Preferred States, Predictability, Classicality and the Environment-Induced Decoherence

Selection of the preferred classical set of states in the process of decoherence is discussed with an emphasis on the role of correlations, information loss and entropy. Persistence of correlations between the observables of two systems (for instance, a record and a state of a system evolved from the initial conditions described by that record) in the presence of the environment is used to define classical behavior. From the viewpoint of an observer (or any system capable of maintaining records and utilising information) predictability is a measure of such persistence. sieve – a procedure which employs both the statistical and algorithmic entropies to systematically explore all of the Hilbert space of an open system in order to eliminate the majority of the unpredictable and non-classical states, and to locate the islands of predictability including the preferred pointer basis, is proposed. The relation between the consistent histories approach and the preferred sets of states is considered. It is demonstrated that histories of sequences of events corresponding to projections onto the states of the pointer basis are consistent. The correspondence between the perceived reality and the evolving state of the quantum Universe based on a prolonged existence of states of individual systems and correlations between them is proposed. This emphasis on predictable existence of states of classical systems and records of classical observers suggests existential interpretation of quantum theory. Existential interpretation addresses the long-standing problem of the “collapse of the wave function” in a manner which goes beyond Everett's relative state and Bohr's Copenhagen interpretations, by identifying the collapse with the decay of non-classical superpositions of states (and of the observers records) into mixtures of preferred states chosen by the environment-induced superselection.