Two faces of Maxwell's demon reveal the nature of irreversibility

Abstract

One of the most delightful thought experiments in the history of physical science is James Clerk Maxwell's sorting demon. Suppose we have two adjacent chambers A and B, with passage between them covered with trap door that can slide open and shut on frictionless bearings. The chambers are filled with gas at uniform temperature and pressure. In letter to Peter Tait in late 1867, Maxwell described a very observant and neat-fingered being that sits by the passage between the chambers and opens the door whenever either relatively fast moving molecule moves towards it from B, or relatively slow moving molecule moves towards it from A. Gradually, the manipulations of the demon lead, without the expenditure of available energy, to sorting of the fast moving into A and the slow moving into B. This lowers the temperature in B relative to A, decreasing the total entropy of the system, apparently violating of the Second Law of Thermodynamics. The exact function of the demon thought experiment remains ambiguous even today. It seems that Maxwell originally invoked the demon to respond to pessimism stimulated by the dissipation of in thermodynamic systems. Although he soon recognized that the demon meant that the Second Law had only statistical certainty, his original concern is clear in an early 1868 letter to William Thomson (Lord Kelvin), in which Maxwell jocularly refers to the demon as pointsman for flying molecules which showed that energy need not be dizzypated as in the present wasteful world. Thomson had established in 1852 that dissipation of mechanical occurred, and was central to the explanation of thermal phenomena in geological context. In 1867, when Maxwell first proposed the demon, the relation between mechanical dissipation and entropy was not clearly understood. Indeed, in defending Thomson's priority in discovering the Second Law, Tait (a close friend of both Maxwell and