Niels Bohr
gigatos | January 23, 2022
Summary
Niels Henrik David Bohr (7 October 1885, Copenhagen – 18 November 1962, the same place) – Danish theoretical physicist and public figure, one of the founders of modern physics. Winner of the Nobel Prize in Physics (1922). Member of the Royal Danish Society (1917) and its president since 1939. He was a member of more than 20 academies of sciences of the world, including a foreign honorary member of the USSR Academy of Sciences (corresponding member – since 1924).
Bohr is known as the creator of the first quantum theory of the atom and an active participant in the development of the foundations of quantum mechanics. He also made a significant contribution to the development of the theory of the atomic nucleus and nuclear reactions, the processes of interaction of elementary particles with the medium.
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Family. Childhood and Youth
Niels Bohr was born into the family of Christian Bohr (1858-1911), a professor of physiology at the University of Copenhagen who was twice a candidate for the Nobel Prize in Physiology or Medicine, and Ellen Adler (1860-1930), daughter of David Baruch Adler (1826-1878), a powerful and very wealthy Jewish banker and liberal parliamentarian, and Jenny Raphael (1830-1902) of the British Jewish banking dynasty, Raphael Raphael & sons. Bohr”s parents married in 1881.
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Youth. Bohr”s Theorem – van Leeuwen (1885-1911)
At school he showed a clear inclination toward physics and mathematics as well as philosophy. This was promoted by regular visits from his father”s colleagues and friends – the philosopher Harald Göffding, the physicist Christian Christiansen, and the linguist Wilhelm Thomsen. A close friend and classmate of Bohr during this period was his third cousin (among the optical illusions he proposed was the so-called Rubin vase (1915). Rubin attracted Bohr to the study of philosophy.
Another of Bohr”s hobbies was soccer. Niels and his brother Harald (who later became a famous mathematician) played for the amateur club “AB” (the first as a goalkeeper and the second as a midfielder). Later Harald successfully played in the Danish national team and won silver in it at the Olympics 1908, where the Danish team lost in the final to Great Britain.
In 1903 Niels Bohr entered the University of Copenhagen, where he studied physics, chemistry, astronomy, and mathematics. Together with his brother, he organized a student philosophical circle, at which its members took turns making reports. At the university, Niels Bohr carried out his first work on the study of the oscillations of a jet of liquid to more accurately determine the value of the surface tension of water. The theoretical study in 1906 was awarded the gold medal of the Royal Danish Society. In subsequent years (1907-1909) it was supplemented by experimental results obtained by Bohr in his father”s physiological laboratory, and published on the presentation of the then coryphaei of physics Ramsay and Rayleigh.
In 1910 Bohr received his master”s degree and in May 1911 he defended his doctoral thesis on the classical electron theory of metals. In his dissertation Bohr, developing the ideas of Lorentz, proved the important theorem of classical statistical mechanics, according to which the magnetic moment of any set of elementary electric charges moving according to the laws of classical mechanics in a constant magnetic field is zero in the stationary state. In 1919, this theorem was independently rediscovered by Hendrika van Leeuwen and is called the Bohr-van Leeuwen theorem. It directly implies that it is impossible to explain the magnetic properties of matter (in particular, diamagnetism) staying within the framework of classical physics. This, apparently, was Bohr”s first confrontation with the limitations of the classical description, bringing him to the questions of quantum theory.
Bohr in England. Bohr Theory (1911-1916)
In 1911 Bohr received a scholarship of 2,500 kroner from the Carlsberg Foundation for an internship abroad. In September 1911, he arrived in Cambridge to work in the Cavendish Laboratory under the direction of the famous J. J. Thomson. However, cooperation did not pan out: Thomson is not interested in a young Dane, who immediately pointed out a mistake in one of his works, and furthermore poorly intelligible in English. Bohr later recalled it this way:
I was disappointed; Thomson wasn”t interested in the fact that his calculations were wrong. It was my fault, too. I did not know English well enough and therefore could not explain myself… Thomson was a genius who, in fact, pointed the way for everyone… On the whole, it was very interesting to work in Cambridge, but it was an absolutely useless occupation.
