= Maxwell's demon
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<Maxwell's demon> is a thought experiment in which a controller measures microscopic motion and selectively operates a barrier, apparently driving a system away from <thermal equilibrium> without the usual thermodynamic cost. For example, admitting fast gas particles preferentially to one side seems to create a <temperature> difference that a <heat engine> could exploit.
The controller must be included in the physical accounting. Its measurement record becomes correlated with the gas, and a repeatable cyclic device must deal with that record rather than treating information storage as an unlimited free resource. <Landauer's principle> connects logically irreversible memory erasure with a thermodynamic cost under its stated conditions. This does not mean that every measurement automatically dissipates exactly one bit's erasure cost. The point is to track the complete cycle, correlations and resources. A controller that reduces one subsystem's <entropy> can still respect the <Second law of thermodynamics> by increasing <entropy> elsewhere.
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