Szilard engine

The Szilard engine is a thought experiment that reduces Maxwell's demon to its simplest form: a single molecule in a box at temperature T. A demon measures which half the molecule occupies, inserts a partition, and lets the molecule push the partition outward as a piston. The resulting isothermal expansion from half the box to the whole box extracts exactly W = k_BT ln 2 of work from the surrounding heat bath.

Repeating the cycle appears to convert heat into work indefinitely, in violation of the second law. Szilard's insight was that the engine runs only because the demon acquired one bit of information — which side the molecule was on — and that this information must eventually be accounted for thermodynamically. The paradox is not resolved by the measurement itself but by what must later be done with the record.

The resolution, completed by Landauer and Bennett, is that erasing the demon's one-bit memory to begin a fresh cycle dissipates at least k_BT ln 2 of heat — exactly the work extracted. The net work over a full cycle is zero, and the second law is preserved. The Szilard engine is thus the founding example linking information to entropy.

Proposed by Leó Szilárd in his 1929 paper 'On the Decrease of Entropy in a Thermodynamic System by the Intervention of Intelligent Beings', the first work to quantify the entropy associated with a measurement and a milestone in the prehistory of information theory.