Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a
Danish 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 and
quantum theory, 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 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, a ...
, 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 n ...
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 ...
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 biophys ...
, which opened in 1920. Bohr mentored and collaborated with physicists including
Hans Kramers,
Oskar Klein,
George de Hevesy, 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 Über quantentheoretische Umdeutung kinematis ...
. He predicted the existence of a new
zirconium-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
Bohrium is a synthetic chemical element with the symbol Bh and atomic number 107. It is named after Danish physicist Niels Bohr. As a synthetic element, it can be created in a laboratory but is not found in nature. All known isotopes of 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. 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 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 Gen ...
and the
Research Establishment Risø of the Danish Atomic Energy Commission and became the first chairman of the
Nordic Institute for Theoretical Physics 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 a ...
, Denmark, on 7 October 1885, the second of three children of
Christian Bohr,
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 chemic ...
at the University of Copenhagen, and Ellen Bohr from the wealthy banking family. He had an elder sister, Jenny, and a younger brother
Harald.
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, rugb ...
who played for the
Danish national team at the
1908 Summer Olympics
The 1908 Summer Olympics (officially the Games of the IV Olympiad and also known as London 1908) were an international multi-sport event held in London, England, United Kingdom, from 27 April to 31 October 1908. The 1908 Games were o ...
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 o ...
(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 ...
.
Bohr was educated at Gammelholm Latin School, starting when he was seven. In 1903, Bohr enrolled as an undergraduate at
Copenhagen University. 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
Thorvald Nicolai Thiele (24 December 1838 – 26 September 1910) was a Danish astronomer and director of the Copenhagen Observatory. He was also an actuary and mathematician, most notable for his work in statistics, interpolation and the three-b ...
, and philosophy under Professor
Harald Høffding, 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
Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water such as razor blades and insects (e.g. water striders) t ...
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. A ...
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 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 int ...
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''.
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 degree at the highest academic level awarded following a course of study. PhDs are awarded for programs across the whole breadth of academic fields. Because it is ...
(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, 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
Hendrika Johanna van Leeuwen (July 3, 1887 – February 26, 1974) was a Dutch physicist known for her early contributions to the theory of magnetism. She studied at Leiden University under the guidance of Hendrik Antoon Lorentz, obtaining her doc ...
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. Bohr resigned his membership in the
Church of Denmark on 16 April 1912, and he and Margrethe were married in a civil ceremony at the town hall in
Slagelse on 1 August. Years later, his brother Harald 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 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 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 Emperor
* Ernest, ...
, 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 ...
for Denmark at the
1948 Summer Olympics
The 1948 Summer Olympics (officially the Games of the XIV Olympiad and also known as London 1948) were an international multi-sport event held from 29 July to 14 August 1948 in London, England, United Kingdom. Following a twelve-year hiatus ...
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 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 o ...
given by
James Jeans 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 influen ...
, and did some research on
cathode rays, but failed to impress Thomson. He had more success with younger physicists like the Australian
William Lawrence Bragg, and New Zealand's
Ernest Rutherford, whose 1911 small central nucleus
Rutherford model 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, a ...
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 ...
. Bohr received an invitation from Rutherford to conduct post-doctoral work at
Victoria University of Manchester, where Bohr met
George de Hevesy 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 a ...
(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 ...
.
Martin Knudsen 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 conf ...
'', 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'' in July, September and November of that year.
He adapted Rutherford's nuclear structure to
Max Planck's quantum theory and so created his
Bohr model 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 ...
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
In physics, a quantum (plural quanta) is the minimum amount of any physical entity ( physical property) involved in an interaction. The fundamental notion that a physical property can be "quantized" is referred to as "the hypothesis of quantizat ...
of discrete energy. This became a basis for what is now known as the
old quantum theory.
In 1885,
Johann Balmer had come up with his
Balmer series 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 ident ...
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-to ...
atom:
:
where λ is the wavelength of the absorbed or emitted light and ''R''
H is the
Rydberg constant. 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:
:
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 ever ...
(1 for hydrogen).
The model's first hurdle was the
Pickering series, 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 i ...
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 ta ...
, 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 theor ...
