Editing T-symmetry (section)

===Time reversal of the known dynamical laws===
[[Particle physics]] codified the basic laws of dynamics into the [[standard model]]. This is formulated as a [[quantum field theory]] that has [[CPT symmetry]], i.e., the laws are invariant under simultaneous operation of time reversal, [[parity (physics)|parity]] and [[charge conjugation]]. However, time reversal itself is seen not to be a symmetry (this is usually called [[CP violation]]). There are two possible origins of this asymmetry, one through the [[CKM matrix|mixing]] of different [[flavour (particle physics)|flavour]]s of quarks in their [[Weak interaction|weak decay]]s, the second through a direct CP violation in strong interactions. The first is seen in experiments, the second is strongly constrained by the non-observation of the [[Neutron electric dipole moment|EDM of a neutron]].

Time reversal violation is unrelated to the [[second law of thermodynamics]], because due to the conservation of the [[CPT symmetry]], the effect of time reversal is to rename [[elementary particle|particle]]s as [[antiparticle]]s and ''vice versa''. Thus the [[second law of thermodynamics]] is thought to originate in the [[initial conditions]] in the universe.