El hijo del Halcon Negro: Faster-Than-Light Neutrinos?

domingo, 25 de septiembre de 2011

Faster-Than-Light Neutrinos?

Probably not. But maybe! Or in other words: science as usual.

For the three of you reading this who haven’t yet heard about it, the OPERA experiment in Italy recently announced a genuinely surprising result. They create a beam of muon neutrinos at CERN in Geneva, point them under the Alps (through which they zip largely unimpeded, because that’s what neutrinos do), and then detect a few of them in the Gran Sasso underground laboratory 732 kilometers away. The whole thing is timed by stopwatch (or the modern high-tech version thereof, using GPS-synchronized clocks), and you solve for the velocity by dividing distance by time. And the answer they get is: just a teensy bit faster than the speed of light, by about a factor of 10^-5. Here’s the technical paper, which already lists 20 links to blogs and news reports.

The things you need to know about this result are:

It’s enormously interesting if it’s right.

It’s probably not right.

By the latter point I don’t mean to impugn the abilities or honesty of the experimenters, who are by all accounts top-notch people trying to do something very difficult. It’s just a very difficult experiment, and given that the result is so completely contrary to our expectations, it’s much easier at this point to believe there is a hidden glitch than to take it at face value. All that would instantly change, of course, if it were independently verified by another experiment; at that point the gleeful jumping up and down will justifiably commence.

This isn’t one of those annoying “three-sigma” results that sits at the tantalizing boundary of statistical significance. The OPERA folks are claiming a six-sigma deviation from the speed of light. But that doesn’t mean it’s overwhelmingly likely that the result is real; it just means it’s overwhelmingly unlikely that the result is simply a statistical fluctuation. There is another looming source of possible error: a “systematic effect,” i.e. some unknown miscalibration somewhere in the experiment or analysis pipeline. (If you are measuring something incorrectly, it doesn’t matter that you measure it very carefully.) In particular, the mismatch between the expected and observed timing amounts to tens of nanoseconds; but any individual “event” takes the form of a pulse that is spread out over thousands of nanoseconds. Extracting the signal is a matter of using statistics over many such events — a tricky business.

The experimenters and their colleagues at other experiments know this perfectly well, of course. As Adrian Cho reports in Science, OPERA’s spokesperson Antonio Ereditato is quick to deny that they have overturned Einstein. “I would never say that… We are forced to say something. We could not sweep it under the carpet because that would be dishonest.” Now there’s a careful and honest scientist for you, I wish we were all so precise and candid. Cho also quotes Chang Kee Jung, a physicist not on the experiment, as saying, “I wouldn’t bet my wife and kids [that the result will go away] because they’d get mad. But I’d bet my house.” A careful and honest husband and father.

Scientists do difficult experiments all the time, of course, and yet we believe their results. That’s simply because it’s proper to be extra skeptical when the results fly in the face of our expectations: extraordinary claims require extraordinary evidence, as someone once paraphrased Bayes’s Theorem. When the supernova results in 1998 suggested that the universe is accelerating, most cosmologists hopped on board fairly quickly, both because we had a simple theoretical model in hand (the cosmological constant) and because the result helped explain several other nagging observational problems (such as the age of the universe). Here that’s not quite true, although we should at least mention that Fermilab’s MINOS experiment also saw evidence for faster-than-light neutrinos, albeit at a woefully insignificant level. More relevant is the fact that we have completely independent indications that neutrinos do travel at the speed of light, from Supernova 1987A. If the OPERA results are naively taken at face value, the SN 87A should have arrived a couple of years before we saw the explosion using good old-fashioned photons. But perhaps we should resist being naive; the SN 87A events were electron neutrinos, not muon neutrinos, and they were at substantially lower energies. If neutrinos do violate the light barrier, it’s completely consistent to imagine that they do so in an energy-dependent way, so the comparison is subtle.

Which brings up a crucial point: if this result is true (which is always a possibility), it is much more surprising than the acceleration of the universe, but it’s not as if we don’t already have ways to explain it. The most straightforward idea is to violate Lorentz invariance, a strategy of which I’m quite personally fond (although I’ve never applied the idea to neutrino physics). Lorentz invariance says that everyone measures the speed of light to be the same; if you violate it, it’s easy enough to imagine that someone (like, say, a neutrino) measures something different. Once you buy into that idea, neutrinos are an interesting place to apply the idea, since our constraints on their properties are relatively weak. It’s an interesting enough topic that there are review articles, and even a Wikipedia page on the idea.

And there are more way-out possibilities. Graininess in spacetime from quantum gravity might affect the propagation of nearly-massless particles; extra dimensions might provide a shortcut through space. This experimental result will probably give a boost to theorists thinking about these kinds of things, as well it should — there’s nothing disreputable about trying to come up with models that fit new data. But it’s still a long shot at this time. I hate to keep saying it over and over in this era of tantalizing-but-not-yet-definitive experimental results, but: stay tuned.

A few of the countless good blog posts on this topic: