Category: physics
Associated thinker(s): James Clerk Maxwell
Original publication or approximate period: Private proposal (1867); Theory of Heat (1871)
Difficulty: Intermediate
Type: thought experiment
Themes: entropy; information; thermodynamics
Related experiments: schrodingers-cat; st-petersburg-paradox
A gas fills two compartments joined by a small doorway. A hypothetical controller can distinguish faster and slower molecules. It admits selected fast molecules to one side and selected slow molecules to the other, apparently creating a temperature difference without the usual expenditure of work.
The gas alone appears to move away from equilibrium. But the controller, gate, memory, power supply, and eventual resetting process also belong in the physical account. A useful analysis must identify a full cycle rather than inspect only the successful sorting stage.
Can microscopic information and selective control be used to extract work in a repeatable cycle without an offsetting thermodynamic cost?
The demon asks where the boundary of a thermodynamic system should be drawn. It connects physical information processing with the difference between a temporarily ordered subsystem and a complete engine. This makes it a productive test of explanations of entropy, even when no literal tiny observer is imagined.
Information-thermodynamic accounts examine measurement, feedback, and especially the resetting of memory; Landauer-style arguments locate a cost in logically irreversible erasure under relevant conditions. Foundational critiques ask whether these arguments assume the thermodynamic principles they are supposed to establish. The debate concerns explanatory details and scope, not an accepted construction of an unlimited perpetual-motion machine.
“Information is energy” is too crude: a bit does not possess a universal energy independent of its implementation and environment. Measurement need not always dissipate the same fixed amount. A local entropy decrease is not automatically a violation of a law about a larger closed system.
Szilard’s one-molecule engine isolates the role of measurement and feedback. Modern feedback experiments implement limited relatives of the scenario, with explicit accounting of resources rather than a supernatural being.
The linked statistical-mechanics entry documents the historical demon literature and explains competing information-based and critical treatments. This article avoids presenting a disputed philosophical “exorcism” as the only settled explanation.
Verified on 2026-10-30. Source coverage: [1] Section 7.2: demon, information, erasure, unresolved foundational issues. [2] Demon’s first mention in the 1867 letter to Tait and information/thermodynamics debate. Reflection questions and illustrative numerical examples are original editorial material.