Niels Bohr
Niels (Henrik David) Bohr (
October 7,
1885 –
November 18,
1962) was a
Danish physicist who made essential contributions to understanding
atomic structure and
quantum mechanics.
*
Bohr's model*The electron's orbital angular momentum is quantized; L=nħ.
*The theory that
electrons travel in discrete
orbits around the atom's
nucleus, with the chemical properties of the element being largely determined by the number of electrons in each of the outer orbits.
*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 the basis for
quantum theory).
*Much work on the
Copenhagen interpretation of
quantum mechanics.
*The principle of
complementarity: that items could be separately analyzed as having several contradictory properties.He received the Nobel Prize for Physics for this work in 1922.
Early years
Bohr was born in
Copenhagen in
1885. His father,
Christian Bohr, was
professor of
physiology at the
University of Copenhagen, while his mother, Ellen Adler Bohr, came from a wealthy
Sephardic Jewish family prominent in
Danish banking and parliamentary circles. His brother was
Harald Bohr, a
mathematician and
Olympic soccer player who played in the Danish national team; Niels Bohr was a passionate soccer player as well, and the two brothers played a number of matches for
Akademisk Boldklub.
Bohr received his doctorate from
Copenhagen University in
1911. He then studied under
Ernest Rutherford in the
Victoria 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.
Middle years
In
1916, Niels Bohr became a
professor at the
University of Copenhagen, and director of the newly constructed "Institute of Theoretical Physics" in
1920. 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.
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 is both a wave and a stream of particles â€" two apparently mutually exclusive properties â€" on the basis of this principle. Bohr also found
philosophical applications for this daringly original principle.
Albert Einstein much preferred the determinism of classical physics over the probabilistic new physics of Bohr (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 debate). One of Bohr's most famous students was
Werner Heisenberg, a crucial figure in the development of
quantum mechanics, who was also head of the German atomic bomb project.
Niels Bohr and his wife Margrethe had six children. Two died young, and most of the others went on to lead successful lives. One,
Aage Niels Bohr, also became a very successful physicist; like his father, he won a
Nobel Prize.
Later years, Death, and Legacy
In
1941, during the
German occupation of Denmark in
World War II, Bohr was visited by Heisenberg in Copenhagen (see next section). In
1943, shortly before he was to be arrested by the German police, Bohr escaped to
Sweden, and then traveled to
London.
He worked at the secret
Los Alamos laboratory in
New Mexico,
USA, on the
Manhattan Project, where, according to
Richard Feynman, he was known by the assumed name of
Nicholas Baker for security reasons. His role in the project was important. He was seen as 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." [
1].
Bohr believed atomic secrets should be shared by the international scientific community. After meeting with Bohr,
J. Robert Oppenheimer suggested Bohr visit
President Franklin 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 England to try to win British approval.
Churchill opposed the idea.
After the war he returned to Copenhagen, advocating the peaceful use of
nuclear energy. He died in Copenhagen in 1962. He is buried in the
Assistens Kirkegård in the
Nørrebro section of Copenhagen.
The element
bohrium is named in his honor. He is pictured on the 500 kr. Danish bank note.In 1965, three years afters Bohr's death, the institute of physics at the university of Copenhagen changed name to the
Niels Bohr Institute.
It is generally accepted that Bohr read the
19th century Danish philosopher
Søren Kierkegaard. 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; 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]". [
2]
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 argues the opposing point of view; by arguing that one can disagree with the content of a theory while accepting its general premise and structure.
Bohr and
Heisenberg enjoyed a strong mentor/mentee relationship up to the onset of World War II. At that point, the relationship became somewhat strained because Bohr, with his Jewish heritage, remained in occupied Denmark, while Heisenberg remained in Germany. Heisenberg made a now-famous visit to Bohr in September 1941, and during a private moment, began to discuss nuclear weapons and the war efforts.
