Unpredictability, information, and chaos

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.