Loschmidt's Paradox
Version 1.0.0 · Updated 2026-07-30
CORE DEFINITION
Since microscopic mechanical laws (Newtonian/quantum) are time-reversible (playing a video backwards also obeys physical laws), why is the macroscopic second law of thermodynamics (entropy increase) time-irreversible?
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Emergent direction of time. Pure microscopic reduction cannot explain macroscopic irreversibility. The direction of time (aging, forgetting, disorder) is a statistical emergent result. In macroscopic management, one must go with this irreversible flow, because microscopic reversibility fails in the face of the law of large numbers.
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Microscopic particle equations of motion are symmetric under time reversal, from which it can be deduced that the evolution of any state can also be reversed; but macroscopically entropy increases irreversibly, questioning how the second law of thermodynamics can be derived from reversible laws, which is the explanatory difficulty of 'reversibility-irreversibility'.
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Source support: Explicit
- en.wikipedia.orghttps://en.wikipedia.org/wiki/Loschmidt%27s_paradoxverified
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