Niels Bohr

• Born: 7 October 1885
• Birthplace: Copenhagen, Denmark
• Died: 18 November 1962
• Best Known As: Danish atomic physicist who won 1922's Nobel Prize

Name at birth: Niels Henrik David Bohr

Niels Bohr was the Danish physicist whose investigations of atomic structure earned him the 1922 Nobel Prize for physics. Bohr's work helped solve the problems classical physics could not explain about the nuclear model of the atom. He postulated that electrons moved in fixed orbits around the atom's nucleus, and he explained how they emitted or absorbed energy. Bohr attended the University of Copenhagen (1903-11), then studied for a time under Ernest Rutherford in Manchester, England. By 1916 he was back at the University of Copenhagen as a professor of physics, and in 1920 he became the first director of the Institute of Theoretical Physics. Bohr's Institute became a gathering place for the world's top physicists, and he is considered one of the foremost scientists of modern physics, along with Albert Einstein, Erwin Schrödinger and Enrico Fermi. During World War II Bohr avoided Adolf Hitler's army and left Denmark in 1943; he ended up in the United States and was sent to Los Alamos, New Mexico to join Robert Oppenheimer and others working to develop the atomic bomb. After the war he returned to Denmark and spent the rest of his career advocating the peaceful uses of atomic energy.

Bohr's son Aage Bohr shared the Nobel Prize for Physics in 1975... Bohrium, a chemical element in the periodic table (symbol Bh, atomic number 107), was first synthesized in 1976... One of Bohr's most famous students was Werner Heisenberg, the physicist who went on to lead Nazi Germany's attempt to develop the atomic bomb.

(1885–1962) Danish physicist. In 1913 he published his explanation of how atoms, with electrons orbiting a central nucleus, achieve stability by assuming that their angular momentum is quantized. Movement of electrons from one orbit to another is accompanied by the absorption or emission of energy in the form of light, thus accounting for the series of lines in the emission spectrum of hydrogen. For this work Bohr was awarded the 1922 Nobel Prize for physics. See Bohr theory.

For more information on Niels Henrik David Bohr, visit Britannica.com.

Danish physicist (1885–1962)

Niels Bohr came from a very distinguished scientific family in Copenhagen, Denmark. His father, Christian, was professor of physiology at Copenhagen and his brother Harald was a mathematician of great distinction. (His own son, Aage, was later to win the 1975 Nobel Prize for physics.) Bohr was educated at the University of Copenhagen where he obtained his PhD in 1911. After four productive years with Ernest Rutherford in Manchester, Bohr returned to Denmark becoming in 1918 director of the newly created Institute of Theoretical Physics.

Under Bohr (who after Albert Einstein was probably the most respected theoretical physicist of the century) the institute became one of the most exciting research centers in the world. A generation of physicists from around the world were to pass through it and eventually it was to bestow on the orthodox account of quantum theory the apt description of the ‘Copenhagen interpretation’.

In 1913 Bohr published a classic paper, On the Constitution of Atoms and Molecules, in which he used the quantum of energy, h, introduced into physics by Max Planck in 1900, to rescue Rutherford's account of atomic structure from a vital objection and also to account for the line spectrum of hydrogen. The first problem Bohr faced was to explain the stability of the atom. Rutherford's 1911 model of the atom with electrons orbiting a central nucleus (the so-called planetary model) was theoretically unstable. This was because, unlike planets orbiting the Sun, electrons are charged particles, which, according to classical physics, should radiate energy and consequently spiral in toward the nucleus.

Bohr began by assuming that there were ‘stationary’ orbits for the electrons in which the electron did not radiate energy. He further assumed that such orbits occurred when the electron had definite values of angular momentum, specifically values h/2π, 2h/2π, 3h/2π, etc., where h is Planck's constant. Using this idea he was able to calculate energies E₁, E₂, E₃, etc., for possible orbits of the electron. He further postulated that emission of light occurred when an electron moved from one orbit to a lower-energy orbit; absorption was accompanied by a change to a higher-energy orbit. In each case the energy difference produced radiation of energy hν, where ν is the frequency. In 1913 he realized that, using this idea, he could obtain a theoretical formula similar to the empirical formula of Johannes Balmer for a series of lines in the hydrogen spectrum. Bohr received the Nobel Prize for physics for this work in 1922. The Bohr theory was developed further by Arnold Sommerfeld.

Bohr also made other major contributions to this early development of quantum theory. The ‘correspondence principle’ (1916) is his principle that the quantum-theory description of the atom corresponds to classical physics at large magnitudes.

