the Technical University of Denmark. He is primarily known for his study of molec ...
put Bohr's name forward for a ''
docent The title of docent is conferred by some European universities to denote a specific academic appointment within a set structure of academic ranks at or below the full professor rank, similar to a British readership, a French " ''maître de con ...
'', which was approved in July 1913, and Bohr then began teaching medical students. His three papers, which later became famous as "the trilogy", were published in ''
Philosophical Magazine The ''Philosophical Magazine'' is one of the oldest scientific journals published in English. It was established by Alexander Tilloch in 1798;John Burnett"Tilloch, Alexander (1759–1825)" Oxford Dictionary of National Biography, Oxford Univer ...
'' in July, September and November of that year. He adapted Rutherford's nuclear structure to
Max Planck Max Karl Ernst Ludwig Planck (, ; 23 April 1858 – 4 October 1947) was a German theoretical physicist whose discovery of energy quanta won him the Nobel Prize in Physics in 1918. Planck made many substantial contributions to theoretical p ...
's quantum theory and so created his
Bohr model In atomic physics, the Bohr model or Rutherford–Bohr model, presented by Niels Bohr and Ernest Rutherford in 1913, is a system consisting of a small, dense nucleus surrounded by orbiting electrons—similar to the structure of the Solar Syst ...
of the atom. Planetary models of atoms were not new, but Bohr's treatment was. Taking the 1912 paper by Darwin on the role of electrons in the interaction of alpha particles with a nucleus as his starting point, he advanced the theory of electrons travelling in
orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...
s of quantized "stationary states" around the atom's nucleus in order to stabilize 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. 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 The old quantum theory is a collection of results from the years 1900–1925 which predate modern quantum mechanics. The theory was never complete or self-consistent, but was rather a set of heuristic corrections to classical mechanics. The theory ...
. In 1885,
Johann Balmer Johann Jakob Balmer (1 May 1825 – 12 March 1898) was a Swiss mathematician best known for his work in physics, the Balmer series of hydrogen atom. Biography Balmer was born in Lausen, Switzerland, the son of a chief justice also named Johan ...
had come up with his
Balmer series The Balmer series, or Balmer lines in atomic physics, is one of a set of six named series describing the spectral line emissions of the hydrogen atom. The Balmer series is calculated using the Balmer formula, an empirical equation discovered b ...
to describe the visible
spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
s of a
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
atom: :\frac = R_\mathrm\left(\frac - \frac\right) \quad \text \ n=3,4,5,... where λ is the wavelength of the absorbed or emitted light and ''R''H is the
Rydberg constant In spectroscopy, the Rydberg constant, symbol R_\infty for heavy atoms or R_\text for hydrogen, named after the Swedish physicist Johannes Rydberg, is a physical constant relating to the electromagnetic spectra of an atom. The constant first aro ...
. 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: : R_Z = where ''m''e is the electron's mass, ''e'' is its charge, ''h'' is Planck's constant and ''Z'' is the atom's
atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of an atomic nucleus. For ordinary nuclei, this is equal to the proton number (''n''p) or the number of protons found in the nucleus of every ...
(1 for hydrogen). The model's first hurdle was the
Pickering series The Pickering series (also known as the Pickering–Fowler series) consists of three lines of singly ionized helium found, usually in absorption, in the spectra of hot stars like Wolf–Rayet stars. The name comes from Edward Charles Pickering and A ...
, lines which did not fit Balmer's formula. When challenged on this by
Alfred Fowler Alfred Fowler, CBE FRS (22 March 1868, in Yorkshire – 24 June 1940) was an English astronomer. Early life and career He was born in Wilsden on the outskirts of Bradford, Yorkshire and educated at London's Normal School of Science, w ...
, Bohr replied that they were caused by
ionised Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
, helium atoms with only one electron. The Bohr model was found to work for such ions. Many older physicists, like Thomson, Rayleigh and Hendrik Lorentz, did not like the trilogy, but the younger generation, including Rutherford, David Hilbert,
Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
, Enrico Fermi, Max Born and Arnold Sommerfeld saw it as a breakthrough. The trilogy's acceptance was entirely due to its ability to explain phenomena which stymied other models, and to predict results that were subsequently verified by experiments. 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. Bohr did not enjoy teaching medical students. He decided to return to Manchester, where Rutherford had offered him a job as a
reader A reader is a person who reads. It may also refer to: Computing and technology * Adobe Reader (now Adobe Acrobat), a PDF reader * Bible Reader for Palm, a discontinued PDA application * A card reader, for extracting data from various forms of ...
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
Tyrol Tyrol (; historically the Tyrole; de-AT, Tirol ; it, Tirolo) is a historical region in the Alps - in Northern Italy and western Austria. The area was historically the core of the County of Tyrol, part of the Holy Roman Empire, Austrian Emp ...
with his brother Harald and aunt Hanna Adler. There, he visited the
University of Göttingen The University of Göttingen, officially the Georg August University of Göttingen, (german: Georg-August-Universität Göttingen, known informally as Georgia Augusta) is a public research university in the city of Göttingen, Germany. Founded ...
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 Christian X ( da, Christian Carl Frederik Albert Alexander Vilhelm; 26 September 1870 – 20 April 1947) was List of Danish monarchs, King of Denmark from 1912 to his death in 1947, and the only List of rulers of Iceland, King of Iceland as ...
, who expressed his delight at meeting such a famous football player.


