Niels Bohr (Dictionary of World Biography: Twentieth Century)
Article abstract: Bohr discovered the fundamental structure and character of the atom, its components and how they interact. For this discovery, he won the Nobel Prize in Physics in 1922. Bohr also made significant contributions to the understanding of how quantum and classical physics unify as a single philosophy in his principle of complementarity.
Niels Henrik David Bohr was born in Copenhagen, Denmark, on October 7, 1885. Bohr’s early environment invited genius; his father, Christian Bohr, was a professor of physiology at the University of Copenhagen, and his mother, Ellen née Adler, came from a family of eminent Danish educators. Bohr’s younger brother, Harald, would become a professor of mathematics.
Bohr attended Gammelholm Grammar School and entered the University of Copenhagen in 1903. Bohr studied under the tutelage of C. Christiansen, a prominent physicist and an original, creative educator. At the University of Copenhagen, Bohr took his master’s degree in physics in 1909 and his Ph.D. in 1911.
Bohr published his first scientific work in 1908. The opportunity arose as a result of a prize offered to the individual who solved an investigation of surface tension by means of oscillating fluid jets. Bohr won the gold medal, and his piece appeared in the Transactions of the Royal Society.
In the fall of 1911, Bohr studied abroad at the University of...
(The entire section is 1798 words.)
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Bohr, Niels (Encyclopedia of Science and Religion)
For the first half of the twentieth century, as both physicist and natural philosopher, Niels Bohr was at the epicenter of the quantum revolution that gave physicists their understanding of the atomic structure of matter. Bohr's Institute for Theoretical Physics (now the Niels Bohr Institute) in Copenhagen, Denmark, was the central headquarters of this revolution, and Bohr was its most senior and respected spokesperson. His influence made this city of his birth the namesake for the position defended by supporters of the revolution: the socalled Copenhagen Interpretation, which became the dominant or orthodox understanding of quantum theory, even while remaining controversial and beset with numerous conceptual difficulties. Although the quantum revolution transformed theoretical physics utterly, making a return to the worldview of classical physics out of the question, Bohr's viewpoint never received unanimous acceptance; several of Bohr's peers, most notably Albert Einstein and Erwin Schrödinger, remained critical and designed various paradoxes to confront the party of Copenhagen. From 1927 onwards, Bohr and Einstein debated these issues, but the precise implications of their differing views remain a matter of intense discussion among historians and philosophers of science.
Early life and work
Born in 1885, Niels Bohr, and his younger brother Harald, a famed mathematician, came to maturity in Danish academic circles. Their father, a professor of physiology at the University of Copenhagen, was a close friend of the philosopher-psychologist Harald Høffding (1843931). The Bohr brothers were auditors and later participants in the intellectual discussions held in the Bohr home with Høffding and their father's other academic friends. Høffding, an eclectic thinker with a broadly Kantian outlook sympathetic to his friend William James's pragmatism, became Bohr's only formal teacher of philosophy.
After receiving his doctorate in physics from the University of Copenhagen in 1911, Bohr found his way to Manchester, England, where Ernest Rutherford had recently discovered a massive positively charged nucleus at the center of the atomic system. The young physicists surrounding Rutherford were eager to develop a theoretical model of a stable atomic system accounting for the then known evidence of atomic behavior. Starting from the assumption that no classical mechanical model would possibly yield a stable system, Bohr quickly sensed that the secret to atomic stability lay in the quantization of action already postulated in 1900 by the German physicist Max Planck (1853947) as a heuristic move toward a formula for blackbody radiation consistent with observation.
