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Neutrinos and the speed of light? Not so fast …

The bartender says, “We don’t serve your kind in here” A faster-than-light neutrino walks into a bar … The media is champing at the bit to proclaim a discovery of faster-than-light travel by a subatomic…

Another week, another rush to proclaim Einstein was wrong. Martial Trezzini/AFP

The bartender says, “We don’t serve your kind in here”

A faster-than-light neutrino walks into a bar …

The media is champing at the bit to proclaim a discovery of faster-than-light travel by a subatomic particle, with some going as far as claiming “Einstein was wrong: relativity theory busted”.

The scientists responsible for the experiment and analysis let slip they have some preliminary data that suggests the particles travelled faster than light, but they seem to be the only ones not jumping to conclusions just yet.

The team at the Oscillation Project with Emulsion-tRacking Apparatus (OPERA) in Italy regularly measures the detection of neutrinos emitted from another experiment at the Large Hadron Collider (LHC) in Switzerland, 730 kilometres away.

Neutrinos (electrically neutral subatomic particles) are rather indifferent to the presence of trivial things such as Earth, and zip through without so much as a passing interest (their cross-section, the probability for interaction, is extremely small). Owing to their small mass, they should do so at approximately the speed of light, c (see video below) – the speed light travels in a vacuum, known quite well to be 299,792,458 metres per second.

Using GPS timing and position data, the OPERA team claim to know the distance between the point at which neutrinos are emitted from the LHC and the point at which they are detected in Italy to a precision that allows them to predict the time the neutrinos should arrive to within ten nano-seconds (a nanosecond being a billionth of a second).

What they claim to have found, though, is neutrinos arriving 60 nano-seconds (0.00000006 seconds) early. If accurate, this would be a six standard-deviation result – enough to convince physicists that something is genuinely awry.

The scientists concerned have released the findings to the scientific community in the hope that, if something has been overlooked, it will be picked up by their peers. The peer-review process is usually quite efficient at eliminating likely sources of error, and in this case there are plenty of possibilities. But on the face of it, it seems the OPERA team has been very careful.

There’s the issue of knowing the exact positions of the source and detector to within the quoted uncertainty – keeping in mind that in the extra 60 nano-seconds the neutrinos are supposedly travelling they will cover a total of 18 metres. This means knowing those two positions – and the geodesic distance between them – to within three metres out of 730,000 metres.

AFP

Traditional civilian grade GPS has an accuracy of about 15 metres. More sophisticated methods are used for proper surveying, such as differential GPS (10-centimetres accuracy). At the very top range is “carrier phase tracking", which can beat one-centimetre accuracy.

This does require the GPS antenna to be above ground, though, so one also needs to take into account the timing for signals to travel along wires to the underground experiments.

The OPERA scientists made use of the more precise GPS system and a cesium atomic clock to ensure their timing and positions were as accurate as possible.

Presuming for now all the possible sources of error are accounted for, what would this result mean? Time-travel seems to be the go-to topic when faster-than-light particles are mentioned, but don’t hold out hope for a TARDIS just yet.

If a particle is able to travel faster than c, a few odd things happen. Critically, it breaks (special) relativity, which states there’s an absolute speed-limit – the speed at which massless particles travel – that doesn’t depend on relative motion.

One practical aspect of relativity is that the concept of simultaneity is frame-dependent. If two events occur at different locations (say, flashing a torch) then, depending on how you are travelling relative to each of those events, you may see them occurring at different times (for instance, if you are accelerating relative to one then you will see it occur later, as if time is slowed). The order in which you observe them to occur depends on the relative motion.

Now, if one of those events was flashing a torch (photons) and the other was flashing super-luminous particles (travelling faster than light) your interpretation would not just be that they occurred at different times, but that one must have travelled back in time.

So, the particles can appear to travel back in time, but there’s still no method of accelerating a cyborg killing-machine to super-luminal speeds.

AFP

The peer-review process is an important step to deciding whether or not to believe a particular result, but is the latest potential finding an isolated incident? Apparently not. In 2007 the MINOS experiment observed the same thing, albeit with a smaller significance (1.8 standard deviations – not enough to get excited about).

Measurements of arrival times of photons and neutrinos from supernova SN1987a in 1987 provided a much better agreement with the speed of light, but those neutrinos were of a much lower energy. The possibility remains that velocity depends on energy.

Somewhat less rigid explanations include neutrinos taking “shortcuts” through extra dimensions. Undoubtedly, many more possible explanations will arise if all conventional sources of error are excluded.