Institute of Physics

In April 1917 Bohr began a campaign to establish an Institute of Theoretical Physics. He gained the support of the Danish government and the Carlsberg Foundation, and sizeable contributions were also made by industry and private donors, many of them Jewish. Legislation establishing the institute was passed in November 1918. Now known as the
Niels Bohr Institute The Niels Bohr Institute (Danish: ''Niels Bohr Institutet'') is a research institute of the University of Copenhagen. The research of the institute spans astronomy, geophysics, nanotechnology, particle physics, quantum mechanics and biophysics ...
, it opened on 3 March 1921, with Bohr as its director. His family moved into an apartment on the first floor. Bohr's institute served as a focal point for researchers into
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
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 Hendrik Anthony "Hans" Kramers (17 December 1894 – 24 April 1952) was a Dutch physicist who worked with Niels Bohr to understand how electromagnetic waves interact with matter and made important contributions to quantum mechanics and statistical ...
from the Netherlands, Oskar Klein from Sweden, George de Hevesy from Hungary,
Wojciech Rubinowicz Wojciech Sylwester Piotr Rubinowicz (February 22, 1889 – October 13, 1974) was a Polish theoretical physicist who made contributions in quantum mechanics, mathematical physics, and the theory of radiation. He is known for the Maggie-Rubinowicz ...
from Poland and Svein Rosseland from Norway. Bohr became widely appreciated as their congenial host and eminent colleague. Klein and Rosseland produced the institute's first publication even before it opened. The Bohr model worked well for hydrogen and ionized single electron Helium which impressed Einstein, but could not explain more complex elements. By 1919, Bohr was moving away from the idea that electrons orbited the nucleus and developed
heuristic A heuristic (; ), or heuristic technique, is any approach to problem solving or self-discovery that employs a practical method that is not guaranteed to be optimal, perfect, or rational, but is nevertheless sufficient for reaching an immediate ...
s to describe them. The
rare-earth element The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides ( yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silv ...
s posed a particular classification problem for chemists, because they were so chemically similar. An important development came in 1924 with
Wolfgang Pauli Wolfgang Ernst Pauli (; ; 25 April 1900 – 15 December 1958) was an Austrian theoretical physicist and one of the pioneers of quantum physics. In 1945, after having been nominated by Albert Einstein, Pauli received the Nobel Prize in Physics ...
's discovery of the
Pauli exclusion principle In quantum mechanics, the Pauli exclusion principle states that two or more identical particles with half-integer spins (i.e. fermions) cannot occupy the same quantum state within a quantum system simultaneously. This principle was formulat ...
, 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 Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...
. (Elements had been predicted and discovered since 1871 by chemical properties) and Bohr was immediately challenged by the French chemist
Georges Urbain Georges Urbain (12 April 1872 – 5 November 1938) was a French chemist, a professor of the Sorbonne, a member of the Institut de France, and director of the Institute of Chemistry in Paris. Much of his work focused on the rare earths, isolating a ...
, who claimed to have discovered a rare-earth element 72, which he called "celtium". At the Institute in Copenhagen,
Dirk Coster Dirk Coster (October 5, 1889 – February 12, 1950), was a Dutch physicist. He was a Professor of Physics and Meteorology at the University of Groningen. Coster was born in Amsterdam. On February 26, 1919, he married Lina Maria Wijsman, who ...
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 Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri M ...
(''Hafnia'' being the Latin name for Copenhagen) turned out to be more common than gold. In 1922 Bohr was awarded the
Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
"for his services in the investigation of the structure of atoms and of the radiation emanating from them". 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 numbers. The discovery of
Compton scattering Compton scattering, discovered by Arthur Holly Compton, is the scattering of a high frequency photon after an interaction with a charged particle, usually an electron. If it results in a decrease in energy (increase in wavelength) of the photon ...
by
Arthur Holly Compton Arthur Holly Compton (September 10, 1892 – March 15, 1962) was an American physicist who won the Nobel Prize in Physics in 1927 for his 1923 discovery of the Compton effect, which demonstrated the particle nature of electromagnetic radi ...
in 1923 convinced most physicists that light was composed of
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...
s, and that energy and momentum were conserved in collisions between electrons and photons. In 1924, Bohr, Kramers and
John C. Slater John Clarke Slater (December 22, 1900 – July 25, 1976) was a noted American physicist who made major contributions to the theory of the electronic structure of atoms, molecules and solids. He also made major contributions to microwave electroni ...
, an American physicist working at the Institute in Copenhagen, proposed the Bohr–Kramers–Slater theory (BKS). It was more a programme than a full physical theory, as the ideas it developed were not worked out quantitatively. 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. 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 Werner Karl Heisenberg () (5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the main pioneers of the theory of quantum mechanics. He published his work in 1925 in a breakthrough paper. In the subsequent serie ...
and Kramers to explore different mathematical models. They led to the development of
matrix mechanics Matrix mechanics is a formulation of quantum mechanics created by Werner Heisenberg, Max Born, and Pascual Jordan in 1925. It was the first conceptually autonomous and logically consistent formulation of quantum mechanics. Its account of quantum j ...
, the first form of modern
quantum mechanics Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
. The BKS theory also generated discussion of, and renewed attention to, difficulties in the foundations of the old quantum theory. 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 Walther Wilhelm Georg Bothe (; 8 January 1891 – 8 February 1957) was a German nuclear physicist, who shared the Nobel Prize in Physics in 1954 with Max Born. In 1913, he joined the newly created Laboratory for Radioactivity at the Reich Physi ...
and
Hans Geiger Johannes Wilhelm "Hans" Geiger (; ; 30 September 1882 – 24 September 1945) was a German physicist. He is best known as the co-inventor of the detector component of the Geiger counter and for the Geiger–Marsden experiment which discover ...
. 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".


