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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.

Simon van der Meer's stochastic cooling uses measured beam fluctuations, a pick-up, amplifier and timed kicker. Finite samples supply the signal; other particles and electronic noise contribute unwanted kicks. Delay and oscillation phase must make the correction oppose the sampled deviation.^[ref]

This active system dissipates energy; it is no one-particle Maxwell's demon individually tracking antiprotons. The stochastic cooling variance model isolates the tradeoff.