Irreversibility and Measurement in Quantum Mechanics

It is argued that a restriction of the set of observables is necessary in order to effect "reduction of the wave packet". This restriction leads to an irreversible evolution for the system plus apparatus (S + A), a possibility which exists only when the apparatus is an infinite quantum system and when the S-A interaction is sufficiently unstable. The key to the resolution of the measurement problem, that is, the unification of the of the unification of the unitary (Schrodinger) evolution and the stochastic evolution of the measurement process, lies in finding a restriction of observables which breaks the time inversion symmetry. It is shown that the restriction of observables is related to rejection of aystem preparations which produce produce "anticausal" behavior. A general, mathematical approach to quantum measurement is presented and a specfic model is discussed which fulfills the general requirements. The relation among the measurement process, quantum K-flows and "internal aging" is explored, and the irreversible evolution (for S + A) is shown to have a monotonic contraction property.