EVOLUTION LAWS TAKING PURE STATES TO MIXED STATES IN QUANTUM-FIELD THEORY

It has been argued that any evolution law taking pure states to mixed states in quantum field theory necessarily gives rise to violations of either causality or energy-momentum conservation in such a way as to have unacceptable consequences for ordinary laboratory physics. We sho w here that this is not the case by giving a simple class of examples of Markovian evolution laws where rapid evolution from pure states to mixed states occurs for a wide class of states with appropriate proper ties at the ''Planck scab,'' suitable locality and causality propertie s hold for all states, and the deviations from ordinary dynamics (and, in particular, violations of energy-momentum conservation) are unobse rvably small for all states which one could expect to produce in a lab oratory. In addition, we argue (via consideration of other, non-Markov ian models) that conservation of energy and momentum for all states is not fundamentally incompatible with causality in dynamical models whi ch pure states evolve to mixed states.