OPERA still wrong, says ICARUS: neutrinos show energy signature for obeying speed limit

A paper by another team at Gran Sasso, ICARUS, contradicts the OPERA neutrino results by measuring the energy spectrum of the supposedly speeding neutrinos and finds that they could not have traveled faster than light.

[T]he ICARUS scientists say, the neutrino beam as tested in their equipment registered an energy spectrum fully corresponding with what it should be for particles travelling at the speed of light and no more.

Physicist Tomasso Dorigo, who works at CERN, the European Organisation for Nuclear Research, and the US Fermilab near Chicago, said in a post on the website Scientific Blogging that the ICARUS paper was “very simple and definitive.”

It says, he wrote, “that the difference between the speed of neutrinos and the speed of light cannot be as large as that seen by OPERA, and is certainly smaller than that by three orders of magnitude, and compatible with zero.”

So something else is causing the differential, otherwise those neutrinos would have been far more energetic. What do you figure it is? I’m still liking the idea of the neutrinos being emitted some distance (e.g. 18m away) from the source. That, or some niggling little miscalibration that nobody’s managed to hit upon yet. A few ideas for testing this again, off the top of my head — build a detector halfway between the start and endpoints and see if the 60ns difference is halved — if not, then it’s a measurement problem, or my new pet hypothesis. Or maybe try some other particles at known speeds and see if the detector gets them too early. Or see if we can produce neutrinos using higher-energy methods and see if the speed differential stays the same. We should definitely find some way to measure where the neutrinos are being emitted from, too.

Perhaps my physics-knowledgeable readers might have some commentary on my suggestions, and how utterly dumb they are for some reason that I in my lay status am completely unaware?

Hat tip to reader Bruce Gorton.


  1. KG says

    A paper by another team at Gran Sasso, ICARUS, contradicts the OPERA neutrino results by measuring the energy spectrum of the supposedly speeding neutrinos and finds that they could not have traveled faster than light.

    Would their result rule out the possibility that the neutrinos are taking a “short cut” through additional dimensions, but not travelling faster than light, which I’ve seen mentioned?

  2. ACN says

    Glashow/Cohen wrote a paper where they show that superluminal neutrinos should have a specific energy signature that would arise from the superluminal equivalent of cherenkov radiation.

    ICARUS did not find this energy signature in their previous data working with the same neutrino beam.

    It doesn’t rule out any extra-dimensional ‘short cuts’, but it should make you VERY suspicious of superluminal neutrinos in the “they just have a v > c” sense.

    As far as I know, there is no reason to believe that the neutrinos are being produced 18m away from the graphite source, nor is it likely, based on the geography of the beam propagation, that it’s feasible to build a NEW neutrino detector at the halfway point.

    I think the safe money is still on systemic errors that no one has tracked down yet.

  3. KG says

    I think the safe money is still on systemic errors that no one has tracked down yet.

    As a complete layperson, that’s still the way I’d bet too, but I’d love to be wrong!

  4. Robert B. says

    Another detector at a different distance is a good idea, though it would probably be easier to re-aim the emitter than build a new detector. (For one thing, neutrinos go in straight lines and the earth is round, so the actual halfway point along the current beamline would be miles underground!)

    You can’t test the current arrangement with other particles, though. This experiment sends the neutrinos through hundreds of miles of rock, and there’s not any other particles that can do that – at least, not particles that we’ve proven to exist. In fact, neutrino detectors are heavily shielded (that is, deeply buried) exactly because neutrinos are the only things that can get through all that rock or ice – it cuts down on false positives.

    Measuring where the neutrinos are emitted from… I’m not sure how practical that is. Neutrino detectors are big, because any given inch of one is highly unlikely to stop a neutrino. Wikipedia says the OPERA detector masses on order 10^6 kg, and that’s lean and efficient as neutrino detectors go. If you put it next to a particle accelerator, it needs serious shielding, too. I’d be very impressed if someone figured out how to put a working neutrino detector within, say, 50 or 100 meters of the neutrino source. (Or maybe I should say, where the neutrino source is supposed to be.)

