Maxwell's Demon

Maxwell's Demon is the thought-experiment introduced by James Clerk Maxwell in 1867 to probe the limits of the Second Law of Thermodynamics. A hypothetical microscopic being — the 'demon' — guards a partition between two chambers of gas and sorts fast molecules from slow ones, apparently decreasing entropy without doing work and violating the second law.

The paradox haunted physics for nearly a century. The resolution, articulated by Charles Bennett in 1982 building on Rolf Landauer's work, is precise: the demon does not pay for measurement; it pays for erasure. The demon can record molecule velocities reversibly, but to continue operating it must eventually clear its memory. That erasure, by Landauer's Principle, costs k_B T ln 2 per bit — exactly enough to preserve the second law.

The demon is therefore not a paradox but a prototype: the foundational demonstration that information processing has thermodynamic consequences. It is the ancestor of the Szilard Engine and the conceptual bridge to the modern thermodynamics of computation.

Maxwell's demon is often presented as a failed attempt to break the second law. The truth is the opposite: the demon is the second law's most elegant proof. By showing that information erasure is the hidden cost, the demon does not defeat thermodynamics — it extends thermodynamics to include information as a thermodynamic variable. The demon lost the battle against the second law but won the war for the physics of information.