Quantum theory of measurement

Quantum theory of measurement

The attempt to reconcile the counterintuitive features of quantum mechanics with the hypothesis that quantum mechanics is in principle a complete description of the physical world, even at the level of everyday objects. A paradox arises because, at the atomic level where the quantum formalism has been directly tested, the most natural interpretation implies that where two or more different outcomes are possible it is not necessarily true that one or the other is actually realized, whereas at the everyday level such a state of affairs seems to conflict with direct experience.

The resolution of this paradox that is probably most favored by practicing physicists proceeds in two stages. At stage 1, it is pointed out that, quite generically, whenever the quantum formalism appears to generate a superposition of macroscopically distinct states it is impossible to demonstrate the effects of interference between them. The reasons for this claim include the facts that the initial state of a macroscopic system is likely to be unknown in detail; the initial state has extreme sensitivity to random external noise; and most important, merely by virtue of its macroscopic nature any such system will rapidly have its quantum-mechanical state correlated (entangled) with that of its environment in such a way that no measurement on the system alone (without a simultaneous measurement of the complete state of the environment) can demonstrate any interference between the two states in question—a result often known as decoherence. Thus, it is argued, the outcome of any possible experiment on the ensemble of macroscopic systems prepared in this way will be indistinguishable from that expected if each system had actually realized one or the other of the two macroscopically distinct states in question. Stage 2 of the argument (often not stated explicitly) is to conclude that if this is indeed true, then it may be legitimately asserted that such realization of a definite macroscopic outcome has indeed taken place by this stage.

Most physicists agree with stage 1 of the argument. However, not all agree that the radical reinterpretation of the meaning of the quantum formalism which is implicit at stage 2 is legitimate; that is, an interpretation in terms of realization, by each individual system, of one alternative or the other, forbidden at the atomic level by the observed phenomenon of interference, is allowed once, on going to the macroscopic level, the phenomenon disappears. Consequently, various alternative interpretations have been developed. See Quantum mechanics