    As for higher-energy neutrinos… hm. It might be worth a shot, if this remains a mystery long enough to build the apparatus. Since there’s no model yet for this effect, whether it’s superluminous or not, it’s unclear what a higher-energy beam would show. Before this whole kerfluffle, I’d expect to see only a tiny, perhaps undetectable, difference in measured speed: that close to the universal speed limit, you could increase kinetic energy by orders of magnitude and only increase the speed by a few hundredths or thousandths of a percent. But it seems very possible that something funky is happening somewhere, and who knows how that funkiness scales with energy?

    (Note: I’m reading the new paper, and the authors cite an earlier measurement of lower-energy neutrinos from a supernova, timed against the light of the same event. They didn’t see any faster-than-light malarkey, and their error bars are small enough that they can rule out anomalies tens of thousands of times smaller than OPERA’s. So if neutrinos really are superluminous, it seems likely that whatever it is causing this effect has in fact increased with energy.)

    For that matter, I’d be happy just to see this experiment repeated on another apparatus, as different as possible while still serving the same function. I’m sure they’ve checked like crazy to see if part of either machine is performing differently than designed, but it’s always possible to miss something.

    @ KG: No, it looks like this result is based on the physics of moving very fast. It doesn’t rule out that the neutrinos are traveling at their expected speed through an unexpected path. But I’ve always been dubious of extra-dimension theories. And why would there be extra space in space that these neutrinos can access, but photons, quarks, other leptons, and even lower-energy neutrinos can’t? (Disclaimer: there are lots of physicists with more degrees and more math than I have who are quite sure that I am wrong and that extra dimensions exist. But I’m not aware that any theory has predicted that extra dimensions would produce these results, so nyah.)

    And there’s a problem with displacing the origin point, too. If you move the neutrino origin point ahead of the generating apparatus, the neutrinos can now obey the speed limit… but the information that it’s time to generate neutrinos would have to get from the apparatus to the origin point faster than light.

    I’m still in favor of a systematic error, with an extradimensional shortcut coming in a fairly distant second.

  5. Eclectic says

    It’s not possible to build a detector halfway, because the beam (which travels in a straight line, not following the earth’s curvature) is 18 km underground at that point.

    There’s probably a problem with the distance or timing somewhere. They were very careful, but as someone mentioned, a single cycle of the 20 MHz master clock at the OPERA detector in LNGS would shift the result from 6 sigma to 1.

    The timing system at LNGS is unfortunately not engineered for single-ns timing resolution.

    And remember, it’s not particularly easy to measure. While the trillions of source protons at CERN are trivial to detect, they detect on average one neutrino per 1016 protons. So you launch a bunch of protons at CERN, and you might detect one neutrino at the far end.

  6. JohnnieCanuck says


    I’m going to go out on a limb here and say that you intended 10^16 to be the number displayed, not 1016. I checked and <sup> is not supported here, if that was what you used. The source html for this page shows only 1016 in your comment.

  7. Poriwoggu says

    should have a specific energy signature that would arise from the superluminal equivalent of cherenkov radiation.

    Cherenkov radiation only applies to charged particles. The ultraviolet glow in reactors is from high energy electrons traveling through purified water (not neutrons). The high energy electrons cause stress (displacement) to the the medium which is relieved (by the medium) through photon emission. The high energy electrons lose energy by doing work to the medium not emitting radiation. Neutrons can travel through water faster than the speed of light in water.

    Neutrinos are detected by the neutral or charged current interactions. Even the Askaryan effect results from the particles generated from neutral/charged current interactions and not a neutrino emission per se, so the Glashow/Cohen theory would have to be labelled as “speculative”.

    ICARUS didn’t describe how they generated the superluminal neutrinos, detected the radiation, and measured the distorted neutrino energy spectrum to prove that the Glashow/Cohen theory is correct; before trying to measure the Opera neutrinos.

    They did measure the energy spectrum of some neutrinos – since they had access to the same timebase (a Symmetricom Cs4000 atomic clock) as the Opera team why didn’t they measure the time displacement?

    That being said however, “sigh” there probably is something in the experimental setup that isn’t connected or performing per published specification.


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