As a result, in March 1912 Bohr moved to Manchester to Ernest Rutherford, with whom he had met shortly before. In 1911 Rutherford published a planetary model of the atom based on his experiments. Bohr was actively involved in the work on this subject, which was facilitated by numerous discussions with the famous chemist Georg Hevesy who was then working in Manchester and with Rutherford himself. The basic idea was that the properties of elements are determined by a whole number, the atomic number, which is the nucleus charge, which can change in the processes of radioactive decay. The first application of Rutherford”s model of the atom for Bohr was the consideration of the interaction of alpha- and beta-rays with matter during the last months of his stay in England. In the summer of 1912 Bohr returned to Denmark.
On August 1, 1912, Bohr and Margaret Nørlund, the sister of Harald”s close friend Niels Erik Nørlund, whom he had met in 1909, were married in Copenhagen. During a wedding trip to England and Scotland, Bohr and his wife visited Rutherford in Manchester. Bohr gave him his ready-to-print article “Theory of inhibition of charged particles during their passage through matter” (it was published in early 1913). At the same time was the beginning of a close friendship between the Bohr and Rutherford families. Communication with Rutherford left an indelible imprint (both scientifically and personally) on the fate of Bohr, who many years later wrote:
Very characteristic of Rutherford was the benevolent interest he took in all young physicists with whom he had to deal long or short. <…> for me Rutherford became a second father.
On his return to Copenhagen Bohr taught at the university, at the same time working intensively on the quantum theory of the structure of the atom. The first results are contained in a draft sent to Rutherford back in July 1912 called the “Rutherford Memorandum”. However, the decisive progress was made in late 1912 and early 1913. The key point was the familiarity in February 1913 with the patterns of spectral lines and the general combinatorial principle for the radiation frequencies of atoms. Bohr himself later said:
As soon as I saw Balmer”s formula, the whole question became immediately clear to me.
In March 1913 Bohr sent a preliminary version of the article to Rutherford, and in April he went to Manchester for a few days to discuss his theory. The result of this work was three parts of the revolutionary article “On the Structure of Atoms and Molecules,” published in Philosophical Magazine in July, October and December 1913, which contained the quantum theory of the hydrogen-like atom. Two main components can be distinguished in Bohr”s theory: general statements (postulates) about the behavior of atomic systems, which have retained their significance and have been comprehensively tested, and a specific model of the atom structure, which is of only historical interest today. Bohr”s postulates contain assumptions about the existence of stationary states and about radiative transitions between them in accordance with Planck”s ideas about energy quantization of matter. Bohr”s model theory of the atom proceeds from the assumption that it is possible to describe the motion of electrons in an atom in a stationary state on the basis of classical physics, on which additional quantum conditions are imposed (for example, quantization of the angular momentum of the electron). Bohr”s theory immediately made it possible to justify the emission and absorption of radiation in the serial spectra of hydrogen, as well as to explain (with correction for the given electron mass) the hydrogen-like spectra observed earlier by Charles Pickering and Alfred Fowler with half-integer quantum numbers as belonging to ionized helium. The brilliant success of Bohr”s theory was the theoretical derivation of the Rydberg constant.
Bohr”s work immediately attracted the attention of physicists and stimulated the rapid development of quantum concepts. His contemporaries appreciated the important step made by the Danish scientist. Thus, in 1936 Rutherford wrote:
I consider Bohr”s original quantum theory of spectra to be one of the most revolutionary that science has ever produced; and I know of no other theory that has been more successful.
In 1949, Albert Einstein recalled his impressions of Bohr”s theory as follows:
All my attempts to adapt the theoretical foundations of physics to these results [that is, the implications of Planck”s law for blackbody radiation] failed utterly. It was as if the ground was gone from under my feet and nowhere to be seen as solid ground on which to build. It has always seemed to me a miracle that this fluctuating and contradictory foundation was enough to allow Bohr, a man of genius intuition and keen intuition, to find the basic laws of spectral lines and electron shells of atoms, including their significance for chemistry. This still strikes me as a miracle. This is the highest musicality in the field of thought.
