Maxwell's demon

Maxwell's demon is a thought experiment devised by the Scottish physicist James Clerk Maxwell in 1867. It imagines a microscopic being — a "demon" — that can observe the individual molecules of a gas and sort fast-moving molecules from slow-moving ones, effectively transferring heat from a cold reservoir to a hot one without doing work. If such a being existed, it would violate the second law of thermodynamics, which states that heat cannot spontaneously flow from cold to hot.

The paradox was resolved not by disputing the demon's ability to measure, but by recognizing the thermodynamic cost of information erasure. Rolf Landauer showed in 1961 that the demon must reset its memory after each measurement, and this erasure dissipates at least kT ln 2 of energy per bit — exactly enough to preserve the second law. The demon can see for free; it cannot forget for free.

The Maxwell demon thought experiment was the direct ancestor of modern information theory and stochastic thermodynamics. It established that information and thermodynamics are not separate domains but two aspects of a single physical reality.