ABSTRACT
Physical processes are viewed as computations, and the difficulty of answering questions about them is characterized in terms of the difficulty of performing the corresponding computations. There is a close correspondence between physical processes and computations. Much of theoretical physics has, however, been concerned with devising shorter methods of calculation that reproduce the outcome without tracing each step. This chapter explores some fundamental consequences of this correspondence. It suggests that it is also common in theoretical physics. Computational reducibility may well be the exception rather than the rule: Most physical questions may be answerable only through irreducible amounts of computation. The chapter suggests that many physical systems are computationally irreducible, so that their own evolution is effectively the most efficient procedure for determining their future. A theoretical model may be considered as a finite specification of the possible behavior of a system. Real-number parameters in classical physics allow infinite information density.
