Maxwell's Demon
Information, entropy, and the second law
First published: J. C. Maxwell, letter to P. G. Tait (1867); published in *Theory of Heat* (1871), ch. 12.
A clever gatekeeper sorts fast molecules from slow, creating a temperature difference for free — apparently violating the second law.
A gas-filled box is divided by a wall with a trapdoor; a tiny demon operates the door, letting fast molecules pass one way and slow ones the other. After a while, one side is hot and the other cold, and useful work can be extracted — without any apparent expenditure of energy. The second law of thermodynamics is in trouble. A century of analysis (Szilard 1929, Landauer 1961, Bennett 1982) resolved the puzzle by locating the entropy cost in *information processing*: erasing the demon's memory dissipates exactly enough heat to save the second law. The case is the founding document of the deep connection between thermodynamics and information.
Formulation
Insulated box, partition with controllable door, two-chamber gas at equilibrium. Demon observes incoming molecules and opens the door selectively, sorting by speed. After many cycles: ΔT ≠ 0, ΔS apparently < 0. Resolution: each bit of information the demon acquires must eventually be erased, and Landauer's principle requires erasure to dissipate kT·ln 2 of heat per bit.
Dimensions Engaged
Energy
Forces Energy · Dispersibility into focus: the second law survives the demon because the apparent decrease in thermodynamic entropy is exactly compensated by the entropy cost of the demon's information-processing.
Information
The canonical argument for Information · Ontological Status as substantival: bits are not free; they have thermodynamic cost. Landauer's "information is physical" descends directly from this thought experiment.
Observer
Engages Observer · Knowledge Extent: the demon's observations are not free of physical consequence. Knowing requires energy.
Responses — How Schools Engage
Affirms / takes the bait 3
Landauer–Bennett resolution: the demon is a physical system, its memory is physical, and erasure is dissipative. The second law is preserved — and information is shown to be a thermodynamic quantity.
The demon is the founding parable: information is not epiphenomenal but constitutive — bits cost energy, and the universe's book-keeping is informational at the deepest level.
The argument shows that thermodynamic entropy is best understood structurally — as a property of the demon-plus-gas system, not the gas alone. Local descriptions miss the relevant cost.
Reframes the question 1
The demon dramatises the empirical content of the second law: it holds *because* the apparent loopholes, on careful operational analysis, cost what they claim to save.
Holds it inconclusive 1
The Landauer-Bennett resolution is widely accepted but remains contested in detail (Earman & Norton, Maroney). The case is a live boundary between physics and metaphysics of information.
Related Experiments
Experiments engaged by an overlapping set of schools — likely to surface the same fault lines.
Further reading
- Leff & Rex (eds.), *Maxwell's Demon 2* (2003)
- Landauer, "Irreversibility and Heat Generation in the Computing Process" (1961)
- Bennett, "The Thermodynamics of Computation" (1982)
Related Historical Debates
Debates that share dimensions and/or aligned schools with this experiment.
Personas Most Aligned With This Experiment
Ranked by total declared-influence weight in the schools that respond to this experiment.
Works Most Aligned With This Experiment
Ranked by total declared-influence weight in the schools that respond to this experiment.
Related Films
Films engaging the same dimensions as this experiment.
Related Contemporary Dilemmas
Dilemmas that engage the same dimensions as this experiment.