Bohr's 1913 presentation of his atomic model astonished physicists by deriving the observed frequencies of the spectrum of the simplest atomic system, hydrogen. Bohr assumed two nonclassical postulates. The first proposed that atomic systems exist in a series of discrete "stationary states" in which, contrary to classical electrodynamics, they neither emit nor absorb radiation. The second postulate stipulated that when atomic systems interact with electromagnetic radiation, the energy emitted or absorbed is determined by the difference between the energy of the stationary states in which the system existed before and after the interaction and is a function of the frequency of the radiation. While Bohr used classical mechanical models of electrons orbiting a nucleus to derive the energy of the stationary states, those same classical pictures imply a radically unstable system, a conclusion explicitly denied by Bohr's first postulate. Moreover, in classical physics, the energy exchanged with radiation should be a function of the orbital characteristics of the electron in each stationary state, rather than the difference between two states. If one imagined the electron in a spatiotemporal trajectory "jumping" from one quantized stationary state to another, the electron would seemingly have to know to which orbit it was going the moment it departed its original orbit. Thus Bohr's 1913 model already gave the interaction between matter and radiation a wholeness, implying that the theoretical representation of such interactions in terms of visualizable, mechanical pictures could not be a realistic picture of microphysical processes.
From 1913 to 1925 Bohr pondered how the classical descriptive concepts were to be used in describing microphenomena while his model became the basis of much new research leading towards building up more complex atomic systems. Although it had many successes, ultimately this "old" quantum theory could not derive the intensities of spectral lines. In 1925 the German physicist Werner Heisenberg (1901976) formulated a matrix mechanics dispensing altogether with spatiotemporal models of atomic systems by replacing single numbered kinematic and dynamic parameters of position and momentum with matrices. A few months later Erwin Schrödinger (1887961) produced wave mechanics, generally held to be mathematically equivalent to Heisenberg's theory, though in a more tractable form. After intense discussions with Bohr and Schrödinger, Heisenberg derived the indeterminacy relations in the spring of 1927; that summer Bohr formulated his new "viewpoint" for understanding this quantum description, and named it complementarity.
Bohr's viewpoint of complementarity, originally presented in 1927 in Como, Italy, remains obscure and controversial, although he repeated the basic argument in many essays. Bohr argues that the use of concepts rests on presuppositions which, upon extending experience into new domains, may be discovered to be of restricted applicability, thus forcing a "rational generalization of classical physics which would permit the harmonious incorporation of the quantum of action" (1987 , p. 2). The quantization of action introduces a feature of "wholeness inherent in atomic processes, going far beyond the ancient idea of the limited divisibility of matter" (1987, p. 2). Thus a visualizable space-time picture of such interactions is merely a conceptual abstraction used for interpreting phenomena as interactions between microphysical particles and macroscopic observing systems that must be classically described. Measurements are interactions, but this indivisibility of interactions implies that the experimental arrangements required for determining both kinematic (space and time) and dynamic (momentum and energy) parameters defining a system's state are physically exclusive, although both are required for a complete definition of the system's state. Heisenberg's indeterminacy relations express formally the physical fact that the indivisibility of interaction prohibits defining the state of the system in terms in which both kinematic and dynamic properties have precise values. Classical deterministic predictions were possible because both properties could be predicated of systems only by neglecting the interaction involved in the measurement, but quantum predictions of observables are statistical because the interaction in which a kinematic parameter is well defined excludes the interaction in which a dynamic parameter can be defined.
Classical determinism requiring predication of a mechanical state with simultaneous arbitrarily precise values of kinematic and dynamic parameters now appears as an idealization attainable only in interactions that are enormous compared to the scale of the quantum of action. The paradoxical fact that, for defining the state of both matter and radiation, the system needs to be characterized using both wave and particle "pictures" has misled some interpreters to misread complementarity as a relation between wave and particle concepts. Classically both kinematic and dynamic measurements can be made in a single experimental arrangement because the effect of the interaction with the measuring system can be left out of the account. Therefore precise values of both position and momentum can be "combined into a consistent picture of the object under investigation" representing its objects as either particles (if matter) or as waves (if radiation). However, because of the wholeness of quantum interactions, Bohr concludes "evidence obtained by different experimental arrangements exhibits a novel kind of complementary relationship . . . which appears contradictory when combination into a single picture is attempted" (1987 , p. 4). Representing the object as a "wave" or a "particle" proves contradictory because defining the state of a material system requires defining the particle's momentum, but in the quantum case to define the particle's momentum one must give it a wavelength, a property defined only by representing it as a wave. To define the state of radiation one must attribute to waves the property of momentum, which requires picturing the object as a particle. Thus wave-particle dualism arises from the complementary relation between the phenomena by which measurements of kinematic and dynamic parameters are empirically determined. Bohr held no other conceptual scheme avoiding this complementarity would be possible because to avoid "ambiguity" one must describe the measuring instruments in classical terms. This unambiguity is the foundation for the objectivity of the quantum description; thus Bohr abandons grounding objectivity in an ontological predication of properties of individuals.