Quantum mechanics

The introduction of spin by
George Uhlenbeck George Eugene Uhlenbeck (December 6, 1900 – October 31, 1988) was a Dutch-American theoretical physicist. Background and education George Uhlenbeck was the son of Eugenius and Anne Beeger Uhlenbeck. He attended the Hogere Burgerschool (High S ...
and
Samuel Goudsmit Samuel Abraham Goudsmit (July 11, 1902 – December 4, 1978) was a Dutch-American physicist famous for jointly proposing the concept of electron spin with George Eugene Uhlenbeck in 1925. Life and career Goudsmit was born in The Hague, Nethe ...
in November 1925 was a milestone. The next month, Bohr travelled to
Leiden Leiden (; in English and archaic Dutch also Leyden) is a city and municipality in the province of South Holland, Netherlands. The municipality of Leiden has a population of 119,713, but the city forms one densely connected agglomeration wit ...
to attend celebrations of the 50th anniversary of Hendrick Lorentz receiving his doctorate. When his train stopped in
Hamburg (male), (female) en, Hamburger(s), Hamburgian(s) , timezone1 = Central (CET) , utc_offset1 = +1 , timezone1_DST = Central (CEST) , utc_offset1_DST = +2 , postal ...
, he was met by Wolfgang Pauli and
Otto Stern :''Otto Stern was also the pen name of German women's rights activist Louise Otto-Peters (1819–1895)''. Otto Stern (; 17 February 1888 – 17 August 1969) was a German-American physicist and Nobel laureate in physics. He was the second most n ...
, 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 Paul Ehrenfest (18 January 1880 – 25 September 1933) was an Austrian theoretical physicist, who made major contributions to the field of statistical mechanics and its relations with quantum mechanics, including the theory of phase transition a ...
and Albert Einstein informed Bohr that Einstein had resolved this problem using relativity. Bohr then had Uhlenbeck and Goudsmit incorporate this into their paper. Thus, when he met Werner Heisenberg and
Pascual Jordan Ernst Pascual Jordan (; 18 October 1902 – 31 July 1980) was a German theoretical and mathematical physicist who made significant contributions to quantum mechanics and quantum field theory. He contributed much to the mathematical form of matri ...
in
Göttingen Göttingen (, , ; nds, Chöttingen) is a university city in Lower Saxony, central Germany, the capital of the eponymous district. The River Leine runs through it. At the end of 2019, the population was 118,911. General information The ori ...
on the way back, he had become, in his own words, "a prophet of the electron magnet gospel". 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
matrices Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...
. This work attracted the attention of the British physicist
Paul Dirac Paul Adrien Maurice Dirac (; 8 August 1902 – 20 October 1984) was an English theoretical physicist who is regarded as one of the most significant physicists of the 20th century. He was the Lucasian Professor of Mathematics at the Univer ...
, who came to Copenhagen for six months in September 1926. Austrian physicist
Erwin Schrödinger Erwin Rudolf Josef Alexander Schrödinger (, ; ; 12 August 1887 – 4 January 1961), sometimes written as or , was a Nobel Prize-winning Austrian physicist with Irish citizenship who developed a number of fundamental results in quantum theo ...
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". When Kramers left the institute in 1926 to take up a chair as professor of theoretical physics at the
Utrecht University Utrecht University (UU; nl, Universiteit Utrecht, formerly ''Rijksuniversiteit Utrecht'') is a public research university in Utrecht, Netherlands. Established , it is one of the oldest universities in the Netherlands. In 2018, it had an enrollme ...
, Bohr arranged for Heisenberg to return and take Kramers's place as a ''
lektor Lector is Latin for one who reads, whether aloud or not. In modern languages it takes various forms, as either a development or a loan, such as french: lecteur, en, lector, pl, lektor and russian: лектор. It has various specialized uses. ...
'' at the University of Copenhagen. Heisenberg worked in Copenhagen as a university lecturer and assistant to Bohr from 1926 to 1927. 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. He conceived the philosophical principle of complementarity: that items could have apparently mutually exclusive properties, such as being a wave or a stream of particles, depending on the experimental framework. He felt that it was not fully understood by professional philosophers. In February 1927, Heisenberg developed the first version of the
