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The fluctuation theorem addresses the probability under [[statistical mechanics]] that the [[entropy]] of a system not at thermodynamic equilibrium (maximum [[entropy]]) will increase or decrease over a period of time.  

The '''fluctuation theorem''' addresses the probability under [[statistical mechanics]] that the [[entropy]] of a system not at thermodynamic equilibrium (maximum [[entropy]]) will increase or decrease over a period of time.  

The [[second law of thermodynamics]] predicts that such entropy should increase until equilibrium.  But under the theory of statistical mechanics there would always be a nonzero probability that the entropy of an isolated system will spontaneously decrease.  The fluctuation theorem precisely quantifies such probability under the theory of statistical mechanics.

The [[second law of thermodynamics]] predicts that such entropy should increase until equilibrium.  But under the theory of statistical mechanics there would always be a nonzero probability that the entropy of an isolated system will spontaneously decrease.  The fluctuation theorem precisely quantifies such probability under the theory of statistical mechanics.