  A source of unpredictability is equivalent to a source of information:
unpredictability means not knowing which of a set of alternatives is the actual
one; determining the actual alternative yields information. The degree of
unpredictability is neatly quantified by the information measure introduced by
Shannon. This perspective is applied to three kinds of unpredictability in
physics: the absolute unpredictability of quantum mechanics, the
unpredictability of the coarse-grained future due to classical chaos, and the
unpredictability of open systems. The incompatibility of the first two of these
is the root of the difficulty in defining quantum chaos, whereas the
unpredictability of open systems, it is suggested, can provide a unified
characterization of chaos in classical and quantum dynamics.
