The Sciences

Superluminal Neutrinos are so 2011

Cosmic VarianceBy Sean CarrollMar 16, 2012 4:30 PM


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We all knew that when the OPERA experiment announced preliminary evidence that neutrinos were traveling faster than the speed of light, the result was so hard to swallow that independent confirmation from other experiments would be necessary before too many people jumped on the bandwagon. In the meantime, a number of theoretical papers pointed out difficulties in accepting the result at face value (probably the cleanest by Cohen and Glashow). And just last month OPERA itself announced that they had located a couple of possible systematic errors in their experiment, without actually backing off the original result. But lets just say things haven't been looking good. Now we have what might be the nail in the coffin: another experiment, ICARUS, at the same laboratory in Gran Sasso in Italy, has reported an independent measurement of the neutrino time-of-flight from CERN. (The CERN twitter feed points to an frustratingly vague press release; more useful info from Tommaso Dorigo.) Answer: spot on the speed of light. They even have a paper on the arxiv, from which we get this lovely plot:

Colloquially, we would say "game over, man." The new measurements sit spot on the speed of light (zero on the plot), and are inconsistent with OPERA. (Actually neutrinos have tiny masses and therefore move just a bit slower than light, but it's close enough as to be invisible in this plot.) Note that ICARUS had previously "refuted" OPERA, but in a much more indirect way, by checking that the neutrinos hadn't lost any energy along the way. This new result is a straight-up check of the original claim, and it falls short. As Tommaso points out, the precision of the ICARUS result is comparable to that of OPERA, so if you live in a mental space free of theoretical priors you could assign 50/50 weight to each one. Those of us in the real world should be ready to accept that the speed of light isn't just a good idea: it's the law.

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