,
Enrico Fermi,
Max Born and
Arnold Sommerfeld
Arnold Johannes Wilhelm Sommerfeld, (; 5 December 1868 – 26 April 1951) was a German theoretical physicist who pioneered developments in atomic and quantum physics, and also educated and mentored many students for the new era of theoretic ...
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 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 King of Denmark from 1912 to his death in 1947, and the only King of Iceland as Kristján X, in the form of a personal union rather ...
, 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 biophys ...
, 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 from the Netherlands,
Oskar Klein from Sweden, George de Hevesy from Hungary,
Wojciech Rubinowicz from Poland and
Svein Rosseland
Svein Rosseland (March 31, 1894, in Kvam, Hardanger – January 19, 1985, in Bærum) was a Norwegian astrophysicist and a pioneer in the field of theoretical astrophysics.
Biography
Svein Rosseland was born in Kvam, in Hardanger, Norway. Rosse ...
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 elements posed a particular classification problem for chemists, because they were so chemically similar. An important development came in 1924 with
Wolfgang Pauli's discovery of the
Pauli exclusion principle
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 formula ...
, which put Bohr's models on a firm theoretical footing. Bohr was then able to declare that the as-yet-undiscovered element 72 was not a rare-earth element, but an element with chemical properties similar to those of
zirconium. (Elements had been predicted and discovered since 1871 by chemical properties) and Bohr was immediately challenged by the French chemist
Georges Urbain, who claimed to have discovered a rare-earth element 72, which he called "celtium". At the Institute in Copenhagen,
Dirk Coster
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, w ...
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 number
In quantum physics and chemistry, quantum numbers describe values of conserved quantities in the dynamics of a quantum system. Quantum numbers correspond to eigenvalues of operators that commute with the Hamiltonian—quantities that can ...
s.
The discovery of
Compton scattering by
Arthur Holly Compton 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 alwa ...
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 Über quantentheoretische Umdeutung kinematis ...
and Kramers to explore different mathematical models. They led to the development of
matrix mechanics, 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 and
Hans Geiger. 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 and
Samuel Goudsmit 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 w ...
to attend celebrations of the 50th anniversary of Hendrick Lorentz receiving his doctorate. When his train stopped in
Hamburg
Hamburg (, ; nds, label=Hamburg German, Low Saxon, Hamborg ), officially the Free and Hanseatic City of Hamburg (german: Freie und Hansestadt Hamburg; nds, label=Low Saxon, Friee un Hansestadt Hamborg),. is the List of cities in Germany by popul ...
, 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 ...
, 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 ...
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 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. 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 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'' 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
Matter waves are a central part of the theory of quantum mechanics, being an example of wave–particle duality. All matter exhibits wave-like behavior. For example, a beam of electrons can be diffracted just like a beam of light or a water w ...
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, presenting it using a
thought experiment where an electron was observed through a
gamma-ray microscope. Bohr was dissatisfied with Heisenberg's argument, since it required only that a measurement disturb properties that already existed, rather than the more radical idea that the electron's properties could not be discussed at all apart from the context they were measured in. In a paper presented at the
Volta Conference 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 ...
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 be abandoned, but
Enrico Fermi's hypothetical
neutrino 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 atomic nucleus, nuclei of atoms. Since protons and ...
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 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 studi ...
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 and
John Wheeler that "I have understood everything." Based on his
liquid drop model of the nucleus, Bohr concluded that it was the
uranium-235 isotope and not the more abundant
uranium-238 that was primarily responsible for fission with thermal neutrons. In April 1940,
John R. Dunning
John Ray Dunning (September 24, 1907 – August 25, 1975) was an American physicist who played key roles in the Manhattan Project that developed the first atomic bombs. He specialized in neutron physics, and did pioneering work in gaseous diffusio ...
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 philosopher
Søren Kierkegaard.
Richard Rhodes 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
''Stages on Life's Way'' ( da, Stadier på Livets Vej; historical orthography: ''Stadier paa Livets Vej'') is a philosophical work by Søren Kierkegaard written in 1845. The book was written as a continuation of Kierkegaard's prior work ''Either/O ...
'' 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
Atheism, in the broadest sense, is an absence of belief in the existence of deities. Less broadly, atheism is a rejection of the belief that any deities exist. In an even narrower sense, atheism is specifically the position that there no ...
.
[: "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 onejudge 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 argument ...
