SUPERSELECTION RULES AND FLUCTUATIONS IN THE MANY-HILBERT-SPACES APPROACH TO QUANTUM MEASUREMENT

The interaction between a macroscopic detector and a quantum system is studied by means of a simple model in the context of the Many-Hilbert-Spaces approach, and it is shown that statistical fluctuations play a determinant role in the description of the detector. An order parameter is introduced, in terms of which a quantitative definition of wave-function collapse can be given. The meaning of the super-selection charges of the Many-Hilbert-Spaces is also clarified, and an inequivalent representation is given in the limit of infinite number of elementary constituents of the detector. Finally, this approach is compared to other proposals in the literature.