uncertainty principle In quantum mechanics, the uncertainty principle (also known as Heisenberg's uncertainty principle) is any of a variety of mathematical inequalities asserting a fundamental limit to the accuracy with which the values for certain pairs of physic ...
, presenting it using a
thought experiment A thought experiment is a hypothetical situation in which a hypothesis, theory, or principle is laid out for the purpose of thinking through its consequences. History The ancient Greek ''deiknymi'' (), or thought experiment, "was the most anc ...
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
Volta Conference The Volta Conference was the name given to each of the international conferences held in Italy by the Royal Academy of Science in Rome, and funded by the Alessandro Volta Foundation. In the interwar period, they covered a number of topics in sci ...
at
Como Como (, ; lmo, Còmm, label= Comasco , or ; lat, Novum Comum; rm, Com; french: Côme) is a city and ''comune'' in Lombardy, Italy. It is the administrative capital of the Province of Como. Its proximity to Lake Como and to the Alps h ...
in September 1927, Bohr emphasized that Heisenberg's uncertainty relations could be derived from classical considerations about the resolving power of optical instruments. Understanding the true meaning of complementarity would, Bohr believed, require "closer investigation". 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 good-natured arguments over such issues throughout their lives. In 1914 Carl Jacobsen, the heir to
Carlsberg breweries Carlsberg A/S (; ) is a Danish multinational brewer. Founded in 1847 by J. C. Jacobsen, the company's headquarters is in Copenhagen, Denmark. Since Jacobsen's death in 1887, the majority owner of the company has been the Carlsberg Foundation. ...
, 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 ( 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. He was elected president of the Academy on 17 March 1939. By 1929 the phenomenon of
beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
prompted Bohr to again suggest that the
law of conservation of energy In physics and chemistry, the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be ''conserved'' over time. This law, first proposed and tested by Émilie du Châtelet, means that ...
be abandoned, but Enrico Fermi's hypothetical
neutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
and the subsequent 1932 discovery of the
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
provided another explanation. This prompted Bohr to create a new theory of the
compound nucleus In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a transformation ...
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. The discovery of nuclear fission by
Otto Hahn Otto Hahn (; 8 March 1879 – 28 July 1968) was a German chemist who was a pioneer in the fields of radioactivity and radiochemistry. He is referred to as the father of nuclear chemistry and father of nuclear fission. Hahn and Lise Meitner ...
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. When Bohr told
George Placzek George Placzek (; September 26, 1905 – October 9, 1955) was a Moravian physicist. Biography Placzek was born into a wealthy Jewish family in Brünn, Moravia (now Brno, Czech Republic), the grandson of Chief Rabbi Baruch Placzek.PDF He studied ...
that this resolved all the mysteries of
transuranic elements The transuranium elements (also known as transuranic elements) are the chemical elements with atomic numbers greater than 92, which is the atomic number of uranium. All of these elements are unstable and decay radioactively into other 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 Léon Rosenfeld (; 14 August 1904 in Charleroi – 23 March 1974) was a Belgian physicist and Marxist. Rosenfeld was born into a secular Jewish family. He was a polyglot who knew eight or nine languages and was fluent in at least five of the ...
and John Wheeler that "I have understood everything." Based on his
liquid drop model In nuclear physics, the semi-empirical mass formula (SEMF) (sometimes also called the Weizsäcker formula, Bethe–Weizsäcker formula, or Bethe–Weizsäcker mass formula to distinguish it from the Bethe–Weizsäcker process) is used to approxi ...
of the nucleus, Bohr concluded that it was the
uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exi ...
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. In the meantime, Bohr and Wheeler developed a theoretical treatment which they published in a September 1939 paper on "The Mechanism of Nuclear Fission".


