90

XKCD

I was quite surprised to read this all over the news today:

Elusive, nearly massive subatomic particles called neutrinos appear to travel just faster than light, a team of physicists in Europe reports. If so, the observation would wreck Einstein's theory of special relativity, which demands that nothing can travel faster than light.

—source

Apparently a CERN/Gran Sasso team measured a faster-than-light speed for neutrinos.

  • Is this even remotely possible?
  • If so, would it be a real violation of Lorentz invariance or an "almost, but not quite" effect?

The paper is on arXiv; a webcast is/was planned here.

News conference video here

Community
  • 1
Sklivvz
  • 13,499
  • 7
  • 64
  • 87
  • Did the automated bot changing HTTP to HTTPS also inline the image, destroying the attribution/citation? Or was that a user edit merged into the bot's edit resulting in a misleading timeline? – Ben Voigt Oct 18 '22 at 16:32

14 Answers14

42

Last (?) Edit: The "problem" is solved: it was mainly a problem in the timing chain, due to a badly screwed optical fibre. A high level description of the problem is given here and a more detailed explanation of the investigation is here.

List of possible systematic biases

I thought it might be a good idea to list the possible systematic biases which could lead xkcd's character to win his bet. As many physicists (including, I guess, many people from the OPERA collaboration), I think it will end like the Pioneer anomaly. Of course, the current list only contains biases which are unlikely, but less unlikely than a causality violation.

Location errors and clocks drifts

The arXiv paper studied them, and seem to exclude it. The distance seems to be known within 20 cm and the synchronisation seems to be within 15 ns (6.9 statistical and 7.4 systematic). If this would however end up to be the explanation, it would be quite boring.

Update: Rumors seems to tell that the boring explanation is the good one.

Not the same neutrinos detected

The neutrinos are emitted on a 10.5 µs window, 175 times longer than the observed effect. It might be possible that the neutrino emitted early are not exactly the same as the one emitted late. Neutrino oscillation might, for example, then make early neutrino more detectable by the distant detector.

However, the detectors were built to measure the oscillation, so I guess that the OPERA collaboration thought about it, and rejected it for whatever reason. I suppose an explanation along these lines would mean interesting new particle physics.

Update: This possibility excluded by a new experiment with 3 ns pulses.

Errors in the statistical timing analysis

The timing itself is based on a quite elaborate statistical analysis. Furthermore, the pulses are quite long (10 μs), so an error in this analysis could easily be of the good order of magnitude.

Update: This possibility excluded by a new experiment with 3 ns pulses.

Community
  • 1
Frédéric Grosshans
  • 10,943
  • 4
    Concerning your #2: they purport to have dealt with this using the shape-shape fitting between the proton current monitor and the timing of the detection. Several of my colleague suspect there may be a subtle effect hiding here, but it is not as if they didn't think of it. – dmckee --- ex-moderator kitten Sep 23 '11 at 18:11
  • 2
    Thanks for making a community wiki reply. This is good because otherwise the voting process could drown out important updates that are otherwise ignored in the media. The researchers who released this data themselves will be one of the most likely sources for resolution of the paradox. – Alan Rominger Sep 23 '11 at 19:53
  • 2
    The new setup (3 ns pulses, 20 times shorter than the observed effect) has eliminated the last two points. The actual timing and positioning hasn't changed, so point one still stands. (However, that's been perhaps the most scruntinized of all explanations). – MSalters Nov 22 '11 at 08:35
  • @MSalters: I agree. But the time and distance measurements have been verified by multiple methods, and the methods are ones that are standard and reliable. E.g., the delay in the 8.3-km optical fiber has been measured both by two-way timing and using a portable clock, and it's been measured repeatedly over time so that one can rule out changes in optical properties due to aging of the plastic. These are simple measurements that could be checked in an afternoon by a competent 2nd-year grad student. I really have a hard time imagining a plausible "goof" explanation at this point. –  Nov 23 '11 at 21:17
  • Given the sheer diversity of possible `goof-up' explanations on this page (all answers combined), I can't help feeling that we are trying to find one plausible way in which this can be MADE to look wrong. I mean, of course, we'll all be very happy if relativity still holds good and there does turn out to be some error, but I hope we are scientific about this whole issue. With due respect to everyone, this reminds of the old EPR remark by Einstein himself - ``everybody says it is wrong for some reason or the other, but curiously, no two people agree on what exactly is wrong with it''. – 299792458 May 30 '14 at 06:47
  • PS - I ask for forgiveness here, even though I'm prepared to get ambushed for an `against the tide' rant. This isn't a mud-slinging remark, it is just a word of caution against sidestepping the virtues that science stands for. Apologies if it angers anyone. – 299792458 May 30 '14 at 06:51
40

You have a few longer answers which were already updated, but here is a concise statement of the situation in mid-2014:

  1. An independent measurement by the ICARUS collaboration, also using neutrinos traveling from CERN to Gran Sasso but using independent detector and timing hardware, found detection times "compatible with the simultaneous arrival of all events with equal speed, the one of light."

