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High Quality Content by WIKIPEDIA articles! Although in restricted contexts one may find this symmetry, the observable universe itself does not show symmetry under time reversal, primarily due to the second law of thermodynamics. Time asymmetries are generally distinguished as either those which are intrinsic to the dynamic laws of nature, and those that are due to the initial conditions of our universe. The T-asymmetry of the weak force is of the first kind, while the T-asymmetry of the second law of thermodynamics is of the second kind. Physicists also discuss the time-reversal invariance of local and/or macroscopic descriptions of physical systems, independent of the invariance of the underlying microscopic physical laws. For example, Maxwell's equations with material absorption or Newtonian mechanics with friction are not time-reversal invariant at the macroscopic level where they are normally applied, even if they are invariant at the microscopic level when one includes the atomic motions into which the "lost" energy is translated.
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