Philosophy

Heisenberg said of Bohr that he was "primarily a philosopher, not a physicist". Bohr read the 19th-century Danish
Christian existentialist Christian existentialism is a theo-philosophical movement which takes an existentialist approach to Christian theology. The school of thought is often traced back to the work of the Danish philosopher and theologian Søren Kierkegaard (1813-1855 ...
philosopher Søren Kierkegaard.
Richard Rhodes Richard Lee Rhodes (born July 4, 1937) is an American historian, journalist, and author of both fiction and non-fiction, including the Pulitzer Prize-winning ''The Making of the Atomic Bomb'' (1986), and most recently, ''Energy: A Human Histor ...
argued in ''
The Making of the Atomic Bomb ''The Making of the Atomic Bomb'' is a contemporary history book written by the American journalist and historian Richard Rhodes, first published by Simon & Schuster in 1987. It won the Pulitzer Prize for General Non-Fiction, the National Book ...
'' that Bohr was influenced by Kierkegaard through Høffding. 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 ... 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 Kierkegaard's philosophy. Some of Bohr's biographers suggested that this disagreement stemmed from Kierkegaard's advocacy of Christianity, while Bohr was an atheist.: "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 han Kierkegaard would say that the good sthe overall lofty goal, as only by being good
an one An, AN, aN, or an may refer to: Businesses and organizations * Airlinair (IATA airline code AN) * Alleanza Nazionale, a former political party in Italy * AnimeNEXT, an annual anime convention located in New Jersey * Anime North, a Canadian an ...
judge according to worth and right.'"
: "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." 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, while Jan Faye argued that one can disagree with the content of a theory while accepting its general premises and structure.


Quantum physics

There has been much subsequent debate and discussion about Bohr's views and philosophy of quantum mechanics. Regarding his ontological interpretation of the quantum world, Bohr has been seen as an
anti-realist In analytic philosophy, anti-realism is a position which encompasses many varieties such as metaphysical, mathematical, semantic, scientific, moral and epistemic. The term was first articulated by British philosopher Michael Dummett in an argume ...
, an
instrumentalist A musical instrument is a device created or adapted to make musical sounds. In principle, any object that produces sound can be considered a musical instrument—it is through purpose that the object becomes a musical instrument. A person who pl ...
, a phenomenological realist or some other kind of realist. Furthermore, though some have seen Bohr as being a
subjectivist Subjectivism is the doctrine that "our own mental activity is the only unquestionable fact of our experience", instead of shared or communal, and that there is no external or objective truth. The success of this position is historically attribute ...
or a positivist, most philosophers agree that this is a misunderstanding of Bohr as he never argued for
verificationism Verificationism, also known as the verification principle or the verifiability criterion of meaning, is the philosophical doctrine which maintains that only statements that are empirically verifiable (i.e. verifiable through the senses) are cogniti ...
or for the idea that the subject had a direct impact on the outcome of a measurement. Bohr has often been quoted as saying that there is "no quantum world" but only an "abstract quantum physical description". This was not said by Bohr, but rather by Aage Petersen attempting to summarize Bohr's philosophy in a reminiscence after Bohr's death.
N. David Mermin Nathaniel David Mermin (; born 30 March 1935) is a solid-state physicist at Cornell University best known for the eponymous Mermin–Wagner theorem, his application of the term " boojum" to superfluidity, his textbook with Neil Ashcroft on sol ...
recalled
Victor Weisskopf Victor Frederick "Viki" Weisskopf (also spelled Viktor; September 19, 1908 – April 22, 2002) was an Austrian-born American theoretical physicist. He did postdoctoral work with Werner Heisenberg, Erwin Schrödinger, Wolfgang Pauli, and Niels Boh ...
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!" Numerous scholars have argued that the philosophy of
Immanuel Kant Immanuel Kant (, , ; 22 April 1724 – 12 February 1804) was a German philosopher and one of the central Enlightenment thinkers. Born in Königsberg, Kant's comprehensive and systematic works in epistemology, metaphysics, ethics, and ...
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.Faye, Jan
"Copenhagen Interpretation of Quantum Mechanics"
''The Stanford Encyclopedia of Philosophy'' (Winter 2019 Edition), Edward N. Zalta (ed.), URL = https://plato.stanford.edu/archives/win2019/entries/qm-copenhagen/.
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. Therefore, for Bohr, we need to use classical concepts to describe experiments that deal with the quantum world. Bohr writes:
It is decisive to recognize that, however far the phenomena transcend the scope of classical physical explanation, the 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).
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 Kantianism is the philosophy of Immanuel Kant, a German philosopher born in Königsberg, Prussia (now Kaliningrad, Russia). The term ''Kantianism'' or ''Kantian'' is sometimes also used to describe contemporary positions in philosophy of min ...
(or
Neo-Kantian In late modern continental philosophy, neo-Kantianism (german: Neukantianismus) was a revival of the 18th-century philosophy of Immanuel Kant. The Neo-Kantians sought to develop and clarify Kant's theories, particularly his concept of the "thi ...
models of
epistemology Epistemology (; ), or the theory of knowledge, is the branch of philosophy concerned with knowledge. Epistemology is considered a major subfield of philosophy, along with other major subfields such as ethics, logic, and metaphysics. Epis ...
in which classical ideas are a priori concepts that the mind imposes on sense impressions);
Pragmatism Pragmatism is a philosophical tradition that considers words and thought as tools and instruments for prediction, problem solving, and action, and rejects the idea that the function of thought is to describe, represent, or mirror reality. ...
(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 Léon Rosenfeld (; 14 August 1904 in Charleroi – 23 March 1974) was a Belgian physicist and Marxist. Rosenfeld was born into a secular Jewish family. He was a polyglot who knew eight or nine languages and was fluent in at least five of the ...
said that we evolved to use); and Experimentalism (which focuses strictly on the function and outcome of experiments which thus must be described classically). These explanations are not mutually exclusive, and at times Bohr seems to emphasize some of these aspects while at other times he focuses on other elements. 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." Therefore, Bohr's theory of complementarity "is first and foremost a semantic and epistemological reading of quantum mechanics that carries certain ontological implications." As Faye explains, Bohr's ''indefinability thesis'' is that
the 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.
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 ''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. 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 transcendental 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 (though quite unintelligible and inaccessible to us) in addition to the classical ones being manifested in measurement."


