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In physics, complementarity is a basic principle of quantum theory closely identified with the Copenhagen interpretation, and refers to effects such as the wave-particle duality, in which different measurements made on a system reveal it to have either particle-like or wave-like properties. Niels Bohr is usually associated with this concept, which he developed at Copenhagen with Heisenberg, as a philosophical adjunct to the recently developed mathematics of quantum mechanics and in particular the Heisenberg uncertainty principle; in the narrow orthodox form, it is stated that a single quantum mechanical entity can either behave as a particle or as wave, but never simultaneously as both; that a stronger manifestation of the particle nature leads to a weaker manifestation of the wave nature and vice versa. In a broader, more philosophical vein Bohr stated: Complementarity or wave-particle duality is considered to be one of the distinguishing characteristics of quantum mechanics, whose theoretical and experimental development has been honoured by more than a few Nobel Prizes for Physics. It has been discussed by prominent physicists for the last 100 years, from the time of Albert Einstein, Niels Bohr and Werner Heisenberg, onwards. On the basis of Bohr's principle of complementarity, it is indeed universally accepted that the observation of two complementary properties, such as position and momentum, requires mutually exclusive experimental measurements. The emergence of complementarity in a system occurs when one considers the circumstances under which one attempts to measure its properties; as Bohr noted, the principle of complementarity "implies the impossibility of any sharp separation between the behaviour of atomic objects and the interaction with the measuring instruments which serve to define the conditions under which the phenomena appear." It is important to distinguish, as did Bohr in his original statements, the principle of complementarity from a statement of the uncertainty principle. For a technical discussion of contemporary issues surrounding complementarity in physics, see, e.g., * (from which parts of this discussion were drawn.) Various neutron interferometry experiments demonstrate the subtleness of the notions of duality and complementarity in an interesting way. In order to pass through the interferometer, the neutron must act as a wave. Yet upon passage, the neutron is subject to gravitation, which one would think only particles, and not waves, are subject to. As the neutron interferometer is rotated through Earth's gravitational field a phase change between the two arms of the interferometer is created, resulting in a change in the constructive and destructive interference of the neutron waves on exit from the interferometer. Note that in order to understand the interference effect one must concede that a single neutron takes both paths through the interferometer at the same time: a single neutron must "be in two places at once", as it were. Since the two paths through a neutron interferometer can be as far as five to 15 cm apart, the effect is hardly microscopic. This is not in contrast to traditional double-slit experiments (or mirror interferometer) where the slits (or mirrors) can be arbitrary far apart. So in interference and diffraction experiments neutron behaves the same way as a photon (or an electron) of corresponding wavelength.
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