Collier, John. “Two faces of Maxwell’s demon reveal the nature of irreversibility.” Studies in the History and Philosophy of Science 21, no. 57-268 (1990): 22J.
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One of the most delightful thought experiments in the history of physical science is James Clerk Maxwell’s sorting demon.1 Suppose we have two adjacent chambers A and B, with a passage between them covered with a trap door that can slide open and shut on frictionless bearings. The chambers are filled with a gas at uniform temperature and pressure. In a 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 a relatively fast moving molecule moves towards it from B, or a relatively slow moving molecule moves towards it from A.2 Gradually, the manipulations of the demon lead, without the expenditure of available energy, to a sorting of the fast moving molecules into A and the slow moving molecules 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.