Nazism and Second World War

The rise of
Nazism Nazism ( ; german: Nazismus), the common name in English for National Socialism (german: Nationalsozialismus, ), is the far-right totalitarian political ideology and practices associated with Adolf Hitler and the Nazi Party (NSDAP) i ...
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 Charles Max Mason (–), better known as Max Mason, was an American mathematician. Mason was president of the University of Chicago (1925–1928) and president of the Rockefeller Foundation (1929–1936). Mason's mathematical research inte ...
, 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 Guido Beck (29 August 1903 in Liberec – 21 October 1988 in Rio de Janeiro) was an Argentinian physicist of German Bohemian origin. Biography Beck studied physics in Vienna and received his doctorate in 1925, under Hans Thirring. He worked in ...
,
Felix Bloch Felix Bloch (23 October 1905 – 10 September 1983) was a Swiss-American physicist and Nobel physics laureate who worked mainly in the U.S. He and Edward Mills Purcell were awarded the 1952 Nobel Prize for Physics for "their development of new ...
,
James Franck James Franck (; 26 August 1882 – 21 May 1964) was a German physicist who won the 1925 Nobel Prize for Physics with Gustav Hertz "for their discovery of the laws governing the impact of an electron upon an atom". He completed his doctorate i ...
, George de Hevesy, Otto Frisch,
Hilde Levi Hilde Levi (9 May 1909 – 26 July 2003) was a German-Danish physicist. She was a pioneer of the use of radioactive isotopes in biology and medicine, notably the techniques of radiocarbon dating and autoradiography. In later life she bec ...
, Lise Meitner, George Placzek,
Eugene Rabinowitch Eugene Rabinowitch (1901–1973) was a Russian-born American biophysicist who is known for his work in photosynthesis and nuclear energy. He was a co-author of the Franck Report and a co-founder in 1945 of the '' Bulletin of the Atomic Scientists ...
,
Stefan Rozental Stefan Rozental (13 August 1903, Łódź – 2 August 1994, Copenhagen), was a Nuclear physics, nuclear physicist, specialising in quantum mechanics. Trapped outside Poland when World War I started, he and his parents ended up in Denmark and spent ...
, Erich Ernst Schneider,
Edward Teller Edward Teller ( hu, Teller Ede; January 15, 1908 – September 9, 2003) was a Hungarian-American theoretical physicist who is known colloquially as "the father of the hydrogen bomb" (see the Teller–Ulam design), although he did not care for ...
, Arthur von Hippel and
Victor Weisskopf Victor Frederick "Viki" Weisskopf (also spelled Viktor; September 19, 1908 – April 22, 2002) was an Austrian-born American theoretical physicist. He did postdoctoral work with Werner Heisenberg, Erwin Schrödinger, Wolfgang Pauli, and Niels Boh ...
. In April 1940, early in the Second World War,
Nazi Germany Nazi Germany (lit. "National Socialist State"), ' (lit. "Nazi State") for short; also ' (lit. "National Socialist Germany") (officially known as the German Reich from 1933 until 1943, and the Greater German Reich from 1943 to 1945) was ...
invaded and occupied Denmark. To prevent the Germans from discovering Max von Laue's and James Franck's gold Nobel medals, 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 Fund for Finnish Relief, and was auctioned off in March 1940, along with the medal of
August Krogh Schack August Steenberg Krogh (15 November 1874 – 13 September 1949) was a Danish professor at the department of zoophysiology at the University of Copenhagen from 1916 to 1945. He contributed a number of fundamental discoveries within severa ...
. The buyer later donated the two medals to the Danish Historical Museum in
Frederiksborg Castle Frederiksborg Castle ( da, Frederiksborg Slot) is a palatial complex in Hillerød, Denmark. It was built as a royal residence for King Christian IV of Denmark-Norway in the early 17th century, replacing an older castle acquired by Frederick ...
, where they are still kept. Bohr kept the Institute running, but all the foreign scholars departed.