  2. In an edited press release (and probably in the peer-reviewed literature as well), all four of the neutrino experiments at Gran Sasso report results consistent with relativity.

  3. The mumblings that begin a few months after the initial report, that a loose cable caused a timing chain error, have been accepted by the experimenters. Frédéric Grosshans links to a nice discussion by Matt Strassler which includes this image: OPERA timing offset vs. date You can clearly see that the timing offset was introduced in mid-2008 and not corrected until the end of 2011.

It's important to remember the scale of the problem here. In vacuum, the speed of light is one foot per nanosecond. In copper/poly coaxial cable it's slower, about six inches per nanosecond, and in optical fiber it's comparable. A bad cable connector can take a beautiful digital logic signal and reflect part of it back to the emitter, in a time-dependent way, turning the received signal into an analog mess with a complicated shape. And a cable can go bad if somebody hits it the wrong way with their butt while they are working in the electronics room.

(I actually had something similar happen to me on an experiment: I had an analog signal splitter "upstairs" that sent a signal echo back to my detectors "downstairs", and a runty little echoed pulse came back upstairs after about a microsecond and got processed like another event. I wound up spending several thousand dollars on signal terminators to swallow the echo downstairs. It was an unusual configuration and needed unusual termination hardware and I must have answered the question "but couldn't you just" a hundred times.)

Gran Sasso is an underground facility for low-background experiments — the detectors can't see GPS satellites directly, because there's a mountain in the way, and their access to the surface is via a tunnel whose main purpose is to carry traffic for a major Italian motorway. I'm quite impressed that they had ~100 ns timing resolution between the two laboratories; the "discovery" came about because they were trying to do ten times better than that.

As an experimentalist I don't begrudge the OPERA guys their error at all. I'm sure they spent an entire year shitting pineapples because they couldn't identify the problem. When they finally did release their result, they had the courage to report it at face value. The community was properly incredulous and the wide interest prompted a large number of other checks they could make. Independent measurements were performed. An explanation was found. Science at its best.

rob
  • 89,569
  • 27
    Indeed, they didn't report "we found superluminal neutrinos" but "we measured data that looks like superluminal neutrinos, but after searching for quite some time still cannot find an error in the experiment, so we now decided to publish so that others can check if we have possibly a real effect; we keep searching for an error anyways." How more honest can you be? (I'm a theorist, BTW; you do not have to be an experimentalist to acknowledge that.) – celtschk May 04 '14 at 08:45
  • 2
    This is inspirational (for theorists and experimentalists alike) :D – Danu Aug 18 '14 at 08:46
  • 1
    MINOS is reporting a completely independent (different beam as well as different detectors) measurement as of July 2015: http://arxiv.org/abs/1507.04328 They also use an actual near detector rather than relying on the pick-off magnet timing as at CERN. – dmckee --- ex-moderator kitten Aug 06 '15 at 22:53
21
  • Is this even remotely possible?

Well... "possible," yes, but kind of like how tunneling through a brick wall is "possible": while you can't definitively prove it impossible, you'd feel pretty safe saying "this will never happen." Relativity is really well-tested, and it's really hard to conceive of a way that neutrinos could travel faster than light without it having other consequences that we would have discovered by now. That being said, I don't know the field inside-out and I'm sure some theorist has come up with some wacky idea that allows it. I asked another question that might come up with something.

  • If so, would it be a real violation of Lorentz invariance or an "almost, but not quite" effect?

If the results from OPERA are accurate, this effect would be a full-blown real Lorentz violation, not just an apparent effect like Cerenkov radiation or astronomical superluminal motion. That's why everyone is so excited about it. (Unless the neutrinos are tachyons; in that case, I guess Lorentz invariance is technically still intact, but the observation of a tachyon would be equally big news.)

The setup of CERN and OPERA is conceptually very simple, basically just two observers located a known distance apart with synchronized clocks. It's a direct measurement of average velocity. There's no complicated theoretical analysis that needs to be done to determine whether the speed of light was exceeded. Either they are wrong about either the distance (mismeasurement, or there is a spacetime "rift" within the Earth :-P) or the time (clock synchronization error or drift), or they have actually discovered superluminal neutrinos.