In the spring of 1914 Bohr was invited by Rutherford to replace Charles Darwin, grandson of the famous naturalist, as lecturer in mathematical physics at Manchester University (Schuster School of Mathematical Physics). He remained in Manchester from the fall of 1914 until the summer of 1916. At this time he tried to extend his theory to multielectron atoms, but soon hit a dead end. Already in September 1914 he wrote:
For systems consisting of more than two particles there is no simple relation between the energy and the number of reversals, and for this reason considerations such as those I used earlier cannot be applied to determine the “stationary states” of the system. I am inclined to believe that there are very considerable difficulties hidden in this problem, which can only be overcome by abandoning the usual notions to an even greater extent than has been required up to now, and that the only reason for the successes achieved is the simplicity of the systems considered.
In 1914 Bohr was able to partially explain the splitting of spectral lines in the Stark and Zeeman effects, but he failed to obtain splitting into more than two components. This showed the limitation of circular orbits considered in his theory. It became possible to overcome it only after Arnold Sommerfeld in early 1916 formulated the generalized quantum conditions, introduced three quantum numbers for the electron orbit and explained the fine structure of spectral lines, taking into account relativistic corrections. Bohr immediately engaged in a fundamental revision of his results in the light of this new approach.
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Further development of the theory. The Principle of Conformity (1916-1923)
In the summer of 1916 Bohr finally returned to his homeland and headed the department of theoretical physics at the University of Copenhagen. In April 1917 he appealed to the Danish authorities to allocate finances for the construction of a new institute for himself and his staff. March 3, 1921, after overcoming many organizational and administrative difficulties in Copenhagen finally opened the Institute for Theoretical Physics, which now bears the name of his first head (Niels Bohr Institute).
Despite being very busy with administrative affairs, Bohr continued to develop his theory, trying to generalize it to the case of more complex atoms, such as helium. In 1918, in his article “On the Quantum Theory of Linear Spectra” Bohr formulated quantitatively the so-called correspondence principle, which links quantum theory with classical physics. The idea of correspondence first emerged as early as 1913, when Bohr used the idea that transitions between stationary orbits with large quantum numbers should produce radiation with a frequency that coincides with the frequency of electron circulation. Since 1918 the correspondence principle became in Bohr”s hands a powerful tool for obtaining new results: it allowed, following the notions of Einstein”s coefficients, to determine the probabilities of transitions and, consequently, the spectral line intensities; to obtain selection rules (to interpret the number and polarization components of the Stark and Zeeman splitting. Later Bohr gave a clear formulation of the correspondence principle:
… “correspondence principle”, according to which the presence of transitions between the stationary states accompanied by radiation is related to the harmonic components of fluctuations in the motion of the atom which determine in the classical theory the properties of radiation emitted due to the motion of the particle. Thus, according to this principle, it is supposed that every process of transition between two stationary states is connected with the corresponding harmonic component so that the probability of presence of transition depends on the amplitude of fluctuation, polarization of radiation is determined by more detailed properties of fluctuation as well as the intensity and polarization of radiation in system of waves emitted by atom according to the classical theory because of presence of the mentioned components of fluctuation is determined by the amplitude and other properties of latter.
The correspondence principle also played a huge role in the construction of consistent quantum mechanics. It was from this principle that Werner Heisenberg started his matrix mechanics in 1925. In a general philosophical sense, this principle, linking new knowledge with the achievements of the past, is one of the main methodological principles of modern science.
In 1921-1923 in a series of papers Bohr for the first time managed to give, based on his model of the atom, spectroscopic data and general considerations about the properties of the elements, an explanation of the periodic system of Mendeleev, by presenting a scheme of filling electron orbits (shells, according to modern terminology). The correctness of the interpretation of the periodic table was confirmed by the discovery in 1922 of the new element hafnium by Dirk Koster and Georg Hevesi, who were working at that time in Copenhagen. As Bohr predicted, this element turned out to be close in its properties to zirconium, and not to the rare-earth elements, as previously thought.
