Szilard engine

Szilard engine is a thought experiment proposed by the Hungarian physicist Leo Szilard in 1929, a direct predecessor to the later paradox of Maxwell's demon. The engine consists of a single molecule trapped in a container divided by a movable partition. The molecule's position — left or right of the partition — constitutes one bit of information. By inserting the partition and measuring which side the molecule occupies, an observer can extract work kT ln 2 from the heat bath by allowing the molecule to push the partition outward.

Szilard's engine was the first explicit demonstration that information has thermodynamic value. One bit of information, properly deployed, can be converted into a definite amount of mechanical work. This established the quantitative bridge between information theory and thermodynamics that Rolf Landauer would later complete with his principle on the thermodynamic cost of erasure.

The resolution of the apparent paradox — why the engine does not violate the second law — mirrors the resolution of Maxwell's demon: the measurement and memory operations required to run the engine have a thermodynamic cost that exactly compensates for the extracted work. Information is not free.