In 1927, Bohr publicly formulated the ‘complementarity principle’. This argued against continuing attempts to eliminate such supposed difficulties as the wave–particle duality of light and many other atomic phenomena. His starting point was the impossibility to distinguish satisfactorily between the actual behavior of atomic objects, and their interaction with the measuring instruments that serve to define the conditions under which the phenomena appear. Examine light with one instrument, the argument went, and it undulates like a wave; select another and it scatters like a particle. His conclusion was that evidence obtained under different experimental conditions cannot be comprehended within a single picture, but must be regarded as complementary in the sense that only the totality of the phenomenon exhausts the possible information about the objects. It was a principle Bohr remained faithful to, even representing it on his coat of arms in 1947 with the motto Contraria sunt complementa above the Yin/Yang symbols. Together with the indeterminancy principle of Werner Heisenberg and the probability waves of Max Born, this principle emerged from the 1930 Solvay conference (the last one Einstein attended) as the most authoritative and widely accepted theory to describe atomic phenomena.

Bohr also made major contributions to the work on radioactivity that led to the discovery and exploitation of nuclear fission. Bohr's liquid-drop model of the nucleus, which was published in 1936, provided the basis for the first theoretical account of fission worked out in collaboration with John Wheeler in 1939. It was also Bohr who, in 1939, made the crucial suggestion that fission was more likely to occur with the rarer isotope uranium–235 than the more common variety uranium–238.

In 1943 Bohr, who had a Jewish mother, felt it necessary to escape from occupied Denmark and eventually made his way to Los Alamos in America where he served as a consultant on the atomic bomb project. He was quick to appreciate the consequences of using such weapons and in 1944 made an early approach to Roosevelt and Churchill proposing that such obvious danger could perhaps be used to bring about a rapprochement between Russia and the West. Scientists were in a unique position, he argued, in having the Soviet contacts and the knowledge to make the first approach. Much of Bohr's time after the war was spent working, among scientists, for adequate controls of nuclear weapons and in 1955 he organized the first Atoms for Peace conference in Geneva.

Bohr was born in Copenhagen, Denmark to affluent, well‐educated parents (his father was a professor of philosophy at the University of Copenhagen). Bohr became a professor at the University of Copenhagen in 1916 and in 1920 established the Institute for Theoretical Physics there, which quickly became a world‐recognized think tank frequented by the best scientific minds of the time. From this institute a new comprehension of the physical world emerged, which would have a profound impact on the remainder of the century and beyond: the observed and the observer were seen to interact; nature was both wave and particle; and philosophy and physics shared subject matter as issues of causality and interdeterminism were raised when Bohr and others deepened their understanding of nuclear fission.

In 1939, Bohr fled Denmark, accepting an invitation from the United States to participate in the Manhattan Project in Los Alamos, New Mexico. There he was instrumental in the development of the atomic bomb but ambivalent about its use as a weapon of mass destruction. Bohr spent the remainder of his life called for international control of nuclear weapons and the peaceful use of atomic energy.

[See also Arms Control and Disarmament: Nuclear; Atomic Scientists.]

Bibliography

• Ruth Moore, Niels Bohr: The Man, His Science, and the World He Changed, 1966.
• Abraham Pais, Niels Bohr's Times: In Physics, Philosophy, and Polity, 1991

The Danish physicist Niels Henrik David Bohr (1885-1962) formulated the first successful explanation of some major lines of the hydrogen spectrum. The Bohr theory of the atom has become the foundation of modern atomic physics.

Niels Bohr was born on Oct. 7, 1885, in Copenhagen, the son of Christian Bohr and Ellen Adler Bohr. He studied physics and philosophy at the University of Copenhagen. His postgraduate work culminated in 1911 in a doctoral dissertation on the electron theory of metals.

In the same year he went to Cambridge University and worked with J. J. Thompson at the Cavendish Laboratory. By the spring of 1912 he was working with Ernest Rutherford at the University of Manchester. It was there that Bohr made some valuable suggestions about the chemical relevance of radioactive decay which proved to be most instrumental in formulating the concept of isotopes.

Secret of the Atom

Bohr's principal interest lay, however, in the planetary model of the atom, which Rutherford proposed in 1911. While pondering the implications of that model, Bohr became acquainted with Johannes Rydberg's studies of spectral lines and with J. J. Balmer's formula. As Bohr himself recalled in 1934, "As soon as I saw Balmer's formula the whole thing was immediately clear to me." The "thing" was the recognition on Bohr's part that basically different laws govern the atom when it is not in its stationary state but is absorbing or emitting radiation. He was no longer at Rutherford's laboratory when he succeeded in developing this revolutionary notion into a consistent and concise picture of the atom.

Meanwhile, in 1912 Bohr married Margrethe Norlund shortly after his return to Copenhagen, where he was appointed assistant professor at the university.

When Bohr asked Rutherford to recommend his now historic paper "On the Constitution of Atoms and Molecules" for publication, Rutherford admitted that Bohr's ideas as to the mode of origin of the spectra of hydrogen were very ingenious and worked very well, but he was unwilling to agree with Bohr's own evaluation of the paper. It took a special trip by Bohr to Rutherford in Manchester and a series of evenings during which the two carefully went over every paragraph in the paper before Rutherford's objections could be overcome. When the paper was published, in three parts in the Philosophical Magazine, June, September, and November 1913, reactions were divided. Some immediately expressed unreserved admiration, but there were doubters as well. In Einstein's eyes the paper was one of the great discoveries.