In 1922 Bohr was awarded the Nobel Prize in Physics “for his achievements in the study of the structure of the atom. In his lecture “On the structure of atoms,” delivered in Stockholm on December 11, 1922, Bohr summed up a decade of work.
However, it was obvious that Bohr”s theory contained an internal contradiction in its basis, since it mechanically combined classical concepts and laws with quantum conditions. In addition, it was incomplete, insufficiently universal, because it could not be used to quantitatively explain the entire diversity of phenomena in the atomic world. For example, Bohr and his assistant Hendrik Kramers never managed to solve the problem of the motion of electrons in the helium atom (the simplest two-electron system), which they were engaged in since 1916. Bohr clearly understood the limitations of the existing approaches (the so-called “old quantum theory”) and the need to build a theory based on entirely new principles:
…the whole approach to the problem was still highly semi-empirical, and it soon became quite clear that for an exhaustive description of the physical and chemical properties of the elements, a new radical departure from classical mechanics was needed in order to combine quantum postulates into a logically consistent scheme.
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Formation of quantum mechanics. The Principle of Complementarity (1924-1930)
The new theory was quantum mechanics, which was created in 1925-1927 in the works of Werner Heisenberg, Erwin Schrödinger, Max Born and Paul Dirac. At the same time, the basic ideas of quantum mechanics, despite its formal successes, remained largely unclear in the early years. For full understanding of physical foundations of quantum mechanics it was necessary to connect it with experience, to reveal the meaning of concepts used in it (because the use of classical terminology was no longer legitimate), i.e. to give an interpretation of its formalism.
It was these questions of physical interpretation of quantum mechanics that Bohr was pondering at that time. The result was the concept of additionality, which was presented at the Congress in memory of Alessandro Volta in Como in September 1927. The starting point in the evolution of Bohr”s views was his adoption of wave-particle dualism in 1925. Prior to this Bohr refused to recognize the reality of Einstein”s quanta of light (photons), which was difficult to reconcile with the principle of correspondence, which resulted in a joint article with Kramers and John Slater, in which the unexpected assumption of non-conservation of energy and momentum in individual microscopic processes (conservation laws assumed a statistical character) was made. However, these views were soon refuted by the experiments of Walter Bothe and Hans Geiger.
It was corpuscular-wave dualism that was the basis of Bohr”s interpretation of the theory. The idea of additionality, developed in early 1927 while on vacation in Norway, reflects a logical relation between two ways of describing or sets of representations that, although mutually exclusive, are both necessary for an exhaustive description of the state of affairs. The essence of the uncertainty principle is that no physical situation can arise in which both additional aspects of a phenomenon appear simultaneously and equally distinctly. In other words, in the microcosm there are no states in which an object would have simultaneously exact dynamic characteristics belonging to two definite classes mutually excluding each other, which finds expression in the Heisenberg uncertainty relation. The measurement data of microcosm objects obtained with different experimental setups, when the interaction between the measuring instrument and the object is an integral part of the measurement process, are in a peculiar complementary relation to each other. Bohr”s ideas, as he himself admitted, were influenced by the philosophical-psychological research of Søren Kierkegaard, Harald Göffding and William James.
The principle of additionality was the basis of the so-called Copenhagen interpretation of quantum mechanics of characteristics of microobjects. According to this interpretation, the dynamic characteristics of a microparticle (its coordinate, momentum, energy, etc.) borrowed from classical physics are not inherent to the particle itself. The meaning and a certain value of one or another characteristic of the electron, for example, its momentum, are revealed in the relationship with the classical objects, for which these quantities have a certain meaning and all at the same time can have a certain value (such a classical object is conventionally called a measuring instrument). The role of the additionality principle turned out to be so essential that Pauli even suggested to call quantum mechanics “additionality theory” by analogy with relativity theory.