, an
instrumentalist, a phenomenological realist or some other kind of realist. Furthermore, though some have seen Bohr as being a
subjectivist or a
positivist, most philosophers agree that this is a misunderstanding of Bohr as he never argued for
verificationism or for the idea that the subject had a direct impact on the outcome of a measurement.
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 recalled
Victor Weisskopf declaring that Bohr wouldn't have said anything of the sort and exclaiming, "Shame on Aage Petersen for putting those ridiculous words in Bohr's mouth!"
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 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.
Epi ...
in which classical ideas are a priori concepts that the mind imposes on sense impressions);
Pragmatism (which focus on how human beings experientially interact with atomic systems according to their needs and interests); Darwinianism (i.e. we are adapted to use classical type concepts, which
Léon Rosenfeld said that we evolved to use); and Experimentalism (which focuses strictly on the function and outcome of experiments 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, 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,
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,
Lise Meitner, George Placzek,
Eugene Rabinowitch,
Stefan Rozental, Erich Ernst Schneider,
Edward Teller,
Arthur von Hippel and
Victor Weisskopf.
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 several ...
. The buyer later donated the two medals to the Danish Historical Museum in
Frederiksborg Castle, 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, 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, one of Heisenberg's students and friends, claimed that the main subject of the meeting was
Carl Friedrich von Weizsäcker, who had proposed trying to persuade Bohr to mediate peace between Britain and Germany.
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
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 a ...
'' explores what might have happened at the 1941 meeting between Heisenberg and Bohr. A
BBC television film version of the play was first screened on 26 September 2002, with
Stephen Rea
Stephen Rea ( ; born 31 October 1946) is an Irish film and stage actor. Rea has appeared in films such as '' V for Vendetta'', '' Michael Collins'', ''Interview with the Vampire'' and '' Breakfast on Pluto''. Rea was nominated for the Academy Aw ...
as Bohr,
Daniel Craig as Heisenberg, and
Francesca Annis as Margrethe Bohr. The same meeting had previously been dramatised by the BBC's ''
Horizon
The horizon is the apparent line that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This line divides all viewing directions based on whether i ...
'' science documentary series in 1992, with
Anthony Bate
Anthony Bate (31 August 1927 – 19 June 2012) was an English actor.
He is possibly best known for his role as Oliver Lacon in the BBC television adaptations of the John le Carré novels ''Tinker Tailor Soldier Spy'' and '' Smiley's People'' an ...
as Bohr, and Philip Anthony as Heisenberg. The meeting is also dramatized in the Norwegian/Danish/British miniseries ''
The Heavy Water War
''The Heavy Water War'' (original title ' and alternative title ''The Saboteurs'' ( UK)) is a six-episode war drama TV miniseries written by Petter S. Rosenlund and produced by Norwegian Broadcasting Corporation.
It is a Norwegian/Danish/Briti ...
''.
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
Gustaf V (Oscar Gustaf Adolf; 16 June 1858 – 29 October 1950) was King of Sweden from 8 December 1907 until his death in 1950. He was the eldest son of King Oscar II of Sweden and Sophia of Nassau, a half-sister of Adolphe, Grand Duke of Luxe ...
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
Frederick Alexander Lindemann, 1st Viscount Cherwell, ( ; 5 April 18863 July 1957) was a British physicist who was prime scientific adviser to Winston Churchill in World War II.
Lindemann was a brilliant intellectual, who cut through bureau ...
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, but for security reasons Bohr was kept out of sight. He was given an apartment at
St James's Palace
St James's Palace is the most senior royal palace in London, the capital of the United Kingdom. The palace gives its name to the Court of St James's, which is the monarch's royal court, and is located in the City of Westminster in London. Alt ...
and an office with the British
Tube Alloys 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
The Institute for Advanced Study (IAS), located in Princeton, New Jersey, in the United States, is an independent center for theoretical research and intellectual inquiry. It has served as the academic home of internationally preeminent schola ...
in
Princeton, New Jersey
Princeton is a municipality with a borough form of government in Mercer County, in the U.S. state of New Jersey. It was established on January 1, 2013, through the consolidation of the Borough of Princeton and Princeton Township, both of w ...
