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Faster-than-light neutrinos explained?

The views expressed are those of the author and are not necessarily those of Scientific American.


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The detector at the Gran Sasso end of the OPERA experiment. Credit: OPERA

The faster-than-light neutrinos seen by the OPERA particle physics experiment last year may have just been explained. By a loose cable. I wish I was joking.

To back up a little, the OPERA collaboration based at the Gran Sasso laboratory underneath the mountain of the same name in Italy published a paper to pre-print server arxiv.org last September saying that they had seen neutrinos, a type of sub-atomic particle, travel faster than the speed of light. They recorded neutrinos, which had travelled from CERN, Geneva, through the Earth to Gran Sasso, Italy, arriving at the laboratory 60 nanoseconds earlier than they would had they travelled at the speed of light.

Since then, scientists around the world have been collectively scratching their heads and publishing papers that tended to fall into one of two categories: suggesting an error with the experiment (such as the clocks at the two laboratories not being synchronised properly), or suggesting an addition to the current theory of particle interactions that could explain the strange result (for example, a new dimension that the neutrinos could have skipped through to make their journey shorter – so they would have never actually travelled faster than light at any point).

But I don’t think anyone expected it to be something as simple as this.

Today, Science is reporting that a fibre optic cable connecting a GPS receiver and an electronic card in a computer was loose. They go on:

After tightening the connection and then measuring the time it takes data to travel the length of the fibre, researchers found that the data arrive 60 nanoseconds earlier than assumed

This news (though still unconfirmed) rather casts a shadow over another recent explanation, involving something slightly less ridiculous.

In a paper published in journal Astronomy & Astrophysics, Claudio Germana of the Astronomical Observatory of Padova, Italy, suggests that there was a problem with the synchronisation of clocks at the two ends of the experiment. His calculations suggest that if the experiment had been run at a different time of year, the neutrinos would in fact have arrived 50 nanoseconds later than light.

I spoke to Carlo Contaldi, a physicist at Imperial College London, who last year published a paper on arxiv.org pointing out a possible problem with clock synchronisation, about the new paper. Though he thought the calculations and the large effect the calculations seemed to show were “interesting”, he had some reservations:

[Germana] does not seem to mention the latest measurements that were carried out by OPERA in November 2012. Those showed a consistent value for the neutrino’s time of flight as the previous results and it would be interesting to see how that time frame fits in with these corrections.

It’s an interesting hypothesis though – and one that is easily testable by running the experiment at a different time of year.

This paper is just the latest in a long string of attempts to explain the faster-than-light neutrinos. For more of the explanations that have been offered over the last few months, have a look at a timeline I made that follows the story right from the beginning until now.

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All of these papers could have been for nothing, of course, if the new report of a loose cable is true. It would be a little disappointing if this turns out to be the case. I’m going to reserve judgement for now, at least until the “sources familiar with the experiment” become something a little more concrete.

I’ll be updating the above timeline as the story unfolds.

UPDATE: The Nature News Blog now has an official statement from OPERA, that says they have “identified two issues that could significantly affect the reported result” – you can read the full statement over there.

UPDATE 23rd Feb: The OPERA experiment has issued an official statement. Here it is in full:

The OPERA collaboration has informed its funding agencies and host laboratories that it has identified two possible effects that could have an influence on its neutrino timing measurement. These both require further tests with a short pulsed beam. If confirmed, one would increase the size of the measured effect, the other would diminish it. The first possible effect concerns an oscillator used to provide the time stamps for GPS synchronizations. It could have led to an overestimate of the neutrino’s time of flight. The second concerns the optical fibre connector that brings the external GPS signal to the OPERA master clock, which may not have been functioning correctly when the measurements were taken. If this is the case, it could have led to an underestimate of the time of flight of the neutrinos. The potential extent of these two effects is being studied by the OPERA collaboration. New measurements with short pulsed beams are scheduled for May.

So, there was a (possible) faulty cable that might have led to an underestimate of the time it took the neutrinos to reach Gran Sasso, which would led to an overestimate of their speed. But there was also another fault that might have led to an underestimate of the speed. Looks like we will have to wait for the new measurements in May to see just how much each of these faults contributed to the early arrival of the neutrinos and whether they can add up to the 60 nanoseconds to fully explain the result.

Kelly Oakes About the Author: Kelly Oakes has a master's in science communication and a physics degree, both from Imperial College London. Now she spends her days writing about science. Follow on Twitter @kahoakes.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Dr. Strangelove 9:55 pm 02/22/2012

    “But I don’t think anyone expected it to be something as simple as this.”

    Kelly, I’ve been saying this in SA blogs all along. If the superluminal result were wrong, it is due to experimental error. Physicists should look for errors in the OPERA experiment rather than use theories to dismiss the experimental result.

    I’m not surprised with this news. As an engineer, I’m not surprised at all if it’s just a loose cable. In engineering, it is often the little things that go wrong.

