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Neutrinos: faster than the speed of light?

By Frank Close

To readers of Neutrino, rest assured: there is no need yet for a rewrite based on news that neutrinos might travel faster than light. I have already advertised my caution in The Observer, and a month later nothing has changed. If anything, concerns about the result have increased.

The response to my article created some waves. There were a couple of cogent remarks on The Observer’s comments section. Firstly, it was questioned that if neutrinos could indeed travel faster than light, the neutrinos from the supernova in 1987 (which had travelled 187,000 light years across space) should have arrived 4 days before the supernova was seen by eye, contrary to what was observed. One person also pointed out that if neutrinos could really go faster than light, then they could have gone at any speed, up to infinite. So the fact that the experiment “only” found them travelling at 1.00005 times light speed suggested that there was some underestimated error somewhere, and that the true answer would turn out to be 1. I agree; finding out what is the challenge.

I already mentioned some of the problems with the experiment – how it measures the time and the distance involved at huge accuracy, and then takes the ratio to get a speed. I read the paper on how they determine the distance, and the methods involved details of geodesy, which are outside my expertise. I admitted in the article that this was a “mystery to me”. One commenter, under the cloak of anonymity, took this to mean that I had not read the paper, and insisted on repeating this. Comments are subject to the laws of libel no less than printed media; the offending item was removed by the moderator.

This aspect of my personal mystery typifies the problems that the actual experimenters have. The team is primarily a group of high-energy physicists, and computer experts; however, the experiment requires expertise in many other areas. This is what inspired the team to go public in the first place: to advertise their concerns in the hope that other experts might come up with ideas. And there have been several.

A neutrino is detected in Italy, 500 miles from CERN, and the time is recorded. Surprisingly the precise time that that particular neutrino left CERN is harder to know than some first thought. CERN does not provide a beam of neutrinos. Instead it makes pions, which decay, producing neutrinos. The pions decay on the average in a hundred-millionth of a second, but some live longer and others die sooner. Precisely when this happens on a case-by-case basis is a statistical result, which is taken into account in the experiment. And the pions themselves are not primary particles, but are themselves produced by collisions between beams of high energy protons and a target. The protons that start all this emerge from the CERN accelerator in bunches – pulses spread over a short period of time. Precisely when in this time interval a proton does its work, leading to the pions and subsequent neutrinos is a big worry in some people’s opinion. The resulting beam of neutrinos then spreads out, over a mile wide by the time they arrive at the target 500 miles away in Italy, and one has to try and match an arriving neutrino with an initial proton back at CERN. This is all done statistically. If you saw the BBC2 television programme Faster than the speed of light on 19 October in the UK, check out Jon Butterworth’s comments about how tricky some of this experimental analysis is, and his questions about whether everything has yet been fully looked into.

More theoretical perhaps, but from a Nobel Laureate, Sheldon Glashow, comes evidence of an inconsistency in the evidence for super-luminal neutrinos. If neutrinos travelled faster than light, they would radiate electrons and positrons, and lose energy – and speed. Nothing like this has been seen. Of course, one can say that if neutrinos could travel beyond the speed at which current theory allows, all bets are off: the implications that they radiate electrons and positrons could also fail. But one has a sense of Occam’s Razor: inventing one excuse on top of another as one is squeezed into ever smaller corners.

Ultimately though, as I said in The Observer article, it is experiment that decides and it doesn’t matter how many theorists say nay. The truth will out. Science is built upon reproducibility. If and when this result is confirmed by independent experiments, and systematic errors ruled out, then I will be excited; but that time is not yet.

Frank Close is a particle physicist, author and speaker. He is Professor of Physics at the University of Oxford and a Fellow of Exeter College, Oxford.
Close was formerly vice president of the British Association for Advancement of Science, Head of the Theoretical Physics Division at the Rutherford Appleton Laboratory and Head of Communications and Public Education at CERN. He is the author of several books, including Neutrino, Nothing: A Very Short Introduction, Particle Physics: A Very Short Introduction, and Antimatter. His latest book, The Infinity Puzzle, publishes this week.

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Recent Comments

  1. Daniel

    Must be-the neutrinos from the supernova in 1987 (which had travelled 187,000 light years across space) should have arrived 4 years before the supernova was seen by eye, contrary to what was observed.

  2. Erwin Unger

    Maybe the neutrinos had already radiated, slowed to sub-light velocity, and the average interval velocity was calculated still to be ftl.

  3. Andrew

    Ever since particle theory was dropped for wave theory, people have stopped asking the right questions. Not being able to accelerate particles past c may not be a speed limit but a breakdown of the force mechanism. The dual particle model of the photon has real potential as well.

  4. Scot Parker

    Well, Forrest Gump did accomplish some surprising things, but I think making him the head of the scientific team at CERN was a mistake.
    When they announce that they’ve found the Higgs boson – I know now not to believe them (I never believed in the Higgs, anyway).

  5. Daniel L. Burnstein

    First, a small correction. If neutrinos travelled at speed comparable to the OPERA neutrinos, neutrinos from the supernova SN1987A would have arrived approximately 4 years before visible light from the same source, and not 4 days.