Copenhagen School

Bohr spent 2 years with Rutherford before returning to Copenhagen, where he began to think that the most effective cultivation of atomic and nuclear physics demanded a special institute, sheltering not only a well equipped laboratory, but also playing host to a large number of physicists from all over the world. In 1917 he approached the university with his plan, and as soon as the war was over the plan was enthusiastically approved. The institute was financed by public subscription, and the city donated a choice site to the Institute for Theoretical Physics, which soon established itself as the world center of theoretical physics.

Bohr's first major scientific award was the Hughes Medal of the Royal Society in 1921. The Nobel Prize followed the next year, but the finest tribute to Bohr was the steady stream of brilliant young physicists to his institute, which was dedicated on Sept. 15, 1920. Among the first to arrive at Bohr's institute was Wolfgang Pauli, and 2 years later, in 1924, came Werner Heisenberg, and shortly afterward Paul Dirac, to mention only some most important names in modern physics. In fact, there was hardly a major theoretical breakthrough in physics in the 1920s without some connection with the so-called Copenhagen school. Heisenberg's matrix mechanics, Erwin Schrödinger's wave mechanics, the demonstration of their equivalence by Max Born, Dirac, and P. Jordan, Pauli's theory of electron spin, Louis de Broglie's wave theory of matter - all entered the mainstream of physics through the animated discussions at Bohr's institute. Reminiscing on the 1920s, Bohr could rightly say that "in these years a unique cooperation of a whole generation of theoretical physicists from many countries created step by step, a logically consistent generalization of quantum mechanics and electromagnetics, and has sometimes been designated as the heroic age in quantum physics."

Principle of Complementarity

To use Bohr's own words, "a new outlook emerged," which put the comprehension of physical experience into radically new perspectives. Bohr contributed an important part to that new outlook when he outlined his principle of Complementarity in 1927. According to Bohr, waves and particles were two complementary aspects of nature which, as far as human perception and reasoning went, represented mutually irreducible aspects of nature. The wider implications of such an outlook were further articulated by Bohr in subsequent years, as he came to grips with such philosophical questions as indeterminism versus causality, and life versus mechanism.

Bohr's famous extension of the principle of complementarity to the question of life versus mechanism came in 1932 in a lecture entitled "Light and Life." In this lecture he first pointed out that an exhaustive investigation of the basic units of life was impossible because those life units would most likely be destroyed by the high-speed particles needed for their observation. For Bohr, the units of life represented irreducible entities similar to the quantum of energy. According to him, the "essential non analyzability of atomic stability in mechanical terms presents a close analogy to the impossibility of a physical or chemical explanation of the peculiar functions characteristic of life." Scientists who, because of the subsequent startling developments in molecular biology, claimed to have come to the threshold of a mechanistic explanation of life found no ally in Bohr. To the end of his life he held fast to the basic message of his nowclassic lecture, as may be seen from his essay "Light and Life Revisited," written in 1962, the year he died.

An even more fundamental aspect of the principle of complementarity was the recognition that the observer and the observed represented a continuous interaction in which the two influenced and altered one another, however slightly. This meant that the rigid line of separation between the subjective and the objective needed some modification. This also meant a radical modification of the physicist's concept of the external world. The impact of the new insight into the correlation of the objective and the subjective was enormous also on the philosophical temper of the age. It seems indeed that the enunciation of the principle of complementarity by Bohr produced an insurmountable stumbling block for a mechanistic or reductionist explanation of the realm of reality as it is conceived and experienced by man.

Compound Nucleus and the Fission Process

With the discovery of the neutron in 1932, attention rapidly turned from electrons, which form the outer part of the atom, to the nucleus. To understand the various phenomena produced when nuclei of atoms were exposed to bombardment by neutrons, physicists first turned to Bohr's atom model. There the electrons moved largely independently of one another and were subject mainly to a field of force that was the average effect of the motion and position of all of them. The great number of nuclear resonances seemed, however, to point toward a rather different situation. The recognition of this came from Bohr himself, who proposed in 1936 that the protons and neutrons in the nucleus should be considered as a strongly coupled system of particles, in a close analogy to molecules making up a drop of water. In such a system there had to be a very large number of resonance levels of energy, and it also followed that a fairly long time could elapse before the available energy would concentrate on a single neutron resulting in its emission.

This picture of the "compound nucleus" formed the basis of Bohr's other crucial contribution to nuclear physics, the analysis of the fission process. In a paper written jointly with John A. Wheeler in 1939, he showed in quantitative detail the behavior of the compound nucleus for the cases of radiation, neutron emission, and fission. On this last point their all-important contribution consisted in arguing that in the fission of uranium it was mainly the isotope U²³⁵ that produced the effect under the impact of slow neutrons. It then became immediately clear that to obtain either a large-scale or a sustained, low-rate energy process by fissioning uranium, one had to achieve a separation of U ²³⁵ in sufficient quantities from uranium ore in which the nonfissionable U ²³⁸ was predominant.

