The Principle of Complementarity
The idea of wave-particle duality has no classical counterpart. In classical physics, a given entity must be exclusively one or the other. But this has come at the expense of great conceptual difficulty. We must somehow accommodate the classically irreconcilable wave and particle concepts. This accommodation involves what is known as the principle of complementarity, first enunciated by Bohr. The wave-particle duality is just one of many examples of complementarity. The idea is the following. Objects in nature are neither particles nor waves; a given experiment or measurement which emphasizes one of these properties necessarily does so at the expense of the other. An experiment properly designed to isolate the particle properties, such as Compton scattering or the observation of cloud chamber tracks, provides no information about the wave aspects. Conversely, an experiment properly designed to isolate the wave properties, for example the diffraction, provides no information about its particle aspects. The conflict is thus resolved in the sense that irreconcilable aspects are not simultaneously observable in principle. Other examples of complementary aspects are the position and linear momentum of a particle, the energy of a given state and the length of time for which the state exists, the angular orientation of a system and its angular momentum, and so on. The quantum mechanical description of the properties of a physical system is expressed in terms of pairs of mutually complementary variables or properties. Increasing precision in the determination of one such variable necessarily implies decreasing precision in the determination of the other.