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  Summer School
The Nature of Entropy 1
From Thermodynamics to Black Holes

22 – 27 July 2019 in Saig, Germany

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"The Impossible Process: Thermodynamic Reversibility"

John D. Norton

Monday, 22 July, 20:00.

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Clausius' original definition of entropy and a standard modern definition depends on the notion of  thermodynamically reversible processes. What are they?

Loosely speaking, the driving forces of a thermodynamically reversible process are so small that the process migrates very slowly through equilibrium states. More precisely, however, this characterization is contradictory. States change if and only if they are not in equilibrium. We cannot have both change and equilibrium. I review nearly 200 years of efforts to evade the problem. The solution, I propose, is that there is no single process that is reversible, but that the properties attributed to them are really the properties of a particular set of irreversible processes.

Getting clear on thermodynamically reversible processes matters, for then we find that they are not always available. Thermal fluctuations preclude them in processes at molecular scales. A neglect of this preclusion has rendered incoherent much of the work on information, thermodynamic entropy and Maxwell's demon.


John D. Norton, "The Impossible Process: Thermodynamic Reversibility," Studies in History and Philosophy of Modern Physics, 55 (2016), pp. 43-61.

John D. Norton, "Thermodynamically Reversible Processes in Statistical Physics," American Journal of Physics, 85 (2017), pp. 135-145.

Further Readings

John D. Norton, collected papers on Maxwell's Demon, Landauer's Principle and the Thermodynamics of Computation