A Towering Figure

After 1939, Bohr's life was largely devoted to humanitarian efforts, such as intervening for the Danish Jews; he had to save human lives, including his own and those of his family. Moreover, he felt duty-bound to prevent science from turning into a tool of wholesale destruction. Following his escape to Sweden in September 1943, he was quickly flown to England and from there to the United States. There he lent his talents to the Manhattan Project, and during his stay at Los Alamos he did work on the initiator phase of the activation of the atomic bomb. He also began to stress the need for international control of atomic weapons and energy. His view and arguments helped shape the Acheson-Lilienthal plan and the Baruch proposals to the United Nations on behalf of the American government. In 1950 he submitted in a letter to the United Nations a plea for an "open world where each nation can assert itself solely by the extent to which it can contribute to the common culture, and is able to help others with experience and resources." In the 1950s Bohr's principal contribution to science consisted in taking a leading part in the development of the European Center for Nuclear Research (CERN). It was at his institute that the decision was made to build the 28-Bev (billion-electron-volt) accelerator near Geneva.

From 1938 until his death he was the president of the Royal Danish Academy of Sciences, acted as chairman of the Danish Atomic Energy Commission, and supervised the first phase of the Commission's program for the peaceful uses of atomic energy. Bohr's last major appearance was to deliver the Rutherford Memorial Lecture in 1961, which gave a fascinating portrayal not only of the great master but also of his equally famous disciple.

Bohr's death came rather suddenly but quietly on Nov. 18, 1962, at his home. Einstein and he were possibly the most towering and influential figures of 20th-century physics.

Further Reading

The best biography of Bohr is Ruth Moore, Niels Bohr: The Man, His Science and the World They Changed (1966). Stefan Rozental, ed., Niels Bohr: His Life and Work as Seen by His Friends and Colleagues (trans. 1967), is a most valuable collection of essays contributed by Bohr's closest friends and associates. On Bohr's role in 20th-century physics one should consult the papers written in his honor on his seventieth birthday, W. Pauli, ed., Niels Bohr and the Development of Physics (1955). See also Niels Hugh de Vaudrey Heathcote, Nobel Prize Winners in Physics, 1901-1950 (1953); Arthur March and Ira Freeman, New World of Physics (1962); and Henry A. Boorse and Lloyd Motz, ed., The World of the Atom (2 vols., 1966).

Bohr, Niels (1885-1962) Danish physicist and Nobel prizewinner, remembered philosophically as the founder of the Copenhagen interpretation of quantum theory. This antirealist interpretation of the quantum world accorded with Bohr's more general views of the perspectival and partial nature of complementary understandings of the world.

Quotes:

"An expert is someone who knows some of the worst mistakes, which can be made, in a very narrow field."

"Prediction is very difficult, especially about the future."

"If anybody says he can think about quantum physics without getting giddy, that only shows he has not understood the first thing about them."

"Technology has advanced more in the last thirty years than in the previous two thousand. The exponential increase in advancement will only continue. Anthropological Commentary The opposite of a trivial truth is false; the opposite of a great truth is also true."

"Never express yourself more clearly than you are able to think."

"The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth."

See more famous quotes by Niels Bohr

A Danish physicist of the twentieth century. Bohr was one of the founders of quantum mechanics and the originator of the Bohr atom.

Niels Bohr
Born	Niels Henrik David Bohr 7 October 1885(1885-10-07) Copenhagen, Denmark
Died	18 November 1962 (aged 77) Copenhagen, Denmark
Nationality	Denmark
Fields	Physics
Institutions	University of Copenhagen University of Manchester
Alma mater	University of Cambridge University of Copenhagen
Doctoral advisor	Christian Christiansen
Other academic advisors	J. J. Thomson Ernest Rutherford
Doctoral students	Hendrik Anthony Kramers
Known for	Copenhagen interpretation Complementarity Bohr model Sommerfeld–Bohr theory BKS theory Bohr-Einstein debates
Influences	Ernest Rutherford
Influenced	Werner Heisenberg Wolfgang Pauli Paul Dirac Lise Meitner Max Delbrück and many others
Notable awards	Nobel Prize in Physics (1922)
Signature
Notes Harald Bohr is his younger brother, and Aage Bohr is his son.

