Violating time-reversal symmetry

Predicted half a century ago, time-reversal asymmetry in particle physics has only now been clearly demonstrated.

The weak interactions of elementary particles have long been known to be asymmetric under CP, the combined operation of parity P and charge conjugation C, the replacement of particles by their antiparticles. But absolute invariance under CPT, the combination of CP with time reversal T, is a bedrock theorem of particle theory's standard model. So it predicts that the weak interactions must violate T invariance to compensate for the CP violation. But only now has the first clear, direct evidence of T violation been reported.