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Complemetarity Principle

According to the uncertainty principle, the pair of conjugate variables such as the position and momentum of a particle is not well defined but exist only as opposing potentialities. These potentialities complement each other, since each is necessary in a complete description of the physical processes through which the particle manifests itself. This is referred as "principle of complementarity". The more general statement reads: At the quantum level, the most general physical properties of any system must be expressed in terms of complementary pairs of variables, each of which can be better defined only at the expense of a corresponding loss in the degree of definition of the other. In particular, particle and wave can be considered as one of those complementary pairs - no experiment can reveal both at once.

However, one experiment seems to contradict this principle. As shown in Figure 12-03g, the apparatus is similar to the
Matter and Wave "double-slit" experiment, but with a lens on the far side of the pinhole screen. The lens refocuses the spreading beams onto two mirrors, which reflect them onto two photon detectors tracking the path of the photons as particles. The interference pattern (wave property) is observed indirectly by placing wires in front of the lens at the "would be" positions of the dark fringes. It is argued that if the photons do not interfere, there will be no dark fringes and the wires will block some of the photons hitting the lens, reducing the photon count at the detectors. Since no such dip in the signal is seen, it implies that the light does form an interference pattern, violating the complementarity principle. Such claim has

Figure 12-03g Particle and Wave [view large image]

raised a storm of criticism initially in 2004, but the publication of the research in 2007 has forced a more cool-headed discussion now.

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