Niels Henrik David Bohr (Danish pronunciation: [nels ˈb̥oɐ̯ˀ]; 7 October 1885 – 18 November 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr mentored and collaborated with many of the top physicists of the century at his institute in Copenhagen. He was part of a team of physicists working on the Manhattan Project. Bohr married Margrethe Nørlund in 1912, and one of their sons, Aage Bohr, grew up to be an important physicist who in 1975 also received the Nobel prize. Bohr has been described as one of the most influential physicists of the 20th century.^[1]

Contents 1 Biography 1.1 Early years 1.2 Physics 1.3 Atomic research 2 Contributions to physics 3 Kierkegaard's influence on Bohr 4 Relationship with Heisenberg 4.1 Tube Alloys 4.2 Speculation 5 Open World 6 Legacy 7 Further reading 7.1 Primary 7.2 Secondary 8 References 9 External links

Biography

Early years

Bohr was born in Copenhagen, Denmark, in 1885. His father, Christian Bohr, a devout Lutheran, was professor of physiology at the University of Copenhagen (it is his name which is given to the Bohr shift or Bohr effect), while his mother, Ellen Adler Bohr, came from a wealthy Jewish family prominent in Danish banking and parliamentary circles. His brother was Harald Bohr, a mathematician and Olympic footballer who played on the Danish national team. Niels Bohr was a passionate footballer as well, and the two brothers played a number of matches for the Copenhagen-based Akademisk Boldklub, with Niels in goal. There is, however, no truth in the oft-repeated claim that Niels Bohr emulated his brother Harald by playing for the Danish national team ^[2].

In 1903 Bohr enrolled as an undergraduate at Copenhagen University, initially studying philosophy and mathematics. In 1905, prompted by a gold medal competition sponsored by the Royal Danish Academy of Sciences and Letters, he conducted a series of experiments to examine the properties of surface tension, using his father's laboratory in the university, familiar to him from assisting there since childhood. His essay won the prize, and it was this success that decided Bohr to abandon philosophy and adopt physics.^[3] As a student under Christian Christiansen he received his doctorate in 1911. As a post-doctoral student, Bohr first conducted experiments under J. J. Thomson at Trinity College, Cambridge. He then went on to study under Ernest Rutherford at the University of Manchester in England. On the basis of Rutherford's theories, Bohr published his model of atomic structure in 1913, introducing the theory of electrons traveling in orbits around the atom's nucleus, the chemical properties of the element being largely determined by the number of electrons in the outer orbits. Bohr also introduced the idea that an electron could drop from a higher-energy orbit to a lower one, emitting a photon (light quantum) of discrete energy. This became a basis for quantum theory.

Niels Bohr and his wife Margrethe Nørlund Bohr had six sons. Their oldest died in a tragic boating accident and another died from childhood meningitis. The others went on to lead successful lives, including Aage Bohr, who became a very successful physicist and, like his father, won a Nobel Prize in physics, in 1975.

Physics

In 1916, Niels Bohr became a professor at the University of Copenhagen. With the assistance of the Danish government and the Carlsberg Foundation, he succeeded in founding the Institute of Theoretical Physics in 1921, of which he became its director.^[4] In 1922, Bohr was awarded the Nobel Prize in physics "for his services in the investigation of the structure of atoms and of the radiation emanating from them." Bohr's institute served as a focal point for theoretical physicists in the 1920s and '30s, and most of the world's best known theoretical physicists of that period spent some time there.

Niels Bohr as a young man. Exact date of photo not known.

Niels Bohr and Albert Einstein debating quantum theory at Paul Ehrenfest's home in Leiden (December 1925).

Bohr also conceived the principle of complementarity: that items could be separately analyzed as having several contradictory properties. For example, physicists currently conclude that light behaves either as a wave or a stream of particles depending on the experimental framework — two apparently mutually exclusive properties — on the basis of this principle. Bohr also found philosophical applications for this daringly original principle.^[specify] Albert Einstein much preferred the determinism of classical physics over the probabilistic new quantum physics (to which Max Planck and Einstein himself had contributed). He and Bohr had good-natured arguments over the truth of this principle throughout their lives (see Bohr–Einstein debates).

Werner Heisenberg worked as an assistant to Bohr and university lecturer in Copenhagen from 1926 to 1927. It was in Copenhagen, in 1927, that Heisenberg developed his uncertainty principle, while working on the mathematical foundations of quantum mechanics. Heisenberg was later to be head of the German atomic bomb project. In 1941, during the German occupation of Denmark in World War II, Bohr was visited by Heisenberg in Copenhagen (see section below). In 1943, shortly before he was to be arrested by the German police, Bohr escaped to Sweden, and then traveled to London.

Atomic research

Niels Bohr worked at the top-secret Los Alamos laboratory in New Mexico, U.S., on the Manhattan Project, where he was known by the assumed name of Nicholas Baker for security reasons.^[5] His role in the project was important and was a knowledgeable consultant or "father confessor" on the project. He was concerned about a nuclear arms race, and is quoted as saying, "That is why I went to America. They didn't need my help in making the atom bomb."^[6]

Bohr believed that atomic secrets should be shared by the international scientific community. After meeting with Bohr, J. Robert Oppenheimer suggested Bohr visit President Franklin D. Roosevelt to convince him that the Manhattan Project should be shared with the Russians in the hope of speeding up its results. Roosevelt suggested Bohr return to the United Kingdom to try to win British approval. Winston 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."^[7]

After the war Bohr returned to Copenhagen, advocating the peaceful use of nuclear energy. When awarded the Order of the Elephant by the Danish government, he designed his own coat of arms which featured a taijitu (symbol of yin and yang) and the Latin motto contraria sunt complementa: opposites are complementary.^[8] He died in Copenhagen in 1962 of heart failure.^[9] He is buried in the Assistens Kirkegård in the Nørrebro section of Copenhagen.