Community
  • 1
David Z
  • 76,371
  • Are the observers using exactly identical detectors? Can't the "timing offset" of detection depend on some build parameters that are different, or is the measured excess velocity simply too large for being caused by something like that? – BjornW Sep 23 '11 at 09:59
  • 3
    No, the detectors are not identical, but the offset they're measuring is not just what they read off their clocks. They account for the time it takes to process the signal and work backwards from their measurements to determine the time at which the neutrino actually interacted with the detector. You can see their analysis in section 6 of the paper. There is some uncertainty in exactly how much time this takes, but it's much smaller than the time difference they detected. So it would appear that differences between the detectors are not the cause of the time difference. – David Z Sep 23 '11 at 10:13
  • 2
    They are not actually using a near detector at all in the usual sense, they are measuring the beam current directly after the pick off magnet, and then correcting for beam TOF down to the target. This is a place that people are examining for subtle effects. I found that odd given that they do have a downstream muon detector system, but they may be concerned about backgrounds. – dmckee --- ex-moderator kitten Sep 23 '11 at 18:13
  • Actually the impossibility of FTL neutrinos is quite different from the impossibility of tunnelling through a brick wall. Tunnelling through a brick wall wouldn't actually violate any known law of physics, it's just sufficiently improbable according to those laws that if we ever observed it, we'd consider it more likely that our theories have to be amended than that we just have observed such an unlikely event. As such, it is comparable to an object spontaneously heating up in a cold environment. FTL OTOH is not just extremely improbable, but forbidden by the currently known laws of physics. – celtschk May 04 '14 at 08:56
  • @celtschk right, but I'm accounting for the small probability that the known laws of physics are wrong. – David Z May 04 '14 at 20:42
12

According to Dr. Phil Plait, there's a rumour that it's been a faulty connection. In summary: nothing is wrong with the calculation, the theoretical assumptions, rotation of the Earth, etc... A hardware problem caused the 60 ns time gap.

It's still gossip, so take this with abundance of caution, but here's what he had to say:

According to sources familiar with the experiment, the 60 nanoseconds discrepancy appears to come from a bad connection between a fiber optic cable that connects to the GPS receiver used to correct the timing of the neutrinos’ flight and an electronic card in a computer. 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.

—source

Sklivvz
  • 13,499
  • 7
  • 64
  • 87
  • This doesn't seem right--- could a hardware problem actually do this? Can you plausibly make a 60ns delay by a loose cable? Usually, you just lose some pulses travelling down the cable. I find it hard to believe its hardware. If I were conspiratorially minded, I would say they are covering up an uncorrected relativistic effect with a bogus story of a hardware error. How could a hardware error cause a systematic bias through two different runs of the same size. They have an incentive to lie, and they are incompetent, and incompetent people lie. – Ron Maimon Feb 23 '12 at 13:52
11

UPDATE 2011-10-15

This phenomena may have been explained. The crux of the problem had to do with differing reference frames - the distance traveled according to the satellites which measured the time was different from the distance traveled according to us on earth. If you're going to measure speed (distance / time), you have to get the distance and time both from the same reference frame. We were getting distance from our reference frame and time from the (very fast) satellite's reference time.

This article explains it in a very accessible way:

To understand how relativity altered the neutrino experiment, it helps to pretend that we're hanging out on one of those GPS satellites, watching the Earth go by underneath you. Remember, from the reference frame of someone on the satellite, we're not moving, but the Earth is. As the neutrino experiment goes by, we start timing one of the neutrinos as it exits the source in Switzerland. Meanwhile, the detector in Italy is moving just as fast as the rest of the Earth, and from our perspective it's moving towards the source. This means that the neutrino will have a slightly shorter distance to travel than it would if the experiment were stationary. We stop timing the neutrino when it arrives in Italy, and calculate that it moves at a speed that's comfortably below the speed of light.

"That makes sense," we say, and send the start time and the stop time down to our colleagues on Earth, who take one look at our numbers and freak out. "That doesn't make sense," they say. "There's no way that a neutrino could have covered the distance we're measuring down here in the time you measured up there without going faster than light!"

And they're totally, 100% correct, because the distance that the neutrinos had to travel in their reference frame is longer than the distance that the neutrinos had to travel in our reference frame, because in our reference frame, the detector was moving towards the source. In other words, the GPS clock is bang on the nose, but since the clock is in a different reference frame, you have to compensate for relativity if you're going to use it to make highly accurate measurements.

The original paper publishing these findings is here: Times of Flight between a Source and a Detector observed from a GPS satelite.

Original Post

Sources: [1] (Associated Press), [2] (Guardian.co.uk), [3] (Original Publication - Cornell University)

Scientists around the world reacted with cautious shock on Friday to results from an Italian laboratory that seemed to show that certain subatomic particles can travel faster than light.

The journey would take a beam of light around 2.4 milliseconds to complete, but after running the Opera experiment for three years and timing the arrival of 15,000 neutrinos, the scientists have calculated that the particles arrived at Gran Sasso 60 billionths of a second earlier, with an error margin of plus or minus 10 billionths of a second. The speed of light in a vacuum is 299,792,458 metres per second, so the neutrinos were apparently travelling at 299,798,454 metres per second.

