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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 (1843–1931). 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 (1853–1947) 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.

Complementarity

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 (1901–1976) 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 (1887–1961) 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 [1963], 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 [1963], 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

HENRY J. FOLSE, JR.