Michael Frayn's play
Copenhagen, which ran on
Broadway for a time, explores what might have happened at the 1941 meeting between Heisenberg and Bohr. The truth of the historical event is still a matter of scholarly debate, as neither Bohr nor
Heisenberg spoke about it in any detail, and they were alone in the woods. While some suggest that the relationship became somewhat strained at this meeting, other evidence suggests that the fracture occurred much 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)." [
3]
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 neither side should help develop 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. [
4] However, Heisenberg acknowledged that his cryptic approach of the subject had so-alarmed Bohr that the discussion failed. Heisenberg nuanced his claims, though, and avoided implication that he and his colleagues had purposely sabotaged the bomb effort. However, this nuance was lost in Jungk's original publication of the book, which strongly implied that the German atomic bomb project was rendered purposely stillborn by Heisenberg.
When Bohr saw this erroneous depiction in the Danish translation of Jungk's book, he disagreed wholeheartedly. He said that while Heisenberg had indeed discussed the subject of nuclear weapons in Copenhagen that Heisenberg never alluded to the fact that Heisenberg might be resisting efforts to build such weapons. He dismissed the idea of any pact as an after-the-fact construction. He drafted several letters to inform Heisenberg about this but never sent any of them.[
5]
"And anyone who thinks they can talk about quantum theory without feeling dizzy hasn't yet understood the first thing about it.""If quantum mechanics hasn't profoundly shocked you, you haven't understood it yet.""Nothing exist until it is measured.""A triviality is a statement whose opposite is false. However, a great truth is a statement whose opposite may well be another great truth.""Your theory is crazy, but it's not crazy enough to be true.""How wonderful that we have met with a paradox. Now we have some hope of making progress!""Einstein, stop telling God what to do." Sometimes quoted including:
"...with his dice."*Alternate version:
"Don't you think caution is needed when using ordinary language to ascribe atributes to God?""The complement of truth is clearness.""It is very difficult to make an accurate prediction, especially about the future.""An expert is a person who has made all the mistakes that can be made in a very narrow field." "Never talk faster than you think."*On
The Simpsons episode "
I Am Furious Yellow," when "When Dinosaurs Get Drunk" was cancelled, it was replaced by "The Boring World of Niels Bohr" much to Homer's discontent.
*A humorous anecdote has been attributed to Bohr in recent years. It is said that as a student at the University of Copenhagen, Bohr answered an exam question on how to measure the height of a tall building with a
barometer, "You tie a long piece of string to the neck of the barometer, then lower the barometer from the roof of the skyscraper to the ground. The length of the string plus the length of the barometer will equal the height of the building." He appealed his failing grade, and an independent arbiter called Bohr in to give a verbal answer demonstrating familiarity with physics. He reportedly suggested dropping the barometer from the roof and measuring the time it takes to reach the ground, measuring the shadows of the barometer and the building, swinging the barometer on a string like a pendulum at the top and bottom of the building and measuring the difference in the gravitational restoring force, walking up an emergency staircase and measuring the building in barometer lengths, using the barometer to measure the air pressure at the roof and at the ground ("if you want to be boring and orthodox"), and paying a janitor one barometer to reveal the height of the building.
*
Niels Bohr: The Man, His Science, and the World They Changed, by Ruth Moore; ISBN 0262631016
*
Niels Bohr's Times, In Physics, Philosophy and Polity, by
Abraham Pais; ISBN 0-19-852049-2
*
Suspended In Language: Niels Bohr's Life, Discoveries, And The Century He Shaped by Jim Ottaviani (graphic novel); ISBN 0966010655
*
Harmony and Unity : The Life of Niel's Bohr, by Niels Blaedel; ISBN 0-910239-14-2
* Richard Rhodes, The Making of the Atomic Bomb, (New York: Simon & Schuster, 1986.), pgs. 528-531.
* Mark Richardson, et al. Religion & Science: History, Method, Dialogue. Routledge 1996, pg.289
*
Bohrium (a chemical element, atomic number 107) is named in honour of Niels Bohr.
*
Asteroid 3948 Bohr is named after him.
*
Bohr's model* When awarded the
Order of the Elephant by the Danish government, he designed his own
coat of arms which featured a
yin-yang.
*
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