Meeting with Heisenberg

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. In September 1941, Heisenberg, who had become head of the
German nuclear energy project The Uranverein ( en, "Uranium Club") or Uranprojekt ( en, "Uranium Project") was the name given to the project in Germany to research nuclear technology, including nuclear weapons and nuclear reactors, during World War II. It went through seve ...
, 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.
Ivan Supek Ivan Supek (8 April 1915 – 5 March 2007) was a Croatian physicist, philosopher, writer, playwright, peace activist and humanist. Early years and education Supek was born in Zagreb, Croatia (still nominally under Austria-Hungary). During hi ...
, one of Heisenberg's students and friends, claimed that the main subject of the meeting was
Carl Friedrich von Weizsäcker Carl Friedrich Freiherr von Weizsäcker (; 28 June 1912 – 28 April 2007) was a German physicist and philosopher. He was the longest-living member of the team which performed nuclear research in Germany during the Second World War, under ...
, who had proposed trying to persuade Bohr to mediate peace between Britain and Germany. 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. 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 never understood the purpose of Heisenberg's visit, was shocked by Heisenberg's opinion that Germany would win the war, and that atomic weapons could be decisive. Michael Frayn's 1998 play ''Copenhagen (play), Copenhagen'' explores what might have happened at the 1941 meeting between Heisenberg and Bohr. A BBC Copenhagen (2002 film), television film version of the play was first screened on 26 September 2002, with Stephen Rea as Bohr, Daniel Craig as Heisenberg, and Francesca Annis as Margrethe Bohr. 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. The meeting is also dramatized in the Norwegian/Danish/British miniseries ''The Heavy Water War''.


Manhattan Project

In September 1943, word reached Bohr and his brother Harald that the Nazis 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. 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. Eventually, over 7,000 Danish Jews escaped to Sweden. 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). 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. 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. Bohr's son Aage followed his father to Britain on another flight a week later, and became his personal assistant. 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 Tube Alloys was the research and development programme authorised by the United Kingdom, with participation from Canada, to develop nuclear weapons during the Second World War. Starting before the Manhattan Project in the United States, the ...
nuclear weapons development team. Bohr was astonished at the amount of progress that had been made. Chadwick arranged for Bohr to visit the United States as a Tube Alloys consultant, with Aage as his assistant. On 8 December 1943, Bohr arrived in Washington, D.C., where he met with the director of the
Manhattan Project The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project w ...
, 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. For security reasons, he went under the name of "Nicholas Baker" in the United States, while Aage became "James Baker". 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. 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. Bohr is quoted as saying, "They didn't need my help in making the atom bomb." Oppenheimer gave Bohr credit for an important contribution to the work on modulated neutron initiators. "This device remained a stubborn puzzle," Oppenheimer noted, "but in early February 1945 Niels Bohr clarified what had to be done." 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. Bohr met Churchill on 16 May 1944, but found that "we did not speak the same language". 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." 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. 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 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". In June 1950, Bohr addressed an "Open Letter" to the United Nations calling for international cooperation on nuclear energy. In the 1950s, after the RDS-1, Soviet Union's first nuclear weapon test, the International Atomic Energy Agency was created along the lines of Bohr's suggestion. In 1957 he received the first ever Atoms for Peace Award.