A month after the Como Congress, at the fifth Solvay Congress in Brussels, the famous Bohr and Einstein discussions about the interpretation of quantum mechanics began. The dispute continued in 1930 at the Sixth Congress, where Bohr explained from the perspective of quantum mechanics Einstein”s photon box paradox, and then resumed with renewed force in 1935 after the appearance of the famous paper by Einstein, Podolsky and Rosen on the completeness of quantum mechanics. The discussion did not stop until Einstein”s death, at times taking on a violent character. However, the participants never ceased to treat each other with the utmost respect, as reflected in the words of Einstein in 1949:
I can see that I was … rather harsh, but after all … only brothers or close friends really fight.
Although Bohr was never able to convince Einstein that he was right, these discussions and solutions to numerous paradoxes allowed Bohr to greatly improve the clarity of his thoughts and formulations and to deepen his understanding of quantum mechanics:
The lesson we have drawn from this has decisively moved us forward in the never-ending struggle for harmony between content and form; it has shown us once again that no content can be grasped without involving a corresponding form, and that every form, however useful it may have been in the past, may be too narrow to embrace new results.
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Nuclear Physics (1930s)
In 1932 Bohr and his family moved to the so-called “House of Honor,” the residence of Denmark”s most respected citizen, built by the founder of the Carlsberg Brewing Company. Here he was visited by celebrities not only of the scientific (e.g., Rutherford), but also of the political world (the royal couple of Denmark, the English Queen Elizabeth, presidents and prime ministers of various countries).
In 1934 Bohr suffered a serious personal tragedy. While sailing on a yacht in the Kattegat Strait, a storm wave washed his oldest son, 19-year-old Christian, overboard; he was never found. Niels and Margaret had a total of six children. One of them, Åge Bohr, also became an outstanding physicist and Nobel Prize winner (1975).
In the 1930s Bohr became interested in nuclear topics, refocusing his institute on them: thanks to his fame and influence he was able to obtain funding for the construction of new facilities at the Institute – the cyclotron, the gas pedal based on the Cockcroft-Walton model, the van de Graaff gas pedal. He himself at this time made a significant contribution to the theory of nuclear structure and nuclear reactions.
In 1936 Bohr, based on the existence of recently observed neutron resonances, formulated a fundamental idea for nuclear physics about the nature of nuclear reactions: he assumed the existence of the so-called compound nucleus (“compound nucleus”), that is, an excited state of the nucleus with a lifetime equal to the time of neutron motion through it. The reaction mechanism, then, not limited to neutron reactions, includes two steps: 1) formation of a compound nucleus, and 2) its decay. In this case, these two stages proceed independently of each other, which is due to the equilibrium redistribution of energy between the degrees of freedom of the compound nucleus. This allowed us to apply a statistical approach to describe the behavior of nuclei, which allowed us to calculate the cross sections of a number of reactions, as well as to interpret the decay of the compound nucleus in terms of particle evaporation, creating at the suggestion of Jacob Frenkel a droplet model of the nucleus.
However, this simple picture takes place only at large distances between resonances (levels of the nucleus), that is, at low excitation energies. As was shown in 1939 in the joint work of Bohr with Rudolf Payerls and Georg Placzek, when the resonances of the compound nucleus overlap, the equilibrium in the system has no time to be established and the two reaction stages cease to be independent, that is, the nature of the decay of the intermediate nucleus is determined by the process of its formation. The development of the theory in this direction led to the creation in 1953 by Victor Weisskopf, Hermann Feshbach, and K. Porter of the so-called “optical model of the nucleus,” which describes nuclear reactions in a wide range of energies.
Simultaneously with the notion of a composite nucleus Bohr (together with F. Calcar) proposed to consider collective motions of particles in nuclei, contrasting them with the picture of independent nucleons. Such vibrational modes of the liquid-drop type are reflected in spectroscopic data (in particular, in the multipole structure of nuclear radiation). Ideas about polarizability and deformations of nuclei were the basis for the generalized (collective) model of the nucleus developed in the early 1950s by Auger Bohr, Ben Mottelson, and James Rainwater.
