First Evidence for an Arrow of Time – or not?

The BaBar detector (UCSB).

In our everyday, macroscopic world, time flows in one direction. People get older, eggs fall of tables and break, hot objects cool, and so on. Yet on the microscopic quantum level, the laws of physics are completely time-symmetric. They do not discriminate between a past direction and a future direction. This discrepancy has been dubbed the problem of the arrow of time, and expounded upon in numerous books such as Sean Carroll’s recent From Eternity to Here.

Particle physicists have expanded considerable efforts to find any asymmetry in properties of fundamental particles. Earlier this week, the first such unambiguous experimental observation was published, and widely reported in the science press (see Physics for a technical exposition, Ars Technica for a laymanish one, the Nature News blog for a quickie, or Phys.org for comments by the researchers).

From Ars Technica:

New results from the BaBar detector at the Stanford Linear Accelerator Center (SLAC) have uncovered this asymmetry in time. Researchers measured transformations of entangled pairs of particles, including the rates at which these transformations occurred. Through analyzing over 10 years of data, they found clear time-reversal asymmetry with an error of only one part in 1043, a clear discovery by any standard. These results are a strong confirmation of predictions of the Standard Model, filling in one of the final missing details of that theory.

The results really are incontrovertible for the case of these particular mesons, and so the Physics article concludes with:

Thus the long wait for an unequivocal time-reversal violation in particle physics is finally over.

That’s all and well by itself. However, Sean Carroll has long argued that actually the low-entropy initial state of the universe is crucial for explaining the Second Law of Thermodynamics, not any particular time-irreversible processes. As he directly comments on the latest results:

This new measurement in the B meson system — indeed, the entire phenomenon of T violation — has absolutely nothing to do with that arrow of time.

The reason why this is a peeve worth keeping as a pet is that the confusion between time reversal and the arrow of time often leads smart working physicists to think they have discovered something interesting about the arrow of time when really they’re addressing a completely different problem. We understand why there is an arrow of time: because the early universe started with a low entropy, and generic evolution from such a state leads to an increase in entropy. If you have a theory that explains why the early universe had a low entropy, you have successfully accounted for the observed arrow of time; likewise, if you have a theory that doesnot explain the low entropy near the Big Bang, you have not successfully accounted for the observed arrow of time. Love the B mesons, but they aren’t the reason why we can’t put Humpty Dumpty back together again.

So it rather sounds the reporting on this study (including the press release, presumably penned with the researchers themselves) was rather misleading all around.

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