# time reversal

Time reversal is the mathematical operation analogous to running a movie film backward, i.e.
replacing time *t* with time -*t*. In other words, a transformation
from agiven physical system undergoing a given sequence of events to a system
in which the exact reverse sequence of states is undergone. The symmetry
of time reversal (symbol T) is known as T invariance. As with CP
violation, T violation occurs in certain weak
interactions, notably kaon decay. See
also CPT theorem.