Maxwell's demon

Maxwell's demon is a thought experiment in which a tiny intelligent being operates a frictionless trap-door in a wall dividing a gas, opening it only to let fast molecules pass one way and slow molecules the other. Without apparently doing any work, the demon builds a temperature difference between the two chambers, lowering the gas's entropy and seeming to violate the second law of thermodynamics.

The resolution, developed over a century, is that the demon must acquire and store information about each molecule, and its memory is finite. Leó Szilárd (1929) recast the demon as a one-molecule engine and identified information as the currency; Rolf Landauer (1961) proved that erasing one bit of memory must dissipate at least k_B ln 2 of entropy; and Charles Bennett (1982) showed that measurement can be reversible, so it is the erasure needed to reset the memory that pays the bill. The entropy the demon removes from the gas is exactly repaid when it clears its records. This is a stub entry; the full resolution is treated alongside free energies in the statistical-mechanics module.

The demon's lasting lesson is that information is physical: logical operations on memory have unavoidable thermodynamic costs.

James Clerk Maxwell introduced the demon in 1867 (in a letter to Tait) and in his 1871 'Theory of Heat.' The information-theoretic resolution runs through Szilárd (1929), Landauer (1961), and Bennett (1982).