Contributions to physics

• The Bohr model of the atom, the theory that electrons travel in discrete orbits around the atom's nucleus.
• The shell model of the atom, where the chemical properties of an element are determined by the electrons in the outermost orbit.
• The correspondence principle, the basic tool of Old quantum theory.
• The liquid drop model of the atomic nucleus.
• Identified the isotope of uranium that was responsible for slow-neutron fission - ²³⁵U.^[10]
• Much work on the Copenhagen interpretation of quantum mechanics.
• The principle of complementarity: that items could be separately analyzed as having several contradictory properties.

Kierkegaard's influence on Bohr

It is generally accepted that Bohr read the 19th century Danish philosopher Søren Kierkegaard. Richard Rhodes argues in The Making of the Atomic Bomb that Bohr was influenced by Kierkegaard via the philosopher Harald Høffding, who was strongly influenced by Kierkegaard and who was an old friend of Bohr's father. 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 ideas]."^[11]

Given this, there has been some dispute over whether Kierkegaard influenced Bohr's philosophy and science. David Favrholdt^[12] argues that Kierkegaard had minimal influence over Bohr's work; taking Bohr's statement about disagreeing with Kierkegaard at face value, while Jan Faye^[13] endorses the opposing point of view by arguing that one can disagree with the content of a theory while accepting its general premises and structure.^[14]

Relationship with Heisenberg

Bohr and Werner Heisenberg enjoyed a strong mentor/protégé relationship up to the onset of World War II. Heisenberg had made Bohr aware of his talent during a lecture in 1922 in Göttingen. During the mid-1920s, Heisenberg worked with Bohr at the institute in Copenhagen. Heisenberg, like most of Bohr's assistants, learned Danish. Heisenberg's uncertainty principle was developed during this period, as was Bohr's complementarity principle.

By the time of World War II, the relationship became strained; this was in part because Bohr, with his partially-Jewish heritage, remained in occupied Denmark, while Heisenberg remained in Germany and became head of the German nuclear effort. Heisenberg made a famous visit to Bohr in September 1941 and during a private moment it seems that he began to address nuclear energy and morality as well as the war. Neither Bohr nor Heisenberg spoke about it in any detail or left written records of this part of the meeting and they were alone and outside.^[15] Bohr seems to have reacted by terminating that conversation abruptly while not giving Heisenberg hints in any direction.

While some suggest that the relationship became strained at this meeting, other evidence shows that the level of contact had been reduced considerably for some time already. Heisenberg suggested that the fracture occurred later. In correspondence to his wife, Heisenberg described the final visit of the trip: "Today I was once more, with Weizsaecker, at Bohr's. In many ways this was especially nice, the conversation revolved for a large part of the evening around purely human concerns, Bohr was reading aloud, I played a Mozart Sonata (A-Major)."^[16] Ivan Supek, one of Heisenberg's students and friends, claimed that the main figure of the meeting was actually Weizsäcker who tried to persuade Bohr to mediate peace between Great Britain and Germany.^[17]

Tube Alloys

"Tube Alloys" was the code-name for the British nuclear weapon program. British intelligence inquired about Bohr's availability for work or insights of particular value. Bohr's reply made it clear that he could not help. This reply, like his reaction to Heisenberg, made sure that if Gestapo intercepted anything attributed to Bohr it would point to no knowledge regarding nuclear energy as it stood in 1941. This does not exclude the possibility that Bohr privately made calculations going further than his work in 1939 with Wheeler.

After leaving Denmark in the dramatic day and night (October 1943) when most Jews were able to escape to Sweden due to exceptional circumstances (see Rescue of the Danish Jews), Bohr was quickly asked again to join the British effort and he was flown to the UK. He was evacuated from Stockholm in 1943 in an unarmed De Havilland Mosquito operated by British Overseas Airways Corporation (BOAC). Passengers on BOAC's Mosquitos were carried in an improvised cabin in the bomb bay. The flight almost ended in tragedy as Bohr did not don his oxygen equipment as instructed and passed out at high altitude. He would have died had not the pilot surmising from Bohr's lack of response to intercom communication that he had lost consciousness, descended to a lower altitude for the remainder of the flight. Bohr's comment was that he had slept like a baby for the entire flight.

As part of the UK team on "Tube Alloys" Bohr went to Los Alamos. Oppenheimer credited Bohr warmly for his guiding help during certain discussions among scientists there. Discreetly, he met President Franklin D. Roosevelt and later Winston Churchill to warn against the perilous perspectives that would follow from separate development of nuclear weapons by several powers rather than some form of controlled sharing of the knowledge, which would spread quickly in any case. Only in the 1950s after the immense surprise that the Soviets developed the weapons independently, was it possible to create the International Atomic Energy Agency along the lines of Bohr's suggestion.

