EMERGENCE OF CLASSICAL PROPERTIES FROM QUANTUM-THEORY

We discuss how classical nature appears from quantum theory using two different points of view. The first employs a coarse-grained dynamics which is obtained by means of tracing over the environment. In this viewpoint, we focus on non-linear quantum systems and examine how this effects their decoherence. In the other viewpoint, we study a formalism which realizes decoherence without the partial trace. The roles of an infinite number of degrees of freedom on the issue are discussed.