Loschmidt's paradox

Loschmidt's paradox states that if there is a motion of a system that leads to a steady decrease of H (increase of entropy) with time, then there is certainly another allowed state of motion of the system, found by time reversal, in which H must increase.

This puts the observed and theoretical reversibility of all known low-level fundamental physical processes at odds with the second law of thermodynamics which descibes the bahavior of macroscopic systems. However, both are well-accepted principles in physics. One possible resolution of Loschmidt's paradox is that there is a so-called arrow of time which disallows processes where entropy increases at a cosmic scale.