What condensed matter physics and statistical physics teach us about the limits of unitary time evolution

The Schrodinger equation for a macroscopic number of particles is linear in the wave function, deterministic, and invariant under time reversal. In contrast, the concepts used and calculations done in statistical physics and condensed matter physics involve stochasticity, nonlinearities, irreversibility, top-down effects, and elements from classical physics. This paper analyzes several methods used in condensed matter physics and statistical physics and explains how they are in fundamental ways incompatible with the above properties of the Schrodinger equation. The problems posed by reconciling these approaches to unitary quantum mechanics are of a similar type as the quantum measurement problem. This paper therefore argues that rather than aiming at reconciling these contrasts one should use them to identify the limits of quantum mechanics. The thermal wave length and thermal time indicate where these limits are for (quasi-)particles that constitute the thermal degrees of freedom.