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Niels Bohr was a Danish physicist who made fundamental contributions to understanding the structure of atoms and to the early development of quantum mechanics. In particular, he developed the Bohr model of the atom (and later the “liquid drop” model) and the principles of correspondence and complementarity. He mentored and collaborated with many of the top physicists of the century at his institute in Copenhagen, where he and Werner Heisenberg developed the “Copenhagen interpretation” of quantum theory. He is recognized as one of the most influential physicists of the 20th Century, and received the Nobel Prize in Physics in 1922 “for his services in the investigation of the structure of atoms and of the radiation emanating from them”. Niels Henrik David Bohr was born in Copenhagen, Denmark on 7 October 1885. His father was a devout Lutheran and a respected professor of physiology at the University of Copenhagen; his mother came from a prominent and wealthy Jewish family of bankers and parliamentarians. Niels’ younger brother, Harald, became a brilliant mathematician as well as an international footballer. Bohr enrolled as an undergraduate at Copenhagen University in 1903, initially studying philosophy and mathematics, but switching to physics in 1905 after winning an essay competition with a report on the properties of surface tension. He completed his doctorate in 1911, under the physicist Christian Christiansen. For his post-doctoral studies, Bohr moved to England, first conducting experiments at Trinity College, Cambridge under J. J. Thomson (the discoverer of the electron), and then at the University of Manchester under Ernest Rutherford (the discoverer of the nucleus and the structure of atoms). On returning to Copenhagen from Manchester in 1912, Bohr took up a position as assistant professor at the University of Copenhagen, and also married Margrethe Nørlund. The couple were to have six children: two died young, but the others went on to lead successful lives, with one, Aage Niels Bohr, also becoming a very successful physicist like his father, winning the Nobel Prize in Physics in 1975. In 1913, on the basis of Rutherford's theories, Bohr developed and published his model of atomic structure, known as the Bohr model, which depicts the atom as a small, positively-charged nucleus surrounded by negatively-charged electrons that travel in circular orbits around the nucleus, similar in structure to the Solar System, but with electromagnetic forces providing attraction, rather than gravity. He also introduced the idea that the electrons travel in discrete orbits around the atom's nucleus, the chemical properties of the particular element being largely determined by the number of electrons in the outer orbits. In addition, he proposed that an electron could drop from a higher energy orbit to a lower one, emitting a photon of discrete energy in the process, which became part of the basis for quantum theory. It was largely for this early work that Bohr was awarded the Nobel Prize in Physics in 1922, "for his services in the investigation of the structure of atoms and of the radiation emanating from them". In 1916, he became a full professor at the University of Copenhagen and continued his research. During this time, he postulated that an atom would not emit radiation while it was in one of its stable states but rather only when it made a transition between states, and that the atom could neither absorb nor emit radiation continuously but only in finite steps or quantum jumps. In 1920, he established the "correspondence principle", the idea that classical physics and quantum physics will give the same answers when the systems become sufficiently large. In 1921, with the assistance of the Danish government and the Carlsberg Foundation, he founded the Institute of Theoretical Physics in Copenhagen, where he served as director for the rest of his life. The Institute soon became an international focal point for theoretical physicists in the 1920s and 1930s, and most of the world's best known theoretical physicists of that period spent at least some time there. Werner Heisenberg worked as Bohr’s assistant at the Institute from 1926 to 1927, and the two men worked closely on the mathematical foundations of quantum mechanics. It was during this fertile period in Copenhagen that Heisenberg developed his famous uncertainty principle. It was also during this period that Bohr developed his principle of complementarity, the idea that that particles could be separately analyzed as having several contradictory, and apparently mutually exclusive, properties (an example being the wave-particle duality of light, where light can either behave as a particle or as wave, but not simultaneously as both). The two physicist also grappled at this time with the philosophical implications of quantum theory, and the extent to which it reflected the reality of the everyday world. Although they were not in complete agreement, their general position was popularly referred to as the “Copenhagen interpretation”, which in broad terms stated that reality could only be ascribed to a measurement, and that quantum effects themselves were essentially characterized by indeterminacy. Bohr, along with John Wheeler, developed the “liquid-drop” model of the atomic nucleus (so called because it likened the nucleus to a droplet of liquid), first proposed by George Gamow. This was a key step in the understanding of many nuclear processes, and it played an essential part in 1939 in explaining the basis of nuclear fission (the splitting of a heavy nucleus into two more or less equal parts, with the consequent release of a tremendous amount of energy). He also identified that it was the rare U-235 isotope that made uranium fissionable, and which made a chain reaction theoretically possible. During the Second World War, Denmark was occupied by the German forces, and Bohr, who was quite aware of German nuclear research (especially given his friendship with Heisenberg, who was intimately involved in German nuclear power research, although he was resisting involvement in the development of nuclear weapons), had to be very careful in his dealings and communications. For example, when the British intelligence services inquired about Bohr's availability for work or insights of particular value, he made it quite clear that he could not help. Eventually, in 1943, Bohr was forced to flee the German authorities, partly because of his Jewish ancestry, and partly due to the anti-Nazi views he made little effort to conceal, escaping to Sweden shortly before he was to be arrested by the German police. From Sweden he traveled on to London, and became involved for a time with Project "Tube Alloys", the code-name for the British nuclear weapon program, and the search for a viable nuclear fission bomb. He also worked at this time on the American equivalent, the Manhattan Project, at the top-secret Los Alamos laboratory in New Mexico, United States, where he was apparently known by the assumed name of Nicholas Baker for security reasons. He gradually assumed an important role as a senior consultant in the Manhattan Project, but he was also concerned about a potential nuclear arms race and he firmly believed that atomic secrets should be shared by the international scientific community. To this end, he had high level discussions with both the U.S. President Franklin D. Roosevelt and the British Prime Minister Winston Churchill. Churchill in particular was vehemently against sharing such secrets with Soviet Russia and considered Bohr as potentially unstable and a dangerous security risk. After the War, Bohr returned to Copenhagen, where he was awarded the Order of the Elephant (Denmark’s highest decoration, the equivalent of a knighthood) by the Danish government in 1947. He continued to serve in many public roles, notably as president of the Royal Danish Academy, a position he held from 1939 until his death in 1962. He also continued to advocate the peaceful use of nuclear energy, and addressed an “Open Letter” to the United Nations on the subject in 1950. However, it was only later in the 1950s, after the immense surprise that the Soviets had developed nuclear weapons independently, that the International Atomic Energy Agency was created, very much along the lines of Bohr's original suggestion. Bohr died in Copenhagen on 18 November 1962, aged 77, and is buried in the Assistens Cemetery in Copenhagen. NIELS BOHR BOOKSSee the additional sources and recommended reading list below, or check the physics books page for a full list. 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