Science Puzzle
Maxwell’s Demon
James Clerk Maxwell imagined a tiny demon sitting at a trapdoor between two halves of a box of gas. The demon watches the molecules and opens the door only to let fast-moving molecules through to the left and slow ones to the right. Over time, the left side gets hot and the right side gets cold.
This seems to create a temperature difference without doing any work, which would violate the Second Law of Thermodynamics. Where is the flaw?
The Answer
The demon must gather information about each molecule in order to decide whether to open the trapdoor. Measuring, storing, and then erasing that information requires physical work.
This resolution was identified by Rolf Landauer in 1961. Every time the demon erases a bit of information from its memory, it must dissipate a minimum amount of energy into the environment as heat (kT ln 2, where k is Boltzmann's constant and T is temperature). This energy cost exactly offsets the thermodynamic gain from sorting the molecules.
Maxwell's Demon therefore reveals a deep connection between information theory and thermodynamics: information has a physical cost. Erasing information generates entropy. This is the principle underlying the field of information thermodynamics.
The Second Law of Thermodynamics is safe: entropy in the total system (including the demon's memory) never decreases.
The principle: Information and thermodynamics are linked. Maxwell's Demon fails because erasing information generates entropy. Landauer's principle shows that every bit erased dissipates a minimum quantity of heat.