Ignoring the boilerplate media hype about the possibilities of time travel and alternate dimensions - I'm looking for academic sources that might suggest how this could be true, or alternatively, how this discrepancy could be accounted for.

I read the published article, Measurement of the neutrino velocity with the OPERA detector in the CNGS beam, with their findings. It looks like they took an insane amount of care with their measurement of distance and time.

One of the most common skepticism of people who no nothing about the experiment is stuff like:

You might worry about[...] have they correctly accounted for the time delay of actually reading out the signals? Whatever you are using as a timing signal, that has to travel down the cables to your computer and when you are talking about nanoseconds, you have to know exactly how quickly the current travels, and it is not instantaneous. [2]

This experiment doesn't use that sort of 'stopwatch' timing mechanism though. There is no 'T=0', and no single firing of neutrinos. What is detected is watermark patterns in the steady stream of particles. The streams at the input and output are time stamped using the same satellites and any position along each stream has a precise time associated with it. By identifying identical patterns at input and output streams, they can identify how long it took particles to travel between the points. [1]

Time

As for distance, they use GPS readings to get the east, north, and altitude position along the path travelled to great precision. So much so that they even detect slow earth crust migration and millimetres of changes in distance between source and destination when something like an earthquake occurs. When your particles are travelling on the scale (730534.61 ± 0.20) metres, this is more than enough precision:

Distance

It's going to take a lot more than grassroots skepticism to think of what could have caused this discrepancy. I've seen suggestions such as the gravity of the Earth being different along the path of the neutrinos, which warps space/time unevenly. The neutrino might not actually be travelling as far as they think if space/time is contracted at one or more points along the path where gravity varies.

Anyway, I'll be interested in seeing how it pans out. Like most scientists, my guess is an unaccounted for systematic error (because they definitely have statistical significance and precision on their side) that has yet to be pointed out, but it probably won't take too long with all the theoretical physicists that will be pouring through this experiment.

