Neutrinos breaking the ultimate speed limit

Only a few things in life are certain. One is that the Detroit Lions will always lose on Thanksgiving. Another is that nothing can move faster than the speed of light — or so I thought. Two months ago, a group of scientists in Europe claimed they had made neutrinos — a neutral, near-zero-mass subatomic particle — go faster than the speed of light. I was skeptical as I skimmed the news reports about the faster-than-light neutrinos in September. Not only did the findings contradict a century of physics, but the scientist group had an enigmatic name, OPERA, seemingly derived from a Cold War spy novel. This was compounded by the weirdness of neutrinos, which seem to belong more in the world of Harry Potter than in our Muggle one. Neutrinos can go through solid objects, are practically invisible because they rarely interact with matter and can interconvert between at least three different forms that include the funnily-named muon neutrino and tau neutrino. I read about the super-speedy neutrinos and thought: no big deal.

But then, just two weeks ago, OPERA, which stands for Oscillation Project with Emulsion-Racking Apparatus, pulsed neutrinos again from Switzerland to Italy in their particle accelerator. And once again, they found that the neutrinos traveled faster than light. The repetition of their unbelievable results has sent physicists and Einstein fans into a tizzy.

The neutrinos arrived at their destination approximately 60 nanoseconds sooner than they would have if they’d been moving at the speed of light. This is an absurdly small difference in time, yet it was substantially more than the margin of error. If true, the shocking finding would go against Einstein’s special theory of relativity. Possible physical explanations for it include a background field unique to the particle accelerator’s vacuum that somehow slowed down light particles more than neutrinos or a different dimension in which the neutrinos traveled. But these seem unlikely, and would still challenge scientific paradigms.

Most physicists have expressed skepticism over the results, saying that there must be an unknown source of error in the measurement of the time or distance of the neutrinos’ journey. The average speed of the neutrinos was determined like any other velocity: the distance the neutrinos traveled divided by the time that it took to reach Italy from Switzerland. Any measurement error in either of these variables might explain the impossible result the experiments seemed to support.

Ultimately, the problem may well lie with two basic terms we all learned about in middle school: accuracy and precision. We know that 1.05 is more precise than 1 because the former contains more digits after the decimal point. In measuring the neutrinos’ and photons’ speeds in these experiments, precision is of the utmost importance. OPERA’s findings may indeed be accurate in that the average neutrino speed that was calculated was close to the true value. Their measurements were very precise, more so than in past neutrino experiments. However, the level of precision in measuring the time and distance — that is, the number of significant figures — may still be insufficient to determine whether neutrinos can truly go faster than the speed of light.

This is science at its best: an unexpected finding that goes against the dogma and generates plenty of discussion without descending into petty disputes. Soon other groups will try to conduct similar experiments, which will either support or refute the findings of OPERA. It is likely that neutrinos are no different from the rest of us — slower than light. So for the time being, I will continue to place my bets on the Lions losing next year and on Einstein’s theories holding true.


  • redman

    Don’t worry, Einsteins theory will be upheld even if it means inventing something unseen and unmeasurable, like dark neutrinos to account for the difference.

  • CharlieWalls

    Your article is pleasant and informative. And you hit an important aspect, it seems to me: the “weirdness of neutrinos.” You mentioned the “near-zero-mass,” which is more massive than in my day. Neutrinos then were thought to probably have zero rest mass that could increase with energy. They paired with beta-particles (much larger and negatively charged; commonly measured in the lab) as the energy of a nuclear event was partitioned between neutrino and beta-particle. Could you explain how the postulated near-zero-mass was deduced? I understand it is the outcome of the interchange of neutrino types that you did mention.

    • PhysicsAlum

      It’s a little complicated (quantum mechanics!), but yeah, it has to due with the interchange or “oscillation” of neutrino states. Basically, we know there are three neutrinos. We can describe these three neutrinos in two ways. First, there are three different “flavors,” or observable types, of neutrino: electron neutrino, muon neutrino, and tau neutrino. There are also presumably three neutrino masses (m1, m2, and m3, which *could* all be zero). But rather than each flavor type corresponding to a single mass (e.g., an electron neutrino having mass m1), each flavor of neutrino is actually a *mixture* of the three masses. If the masses are each different and not equal to zero, you get interference between the various flavor terms and mass terms as the neutrino travels through time and space. This appears as “oscillation,” which looks like, for example, a muon neutrino turning into an electron neutrino. We’ve seen this oscillation happen many times in the past ten years in lots of experiments. This means that there must be three non-zero, different, neutrino masses! Still really really tiny, though. And we’re still figuring out *why* it’s non-zero (well, we *do* have some pretty good ideas, but they’re A) really hard to test and B) way more complicated to explain than the above…google the Seesaw mechanism and Majorana neutrinos if you’re curious/have too much free time).

      For a better-explained version of the the above, here’s a good link: