CONCEPT
Maxwell's Demon
Maxwell's 1867 thought experiment: a tiny intelligence stationed at a door between two chambers of gas that sorts fast molecules from slow without doing work, apparently defeating the second law of thermodynamics—a puzzle resolved a century later when Landauer proved that the price of intelligence is not knowing but forgetting.
In a letter to Peter Guthrie Tait in 1867, James Clerk Maxwell imagined a being “whose faculties are so sharpened that he can follow every molecule in its course” and stationed it at a tiny door between two chambers of gas at uniform temperature. The demon opens the door only to let fast molecules pass one way and slow ones the other; one chamber grows hot, the other cold; a temperature difference has appeared that could drive an engine—from no external work, in apparent violation of the second law of thermodynamics. Maxwell did not believe he had broken the law; he was exposing its statistical character, the fact that it is true of the large and clumsy but not necessarily of the sufficiently attentive. The demon pressed physics for a century until Rolf Landauer in 1961 located the answer: the price is not in measurement but
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