Later years

With the war now ended, 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. At a memorial meeting of the Academy on 17 October 1947 for King Christian X of Denmark, Christian X, who had died in April, the new king, Frederick IX of Denmark, Frederick 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. Bohr designed his own coat of arms which featured a taijitu (symbol of yin and yang) and a motto in la, contraria sunt complementa, links=no, "opposites are complementary". 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 The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
, 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. 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 European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...
. The enthusiasm and ideas of the other people would not have been enough, however, if a man of his stature had not supported it." Meanwhile, Scandinavian countries formed the
Nordic Institute for Theoretical Physics The Nordic Institute for Theoretical Physics, or NORDITA, or Nordita ( da, Nordisk Institut for Teoretisk Fysik), is an international organisation for research in theoretical physics. It was established as Nordisk Institut for Teoretisk Atomfys ...
in 1957, with Bohr as its chairman. He was also involved with the founding of the Research Establishment Risø of the Danish Atomic Energy Commission, and served as its first chairman from February 1956. Bohr died of heart failure at his home in Carlsberg (district), Carlsberg on 18 November 1962. 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. 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.


Accolades

Bohr received numerous honours and accolades. In addition to the Nobel Prize, he received the Hughes Medal in 1921, the Matteucci Medal in 1923, the Franklin Medal in 1926, the Copley Medal in 1938, the Order of the Elephant in 1947, the Atoms for Peace Award in 1957 and the Sonning Prize in 1961. He became foreign member of the Royal Netherlands Academy of Arts and Sciences in 1923, and of the
Royal Society The Royal Society, formally The Royal Society of London for Improving Natural Knowledge, is a learned society and the United Kingdom's national academy of sciences. The society fulfils a number of roles: promoting science and its benefits, re ...
in 1926. 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: h\nu = \epsilon_2 - \epsilon_1. Several other countries have also issued postage stamps depicting Bohr. 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. On 7 October 2012, in celebration of Niels Bohr's 127th birthday, a Google Doodle depicting the Bohr model of the hydrogen atom appeared on Google's home page. An asteroid, 3948 Bohr, was named after him, as was the Bohr (crater), Bohr lunar crater and bohrium, the chemical element with atomic number 107.


Bibliography

* * * * * * * * * * * * * *


See also

* EPR paradox


Notes


References

* * * * * ** Excerpted from: * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *


Further reading

* * * Bohr's researches on reaction times. * * * * *


External links


Niels Bohr Archive
* * *
Author profile
in the database Zentralblatt MATH, zbMATH * * * * including the Nobel Lecture, 11 December 1922 ''The Structure of the Atom''
Oral history interview transcript for Niels Bohr on 31 October 1962, American Institute of Physics, Niels Bohr Library & Archives
- interviews conducted by Thomas Kuhn, Thomas S. Kuhn, Léon Rosenfeld, Leon Rosenfeld, Erik Rudinger, and Aage Petersen
Oral history interview transcript for Niels Bohr on 1 November 1962, American Institute of Physics, Niels Bohr Library & Archives

Oral history interview transcript for Niels Bohr on 7 November 1962, American Institute of Physics, Niels Bohr Library & Archives

Oral history interview transcript for Niels Bohr on 14 November 1962, American Institute of Physics, Niels Bohr Library & Archives

Oral history interview transcript for Niels Bohr on 17 November 1962, American Institute of Physics, Niels Bohr Library & Archives
{{DEFAULTSORT:Bohr, Niels Niels Bohr, 1885 births 1962 deaths Nobel laureates in Physics Danish Nobel laureates Jewish physicists Academics of the Victoria University of Manchester Akademisk Boldklub players Alumni of Trinity College, Cambridge Association football goalkeepers Atoms for Peace Award recipients Bohr family, Niels Corresponding Members of the Russian Academy of Sciences (1917–1925) Corresponding Members of the USSR Academy of Sciences Danish atheists Jewish atheists Danish Jews Danish expatriates in England Danish expatriates in the United States Danish footballers Jewish footballers Danish nuclear physicists Danish people of Jewish descent Danish people of World War II Jewish philosophers 20th-century Danish physicists Faraday Lecturers Foreign Members of the Royal Society Foreign associates of the National Academy of Sciences Foreign Fellows of the Indian National Science Academy Grand Crosses of the Order of the Dannebrog Honorary Members of the USSR Academy of Sciences Institute for Advanced Study visiting scholars Manhattan Project people Members of the Pontifical Academy of Sciences Members of the Prussian Academy of Sciences Members of the Royal Netherlands Academy of Arts and Sciences Members of the German Academy of Sciences Leopoldina Members of the German Academy of Sciences at Berlin Niels Bohr International Gold Medal recipients People from Gribskov Municipality People associated with CERN People associated with the nuclear weapons programme of the United Kingdom Philosophers of science Quantum physicists Recipients of the Copley Medal Recipients of the Pour le Mérite (civil class) Scientists from Copenhagen Theoretical physicists University of Copenhagen alumni Winners of the Max Planck Medal 20th-century Danish philosophers Burials at Assistens Cemetery (Copenhagen) Recipients of the Matteucci Medal Manchester Literary and Philosophical Society Members of the Göttingen Academy of Sciences and Humanities