Fundamental Physical Limits on Computation

Current (May 1995) revision of 1992 report. We consider limitations on the performance of computers arising from thermodynamics and the laws of physics. We provide upper bounds on three quantities: sustained information flux, information storage density, and sustained computational speed. All of these upper bounds are "tight" in the sense that they could be approached by plausible-sounding physical systems, and they all arise from a single unified point of view. We also make a conjecture about the rate of inevitable decay of stored information. This conjecture may be thought of as a quantitative extension of the second law of thermodynamics. It leads to a bound on the density of stable information. We carefully elucidate the assumptions behind these bounds. We give a list of 4 open problems at the end. KEYWORDS: thermodynamics, computation, reversible Turing machines, blackbody radiation, decay of information, physics, entropy, information transmission and storage, cooling requirement..