Community
  • 1
Alain
  • 285
  • 2
    One possibility is that the widespread use of GPS for measurments of earth has redefined the meter. The meter is defined as a specific fraction of the speed of light in vacuum. The GPS is not working in vacuum but its electromagnetic pulses go through the atmosphere and ionosphere and are corrected for that. If a systematic error enters there though, the fact of the precision of measurement with GPS, not disputed, would be a demonstration of the difference between accuracy and precision. The neutrinos are little affected by matter and seem to be covering more "meters" than vacuum meters. – anna v Sep 25 '11 at 12:55
  • 3
    The problem with the GPS position measurements (I think that the time measurements are accurate) is that the relative position is not subject to the same systematics as the aboslute position. The different rotational velocity at Geneva vs. Central Italy gives diurnal abberation which must be corrected for to get an accurate absolute distance. You must convince yourself that the absolute measurements have the same error bars as the relative measurements, and I did not see that in the arxiv paper. – Ron Maimon Sep 25 '11 at 20:32
  • 3
    Note that the author of the pre-print you link in you edit has issues a partial retraction and appology. – dmckee --- ex-moderator kitten Oct 22 '11 at 21:11
  • So does this solution have anything to do with the rotation of the Earth as I have read else where? It says that from the satellites perspective, The detector was moving towards the source, how is that known? Surely depending on the satelitte the detector could have been moving away from the source? – Jonathan. Oct 28 '11 at 08:51
  • 1
    There have been plenty of papers (well, preprints) have been put forward offering various explanations of the OPERA results, but none of them has been widely accepted yet as far as I know so it's rather premature to say the results have been explained. It's a nice answer other than that, though. – David Z Nov 04 '11 at 03:16
  • 1
    @David: the explanation that they did not account for the failure of simultaneity in the transformation from the satelite frame to the Earth's rotating frame fits the systematic error perfectly, and I think it is naive to expect them to acknowledge that this is their error, they will just keep quiet about the error and incorporate the fix. The effect is clearly of the order of the diurnal abberation, same order as failure of simultaneity, and all of order rotational velocity corrections, and independent of neutrino energy, as expected for a systematic. – Ron Maimon Nov 04 '11 at 03:42
  • 3
    @dmckee: The "partial apology and retraction" is not an apology or a retraction. The explanation for the error provided is cogent, clear, and almost certainly correct. The author is only clarifying that the GPS community doesn't need to read his paper, because it has no impact GPS best-practices, since the issue of precise time-of-flight is not relevant for most GPS uses. – Ron Maimon Nov 04 '11 at 03:45
  • 1
    Van Elburg's paper is completely wrong. He simply doesn't have a clue about GPS. By assuming incorrect things about how GPS incorporates or doesn't incorporate relativity, he reaches incorrect conclusions. It's a sad commentary on science journalism that this paper was given such prominence in the popular press. Van Elburg assumes that GPS doesn't correct for SR or GR effects. But this is total hogwash: http://relativity.livingreviews.org/Articles/lrr-2003-1/ I suppose people are apt to latch onto the van Elburg paper because it only uses freshman relativity. –  Nov 05 '11 at 22:22
  • @Ben: Van Elburg's paper is completely correct. The corrections for relativity in GPS are fine, and this is the reason for his later clarification--- he is not saying that GPS is off. In fact, it is because GPS is so good that the systematic is there--- the distances are measured on the corrected Earth frame, and then the times are off by the failure of simultaneity relative to the satellite frame. This doesn't affect anyone who is only doing pure GPS, and not trying to synchronize distant clocks at the same time. There is nothing in the link you provide that contradicts his paper. – Ron Maimon Nov 06 '11 at 07:50
  • @Ron: "the distances are measured on the corrected Earth frame, and then the times are off by the failure of simultaneity relative to the satellite frame." No, the satellites' frame is irrelevant. This is what van Elburg apparently didn't understand, because he apparently wrote his paper without taking the time to learn basic facts about GPS. As explained in the review paper I linked to, the GPS time coordinate is defined in a nonrotating frame tied to the center of the earth. This differs from the frame of the accelerator and detectors by the rotational velocity of the earth's surface[...] –  Nov 06 '11 at 17:05
  • [...] This velocity is on the order of 400 m/s, and it causes violation of simultaneity by $vx/c^2$, which is about 3 ns. Even if the OPERA group had neglected this correction (which I'm sure they didn't), it would be 20 times too small to account for the anomaly. –  Nov 06 '11 at 17:11
  • @Ben: I have to admit I didn't read Van Elberg, I just personally calculated the diurnal abberation error for a sattelite 25,000 miles away, and it is 20 m, and all other effects are same order, so result is garbage. The failure of simultaneity error is indeed small by a factor of 10, but without OPERA cooperation, what else can you say? It is obviously an effect of the rotation of the Earth and the finite speed of light. If you take an absolute frame rotating with the Earth, then the neutrinos have to traverse 20m less distance just because the earth rotated that much during that time. – Ron Maimon Nov 06 '11 at 20:24
  • I watched the original presentation of the results, and they say specifically that the clocks are originally synced by using a portable atomic clock, twinned with the ones in the satellites. So this allows for a very strict (original) sync without relying on GPS directly. They further said that each site syncs with the GPS clocks to keep the two sites with max ~1ns difference. So I as far as I see, it would be very hard for the clocks to go out by 60ns. Finally GPS is used to find the site's positions. But these are two independent measurements. – Sklivvz Nov 21 '11 at 13:19
  • @Sklivvz The above preprint isn't suggesting that the clocks came out of sync. It is suggesting that the measurements were not correctly translated from the satellites reference frame to the earth's reference frame. – Alain Nov 21 '11 at 16:26
  • @Alain I am a bit confused: if the clocks are in sync, the time measurement does not depend on the GPS movement. Or are you talking about the distance measurement (which is done once)? – Sklivvz Nov 21 '11 at 17:13
  • @Sklivvz According to R.A.J. van Elburg: "I propose the experiment was set in the satellite reference frame and has been treated (at least partially) as if it were set in the CERN-Gran Sasso reference frame." REF – Alain Nov 21 '11 at 18:10
  • The satellite reference frame? According to the Physikalisch-Technische Bundesanstalt (who have done the time transfer check) the GPS synchronization has been done using all satellites visible. Given the short baseline, you'd expect that to include at least 8 satellites in at least two different orbits. – MSalters Nov 22 '11 at 08:48
5

Suppose this is real, that the neutrinos arrive very slightly faster than light would through the vacuum. Wouldn't that point to there being a slightly higher c which actually limits speeds, and some slight slow-down for light from this maximum due to interactions of the electromagnetic field with other particles, including virtual particles?

After all, you can move an electron faster than a photon in glass, and we don't call it the end of relativity, we call it Cherenkov radiation.

So the definition of refractive index might need adjusting, but effectively the vacuum has a non-zero refractive index, or rather the vacuum is not entirely empty. Which we know.

It makes sense that a neutrino is not subject to the same interactions, given its famed reluctance to interact with anything. Perhaps it is just an indication that the particles in a vacuum are more likely to be electromagnetic-interacting than weak-interacting.

Or am I labouring under a false premise? Is the speed of light in a vacuum already adjusted for virtual particle interactions?

