time reversal (T)
/ tee (the letter T) /
Film a collision of two billiard balls, then play the movie backwards. The reversed clip — balls flying together and bouncing apart in the opposite order — still looks like a perfectly ordinary collision that could really happen. Time reversal, written with the letter T, is exactly this: run a process backwards in time and ask whether the laws of physics still permit it. For the basic mechanics of a few particles, the answer is yes; the microscopic rules do not care which way the clock runs.
More precisely, T reverses the direction of time, which means every motion runs in reverse: velocities flip, and so do spins. A symmetry under T means that for any allowed process, the exactly time-reversed process is also allowed and follows the same rules. This is subtle because it clashes with everyday experience — a shattered glass never reassembles, a drop of ink never un-mixes from water. But that one-way feeling of time comes from statistics, from there being overwhelmingly more messy arrangements than tidy ones, not from the underlying particle laws themselves, which are very nearly time-symmetric.
Almost — but not quite. The weak force violates T symmetry by a tiny amount, which experiments have directly detected: certain particle transitions happen at a slightly different rate forwards than backwards. This matters enormously because of the CPT theorem, a deep result saying the combination of charge conjugation, parity, and time reversal together must be an exact symmetry of any sensible theory. If T is violated, then CP (charge-and-mirror) must be violated by exactly the same amount to keep CPT intact — linking the arrow of microscopic time directly to the small difference between matter and antimatter.
Experiments with neutral kaons and B mesons have measured tiny differences between a particle turning into its antiparticle and the reverse process running backwards in time — a direct sighting of T violation, exactly mirroring the CP violation seen in the same systems, just as the CPT theorem demands.
Measured T violation mirrors CP violation, as the CPT theorem requires.
Microscopic T violation is real but minuscule, and it is not the reason eggs do not unscramble: the everyday arrow of time comes from thermodynamics and probability, not from this tiny breakdown of T in the weak force.