# Foundations of statistical physics

## Relation between quantum mechanics and statistical mechanics

While the description of many-particle systems by the Schrodinger equation is deterministic, time symmetric and linear, their description by statistical mechanics is stochastic and irreversible. The decoherence theory and the theory of open quantum systems explain this difference by referring to a large environment, the detailed state of which is not observed. In contrast to this, we pursue the thesis that statistical mechanics reveals true limits of validity of unitary time evolution and linear superposition. This means that statistical mechanics gives useful hints for the explanation of the quantum measurement process. We work on this topic by critically analyzing the methods used in many particle physics and showing that they always mix unitary and nonunitary elements, and by studying models that are similar to spontaneous collapse models.