Maxwell's demon

A thought experiment in which an imagined agent sorts fast and slow gas molecules, apparently decreasing entropy without expending work. Its resolution connects information processing to the thermodynamics of measurement and memory.

Second law of thermodynamics: A law stating that the total entropy of an isolated system never decreases. The demon seems to violate this law by creating a temperature difference through sorting.
James Clerk Maxwell: A Scottish physicist whose work unified electricity, magnetism, and light and advanced kinetic theory. Maxwell introduced the imagined gatekeeper to expose a puzzle about entropy and molecular motion.
Leo Szilard: A Hungarian-American physicist who introduced a one-molecule engine linking information to thermodynamic work. His 1929 analysis made the demon’s information problem precise using a single-particle engine.
Perpetual motion machine of the second kind: A hypothetical device that extracts work from a single heat reservoir without other changes. A successful demon cycle would amount to this forbidden thermodynamic machine.
Feedback cooling: A technique that uses measurements and controlled forces to reduce a system’s random motion. It uses demon-like measurement and feedback to cool microscopic particles.
Landauer's principle: A lower bound requiring heat dissipation when logically irreversible information is erased. Erasing the demon’s recorded outcomes prevents a cycle from producing work without cost.
Kinetic theory of gases: A theory that explains gas properties through the motion and collisions of microscopic particles. The demon operates on individual molecules, making the theory’s microscopic view central to the puzzle.
Rolf Landauer: A physicist who established a thermodynamic cost for logically irreversible computation. Landauer identified memory erasure as a physical process that can restore the second law.
Brownian ratchet: A hypothetical mechanism that rectifies random thermal motion into directed work. Like the demon, it appears to extract work from equilibrium fluctuations until its own thermal behavior is counted.
Single-electron box: A nanoscale device that controls electron transfer onto a small conducting island. Experiments with this device demonstrate information-assisted work extraction and Landauer erasure costs.

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