WHAT IS WAVE-FUNCTION COLLAPSE BY MEASUREMENT

On the basis of the many-Hilbert-space approach to quantum measurement s, we reformulate the notion of wave-function collapse as a dephasing process, without making use of von Neumann's projection postulate. A ' 'detector'' is described as an open quantum system, characterized by t hermodynamical properties, and endowed with a many-Hilbert-space struc ture: These features enable us to provoke decoherence and to perform q uantum measurements. Our approach is compared to other Copenhagen-base d theories in some detail.