Speculation

In 1957, while the author Robert Jungk was working on the book Brighter Than a Thousand Suns, Heisenberg wrote to Jungk explaining that he had visited Copenhagen to communicate to Bohr his view that scientists on either side should help prevent development of the atomic bomb, that the German attempts were entirely focused on energy production and that Heisenberg's circle of colleagues tried to keep it that way.^[18] Heisenberg acknowledged that his cryptic approach of the subject had so alarmed Bohr that the discussion failed. Heisenberg nuanced his claims and avoided the implication that he and his colleagues had sabotaged the bomb effort; this nuance was lost in Jungk's original publication of the book, which implied that the German atomic bomb project was obstructed by Heisenberg.

When Bohr saw Jungk's erroneous depiction in the Danish translation of the book, he disagreed. He drafted (but never sent) a letter to Heisenberg, stating that while Heisenberg had indeed discussed the subject of nuclear weapons in Copenhagen, Heisenberg had never alluded to the fact that he might be resisting efforts to build such weapons. Bohr dismissed the idea of any pact as hindsight.^[19]

Michael Frayn's play Copenhagen, which was performed in London (for five years), Copenhagen, Gothenburg, Rome, Athens, Geneva and on Broadway in New York, explores what might have happened at the 1941 meeting between Heisenberg and Bohr. Frayn points in particular to the onus of being one of the few to understand what it would mean to create a nuclear weapon.

Open World

Bohr advocated informing the Soviet authorities that the atomic bomb would soon be in use. In 1944 he obtained an audience with Winston Churchill, who became worried about whether Bohr was a security risk.^[20] In 1950 he addressed an 'Open Letter' to the United Nations.^[21]

Legacy

• Received the first ever Atoms for Peace Award in 1957.
• In 1965, three years after Bohr's death, the Institute of Physics at the University of Copenhagen changed its name to the Niels Bohr Institute.
• The Bohr models semicentennial was commemorated in Denmark on 21 November 1963 with a postage stamp depicting Bohr, the hydrogen atom and the formula for the difference of any two hydrogen energy levels: .
• Bohrium (a chemical element, atomic number 107) is named in honour of Niels Bohr.
• Hafnium, another chemical element, whose properties were predicted by Niels Bohr, was named by him after Hafnia, Copenhagen's Latin name.
• Asteroid 3948 Bohr is named after him.
• The Centennial of Bohr's birth was commemorated in Denmark on 3 October 1985 with a postage stamp depicting Bohr with his wife Margrethe.
• In 1997 the Danish National Bank started circulating the danish five-hundred-kroner bill with the portrait of Niels Bohr smoking a pipe.^[22]

Further reading

Primary

• Niels Bohr (1913). "On the Constitution of Atoms and Molecules, Part I". Philosophical Magazine 26: 1–24.  The landmark paper laying the Bohr model of the atom and molecular bonding.
• 1999. Causality and Complementarity: Epistemological Lessons of Studies in Atomic Physics. Ox Bow Press. ISBN 1-881987-13-2. The 1949–50 Gifford lectures.
• 1987 (1958). Atomic Physics and Human Knowledge. Ox Bow Press. ISBN 0-91802452-8. Seven essays written 1933-57. 1958 ed., Wiley Interscience.

Secondary

• Niels Bohr Collected Works 13-Volume Limited Edition Set, General Editor, Finn Aaserud; ISBN 978-0-444-53286-2.
• Niels Bohr: The Man, His Science, and the World They Changed, by Ruth Moore; ISBN 0-262-63101-6.
• Niels Bohr's Times, In Physics, Philosophy and Polity, by Abraham Pais; ISBN 0-19-852049-2.
• Niels Bohr: His Life and Work As Seen by His Friends and Colleagues, edited by Stefan Rozental, John Wiley & Sons, 1964.
• Suspended In Language: Niels Bohr's Life, Discoveries, And The Century He Shaped by Jim Ottaviani (graphic novel); ISBN 0-9660106-5-5.
• Harmony and Unity : The Life of Niel's Bohr, by Niels Blaedel; ISBN 0-910239-14-2.
• Niels Bohr: A Centenary Volume, edited by A. P French and P.J. Kennedy. ISBN 0-674-62415-7.
• Copenhagen Michael Frayn ISBN 0 413 72490 5.
• Faust in Copenhagen: A Struggle for the Soul of Physics by Gino Segre; ISBN 0-670-03858-X.