Influence beyond physics
Bohr ventured beyond atomic physics to suggest that in other cases where the "analysis and synthesis" of experience encountered indivisible interactions analogous to quantum interactions, one must expect to employ complementary descriptions. In biology the wholeness of the organism-environment interaction necessary for displaying the phenomena of life, excludes the sort of isolation necessary for unambiguously defining the organism's state as a mechanical system, thereby leading to the necessity for a complementary combination of functional descriptions with mechanistic ones. Psychological descriptions of conscious experiences require the well known distinction between empirical ego (the object) and the transcendental ego (the subject) leading to the complementarity between deterministic description and that employing the notion of free will. In the human sciences Bohr called attention to the complementary relationship between descriptions of experience by persons within a culture or religious tradition and those of observers from another culture standing outside the cultural tradition being described, leading him to speak of different cultures and religious traditions as "complementary."
Bohr has often been presumed to be a positivist holding an antirealist or instrumentalist interpretation of atomic physics; however, his viewpoint arises from a physical discovery expressed in the quantization of action rather than an epistemological analysis along positivistic lines. He agrees that quantum physics bars a realistic visualizing of microphysical interactions, but it is clear that he regards atomic systems as independently real entities in nature, not as theoretical constructions. He seeks a radical revision of the conception of physical reality rather than its elimination from atomic physics. Although Bohr emphasized the epistemological lesson following from the indivisibility of observational interactions at the atomic level, he left unexplored the ontological implications of combining complementary descriptions of the same object appearing in different phenomena, thus inviting widely divergent philosophical interpretations of complementarity that continue to be debated.
See also COMPLEMENTARITY; COPENHAGEN INTERPRETATION; DETERMINISM; EINSTEIN, ALBERT; PHYSICS, QUANTUM; WAVE-PARTICLE DUALITY
Beller, Mara. Quantum Dialogue: The Making of a Revolution. Chicago: University of Chicago Press, 1999.
Bohr, Niels. Atomic Theory and the Description of Nature. New York: Macmillan, 1934; Reprinted as The Philosophical Writings of Niels Bohr: Volume 1. Woodbridge, Conn.: Ox Bow Press, 1987.
Bohr, Niels. Atomic Physics and Human Knowledge. New York: Wiley, 1957; Reprinted as The Philosophical Writings of Niels Bohr: Volume 2, Essays 1932957 on Atomic Physics and Human Knowledge. Woodbridge, Conn.: Ox Bow Press, 1987.
Bohr, Niels. Essays 1958962 on Atomic Physics and Human Knowledge. New York: Wiley, 1963; Reprinted as The Philosophical Writings of Niels Bohr: Volume 3. Woodbridge, Conn.: Ox Bow Press, 1987.
Bohr, Niels. The Philosophical Writings of Niels Bohr: Volume 4, Causality and Complementarityupplementary Papers, eds. Jan Faye and Henry J. Folse. Woodbridge, Conn.: Ox Bow Press, 1998.
Faye, Jan. Niels Bohr: His Heritage and Legacy. An Anti- Realist View of Quantum Mechanics. Dordrecht, Netherlands: Kluwer Academic, 1991.
Faye, Jan, and Folse, Henry J., eds. Niels Bohr and Contemporary Philosophy. Dordrecht, Netherlands: Kluwer Academic, 1994.
Folse, Henry J. The Philosophy of Niels Bohr: The Framework of Complementarity. Amsterdam: North Holland Physics, 1985.
Honner, John. The Description of Nature: Niels Bohr and the Philosophy of Quantum Physics. Oxford: Clarendon Press, 1987.
Hooker, C. A. "The Nature of Quantum Mechanical Reality: Einstein vs Bohr." In The Pittsburgh Studies in the Philosophy of Science, Vol. 5, ed. R. G. Colodny. Pittsburgh, Pa.: University of Pittsburgh Press, 1972.
