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Next: Derivation of Collision Time Up: Maxwell's Demon and the Previous: The Stochastic Gas Model


It does seem to be possible in principle to create devices which are capable of reducing the entropy of a gas without actually doing work on it. This seems to be a contradiction of the second law of thermodynamics until we realize that, although no work is done on the gas, it is still not an ``isolated system;'' the entropy of the demon must be taken into account. The entropy of the demon is not in a form calculable by Clausius's formulation, as the demon need not change in temperature or energy so long as the operations of the demon are reversible. There is, however, an increase in the number of memory states accessible by the demon, and so Boltzmann's formulation of entropy can be adopted to measure the entropy of the demon as well. Once we begin measuring the demon-gas system taking into account the demon's memory, it becomes apparent that the demon cannot reduce the entropy of the overall system, except in rare fluctuations that cannot be predicted and used to violate the second law.

The demon and hard disk gas program was designed to test this, and to as nearly simulate an actual demon in a physical system as was practical. The gas simulated acted in accordance with known laws of statistical mechanics. After the proposed demon was added to the system, the predictions of the extension of the statistical law were followed. Even after the intervention of intelligently acting beings, the second law still holds.

Eric H. Neilsen
Mon Jun 16 13:53:44 EDT 1997