Macroscopic systems in quantum mechanics

A macroscopic system is a quantum system with numerous degrees of freedom. As a consequence the description in terms of individual levels and eigenstates is inappropriate. Rather, the relevant concepts are macrostates and macrovariables. It is shown how to construct these. It is found that on this macroscopic level the state of the system can be described as a classical ensemble. This result provides the answer to the paradoxes of quantum measurement theory. It is also shown that the evolution of the ensemble is governed by a classical master equation.