Murdoch, Dugald. Niels Bohr's Philosophy of Physics. Cambridge,UK: Cambridge University Press, 1987.
Pais, Abraham. Niels Bohr's Times: in Physics, Philosophy, and Polity. Oxford: Clarendon Press, 1991.
Petruccioli, Sandro. Atoms, Metaphors, and Paradoxes: Niels Bohr and the Construction of a New Physics. Cambridge, UK: Cambridge University Press, 1992.
HENRY J. FOLSE, JR.
Bohr, Niels (1885-1962) (World of Earth Science)
Niels Bohr received the Nobel Prize in physics in 1922 for the quantum mechanical model of the atom that he had developed a decade earlier, the most significant step forward in scientific understanding of atomic structure since English physicist John Dalton first proposed the modern atomic theory in 1803. Bohr founded the Institute for Theoretical Physics at the University of Copenhagen in 1920, an Institute later renamed for him. For well over half a century, the Institute was a powerful force in the shaping of atomic theory. It was an essential stopover for all young physicists who made the tour of Europe's center of theoretical physics in the mid-twentieth century. Also during the 1920s, Bohr thought and wrote about some of the fundamental issues raised by modern quantum theory. He developed two basic concepts, the principles of complementarity and correspondence, both of which he held must direct all future work in physics. In the 1930s, Bohr became interested in problems of the atomic nucleus and contributed to the development of the liquid-drop model of the nucleus, a model used in the explanation of nuclear fission.
Niels Henrik David Bohr was born on in Copenhagen, Denmark, the second of three children of Christian and Ellen Adler Bohr. Bohr's early upbringing was enriched by a nurturing and supportive home atmosphere. His mother had come from a wealthy Jewish family involved in banking, government, and public service. Bohr's father was a professor of physiology at the University of Copenhagen. His closest friends met every Friday night to discuss events, and often, young Niels listened to the conversations during these gatherings.
Bohr became interested in science at an early age. His biographer, Ruth Moore, has written in her book Niels Bohr: The Man, His Science, and the World They Changed that as a child he "was already fixing the family clocks and anything else that needed repair." Bohr received his primary and secondary education at the Gammelholm School in Copenhagen. He did well in his studies, although he was apparently over-shadowed by the work of his younger brother Harald, who later became a mathematician. Both brothers were also excellent soccer players.
On his graduation from high school in 1903, Bohr entered the University of Copenhagen, where he majored in physics. He soon distinguished himself with a noteworthy research project on the surface tension of water as evidenced in a vibrating jet stream. For this work, he was awarded a gold medal by the Royal Danish Academy of Science in 1907. In the same year, he was awarded his bachelor of science degree, to be followed two years later by a master of science degree. Bohr then stayed on at Copenhagen to work on his doctorate, which he gained in 1911. His doctoral thesis dealt with the electron theory of metals and confirmed the fact that classical physical principles were sufficiently accurate to describe the qualitative properties of metals but failed when applied to quantitative properties. Probably the main result of this research was to convince Bohr that classical electromagnetism could not satisfactorily describe atomic phenomena. The stage had been set for Bohr's attack on the most fundamental questions of atomic theory.
Bohr decided that the logical place to continue his research was at the Cavendish Laboratory at Cambridge University. The director of the laboratory at the time was English physicist J. J. Thomson, discoverer of the electron. Only a few months after arriving in England in 1911, however, Bohr discovered that Thomson had moved on to other topics and was not especially interested in Bohr's thesis or ideas. Fortunately, however, Bohr met English physicist Ernest Rutherford, then at the University of Manchester, and received a much more enthusiastic response. As a result, he moved to Manchester in 1912 and spent the remaining three months of his time in England working on Rutherford's nuclear model of the atom.
On July 24, 1912, Bohr boarded ship for his return to Copenhagen and a job as assistant professor of physics at the University of Copenhagen. Also waiting for him was his bride-to-be Margrethe Nørlund, whom he married on August 1. The couple later had six sons. One son, Aage, earned a share of the 1975 Nobel Prize in physics for his work on the structure of the atomic nucleus.
