T-symmetry  

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T-symmetry or time reversal symmetry is the theoretical symmetry of physical laws under the transformation of time reversal,

<math> T: t \mapsto -t.</math>

Since the second law of thermodynamics states that entropy increases as time flows toward the future, in general, the macroscopic universe does not show symmetry under time reversal. In other words, time is said to be non-symmetric, or asymmetric, except for special equilibrium states when the second law of thermodynamics predicts the time symmetry to hold. However, quantum noninvasive measurements are predicted to violate time symmetry even in equilibrium, contrary to their classical counterparts, although this has not yet been experimentally confirmed.

Time asymmetries generally are caused by one of three categories:

1. intrinsic to the dynamic physical law (e.g., for the weak force)
2. due to the initial conditions of our universe (e.g., for the second law of thermodynamics)
3. due to measurements (e.g., for the noninvasive measurements)

See also

• The second law of thermodynamics, Maxwell's demon and the arrow of time (also Loschmidt's paradox).
• Microscopic reversibility
• Detailed balance
• Applications to reversible computing and quantum computing, including limits to computing.
• The standard model of particle physics, CP violation, the CKM matrix and the strong CP problem
• Neutrino masses and CPT invariance.
• Wheeler–Feynman absorber theory
• Tesla valve