The Duality in Matter and Light; December 1994; Scientific American Magazine; by Englert, Scully, Walther; 6 Page(s)

In the microcosmos of quantum mechanics, phenomena abound that fly in the face of common sense. Many of these effects are a consequence of the principle of complementarity. Its most popular manifestation is the waveparticle duality. A microscopic object, such as a photon, an atom or an electron, can appear to behave as a water wave in one instance and as a discrete particle in another. Both features complement one another as a complete description of the object. Since the idea of complementarity was first enunciated more than 70 years ago, a belief common among many physicists has been that it is simply a consequence of the uncertainty relation. According to this rule, two complementary variables, such as position and momentum, cannot simultaneously be measured to less than a fundamental limit of accuracy. The uncertainty relation normally prevents one from learning everything about the behavior of a quantum object. As a result, we can never see the object acting both as a particle and as a wave.

Recently we and our colleagues have worked to show that uncertainty is not the only enforcer of complementarity. We devised and analyzed both real and thought experiments that bypass the uncertainty relation, in effect, to "trick" the quantum objects under study. Nevertheless, the results always reveal that nature safeguards itself against such intrusions--complementarity remains intact even when the uncertainty relation plays no role. We conclude that complementarity is deeper than has been appreciated: it is more general and more fundamental to quantum mechanics than is the uncertainty rule.