, and went to
Los Alamos in
New Mexico
)
, population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano)
, seat = Santa Fe, New Mexico, Santa Fe
, LargestCity = Albuquerque, New Mexico, Albuquerque
, LargestMetro = Albuquerque metropolitan area, Tiguex
, Offi ...
, 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
Moscow ( , US chiefly ; rus, links=no, Москва, r=Moskva, p=mɐskˈva, a=Москва.ogg) is the capital and largest city of Russia. The city stands on the Moskva River in Central Russia, with a population estimated at 13.0 million ...
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 Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nationa ...
. 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
The United Nations (UN) is an intergovernmental organization whose stated purposes are to maintain international peace and security, develop friendly relations among nations, achieve international cooperation, and be a centre for harmoni ...
calling for international cooperation on nuclear energy. In the 1950s, after the
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
Christian X ( da, Christian Carl Frederik Albert Alexander Vilhelm; 26 September 1870 – 20 April 1947) was King of Denmark from 1912 to his death in 1947, and the only King of Iceland as Kristján X, in the form of a personal union rather ...
, who had died in April, the new king,
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
A coat of arms is a heraldic visual design on an escutcheon (i.e., shield), surcoat, or tabard (the latter two being outer garments). The coat of arms on an escutcheon forms the central element of the full heraldic achievement, which in its ...
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 Gen ...
, 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 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
Geneva ( ; french: Genève ) frp, Genèva ; german: link=no, Genf ; it, Ginevra ; rm, Genevra is the second-most populous city in Switzerland (after Zürich) and the most populous city of Romandy, the French-speaking part of Switzerland. Situa ...
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
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 Gen ...
. 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 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 on 18 November 1962. He was cremated, and his ashes were buried in the family plot in the
Assistens Cemetery
Assistens Cemetery ( da, Assistens Kirkegård) is the name of a number of cemeteries in Denmark. The common nominator is, as the first part of the name implies (Latin: ''assistens'' meaning assisting), an assisting cemetery for a town's churches. ...
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
The Hughes Medal is awarded by the Royal Society of London "in recognition of an original discovery in the physical sciences, particularly electricity and magnetism or their applications". Named after David E. Hughes, the medal is awarded wit ...
in 1921, the
Matteucci Medal in 1923, the
Franklin Medal
The Franklin Medal was a science award presented from 1915 until 1997 by the Franklin Institute located in Philadelphia, Pennsylvania, U.S. It was founded in 1914 by Samuel Insull.
The Franklin Medal was the most prestigious of the various awar ...
in 1926, the
Copley Medal
The Copley Medal is an award given by the Royal Society, for "outstanding achievements in research in any branch of science". It alternates between the physical sciences or mathematics and the biological sciences. Given every year, the medal is t ...
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
postage stamp
A postage stamp is a small piece of paper issued by a post office, postal administration, or other authorized vendors to customers who pay postage (the cost involved in moving, insuring, or registering mail), who then affix the stamp to the f ...
depicting Bohr, the hydrogen atom and the formula for the difference of any two hydrogen energy levels:
. Several other countries have also issued postage stamps depicting Bohr. In 1997, the
Danish National Bank began circulating the
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 lunar crater and
bohrium
Bohrium is a synthetic chemical element with the symbol Bh and atomic number 107. It is named after Danish physicist Niels Bohr. As a synthetic element, it can be created in a laboratory but is not found in nature. All known isotopes of bohrium ...
, the chemical element with atomic number 107.
Bibliography
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See also
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EPR paradox
EPR may refer to:
Science and technology
* EPR (nuclear reactor), European Pressurised-Water Reactor
* EPR paradox (Einstein–Podolsky–Rosen paradox), in physics
* Earth potential rise, in electrical engineering
* East Pacific Rise, a mid-oce ...
Notes
References
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Further reading
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* Bohr's researches on reaction times.
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External links
Niels Bohr Archive*
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Author profilein the database
zbMATH
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* 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 S. Kuhn,
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 & ArchivesOral history interview transcript for Niels Bohr on 7 November 1962, American Institute of Physics, Niels Bohr Library & ArchivesOral history interview transcript for Niels Bohr on 14 November 1962, American Institute of Physics, Niels Bohr Library & ArchivesOral history interview transcript for Niels Bohr on 17 November 1962, American Institute of Physics, Niels Bohr Library & Archives
{{DEFAULTSORT:Bohr, Niels
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