Thursday, November 29, 2012

Time Reversal Asymmetry

Today's post is by David Zaslavsky, a physics graduate student who writes a blog called ellipsix informatics. It's a fairly advanced physics blog, but if you have the stomach for some math I highly recommend it.

About a week ago, a particle physics experiment called BaBar published a big discovery. Not as big as the Higgs boson, so it's probably not the kind of thing you would have heard of if you're not a physicist. But it's still pretty interesting, and it does have the exotic-sounding name of "time reversal asymmetry."

If you love science fiction as much as I do (and who doesn't?), you might think "Whoa, they figured out how to reverse time?" That would be awesome. But no, sadly, that's not what happened. Time reversal asymmetry just means that, if you could reverse time, certain particles would behave differently than you'd expect. Of course, the fact that you can't reverse time means that the scientists had to get pretty creative to figure out what would happen if you did. They were lucky enough to find two sequences of particle decays that are time-reversed versions of each other — that is, if you could reverse time, you'd turn each one into the other.

The special sequences start with an exotic particle called a "neutral B meson" and its antiparticle, linked together by quantum entanglement. Each one of the two particles can decay in one of two ways (that this experiment cares about): either "hadronically," into two lighter but still somewhat exotic particles, or "leptonically," into something like an electron and other stuff which we don't care about. Since the particles are entangled, if they were normal particles, both of them would always decay the same way, either both hadronically or both leptonically. So, for example, if you see the first B meson decay leptonically, then normally that tells you the other one will also decay leptonically.

However, B mesons (and their antiparticles) are not normal. They can oscillate, which means that they spontaneously flip back and forth between "wanting" to decay hadronically vs. leptonically. This oscillation is what the scientists at BaBar measured. They picked out the times when the first B meson in a pair decayed leptonically, thus forcing its partner into a leptonic decay state, but the partner oscillated back to decaying hadronically, and also the times when the first B meson in a pair decayed hadronically and the partner oscillated back to decaying leptonically, and they compared how many times each one happened. It turns out that the leptonic to hadronic oscillation occurs significantly more often than the hadronic to leptonic oscillation in the time before the B meson decays. Presto, time reversal asymmetry!
Time reversal symmetry violation in B meson oscillations
Although this didn't come as a surprise, not even a little, it's good to know that it works the way it does, because it helps confirm that the theory we use to understand how particles behave is correct, in a way that hasn't ever been directly tested before.

For a full-length explanation of this experiment and what it means for theoretical physics, check out the original post on my website!