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I Don’t Want to Write About Neutrinos

As usual, xkcd gets it right.

(This title sounds like a Ramones parody written by Weird Al Yankovic.)

Yesterday afternoon, a big news story broke about a possible violation of relativity by neutrinos. To summarize, neutrinos produced at CERN (English version: European Organization for Nuclear Research) are detected at a facility in Italy known as OPERA (Oscillation Project with Emulsion-tRacking Apparatus — I think Calvin works for them). The distance between the two labs is about 730 kilometers (about 450 miles) through the crust of the Earth; the research team claims the actual distance is known to 20 centimeters. With this measurement, they found neutrinos to move just slightly faster than the speed of light by a factor of about 0.000025, or 0.0025%.

Neutrinos are difficult particles to study for a large number of reasons. First, they are electrically neutral and non-magnetic, so steering them using magnets and the like doesn’t work. (For more on how neutrinos are detected, see Ethan Siegel’s post on the same subject.) Also, the standard techniques for finding mass fail: to this day, though we know they have mass, we don’t know exactly what that mass is, only that it is very tiny. This combination of factors means that neutrinos move very close to the speed of light, according to what we know about particles (both from basic relativity and from quantum field theory). In Einstein’s theory of relativity, the speed of light is not just a speed limit for particles with mass — it’s a speed barrier. You can’t even reach it, much less pass it.

How the timing was accomplished in the OPERA neutrino measurement. Graphic is borrowed from the original paper.

Now we can see why this story is a big deal: if the neutrinos are actually traveling faster than light, then relativity will need modifications, much as Newtonian physics needed the changes relativity itself provided to account for electromagnetism, especially light. But here’s also why I don’t want to write about the whole situation (despite the fact that I’m doing it anyway, ’cause I’m dumb): it’s simply too soon to make any concrete claims based on the available data. Even the researchers don’t claim relativity violations: they say specifically that they draw no theoretical conclusions. Now that I’ve started, though, I should finish.

  • This is not as simple as “Einstein was wrong”. For one thing, other experiments don’t show neutrinos moving faster than light over long distances. Ben Still points out that if neutrinos consistently move as quickly as was measured, the neutrinos from Supernova 1987a (a massive star explosion in the Large Magellanic Cloud) would have arrived 4 years before the light did. However, the neutrinos only were 3 hours ahead of the light, and that discrepancy is because light scatters off the gas in the supernova, while neutrinos don’t interact strongly with anything and so can go right through. Supernova 1987a immediately rules out the idea that neutrinos always move faster than the speed of light.
  • Although the effect is larger than the experimenters’ estimated errors, this is a complex chain of measuring apparatus. Because the relative deviation from the speed of light is so tiny, a relatively small error missing from their analysis would push the results into the regime where relativity isn’t violated. (One possible problem is with the time of the neutrino emission.) I’m not sophisticated enough to understand everything in the paper, so I’m not going to try to find flaws. Others will do so, not necessarily to “show OPERA is wrong” — science operates by testing claims, and in fact the OPERA researchers invite people to check their results. As I say over and over again on this blog, it’s about evidence.
  • Even with everything taken into account (and they were careful!) the result is still ambiguous. In an email exchange with my Ph.D. advisor, Arthur Kosowsky, he said that he would be more likely to believe a claim of neutrinos moving 100 times faster than light than 1.0025 times faster. Although that obviously falls under the umbrella of my first comment, I have to say I agree: a result so incredibly close to light speed feels too close for comfort. Science doesn’t work on feelings, but I think everyone would be more comfortable declaring the discovery of new physics if the speed was a lot bigger!

So what are we left with? First, time and further checks will tell. Many seemingly ground-breaking results evaporate under scrutiny, and this may be one of them. Second, the press releases were incredibly over-hyped: Einstein’s relativity is not overthrown, though if the OPERA results are borne out, relativity will effectively be shown incomplete. (For a couple of alternative ideas, listen to particle physicist/rock star Brian Cox’s interview on the BBC.) Any alternative explanation will not involve allowing just any particle to go faster than the speed of light, and won’t let neutrinos go faster than light in a consistent manner, which means something is special about this particular set of circumstances. Third, automatically rejecting these results is also a mistake: the hype is there, but the paper itself seems to strike the right tone. Researchers are allowed to believe their results are correct, after all! There are many neutrino detectors in use around the world, and although all neutrino experiments are challenging, I have no doubt we’ll have follow-up tests very soon.

OK, I’ve done my duty and blogged about the neutrinos. Can I do something else now?

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25 Responses to “I Don’t Want to Write About Neutrinos”


  1. 1 Ruslan September 24, 2011 at 13:37

    I always thought that Einstein’s second postulate only says that light travels at a constant speed in the vacuum and that it is the same for all observers. It doesn’t exclude possibility of traveling faster than c. However, my question is regarding Lorentz transforms, if muon neutrinos do have mass and they travel faster than c then what would happen to gamma if v >c then gamma has negative number in the square root? Will we have to modify Lorentz equations? I thought that Lorentz equations make Maxwell’s equations covariant and the way they’re derived are true regardless of Einstein’s theory.

    • 2 Matthew R. Francis September 25, 2011 at 07:30

      Faster-than-light travel by a massive object is also known as Lorentz violation because, as you point out, the Lorentz transforms fail when v > c . Lorentz transformations are a direct consequence of relativity (even though they were derived first), and all theories that are Lorentz-covariant will be affected, including electromagnetism.

      Also, relativity doesn’t technically forbid traveling faster than light if whatever it is does that all the time. Tachyons (faster-than-light particles) don’t violate Lorentz invariance because they never cross the speed of light; they pop up in theories every now and then, but because of causality and energy difficulties are usually seen to be a problem. (One earlier version of string theory had to be rejected entirely because of the way tachyons entered into it.) Neutrinos definitely are not tachyons, though: they usually move at sub-light velocities, as the Supernova 1987a results (among others) indicate. That’s why if this result holds up, Lorentz-invariance is violated and relativity (with all its dependent theories like electromagnetism) will have to be modified.

      • 3 Ruslan September 25, 2011 at 13:14

        I really hope other scientists now will find some kind of error in the measurements. This has to be a mistake, right? Besides, we don’t fully understand neutrinos, and how much mass they have and how they’re able to oscillate from one flavor to another, so I hope it has to do more with nature of neutrinos rather than relativity.

      • 4 Matthew R. Francis September 25, 2011 at 14:30

        My guess is that there will turnout to be some problem. Neutrino experiments are notoriously treacherous, and while the OPERA team has done a very good job, they themselves acknowledge that something may have escaped their analysis.

        On the other hand, if these results hold up under scrutiny and are replicated by other groups, physics will go on. It always has! What we’ll find is relativity is a good approximation in 99% (or more) of cases, and the exceptions lead to interesting new theories. Again, I suspect that’s not the case here. It seems odd to me that new physics would turn up under these particular circumstances. However, I could be wrong. Time — and further experiments — will tell.

  2. 6 nimmy June 9, 2012 at 08:25

    hi sir ,this blog gave me a lot of informations on nuetrinos


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