    Link to this
  2. 2. Kelly Oakes in reply to Kelly Oakes 4:46 am 02/23/2012

    Dr. Strangelove: A lot of people have also said it was due to an experimental error, but I think many people expected something a little bit more complicated than “dodgy wiring”…! Of course, it could be the nature of the result – how massive it would be if it were true – that makes people think OPERA would have checked and double checked things like this before publishing, even on arXiv.

    Link to this
  3. 3. G_mac 8:09 am 02/23/2012

    “But I don’t think anyone expected it to be something as simple as this.”

    Well that’s not accurate.

    The Opera paper that stirred up all that fuss reported the delay of the optical fiber cable to be 40000 ± 1 ns, that had more than one person scratching their head, see here for instance:

    http://www.science20.com/quantum_diaries_survivor/few_additional_technicalities_opera_measurement-84788

    “At this point I refuse to discuss any explanation of the 60ns anomaly until new data on the light fiber time delay measurement arrives. I am convinced that the uncertainty on that 40000 ns delay cannot be 1ns (MINOS has a much larger uncertainty on a similar fiber, and they measured it with four methods, finding four different values, while Opera did it only with one method!).”

    T. Dorigo

    Moreover, what made impossible for the Opera team to.. “have checked and double checked things like this before publishing, even on arXiv.” … is the mere existence of Internet and social networks.
    Opera is a big collaboration with hundreds of people involved. Rumors were already leaking out at the time they published on Arxiv, they could not delay it any longer.
    Maybe twenty years ago they might have sealed their results a little longer, but these days that is no longer possible. In their position, I think they had no choice but publishing the results

    Besides, they’ve always warned about their results to be still verified and checked independently. It was the media system that jumped on it, bloating about “faster than light neutrinos”, “Relativity to be rewritten from scratch” and so on.

    I see all this story as a failure in communication rather than physics.

    Link to this
  4. 4. Kelly Oakes in reply to Kelly Oakes 5:53 pm 02/23/2012

    G_mac: I don’t see this story as a failure of communication. It’s more like a look at how physics/science actually works.

    But are you saying that putting this whole episode down to dodgy wiring, essentially, doesn’t seem just a tiny bit ridiculous? When Science first reported it that was very much what it sounded like, which was when I made the “no-one was expecting it to be this simple” comment – since then it’s become obvious that it’s not actually that simple. I think to pick holes in that one statement is just silly.

    And I wasn’t suggesting that they shouldn’t have published – I think OPERA did the right thing, and were very cautious in what they said (and didn’t say) when they published the paper in September. Ben Still has a blog post in which he talks about the openness and honesty of the OPERA collaboration, and I fully agree with what he says there about how modest researchers have been. I would also add that the press (bar some, I’m sure) have also handled this surprisingly well.

    Link to this
  5. 5. Kelly Oakes 5:57 pm 02/23/2012

    Let me just point something out: I’m going to be moderating comments and deleting ones that I deem to be too off-topic. Previous comment threads about the neutrino result have tended to spiral into nonsense, so I’m going to try to stop this one going the same way.

    Link to this
  6. 6. S. Dinowitz 9:35 am 03/5/2012

    At this late juncture I probably won’t get an answer but was this statement from Science true: “After tightening the connection and then measuring the time it takes data to travel the length of the fibre, researchers found that the data arrive 60 nanoseconds earlier than assumed” If so, it seems that OPERA is just going through the motions re-running the velocity measuements in May. I think they should regardless, but I hadn’t read the Science statement previously and was just wondering if it was true.

    Link to this
  7. 7. Kelly Oakes in reply to Kelly Oakes 9:02 am 03/16/2012

    @S. Dinowitz,
    The only official word I’m aware of is the OPERA statement I copied at the end of the post, which states – “The potential extent of these two effects is being studied by the OPERA collaboration.” – so it sounds to me like it’s not yet clear whether what Science said is exactly true. It sounds to me like it was a bit of an oversimplification.

    Link to this
  8. 8. jtdwyer 2:14 pm 03/17/2012

    Kelly Oakes,
    The original SienceInsider report [http://news.sciencemag.org/scienceinsider/2012/02/breaking-news-error-undoes-faster.html?ref=hp] states:
    “After tightening the connection and then measuring the time it takes data to travel the length of the fiber, researchers found that the data arrive 60 nanoseconds earlier than assumed. Since this time is subtracted from the overall time of flight, it appears to explain the early arrival of the neutrinos. New data, however, will be needed to confirm this hypothesis.”

    As you indicated in the official statement, two potential faults were identified that might have opposing effects on the final measurement. The first possible fault that “concerns an oscillator used to provide the time stamps for GPS synchronizations. It could have led to an overestimate of the neutrino’s time of flight.”

    The loose fiber connection could account for the entire measurement error as Science’s sources indicated if the second potential fault has no measurable effect. Determining the net effect of both faults reportedly requires the additional tests and analyses.

    Link to this
  9. 9. Kelly Oakes in reply to Kelly Oakes 9:19 am 03/19/2012

    jtdwyer,
    Yes, it could account for the entire error. But it might not. Nobody knows yet, only time will tell…

    Link to this

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