Maxwell's Demon

Maxwell's Demon is a thought experiment proposed by James Clerk Maxwell in 1867 to challenge the second law of Thermodynamics. Maxwell imagined a microscopic intelligence — the 'demon' — stationed at a small door between two chambers of gas. By selectively opening the door for fast molecules moving right and slow molecules moving left, the demon could drive a temperature gradient between the chambers without expending work. If successful, the demon would violate the second law by decreasing entropy without a compensating energy cost.

The thought experiment resisted resolution for nearly a century. Leo Szilard's 1929 analysis correctly identified that the demon's act of measurement must cost entropy — but placed the cost in the wrong location. The resolution, provided by Rolf Landauer in 1961 and clarified by Charles Bennett in 1982, is precise: the cost falls on erasure, not measurement. The demon can measure which molecules are fast or slow without thermodynamic penalty, provided the measurement is performed reversibly. But to reset its memory between cycles — to erase the record of the previous measurement — it must pay Landauer's minimum cost of kT ln 2 per bit erased. The second law is saved not by the impossibility of knowing but by the impossibility of forgetting for free.

Maxwell's Demon is thus not a failure of thermodynamics — it is a proof that information is physical. The demon's memory is a thermodynamic system. Its records are physical configurations. The substrate of knowledge has energy costs that no abstract description can wish away.