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Quantum Theory and the Appearance of a Classical World

It is a puzzling fact that the most successful theory of this century, namely, quantum theory, until now has been bothered by severe interpretation problems. Every physicist knows how to manage with the symbols of quantum theory-wave functions, operators, probabilities, and so on. On these points, there is complete agreement. If, however, a physicist is urged to explain what these symbols mean, then it seems that there are not even two physicists professing to the very same interpretation of these symbols. From a practical point of view, quantum mechanics talks about measurement results. However, what is a measurement? Since the analysis given by John von Neumann, we are quite sure that a measurement process cannot be described by a Schrodinger equation. The Copenhagen school strongly insisted that measurement results had to be described in classical terms. This leads to a new problem: What is or when is a classical object? Is there some way, a t least some hint, how to understand classical concepts out of quantum concepts? This is the topic of the following discussion.