Macroscopic and mesoscopic changes of entropies in detectors and collapse in quantum-measurement processes.

Abstract

Two different mechanisms for the wave-function collapse represented by the disappearance of the off-diagonal terms in a density matrix are investigated from the viewpoint of a change of entropies in detectors by using a simple calculable model. In the mechanism called quantum collapse, the difference between the entropy of the initial detector state and that of the final one becomes a macroscopic value in the macroscopic limit where the collapse is realized, while there is no such difference in the other mechanism called pure statistical collapse, induced by thermal fluctuations of active particle numbers in detectors