The field of atomic physics was going through a difficult phase in 1912. Rutherford had only recently discovered the atomic nucleus, which had created a profound problem for theorists. The existence of the nucleus meant that electrons must have been circling it in orbits somewhat similar to those traveled by planets in their motion around the Sun. According to classical laws of electrodynamics, however, an electrically charged particle would continuously radiate energy as it traveled in such an orbit around the nucleus. Over time, the electron would spiral ever closer to the nucleus and eventually collide with it. Although electrons clearly must be orbiting the nucleus, they could not be doing so according to classical laws.
Bohr arrived at a solution to this dilemma in a somewhat roundabout fashion. He began by considering the question of atomic spectra. For more than a century, scientists had known that the heating of an element produces a characteristic line spectrum; that is, the specific pattern of lines produced is unique for each specific element. Although a great deal of research had been done on spectral lines, no one had thought very deeply about what their relationship might be with atoms, the building blocks of elements.
When Bohr began to attack this question, he decided to pursue a line of research begun by the German physicist Johann Balmer in the 1880s. Balmer had found that the lines in the hydrogen spectrum could be represented by a relatively simple mathematical formula relating the frequency of a particular line to two integers whose significance Balmer could not explain. It was clear that the formula gave very precise values for line frequencies that corresponded well with those observed in experiments.
When Bohr's attention was first attracted to this formula, he realized at once that he had the solution to the problem of electron orbits. The solution that Bohr worked out was both simple and elegant. In a brash display of hypothesizing, Bohr declared that certain orbits existed within an atom in which an electron could travel without radiating energy; that is, classical laws of physics were suspended within these orbits. The two integers in the Balmer formula, Bohr said, referred to orbit numbers of the "permitted" orbits, and the frequency of spectral lines corresponded to the energy released when an electron moved from one orbit to another.
Bohr's hypothesis was brash because he had essentially no theoretical basis for predicting the existence of "allowed" orbits. To be sure, German physicist Max Planck's quantum hypothesis of a decade earlier had provided some hint that Bohr's "quantification of space" might make sense, but the fundamental argument for accepting the hypothesis was simply that it worked. When his model was used to calculate a variety of atomic characteristics, it did so correctly. Although the hypothesis failed when applied to detailed features of atomic spectra, it worked well enough to earn the praise of many colleagues.
Bohr published his theory of the "planetary atom" in 1913. That paper included a section that provided an interesting and decisive addendum to his basic hypothesis. One of the apparent failures of the Bohr hypothesis was its seeming inability to predict a set of spectral lines known as the Pickering series, lines for which the two integers in the Balmer formula required half-integral values. According to Bohr, of course, no "half-orbits" could exist that would explain these values. Bohr's solution to this problem was to suggest that the Pickering series did not apply to hydrogen at all, but to helium atoms that had lost an electron. He rewrote the Balmer formula to reflect this condition.
Within a short period of time spectroscopists in England had studied samples of helium carefully purged of hydrogen and found Bohr's hypothesis to be correct. Although a number of physicists were still debating Bohr's theory, at least oneutherfordas convinced that the young Danish physicist was a highly promising researcher. He offered Bohr a post as lecturer in physics at Manchester, a job that Bohr eagerly accepted and held from 1914 to 1916. He then returned to the University of Copenhagen, where a chair of theoretical physics had been created specifically for him. Within a few years he was to become involved in the planning for and construction of the University of Copenhagen's new Institute for Theoretical Physics, of which he was to serve as director for the next four decades.
In many ways, Bohr's atomic theory marked a sharp break between classical physics and a revolutionary new approach to natural phenomena made necessary by quantum theory and relativity. He was very much concerned about how scientists could and should now view the physical world, particularly in view of the conflicts that arose between classical and modern laws and principles. During the 1920s and 1930s, Bohr wrote extensively about this issue, proposing along the way two concepts that he considered to be fundamental to the "new physics." The first was the principle of complementarity that says, in effect, that there may be more than one true and accurate way to view natural phenomena. The best example of this situation is the wave-particle duality discovered in the 1930s, when particles were found to have wavelike characteristics and waves to have particle-like properties. Bohr argued that the two parts of a duality may appear to be inconsistent or even in conflict and that one can use only one viewpoint at a time, but he pointed out that both are necessary to obtain a complete view of particles and waves.
