Maxwell's demon is a thought experiment proposed by the physicist James Clerk Maxwell in 1867. It illustrates a concept in thermodynamics, specifically relating to the second law of thermodynamics, which states that the total entropy of an isolated system can never decrease over time. In the thought experiment, imagine a container divided into two parts by a wall with a small door that can be opened and closed. The container is filled with gas molecules at an equal temperature.
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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.