Time-Reversal Violation Is Not the “Arrow of Time”

Sean Carroll in Cosmic Variance:

Looks like the good folks at the BaBar experiment at SLAC, feeling that my attention has been distracted by the Higgs boson, decided that they might be able to slip a pet peeve of mine past an unsuspecting public without drawing my ire. Not so fast, good folks at BaBar!

They are good folks, actually, and they’ve carried out an extremely impressive bit of experimental virtuosity: obtaining a direct measurement of the asymmetry between a particle-physics process and its time-reverse, thereby establishing very direct evidence that the time-reversal operation “T” is not a good symmetry of nature. Here’s thetechnical paper, the SLAC press release, and a semi-popular explanation by the APS. (I could link you to the Physical Review Letters journal server rather than the arxiv, but the former is behind a paywall while the latter is free, and they’re the same content, so why would I do that?)

The reason why it’s an impressive experiment is that it’s very difficult to directly compare the rate of one process to its precise time-reverse. You can measure the lifetime of a muon, for example, as it decays into an electron, a neutrino, and an anti-neutrino. But it’s very difficult (utterly impractical, actually) to shoot a neutrino and an anti-neutrino directly at an electron and measure the probability that it all turns into a muon. So what you want to look at are oscillations: one particle turning into another, which can also convert back.

More here.