The second principle, the correspondence principle, was intended to show how the laws of classical physics could be preserved in light of the new quantum physics. We may know that quantum mechanics and relativity are essential to an understanding of phenomena on the atomic scale, Bohr said, but any conclusion drawn from these principles must not conflict with observations of the real world that can be made on a macroscopic scale. That is, the conclusions drawn from theoretical studies must correspond to the world described by the laws of classical physics.
In the decade following the publication of his atomic theory, Bohr continued to work on the application of that theory to atoms with more than one electron. The original theory had dealt only with the simplest of all atoms, hydrogen, but it was clearly of some interest to see how that theory could be extended to higher elements. In March, 1922, Bohr published a summary of his conclusions in a paper entitled "The Structure of the Atoms and the Physical and Chemical Properties of the Elements." Eight months later, Bohr learned that he had been awarded the Nobel Prize in physics for his theory of atomic structure, by that time universally accepted among physicists.
During the 1930s, Bohr turned to a new, but related, topic: the composition of the atomic nucleus. By 1934, scientists had found that the nucleus consists of two kinds of particles, protons and neutrons, but they had relatively little idea how those particles are arranged within the nucleus and what its general shape was. Bohr theorized that the nucleus could be compared to a liquid drop. The forces that operate between protons and neutrons could be compared in some ways, he said, to the forces that operate between the molecules that make up a drop of liquid. In this respect, the nucleus is no more static than a droplet of water. Instead, Bohr suggested, the nucleus should be considered to be constantly oscillating and changing shape in response to its internal forces. The greatest success of the Bohr liquid-drop model was its later ability to explain the process of nuclear fission discovered by German chemist Otto Hahn, German chemist Fritz Strassmann, and Austrian physicist Lise Meitner in 1938.
Bohr continued to work at his Institute during the early years of World War II, devoting considerable effort to helping his colleagues escape from the dangers of Nazi Germany. When he received word in September 1943 that his own life was in danger, Bohr decided that he and his family would have to leave Denmark. The Bohrs were smuggled out of the country to Sweden aboard a fishing boat and then, a month later, flown to England in the empty bomb bay of a Mosquito bomber. The Bohrs then made their way to the United States, where both Bohr and his son became engaged in work on the Manhattan Project to build the world's first atomic bombs.
After the War, Bohr, like many other Manhattan Project researchers, became active in efforts to keep control of atomic weapons out of the hands of the military and under close civilian supervision. For his long-term efforts on behalf of the peaceful uses of atomic energy, Bohr received the first Atoms for Peace Award given by the Ford Foundation in 1957. Meanwhile, Bohr had returned to his Institute for Theoretical Physics and become involved in the creation of the European Center for Nuclear Research (CERN). He also took part in the founding of the Nordic Institute for Theoretical Atomic Physics (Nordita) in Copenhagen. Nordita was formed to further cooperation among and provide support for physicists from Norway, Sweden, Finland, Denmark, and Iceland.
Bohr reached the mandatory retirement age of seventy in 1955 and was required to leave his position as professor of physics at the University of Copenhagen. He continued to serve as director of the Institute for Theoretical Physics until his death in Copenhagen at the age of 77.
Bohr was held in enormous respect and esteem by his colleagues in the scientific community. American physicist Albert Einstein, for example, credited him with having a "rare blend of boldness and caution; seldom has anyone possessed such an intuitive grasp of hidden things combined with such a strong critical sense." Among the many awards Bohr received were the Max Planck Medal of the German Physical Society in 1930, the Hughes (1921) and Copley (1938) medals of the Royal Society, the Franklin Medal of the Franklin Institute in 1926, and the Faraday Medal of the Chemical Society of London in 1930. He was elected to more than twenty scientific academies around the world and was awarded honorary doctorates by a dozen universities, including Cambridge, Oxford, Manchester, Edinburgh, the Sorbonne, Harvard, and Princeton.
See also Bohr Model