The much more likely scenario is that the analysis has overlooked some seemingly insignificant but critical aspect, and that re-analysis will led to a very good agreement with the speed of light.

Should that be the case, the follow-up press-release will no doubt refer to the “Phantom of the OPERA”.

Join the conversation

8 Comments sorted by

  1. Phillip Ebrall

    Professor of Chiropractic at Central Queensland University

    How refreshing to have heavy science explained with a sense of humour. Here I was thinking a neutrino was a new-fangled serve of coffee, like a babycino or such. Now I know better. Thanks Jonathan.

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  2. John Nicol

    logged in via Facebook

    A very interesting piece of news Jonathan and thankyou for writing it in such a clear and engaging way. I guess it will stand a few theories on their head should their measurments prove to be correct, and open up a lot of channels for research in physics. Having observed some exciting "discoveries" in physics over the years and found them to be artifacts of either the apparatus or the imagination, I believe your last couple of comments to be quite prescient. However, one must always allow for the possiblity that one day, such an unexpected result will be confirmed and this may be that day. Nevertheless, the early history of the N-rays in France, the "discovery" of a single quark in Sydney and the "fifth force" in the late 1980s do sometimes encourage sceptism which may not always continue to be justified.
    John Nicol jonicol18@bigpond.com

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  3. Craig Savage

    Professor of Theoretical Physics at Australian National University

    The OPERA result has followed the usual scientific path of building on previous experimental work, supported by a theoretical explanation. Previously, the American MINOS collaboration reported observing faster-than-light neutrinos, but their result was statistically unreliable. However, they pioneered the required experimental techniques and data analysis methods.

    Faster-than-light neutrinos are predicted by various theories of quantum gravity. Faster-than-light elementary particles may emerge when…

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  4. Policarpo Uliana

    logged in via Facebook

    In the OPERA experiment , the neutrino seem to be traveling faster than light because the synchronized clocks used in the experiment generate a system in which the time information spreads faster than light.
    So is the Earth which moves in space with a speed of at least 7 km / s (in the north / south line that runs from Switzerland to Italy). Thus the neutrinos are not coming soon in Italy, but Italy that travel to the neutrinos while they are traveling by the Earth.
    See the full explanation on:
    http://www.atomlig.com.br/poli/Neutrino-IG.pdf

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    1. Jonathan Carroll

      Post Doctoral Research Associate, Centre for the Subatomic Structure of Matter at University of Adelaide

      In reply to Policarpo Uliana

      The point of this article was to advocate caution in interpreting scientific data/results. Your use of an unqualified 'because' suggests that you are 100% sure this is the source of error.

      Perhaps "might be because" would be a better statement. Then people would have an opportunity to refute your argument, such as by noting that the measurements were performed in an Earth-centered inertial (ECI) coordinate frame, or that GPS satellites already take into account the Sagnac effect when calculating timing.

      In any case, a repeat experiment could rule out your suggestion quite easily if it has a baseline not parallel to the current one (even anti-parallel would do).

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    2. Policarpo Uliana

      logged in via Facebook

      In reply to Jonathan Carroll

      Dear Jonathan
      I'm 100% sure this is the source of error in order to defend my point of view. Obviously I could be wrong ...
      I developed a work on the theory of general relativity, where the matter expands the space instead of shrinking it. This model allows calculate the equation of a simple black hole without rotation (see it on http://www.atomlig.com.br/poli/USN-PT-V7.pdf).
      But my theory uses a type of ether and because of that physicists do not even want to read the work, even with the math…

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  5. Lindsay Gilmour

    Ordinary citizen

    Hi boffins . . . as a keen science student in high school many years ago (early seventies), I remember reading in a physics book - the title of which escapes me - about tachyons. This book suggested that tachyons may indeed travel faster than light. Have tachyons gone out of fashion, been disproved, or what? Where do they fit into the subatomic hierarchy? Thanks.

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    1. Jonathan Carroll

      Post Doctoral Research Associate, Centre for the Subatomic Structure of Matter at University of Adelaide

      In reply to Lindsay Gilmour

      Tachyon is the general term for a faster-than-light particle (from the Greek: ταχύς or tachys: “swift, quick, fast, rapid”... thanks Wikipedia!). For the reasons explained in this article, they're not particularly popular (breaking relativity = bad).

      Of course, it's impossible to prove that something doesn't exist, but at the moment tachyons are inconsistent with several well-studied theories. At present, there's no evidence that neutrinos are tachyons; more on that in an epilogue shortly.

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