Neutrinos don't travel faster than light

Last September, those who pay attention to discoveries in physics were rocked by an experimental result that suggested that neutrinos had been observed moving faster than the speed of light. Neutrinos produced at CERN in Switzerland (not at the LHC, but at another accelerator there) were detected at Grand Sasso in Italy by the OPERA experiment. The result only showed them moving a couple hundredths of a percent faster than light. That doesn't sound like much, but Special Relativity, one of the pillars of modern physics, indicates that the speed of light is an absolute limit, and that it takes an infinite amount of energy for a massive particle to reach that limit. As such, if a particle is going even a little bit faster— and the reported uncertainties on the reported result were much less than the difference between the measured speed and the speed of light— it would be an important result, indicating that in at least some regimes, Special Relativity breaks down.

Most physicists were dubious of the result. Special Relativity (SR) has withstood a lot of experimental tests. As such, any result that indicates that it isn't absolutely true is going to be subject to a lot of scrutiny, and is going to require robust reproduction before we really take it seriously. Also, the press reports and popular perception of the result missed one of the most important points. "Was Einstein wrong?" was the question asked. There were statements or implications that SR would have to be thrown out. The thing is, even if we'd seen a case where SR was violated, all the other experiments that show that it works still stand. It would only mean that SR isn't an absolute theory, that it's an approximation that works frequently but not always. We have many other theories like that. Newton's theory of gravity is a great theory that we use for a lot of things, but we know that there are situations where it isn't quite right (black holes, gravitational lensing of light, the precise orbit of Mercury, the precision required by the GPS), and we have to go to another theory (Einstein's General Relativity).

As I said, though, most of us suspected that the results from the OPERA experiment indicating neutrinos moved faster than light weren't correct. There was no scientific fraud involved, nor did anybody at OPERA do anything wrong from a moral or professional point of view. What they did was exemplary science. And, the rest of us, in doubting their results and wanting confirmation, also behaved as scientists are supposed to. We suspected that there would be some sort of systematic error that explained the apparent detection of faster-than-light neutrinos. It wouldn't be easy to find, because if it were, the scientists at OPERA would have already found it and would never have reported their result. Indeed, a few weeks ago there were reports that a loose cable, as well as possible shortcomings in corrections to GPS timings, may have been the source of the measured too-short travel time for the Neutrinos from CERN in Switzerland to OPERA in Italy.

However, even if the systematic error were never identified, for scientists as a whole to take the result seriously, there would need to be an independent verification by other scientists using separate techniques and separate equipment. A couple of days ago, a report from the ICARUS team indicated a failure to reproduce the OPERA results. This team was looking at the same source of neutrinos at CERN as the OPERA team, but it was a second team using a different detector. This is exactly what we needed. If they had come up with a measurement consistent with OPERA's measurement (and if the OPERA team hadn't identified the likely culprit systematic), then the scientific community would have to start taking faster-than-light neutrinos seriously. (It would be nice to reproduce the result also with a different source, in case there was some systematic involved with the timing on that side that would affect both results, but this would already be enough that we'd have to sit up and pay attention.) However, the ICARUS team reports timings that are entirely consistent with neutrinos moving at the speed of light. (In fact, they aren't moving at the speed of light. However, because their mass is so low, at the energies of these neutrinos they are moving a tiny, tiny fraction slower than the speed of light, a difference small enough that the experiment wouldn't be able to detect it.)

The story isn't completely over. Because SR is one of the foundations of modern physics, it's always worth testing its fundamental postulates. We'll keep pushing at it and looking for cracks in the theory, trying to see if it breaks down in places where we hadn't yet looked. However, at the moment, there's no indication that the theory breaks down anywhere. The brief suggestion that neutrinos might somehow be able to violate SR (which would have lead to all sorts of potentially cool consequences) has passed, as most of us suspected it would.

2 responses so far

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  • Clif says:

    I remember reading an article about this on BBC News and was fascinated. I think that it presents a very interesting caveat in dealing with new and edgy scientific facts. As a person who is not professionally trained in physics, I cannot fully appreciate the impact of the violation of Special Relativity. However, I can appreciate the impact of changing the primary theory of physics of the last century. I think that we have to be careful after one test to throw out a theory that has been so solid. Rather in order to know the truth in a scientific fashion, we should always conduct a cautious multiplicity of reviews of the findings before pronouncing them as ground-breaking. I think that approaching the makeup of the universe demands more of a search for truth, and less of a search for more grant money. Personally, I believe that much (especially in the biological field) has been influenced more by the pursuit of money than of science. With the proliferation of staunch, unshakeable evolutionists, the idea that groundbreaking theories should be taken quickly is overused. Rather, those who claim to have open minds ought to have that same mind in every issue that is raised. Regardless, the failing of the neutrino to beat light speed was very interesting and enlightening for me as an outsider to the world of physics.