References

1. ^ Murdoch, Dugald (2000) "Bohr" in Newton-Smith, N. H. (ed.) A Companion to the Philosophy of Science. Great Britain: Blackwell Publishers, p. 26. ISBN 0-631-23020-3.
2. ^ http://www.guardian.co.uk/football/2005/jul/27/theknowledge.panathinaikos Bohr's footballing career
3. ^ Rhodes, Richard (1986). The Making of the Atomic Bomb. New York: Simon and Schuster. pp. 62–63. ISBN 0-671-44133-7. 
4. ^ Finn Aaserud. "History of the institute: The establishment of an institute". Niels Bohr Institute. http://www.nbi.ku.dk/english/about/history/. Retrieved 2008-05-11. 
5. ^ Pais, Abraham. Niels Bohr's Times, In Physics, Philosophy and Polity p.496
6. ^ Long, Doug. "Niels Bohr - The Atomic Bomb and beyond". Hiroshima - was it necessary?. http://www.doug-long.com/bohr.htm. Retrieved 2006-12-21. 
7. ^ Rhodes (1986:528-538)
8. ^ "Bohr crest". University of Copenhagen. 1947-10-17. http://www.nbi.dk/hehi/logo/bohr_crest.png. Retrieved 2007-03-16. 
9. ^ Pais, Abraham. Niels Bohr's Times, In Physics, Philosophy and Polity p.529
10. ^ Rhodes (1986:282-88)
11. ^ Register, Bryan (1997-12-01). "Complementarity: Content, Context and Critique". http://enlightenment.supersaturated.com/essays/text/bryanregister/bohr_compliementarity.html. Retrieved 2006-12-21. 
12. ^ Favrholdt, David. Niels Bohr’s Philosophical Background. Copenhagen: Munksgaard (1992): pp. 42-63.
13. ^ Faye, Jan. "Niels Bohr: His Heritage and Legacy." Dordrecht: Kluwer Academic Publishers (1991).
14. ^ Mark Richardson, et al. Religion & Science: History, Method, Dialogue. Routledge 1996, pg.289
15. ^ Heisenberg, Elisabeth (1984). Inner Exile: Recollections of a Life With Werner Heisenberg. Boston MA: Birkhauser. p. 77 et seq. ISBN 0817631461. 
16. ^ Heisenberg, Werner. "Letter from Werner Heisenberg to his wife Elisabeth written during his 1941 visit in Copenhagen". Heisenberg, Jochen. http://werner-heisenberg.unh.edu/copenhagen.htm. Retrieved 2006-12-21. 
17. ^ Jutarnji list. "A March 2006 interview with Ivan Supek relating to 1941 Bohr - Heisenberg meeting (Croatian)". Jutarnji list. http://jutarnji.hr/clanak/art-2006,3,19,supek_intervju,17440.jl?artpg=1. Retrieved 2007-08-13. 
18. ^ Heisenberg, Werner. "Letter From Werner Heisenberg to Author Robert Jungk". The Manhattan Project Heritage Preservation Association, Inc.. http://www.childrenofthemanhattanproject.org/MP_Misc/Bohr_Heisenberg/bohr_2.htm. Retrieved 2006-12-21. 
19. ^ Aaserud, Finn (2002-02-06). "Release of documents relating to 1941 Bohr-Heisenberg meeting". Niels Bohr Archive. http://www.nba.nbi.dk/release.html. Retrieved 2007-06-04. 
20. ^ Niels Bohr’s mission for an ‘open world’ accessed 15 November 2008
21. ^ To the United Nations by Niels Bohr accessed 15 November 2008
22. ^ "The coins and banknotes of Denmark". National Bank of Denmark. 2005. p. 20. http://www.nationalbanken.dk/C1256BE900406EF3/sysOakFil/Danmarks_penge_2005_ENG/$File/Coins_Banknotes.pdf. Retrieved 2008-05-03. 

External links

Wikimedia Commons has media related to: Niels Henrik David Bohr

Wikiquote has a collection of quotations related to: Niels Bohr

• Niels Bohr Collected Works.
• Niels Bohr on the 500 Danish Kroner banknote.
• Encyclopaedia Britannica article on Niels Bohr
• Niels Bohr Archive
• Nobel Foundation: Niels Bohr
• Annotated bibliography for Niels Bohr from the Alsos Digital Library for Nuclear Issues
• Quantum Chemistry I Lecture - Bohr's Model of the Atom
• The Bohr-Heisenberg meeting in September 1941
• Werner Heisenberg Ausstellung: Vom Frieden zum Krieg: Kernphysik und Kernenergie
• Oral History interview transcript with Niels Bohr 31 October 1962, American Institute of Physics, Niels Bohr Library and Archives
• Short video documentary about Niels Bohr
• The gunfighter's dilemma BBC News story about Bohr's researches on reaction times.

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Persondata
NAME	Bohr, Niels Henrik David
ALTERNATIVE NAMES	Bohr, Niels
SHORT DESCRIPTION	Danish physicist
DATE OF BIRTH	7 October 1885
PLACE OF BIRTH	Copenhagen, Denmark
DATE OF DEATH	18 November 1962
PLACE OF DEATH	Copenhagen, Denmark

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