ABSTRACT
In this chapter we shall consider one of the most difficult problems
of quantum mechanics, that is, how it is possible that we have a
dynamics ruled by a reversible equation but we obtain random
irreversible events when measuring. Many different interpretations
have been provided for solving this puzzle. We will discuss some of
them by starting with the von Neumann’s projection postulate, one
of the first attempts to address the measurement problem and its
implications. The role played by the environment in the concept of
measurement is then emphasized. We shall also introduce entropy
as a measure of the average information content. Finally, the famous
Schro¨dinger’s cat paradox will be examined.