Bohr”s contribution to the explanation of the mechanism of nuclear fission, in which enormous amounts of energy are released, is great. Fission was experimentally discovered at the end of 1938 by Otto Hahn and Fritz Strassmann and correctly interpreted by Lisa Meitner and Otto Frisch during the Christmas vacations. Bohr learned of their ideas from Frisch, then working in Copenhagen, just before he left for the United States in January 1939. At Princeton, together with John Wheeler, he developed a quantitative theory of nuclear fission based on the composite nucleus model and ideas about the critical deformation of the nucleus leading to its instability and disintegration. For some nuclei this critical value can be zero, which is reflected in the decay of the nucleus at arbitrarily small deformations. The theory made it possible to obtain the energy dependence of the fission cross section, which coincides with the experimental one. In addition, Bohr was able to show that the fission of uranium-235 nuclei is caused by “slow” (low-energy) neutrons, while uranium-238 is caused by fast neutrons.
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Opposition to Nazism. The war. The struggle against the atomic threat (1940-1950)
After the Nazis came to power in Germany, Bohr took an active part in the management of the fate of many emigrant scientists who moved to Copenhagen. In 1933, through the efforts of Niels Bohr, his brother Harald, the director of the Institute of Vaccines Torvald Madsen, and the lawyer Albert Jorgensen, a special Committee for Assistance to Refugee Scientists was established.
After the occupation of Denmark in April 1940, there was a real danger that Bohr would be arrested because of his semi-Jewish background. Nevertheless, he decided to remain in Copenhagen as long as possible in order to guarantee the protection of the institute and his collaborators from the encroachments of the occupation authorities. In October 1941, Bohr was visited by Heisenberg, at that time the head of the Nazi atomic project. Between them they had a conversation about the possibility of realizing nuclear weapons, about which the German scientist wrote as follows:
I visited Copenhagen in the fall of 1941, I think it was the end of October. By that time, we at the “Uranium Society” had concluded through experiments with uranium and heavy water that it was possible to build a reactor using uranium and heavy water to produce energy. <…> At that time we overestimated the scale of the technical costs required. <…> Under the circumstances, we thought a conversation with Bohr would be useful. Such a conversation took place during an evening walk in the Nielsberg area. Knowing that Bohr was under the surveillance of the German political authorities and that his remarks about me would probably be transmitted to Germany, I tried to conduct this conversation in such a way as not to endanger my life. The conversation, as I recall, began with my question as to whether physicists should deal with the uranium problem in wartime, since progress in this field could lead to serious consequences in the techniques of warfare. Bohr immediately understood the significance of this question, as I was able to catch his reaction of slight consternation. He responded with a counter-question: “Do you really think that uranium fission can be used to create weapons?” I replied, “It is possible in principle, but it would require such an incredible technical effort that hopefully could not be accomplished in a real war.” Bohr was stunned by my reply, apparently assuming that I intended to inform him that Germany had made enormous progress in the production of atomic weapons. Although I tried afterwards to correct this mistaken impression, I still failed to win Bohr”s confidence…
Thus, Heisenberg hints that Bohr did not understand what he meant. However, Bohr himself disagreed with such an interpretation of his conversation with Heisenberg. In 1961, in a conversation with Arkady Migdal, he stated:
I understood him perfectly. He suggested that I cooperate with the Nazis…
In 2002 Bohr”s descendants published unsent letters from Bohr to Heisenberg, probably written in the same year, 1957. In the first of them Bohr writes that he remembers perfectly their conversation, in which Heisenberg expressed confidence in the final victory of Germany and invited Bohr to join in the development of the atomic bomb.
By the fall of 1943, it had become impossible to remain in Denmark, so Bohr and his son Åge were transported by Resistance forces, first by boat to Sweden and then by bomber to England, almost dying in the process. Bohr”s aunt (his mother”s older sister), the famous Danish educator Hannah Adler (1859-1947), was deported to a concentration camp despite her 84-year-old age and government protection. In Great Britain and the United States, where he soon moved, the scientist became involved in the work on the atomic bomb and participated in it until June 1945. In the United States, he and his son were named Nicholas and Jim Baker.
