At the level of tiny particles, the laws of physics are symmetrical in time. A reaction that proceeds in one direction (such as particle A transforming into particle B) is just as likely to occur in the reverse direction (particle B transforming into A).
It’s not too strange a concept: A video showing a billiard ball’s initial bounce off a pool table’s cushion would look the same whether it was running backward or forward in time. The physics works just as well, and identically, either way.
Yet experiments since the 1960s have suggested there should be exceptions to this rule — special cases of so-called “time-reversal violation.” The first conclusive evidence of such a violation was finally discovered late last year by collaborators on an experiment named BaBar at Stanford University’s SLAC National Accelerator Laboratory. Read more