    That said, this doesn’t not necessarily constitute counter-evidence against the OPERA results. Not if, the difference between the superluminal speed and c corresponds to the speed of the Earth relative to the quantum-geometrical space background along the axis connecting the source and target of the neutrinos.

    As for Prof. Glashow’s paper, all it really says is that the OPERA results do not agree with what the theory predicts would happen if neutrinos travelled as superluminal speed. It is note evidence against the OPERA results, it is only a theoretical argument.

    The only thing that can constitute counter-evidence is evidence that is counter to the results. That is, only experimental evidence from an independent experiment can constitute counter evidence. Everything else is tautological.

    Daniel. L. Burnstein

  6. Cira Goldies

    It wouldnt be the first time Einstein has been wrong. He never accepted the uncertainty principal, and ended up marginalizing himself as a result. However even in that opposition, he provided a valuable service to science. In his attempt to disprove uncertainty where he stated that “G-d does not play dice with the universe”, he developed a large number of challenges which those supporting uncertainty (among them including Heisenberg, Pauli, and Bohr — who said, “Einstein should stop telling God what to do”) had to refute. They did find answers for all of Einstein’s challenges, helping support and prove the principle of uncertainty. But even if the neutrino expierment proces to be correct, which in my opinion is a long way off, it is as unfair to say that Enstein was wrong about Special Relativity as it is to say that Newton was wrong about classical physics. It is known that at times different addumptions have to be added to a theory in order to gain a comprehensive understanding. And this is a legitimate limitation and is often seen in science. The computers that took the Saturn V to the Moon use the assumption that the Earth was flat and that the Earth was the center of the universe – both of which the scientists and engineers knew not to be true. Why? Because a flat-plate model gave accurate results for first stage guidance (treating the Earth as flat), while using the Aries Mean of 1950 coordinate system (which has the Earth’s center as the system’s origin — making the Earth the center of the universe) permitted accurate navigation while simplifying the calculations needed. Similarly, we use Newtonian mechanics for most Earthside applications, even though Newton was superceded by Einstein. And no doubt, if Einstein is superceded, we will still use special relativity when it makes sense to do so.

  7. Ashbert

    Yes the neutrino-faster-than-light experiment will NOT stand. So “If nothing travels faster than light then Einstein’s theory is right!” Nice rhyme… most mediocre minds (i.e. just about everyone in today’s world of physics) would dance to the line. A dance of the non-thinking crowd!

    Reality is more complex — a COUNTER-EXAMPLE exists that PROVES that (though Einstein’s postulates are correct), Einstein’s claim of having derived the Lorentz transformations is wrong, yes a COUNTER-EXAMPLE — and at least one Nobel prize winner takes this realization seriously.See http://physicsnext.org/ for details, a very simple read, but majority be warned… facing physics reality regarding the foundations could disturb a mediocre mind and make you react emotionally…for example, Howard Georgi got very angry!

  8. Marek Radzikowski

    Dr. Close–

    Thank you for posting this article, since I am quite interested in this topic.

    I wish to note a possible correction: if a superluminal particle loses energy, its speed would actually *increase*, not decrease. Therefore, if Cohen and Glashow are obtaining their conclusions by using some misconception about the kinematics of tachyons (whose energy-momenta transform in the usual way under Lorentz transformations), then I wonder if what they are dealing with (and debunking) is really close to a legitimate tachyonic neutrino model at all. In the same vein, there are a number of choices which must be made in attempting to construct such a model, and CG do not clearly describe what choices they are making.

    I would admit that I do not understand most of Cohen and Glashow’s argument, in part because I haven’t studied (classical or quantum) bremsstrahlung very much, but it would be nice if they could give more details & provide more clarity about the model(s) they are debunking, otherwise I would not be inclined to accept the conclusions of their paper as significant. Also, there are a few mass parameters (including a tachyonic one) which might replace the E’s in their dimensional argument. It’s not obvious to me why those should be ruled out, as they seem to do.

    I’d agree that experiment does seem to have the final word with regards to whether an idea is accepted or not as being physical, but theory does play an important role. For example, how would you determine the details of an effect if you didn’t have a set of competing theoretical models to test (e.g., to see which one works the best)? Also, some things just aren’t looked for in experiments, unless a theory suggests it as a possibility. For example, consider the case of the discovery of positrons (anti-particles of the electron). They were seen by people studying cosmic rays before Dirac’s prediction, but were discarded as some sort of optical illusion or who knows what reason. Then Carl Anderson noticed them in his cosmic ray experiments, followed up on them (presumably having some knowledge of Dirac’s prediction), proved they had the right properties to be positrons, and his work earned him the Nobel Prize.

    Best regards,
    Marek Radzikowski

  9. Physics_breaker

    Can anyone tell me how can the scientist be sure that neutrino is from their source?

  10. [...] and Public Education at CERN. Read more of what Frank Close has to say about neutrinos here and [...]

  11. [...] and Public Education at CERN. Read more of what Frank Close has to say about neutrinos here and here. Read Frank’s reflections on the Nobel Prize nominations for the 4 July [...]

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