At the same time, as early as 1944, Bohr was aware of the danger of the atomic threat. The meeting with the prime minister of Great Britain on May 16, 1944 did not lead to any results. After that, Niels Bohr began to seek an audience with U.S. President F. Roosevelt. In his memorandum to President Roosevelt (July 3, 1944) he called for a complete ban on the use of nuclear weapons, to ensure strict international control over them, and at the same time to destroy any monopoly on the peaceful use of atomic energy. Later he sent two more memorandums to the U.S. leaders – on March 24, 1945 and May 17, 1948. Bohr tried to bring his thoughts to Churchill and Roosevelt also during personal meetings with them, but to no avail. Moreover, this activity, as well as an invitation to come to the Soviet Union for the duration of the war, received from Peter Kapitsa in early 1944, led to suspicions of espionage in favor of the USSR.
In November 1945, Bohr was visited by Soviet physicist Ya. Terletsky on the instructions of Soviet intelligence and on the recommendation of P. Kapitsa, who asked him a number of questions about the American atomic project (about atomic reactors). Bohr told only what had by that time been published in open sources and reported Terletsky”s visit to the counterintelligence services.
In 1950 Bohr published an open letter to the UN, insisting on peaceful cooperation and the free exchange of information among states as a prerequisite for an “open world. Thereafter, he repeatedly spoke out on the subject, backing up calls for peace and the prevention of the threat of nuclear war with his authority.
Recent years
In recent years Bohr was engaged mainly in social activities, gave lectures in various countries and wrote articles on philosophical topics. Directly in the field of physics in the 1940′s and 1950′s he continued to work on the problem of the interaction of elementary particles with the medium. Bohr himself considered the principle of additionality as his most valuable contribution to science. He tried to expand its application to other areas of human activity – biology, psychology, culture, thinking a lot about the role and importance of language in science and life.
Niels Bohr died on November 18, 1962, of a heart attack. The urn with his ashes is in the family grave at Assistens Cemetery in Copenhagen.
Bohr created a major international school of physicists and did much to promote cooperation between physicists around the world. Since the early 1920s Copenhagen became the “center of attraction” for the most active physicists: most of the founders of quantum mechanics (Heisenberg, Dirac, Schrödinger and others) worked there at one time or another, their ideas crystallized in long grueling conversations with Bohr. His lecture visits to different countries were of great importance for the dissemination of Bohr”s ideas. For example, seven lectures given by Bohr in June 1922 at the University of Göttingen (the so-called “Bohr Festival”) played a great role in the history of science. It was then that he met the young physicists Wolfgang Pauli and Werner Heisenberg, pupils of Sommerfeld. Heisenberg expressed his impressions of his first conversation with Bohr during the walk as follows:
This walk had the strongest influence on my subsequent scientific development, or perhaps it is better to say that my own scientific development only began with this walk.
Later the Bohr group”s connection with the Göttingen group, led by Max Born, was not interrupted and produced many outstanding scientific results. Naturally, Bohr”s ties with the Cambridge group headed by Rutherford were very strong: Charles Darwin, Paul Dirac, Ralph Fowler, Douglas Hartree, Neville Mott and others worked in Copenhagen at different times. Bohr also hosted Soviet scientists in his institute, many of whom worked there for long periods of time. He visited the USSR several times, most recently in 1961.
Such scientists as Hendrik Kramers, Oskar Klein, Lev Landau, Viktor Weisskopf, Leon Rosenfeld, John Wheeler, Felix Bloch, Auger Bohr, Hendrik Casimir, Yoshio Nishina, Christian Moeller, Abraham Pace and many others can be referred to the Niels Bohr school. The nature of Bohr”s scientific school and his relationship with his students can be made clear by the following episode. When Landau, during Bohr”s visit to Moscow in May 1961, asked his mentor, “What secret did you possess that allowed you to concentrate creative theoretical youth around you to such an extent?” the latter replied:
There wasn”t much of a secret, except that we weren”t afraid to look stupid in front of the young people.
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