On Evolution Laws Taking Pure States to Mixed States in Quantum Field Theory

TL;DR

This paper demonstrates that quantum field theory can have evolution laws transforming pure states into mixed states without violating causality or energy-momentum conservation, challenging previous assumptions.

Contribution

It introduces simple Markovian models allowing pure-to-mixed state evolution with negligible observable deviations, maintaining causality and conservation laws.

Findings

Markovian evolution laws can produce rapid pure-to-mixed state transitions

Deviations from standard physics are unobservably small in laboratory conditions

Energy-momentum conservation and causality can coexist with pure-to-mixed state evolution

Abstract

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 show 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 properties at the ``Planck scale", suitable locality and causality properties hold for all states, and the deviations from ordinary dynamics (and, in particular, violations of energy-momentum conservation) are unobservably small for all states which one could expect to produce in a laboratory. In addition, we argue (via consideration of other, non-Markovian models) that conservation of energy and momentum for all states is not fundamentally incompatible with causality in dynamical models in which pure states evolve to mixed states.