Niels Bohr

Niels Henrik David Bohr (Danish pronunciation: [nels ˈboɐ̯ˀ]; 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 scientists of the 20th century.

Niels Bohr
Born	Niels Henrik David Bohr (1885-10-07)7 October 1885 Copenhagen, Denmark
Died	18 November 1962(1962-11-18) (aged 77) Copenhagen, Denmark
Nationality	Danish
Alma mater	University of Copenhagen
Known for	Copenhagen interpretation Complementarity Bohr model Sommerfeld–Bohr theory BKS theory Bohr-Einstein debates Bohr magneton
Awards	Nobel Prize in Physics (1922) Franklin Medal (1929)
Scientific career
Fields	Physics
Institutions	University of Copenhagen University of Cambridge University of Manchester
Doctoral advisor	Christian Christiansen
Other academic advisors	J. J. Thomson Ernest Rutherford
Doctoral students	Hendrik Anthony Kramers
Signature
Notes
Harald Bohr is his younger brother, and Aage Bohr is his son.

Biography

Early years

Bohr was born in Copenhagen, Denmark, in 1885. His father, Christian Bohr, 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.

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. 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. In 1912 he joined Ernest Rutherford at Manchester University and he adapted Rutherford's nuclear structure to Max Planck's quantum theory and so obtained a theory of atomic structure which, with later improvements, mainly as a result of Heisenberg's concepts, remains valid to this day. 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 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. 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.

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, 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 director. 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 unknown.

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 found philosophical applications for this daringly original principle. 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 later became 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

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. His role in the project was important as he 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."

Coat of arms

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. He died in Copenhagen in 1962 of heart failure. He is buried in the Assistens Kirkegård in the Nørrebro section of Copenhagen.

Political activity

As regards the Occupation of Denmark during World War II, and especially the events surrounding the Danish policy of cooperation with Nazi Germany and the treatment of Danish Jews, most Danish archives remained sealed until 1998, many remain sealed to this day, and opinions of Danish historical scholars and politicians on these dark topics remain deeply divided. So too, scholarly opinions remain divided on the importance of the political activities of Niels Bohr during this period. All sources agree that almost as soon as Hitler had taken power in Germany, Bohr played an active role in rescuing Jewish physicists out of Germany, typically offering them haven in Copenhagen before they could take up permanent residence elsewhere. As for Sweden during World War II and especially in the autumn of 1943, it was far from certain that they would accept Danish Jews attempting to escape Hitler's deportation order. As related by Bohr's friend Stefan Rozental and the historian Richard Rhodes (sources cited in note 11), Bohr was immediately smuggled out of Denmark in order to secure his services for the Manhattan project. But rather than proceeding promptly to the United States, as had been planned for him, on 30 September 1943 Bohr persuaded King Gustav of Sweden to make public Sweden's willingness to provide asylum, on 2 October 1943 Swedish radio broadcast that Sweden was ready to offer asylum, and there followed quickly thereafter the mass rescue of the Danish Jews by their countrymen. Historians are divided not on Bohr's political actions in Sweden, but rather on the implications and impacts of those actions. Some argue that Bohr was among those rescued and therefore could have played no role in facilitating the mass rescue, whereas Richard Rhodes and others (note 11) interpret Bohr's political action in Sweden as being a decisive event without which that mass rescue could not have occurred. Whether or not the mass rescue could have happened without Bohr's political activity in Sweden, there is no doubt that he did all that he could for his countrymen.

Contributions to Physics and Chemistry

• 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.
• 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 Christian existentialist 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]."

Given this, there has been some dispute over whether Kierkegaard influenced Bohr's philosophy and science. David Favrholdt argues that Kierkegaard had minimal influence over Bohr's work, taking Bohr's statement about disagreeing with Kierkegaard at face value; while Jan Faye 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.

Relationship with Heisenberg

Bohr and Werner Heisenberg enjoyed a strong mentor/protégé relationship up to the onset of World War II. Bohr became aware of Heisenberg's talent during a lecture Heisenberg gave in Göttingen in 1922. 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, who was of half-Jewish ancestry, 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. 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 Weizsäcker, 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)." 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.

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 Soviet Union's first nuclear weapon test, 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 (See letter page 100 in this book), Heisenberg wrote to Jungk explaining that he had visited Copenhagen to communicate to Bohr the views of the german scientists, that production of an atomic weapon was possible with great efforts and this raised enormous responsibilities on the worlds scientists on either side. But Bohr was chocked by the news, that Heisenberg was involved in such studies. And the conversation went wrong because of this. The German attempts were mostly focused on energy production and Heisenberg's circle of colleagues tried probably to keep it that way. 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. The production of the bomb in USA showed that it craved enormous ressources, which was never considered in this scale in Germany during the war.

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 he never understood the purpose of Heisenbergs visit. And was chocked by Heisenbergs opinion, that Germany would win the war. And atomic weapons could be decisive in a long war. Bohr had described the possibility of an atomic weapon in a lecture in England 1939. .

Michael Frayn's play Copenhagen (1998), 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. In 1950 he addressed an 'Open Letter' to the United Nations.

Legacy

• He was one of the founding fathers of CERN in 1954.
• 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: $h\nu =\epsilon _{2}-\epsilon _{1}\,$.
• Bohrium (a chemical element, atomic number 107) is named in honour of Bohr.
• Hafnium, another chemical element, whose properties were predicted by 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 500-krone banknote with the portrait of Bohr smoking a pipe.
• Bohr has been a common name in Europe since the Middle Ages. It remains fairly common in Europe and spread to the U.S. with pilgrims named Bohr settling there. There was an notable increase in the middle name Bohr throughout Europe and America following Bohr's death.
• Bohr was referenced in The Simpsons thirteenth season episode entitled I Am Furious Yellow. In the episode, Homer Simpson is looking forward to watching a television show called When Dinosaurs Get Drunk, when it is suddenly announced that it will be replaced with another called The Boring World of Niels Bohr. The opening image is of the scientist and an expanded version of the Bohr model of the atom.

See also

• List of Jewish Nobel laureates

Further reading

Niels Bohr (1913). "On the Constitution of Atoms and Molecules, Part I" (PDF). 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.
• 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 0413724905
• Faust in Copenhagen: A Struggle for the Soul of Physics by Gino Segre; ISBN 0-670-03858-X.

External links

• 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.