Foundations of Physics

, Volume 25, Issue 8, pp 1221-1229

First online:

A quantum-mechanical treatment of Szilard's engine: Implications for the entropy of information

  • L. C. BiedenharnAffiliated withCenter for Particle Physics, Department of Physics, University of Texas at Austin
  • , J. C. SolemAffiliated withTheoretical Division, Los Alamos National Laboratory

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We present a quantum-mechanical analysis of Szilard's famous single-molecule engine, showing that it is analogous to the double-slit experiment. We further show that the energy derived from the engine's operation is provided by the act of observing the molecule's location. The engine can be operated with no increase in physical entropy, and the second law of thermodynamics does not compel us to relate physical entropy to informational entropy. We conclude that information per seis a subjective, idealized, concept separated from the physical realm. Physical entropy depends on physical objects and physical interactions, and any entropy change owing to observations is entirely a result of the entropy created in the physical apparatus by the process of observation.