Peter Mortensen
  • 2,352
Phil H
  • 1,035
  • You would still need to explain why a massive particle (the neutrino) moves faster than a massless particle (the photon). – Sklivvz Sep 23 '11 at 12:37
  • @Sklivvz The mass of the neutrino is so small that it is irrelevant in the argument, if the refraction is of the order of magnitude of the measurement. We end up with statistical errors. I think what is true is that the group velocity of light as assumed by the experimenters is shown to be smaller than the group velocity of the neutrinos as measured by them. "Assumed" because there is no discussion of the effect of the collective refraction index due to the atmosphere, ionosphere, magnetic field (and maybe etc) of the earth in the measure of time they use. – anna v Sep 23 '11 at 13:21
  • 2
    @Sklivvz: a massive particle moving faster than massless photons is what also happens in Cherenkov radiation. What one would need to explain is why hadrons and non-neutrino leptons experience exactly the same "braking" effekt as photons do. – leftaroundabout Sep 23 '11 at 13:33
  • @leftaroundabout How precisely has this "braking" effect been measured for other particles? – starblue Sep 23 '11 at 17:58
  • 1
    This is not supported by the supernova data. – dmckee --- ex-moderator kitten Sep 23 '11 at 18:15
  • @starblue: well – as far as we know there is no braking mechanism apart from relativity itself, so all these particles simply approach $c$. That's being measured very precisely in every synchrotron. If it should now turn out that this is in fact not the fundamentally highest speed possible but just the speed that photons, hadrons etc. are being throttled down to, it would be quite remarkable that they are all throttled down to precisely $c$. – leftaroundabout Sep 23 '11 at 18:37
  • @leftaroundabout Instead of handwaving, could you please tell me to which precision the limiting speed has been measured? Much better than 20 ppm? – starblue Sep 23 '11 at 18:46
  • 1
    @leftaroundabout: we can only measure the speed of light in a vacuum through a vacuum. So given a constant density of vacuum particles, the speed of light through the vacuum would always be constant. Only with a different particle (e.g. a neutrino) would we be able to measure a higher speed. Inevitably, if this turned out to be the case, the real upper limit is slightly higher again, since neutrinos are massive and thus move below the maximum speed. – Phil H Sep 26 '11 at 11:47
  • @Phil: yes, only with a different particle. Hadrons, or electrons are quite different from photons though, aren't they? That's my point. Though I can't give a concrete error margin for how precise the speed of the charged particles in a synchrotron is measured, I'm quite sure it's better than 20 ppm (sorry, handwaving again @starblue) because the whole working principle of a synchrotron relies on exactly knowing where the particles are at which time. And electrons had the energy to reach $(1-10^{-11})c$ in the LEP, if they could go any faster than light they certainly would have. – leftaroundabout Sep 26 '11 at 12:22
  • There is interstellar gas that slows the photons' motion. At the same time, neutrinos virtually do not interact with the gas so do not get slowed. – Anixx Feb 24 '12 at 00:31
4

There are strong reasons for disbelieving this result.

[This paragraph is disproved by the Nov. 17 result.] It uses an experimental design that was never intended for this purpose, and that is inherently poorly suited to it; the beam pulses were 10,000 ns wide, and the shift they claim to have measured is only 60 ns. This means that the shift can only be detected statistically, and it makes the result extremely vulnerable to unanticipated systematic errors, e.g., correlations between the time of emission of the neutrinos and their energy (which strongly affects the efficiency of detection) or the direction of emission. They did another run at the end of October, with beam pulses 1-2 ns wide. That's the correct design if you want to measure the speed of the neutrinos reliably. They should have simply waited until after they had those data before announcing their results. (In fact, five senior members of the collaboration did not put their names on the paper.) I have a bet running with a colleage, for a six-pack of Fat Tire, that the new run will show that the original result was bogus. The result may be announced as soon as November or December. [The result was announced Nov. 17, and I lost my six-pack.]

Another reason to disbelieve it is that there are strong and fairly model-independent reasons to believe that it cannot be correct. A superluminal neutrino beam would have lost a lot of its energy via radiation, but a measurement by another detector shows that this was not the case: http://arxiv.org/abs/1110.3763 Superluminal motion for neutrinos would also cause superluminal motion for electrons, which is contrary to observation http://arxiv.org/abs/1109.5682 , and it would also have caused a suppression of pion decay, so that the beam could never have been produced in the first place http://arxiv.org/abs/1109.6630 . All of this holds regardless of the details of the model. E.g., it holds both for tachyonic neutrinos without a preferred frame and for models in which neutrinos are not tachyonic and there is a preferred frame.

Yet another reason for disbelief is that the velocity of propagation of neutrinos has been measured to much higher precision by other techniques, so if you want to believe the OPERA result, you have to posit a very strange energy-dependence of the velocity.

