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:

95 Comments

95 thoughts on “Faster-Than-Light Neutrinos?”

  1. Does c actually have to be the same as the speed of light that we can measure? Is it conceivable that photons have an extremely small rest mass, and so don’t travel exactly at c?

    I’m sure there’s a reason which I’ve forgotten about since my university physics days, particularly as no-one else seems to be suggesting it. I think it may mess up QED for instance, but is it absolutely inconceivable? I’d be grateful if someone could explain why this couldn’t possibly be what’s going on here.

  2. Interested observer here, not easily keeping up with things like this, but having lots of fun trying. BUT – I thought neutrinos were shown to have mass. How do they even travel at the speed of light? Or does that only apply to photons (electromagnetic radiation I guess). And one question above… if light travels slower in glass e.g., why wouldn’t neutrinos travel (at least microscopically) slower thru mass? Please don’t yell at me.

  3. Re Doug #13: neutrinos propagate as mass eigenstates. It’s possible that there is one mass eigenstate with zero or very small positive mass-squared (which is the one that was observed on the day of SN1987A), and another eigenstate with negative mass-squared. Due to its nonzero mass, the second eigenstate could arrive long before 1987. Furthermore, negative-mass-squared neutrinos could be dispersed over a period of, say, a year, and be completely lost in background noise.

    Even saying that “the SN 87A should have arrived a couple of years before we saw the explosion” is not quite right, because we don’t know the dispersion relations (speed vs energy) for this weird neutrino.

    If what we have is a tachyonic/negative-mass-squared effect, (v-c)/c would scale as E^{-2}. That effect would get stronger as we go down the energy scale, and I’m pretty sure that it would’ve been detected by now. (It’s easy to calculate that OPERA-like neutrinos with 100 MeV energy would travel at twice the speed of light).

    On the other hand, if it is a Lorentz-violating effect, (v-c)/c would scale as some positive power of E, and the violating eigenstate could possibly get to Earth just before the well-behaved eigenstate. There is an article on arxiv (hep-ph/9712265) that claims that such a burst was seen by a neutrino detector in France about 5 hours ahead of the main bunch.

  4. @Paul nah, that doesn’t work as it’s not about THIS experiment. Nameless at #31 comes closer. My conclusion is that this result is so unlikely to hold up that even theorists can’t be bothered to probe the implications (though surely someone will prove me wrong, momentarily…)

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  6. Thank you for discussing this, Sean. This thread doesn’t seem to contain the usual spate of insightful/provocative comments. Maybe we are mostly dazed and nonplussed. Oh well, “systematic error” has been a good horse so far. I’ll check back tomorrow.

  7. What would be the effect of photons ‘following’ the curvature of space and neutrino’s travelling in a truly straight line?

  8. Suppose for argument’s sake that the result is sound. Do we really need Lorentz violation, or massive photons, or quantum gravitational interactions, in order to account for it? How does the effective index of refraction of the vacuum for photons compare to muon neutrinos? The sense of this effect is at least correct: electron/positron pairs pop out of the vacuum far more often than do muon/anti-muon pairs simply because electrons are 200 times lighter than muons. Moreover, the scattering cross-section of photons off of electrons is many orders of magnitude larger than that the scattering cross-section of a neutrino off of its associated lepton. So couldn’t this be accounted for by photons failing to travel on null geodesics in vacuum due to ordinary QED scattering events?

  9. Why do you people care so much? Go have fun outside, maybe share a laugh with a real person before oblivion comes.

  10. Iain, for what it’s worth, the order of magnitude of an general-relativistic effects should be three or four orders of magnitude smaller than the claimed signal.

  11. Can anybody answer this question because I am not a scientist. I thought that as long as something does not accelerate faster than the speed of light it would not violate special relativity. If the particle starts out faster than light…would that still be a problem? Like Tachyons ( spelling?) ..theoretical particles that travel faster than light?

    How about the vacuum thing? I thought the speed of light constant was for a vacuum??

    How about this . Can special relativity handle something like. “Almost constant”..”give or take ” ? Sort of like trying to predict prime numbers, although they may exist you can only be so accurate( Riemann Hypothesis. )

  12. sansar swaroop saxena

    it is only possible if muon nutrino is mass less particle but as it is a part of atom it may contribute sme mass possibly may be negligible or very less but einstein might had considered its presence but if its true then relativity fails………..

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  15. Very cool, but I remain skeptical because of the size of the effect – if Lorentz invariance can be violated, is there any reason to think it would be… just ever so slightly violated? If neutrinos are truly not bound by the speed of light limit, then why not expect to see neutrinos moving convincingly faster than the speed of light rather than just barely measurably faster?

    Anyway, very interested in watching this unfold.

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  17. IF this result is confirmed (and that’s a big IF), perhaps quantum tunneling may offer an explanation? Some experiments show propagation far in excess of C because tunneling is a position uncertainty thing, no space to go through (see arxiv 0204043v1). However causality is preserved, information has not been shown to travel faster than C. Tunneling is statistical, it takes an accumulation of events to reveal the information, thus any single event cannot carry information. What makes me think of tunneling is the scale seems about right – flash memory chips work by tunneling electrons through a tiny glass barrier using a few tens of volts of energy, enough to spread out the “jumping electrons” enough to cross the barrier, where they become trapped when the energy is lowered. In the neutrino experiment, there is a 200mhz component, that would be a wavelength of about 5 feet, or cresting every 2 1/2 feet. The energies in the experiment are are far higher than what takes place in a thumb drive, so to my feeble brain seems like the average tunneling distance, if that’s what’s going on, combined with the high frequency of oscillation, might be enough to account for the discrepancy. Maybe, big shot in the dark. The main point is just because the statistical average velocity of something exceeds C, doesn’t mean that information can exceed C. If tunneling is at play here (or any phenomena involving quantum uncertainty), I’d expect the spread of detection events to increase as distance increases, and over large distances would be indistinguishable from noise.

  18. It’s all relative. To make sure, I just verified it in my basement with a shoebox, a flashlight and couple of bricks (a good approximation to rocks): according to Bahcall’s modeling, during my experiment, about 5 x 106 solar neutrinos/ cm^2-s zoomed through the bricks, but NO photons made it through! I therefore conclude that through the rocks under the Alps, neutrinos moved not only faster than light by a factor of 1 in 40,000, they move infinitely faster.

  19. Nalliah Thayabharan

    All of my investigations seem to point to the conclusion that they are small particles, each carrying so small a charge that we are justified in calling them neutrons. They move with great velocity, exceeding that of light – Nikola Tesla 1932

    Experimental tests of Bell inequality have shown that microscopic causality must be violated, so there must be faster than light travel. According to Albert Einstein’s theory of relativity, nothing with nonzero rest mass can go faster than light. But zero rest mass particles can go faster than the light. Neutrinos have a small nonzero rest mass. Faster than light interactions are a necessity and they provide the non local structure of the universe. We should understand the relation between local and nonlocal events like the dynamics of universal structure. In any physical theory, it is assumed that there is some kind of nonlocal structure violates causality. If neutrinos are traveling faster than light, then neutrinos must be on the otherside of the light barrier going backwards in time, where the future can interact with the past.

    – Nalliah Thayabharan

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