DECOHERENCE AND THE TRANSITION FROM QUANTUM TO CLASSICAL.

This article explains the process of decoherence and the emergence of classical behavior from quantum dynamics. An analysis of quantum measurements due to John von Neumann is presented in this article. The quantum mechanics of the universe must allow for possible large quantum fluctuations of space-time geometry at certain epochs and scales. In particular, it may include important effects of quantum gravity early in the expansion of the universe. Significant progress in the study of decoherence in this context has been reported by Murray Gell-Mann and James B. Hartle, who are pursuing a program suitable for quantum cosmology that may be called the many-histories interpretation. A set of coarse-grained alternatives for a quantum system at a given time can be represented by a set of mutually exclusive projection operators, each corresponding to a different range of values for some properties of the system at that time. The definition of consistent histories for a closed quantum system was first proposed by Robert Griffiths. Gell-Mann and Hartle pointed out that in practice somewhat stronger conditions than Griffiths' tend to hold whenever histories decohere. The origin of the question about the interpretation quantum physics can be traced to the clash between predictions of the Schrödinger equation and the perceptions presented in this paper. INSET: Classical and quantum correlations..