  • is this the result of the experiment you're talking about? http://arxiv.org/abs/1109.4897v2 ... http://www.science20.com/quantum_diaries_survivor/opera_confirms_neutrinos_travel_faster_light-84763 – Hrant Khachatrian Nov 18 '11 at 14:02
  • @Hrant Khachatrian: Yes. It shows that the effect was not a statistical artifact as I proposed above. IMO what really needs to happen now is two things: (1) Other groups will try to reproduce the anomaly. (2) OPERA should try to verify that the anomaly has an energy dependence. (If the result is wrong, then it should be independent of the energy.) –  Nov 23 '11 at 21:05
  • @BenCrowell, if this were verified, what energy-dependent effects would we see in Nature? By analogy, if Einstein relativised the classical picture, how would this result "relativize" Einstein's theory of gravity? –  Nov 23 '11 at 22:30
3

•Is this even remotely possible?

Well yes, of course it's possible in the same way that it's possible that invisible neutrino fairies are messing around with the neutrinos underground and hence causing havoc with the mental health of physicists around the world. It's just... unlikely, very unlikely, just as the 4-sigma evidence for new CP violation in like-sign dimuons was possible, only to fall flat on its face when ATLAS and CMS failed to see the same thing. But, it's still possible! Even so, let's focus on what's more likely: There are no neutrino fairies, and the conflict between data and special relativity lies with >> 6-sigma likelyhood of it being an error with the experiment.

•If so, would it be a real violation of Lorentz invariance or an "almost, but not quite" effect?

It would be one hell of a kick-up-Einsten's-backside violation of Lorentz invariance. All particles show the same speed limit as light, yet neutrinos with a rest mass greater than light possess a larger speed limit?

Community
  • 1
Physiks lover
  • 3,098
  • 4
    No one has forgotten this. I can assure you that the OPERA people are acutely and painfully aware of the long history of highly "significant" bumps just going away. // also in Big Physics (tm) – dmckee --- ex-moderator kitten Sep 22 '11 at 21:01
  • 3
    The archive exists: http://arxiv.org/ftp/arxiv/papers/1109/1109.4897.pdf – anna v Sep 23 '11 at 05:01
  • 1
    Given how big this question is, maybe it would be best to delete this answer? – Jonathan. Sep 25 '11 at 20:16
  • @jonathan I'll delete my answer if neutrinos travelling faster than c is confirmed, big question or not ;) – Physiks lover Sep 25 '11 at 23:19
  • @Physics lover: You should not delete your answer. Irresponsible douches exploit politeness to publish rubbish for publicity. – Ron Maimon Sep 27 '11 at 23:52
  • @Ron so just ridicule them and when someone else down the line comes out with equally a ridiculous result or theory they won't say anything. And then we're back to the drak ages where science was discouraged. This answer does not answer the question, and under normal StackExchange rules would be deleted. – Jonathan. Sep 28 '11 at 15:01
  • @ron also they only published results so that the wider community could pick it apart to find the cause, errors or whatever. It was not to say "oh look how clever are we, give us publicity" – Jonathan. Sep 28 '11 at 15:05
  • @Jonathan ok, I'll edit my answer to answer the question more clearly – Physiks lover Sep 28 '11 at 15:11
  • @Jonathan: I get your point--- but I don't want to keep them quiet, only that they should do at least one independent test of the systematic time/position uncertainty, by perhaps bouncing radio from a tower they can see, whose distance they calibrate with GPS at 50 km or 100km, and make sure their procedure is accurate. This doesn't seem so expensive to do, and it would give a lot more confidence in the result. Unfortunately, they know that this investment of money will most likely lead the result to go away, since it will reveal the systematic error. Similar anomalies had no press release. – Ron Maimon Sep 28 '11 at 18:05
  • @Ron I believe they also measured the distance using a fibre optic cable. And the GPS system can measure continental drift and earthquakes. They've had the results for 3 years and decided to ask others for help. It was the news papers/etc that took it further than they wanted. – Jonathan. Sep 28 '11 at 18:31
  • @Jonathan: The business about measuring continental drift and earthquakes is exactly why I don't trust the measurement. They can see if a spot moves by 1cm doesn't mean that the distance to 700km is accurate to 1cm. The systematic errors for long distance measurement are unrelated to the purely statistical errors for local comparisons. The long description they give to their local measurements is not appropriate and distracting--- who cares about local measurements? The global measurements are the ones in question. As for the cable, it doesn't go through the Earth, so how can it verify this? – Ron Maimon Sep 28 '11 at 20:14
  • 2
    @jonathan light travels at a velocity below c in fibre optic cable. – Physiks lover Sep 28 '11 at 20:50
  • @Physikslover, as long as the speed is known and the route of the cable it doesn't matter – Jonathan. Sep 29 '11 at 00:18
0

May be the case that this problem has to do with the «one-way» light speed and the referential that is used. Afaik the only known measures of the c are done in a «two-way» version (mean value in a closed path). When a photon is released in space it starts its journey at c speed independently of the source and of the receiver. The CMB referential clearly is the only referential to «observe» the light as isotropic. As the Earth moves we observe a dipole, and in different directions we measure different wavelengths for the same physical object (photon).
Anisotrophy

This paper (Cosmological Principle and Relativity - Part I) analyses the anisotropy of light speed for a moving observer.

