Complementarity (physics)

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        In physics, complementarity is a basic principle of quantum theory, 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; in the orthodox form, it is stated that a quantum mechanical system 
        consisting of a boson or fermion can either behave as a particle or as wave, but never 
        simultaneously as both. A less orthodox interpretation is the "duality condition," 
        described by the inequality due to Englert (see Phys. Rev. Lett., Vol. 77, 2154 (1996)), 
        which allows wave and particle attributes to co-exist, but postulates that a stronger 
        manifestation of the particle nature leads to a weaker manifestation of the wave nature 
        and vice versa.
      
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        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., [1] (from which parts of this discussion were 
        drawn.)
      
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        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 in contrast to traditional double-slit experiments where the slits 
        are less than one mm apart. Adding to the intellectual drama is that the neutron is a 
        baryon, a rather massive, slow-moving fermion, and thus not ephemeral in the way that a 
        photon (a boson) is.
      
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        The Afshar experiment is claimed to question the validity of the principle of 
        complementarity in quantum mechanics. However, see rebuttals by Unruh [2] and Kastner [3].
      
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