Fig.3 and eq. 18, pag 14

The one-way light speed is : $c_{A}^{r}=\frac{c_{0}}{1+V/c_{0}\cdot\cos\phi_{A}}$

Using $c_0=299792.458$ Km/s is two-way light speed, $V\;$ is the speed of the lab in relation to the CMB: $V=V_{SS}+V_E$=369$\pm$30 km/s (data from here)
gives the max value of $\frac{\left|c_{V\pm\delta V}-c_{V}\right|}{c_{V}}\cdot10^{5}$=10.2.

All experimental measures of |v-c|/c are within this limit.

Anyway Einstein is correct.

I suspect that the syncronization used in the GPS is in the same as in the above paper and not as Einstein did.
In the pic GPS sync Sat A must be synchronized with C at the same time thru the shortest red path and thru the longest blue path. At the same time B is in sync with C thru other paths with different lengths. IMO this is only possible if they are synchronised as in the above paper (instant observer) and not in the Einstein way that only considers one path between the observer and any other point (Synchronisation around the circumference of a rotating disk gives a non vanishing time difference that depends on the direction used).

Anyway Einstein is correct, and the neutrinos are not superluminal.
A careless reading of the paper might make you think that it is contrary to Einstein, but it is not.

Helder Velez
  • 2,643
0

Did anybody catch this?

Speedy neutrino result may be due to instrument glitch

http://www.newscientist.com/blogs/shortsharpscience/2012/02/speedy-neutrino-result-may-be.html

Loose Cable Explains Faulty 'Faster-than-light' Neutrino Result

http://www.space.com/14654-error-faster-light-neutrinos.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+spaceheadlines+%28SPACE.com+Headline+Feed%29

Antillar Maximus
  • 1,301
  • 7
  • 14
-1

I will bet all my beans into the idea that they didn't estimate the spacetime curvature inside the earth well and over the beam trajectory, and what they actually discovered is a great way to measure space-time inside the Earth.

EDIT it seems this effect is settled to be a missing correction due to sattelite-speed terms: http://arxiv.org/abs/1110.2685. Until i hear or read any counter-claims to that paper, i'll consider this to be a settled matter

lurscher
  • 14,423
  • 1
    Maybe a control would be to send photons along the same trajectory and measure THEIR speed? – Lagerbaer Sep 23 '11 at 17:38
  • Could this really account for a 20 ppm difference? – starblue Sep 23 '11 at 17:48
  • 5
    General relativistic effects near the surface of the Earth are of order $(9\text{ mm})/(6400\text{ km}) \approx 10^{-9}$. It is less important that the rotation of the Earth. – dmckee --- ex-moderator kitten Sep 23 '11 at 18:08
  • 1
    @Lagerbaer I think the trajectory is all underground... it starts in a deep tunnel at CERN and ends under a mountain at Gran Sasso :-) – Sklivvz Sep 23 '11 at 20:40
  • 1
    @nominator: Any relativistic effect cannot make the speed superluminal. The only explanation is systematic errors in GPS position, GPS time, or bunching statistics. – Ron Maimon Sep 25 '11 at 20:34
  • 1
    @Ron, any (general) relativistic effect cannot make the speed superluminal, but it can make your length measurement based on GPS incorrect. Read again what i wrote – lurscher Sep 26 '11 at 19:36
  • Explanations involving GR have been investigated: http://arxiv.org/abs/1109.6312 http://arxiv.org/abs/1109.5687 They can in principle be made to produce an effect in which neutrinos arrive earlier than expected at for motion at c, but they aren't workable because they require the violation of energy conditions. –  Nov 05 '11 at 22:57
  • @Ben, actually i had the impression that this issue was mostly settled as a sattelite-velocity correction terms missing from the time-of-flight estimation: http://arxiv.org/abs/1110.2685 – lurscher Nov 07 '11 at 19:16
  • @lurscher: No, van Elburg is incorrect. He wrote his paper without understanding elementary facts about the role of relativity in GPS or the way in which the GPS coordinate system is defined. For information about how the GPS coordinate system is defined, see http://relativity.livingreviews.org/Articles/lrr-2003-1/ –  Nov 23 '11 at 20:58
-1

This is not a true answer... none is knowing the explanation, so far. However, I will post this "consideration" anyway... Now, November 21, 2011, with 3ns pulses, the new value for the "missing time" is 62.1ns +/-3.7 (only 20 events). My answer is only a "would-be" consideration that, if read by the experimenters, could give them some "debug" clues.

First of all my assumptions:

it is unlikely that the neutrinos go superluminal or SR is not holding true anymore

  • it is unlikely that the distance is measured incorreclty

  • it is unlikely that the GPS setup/usage is incorrect

  • it is also unlikely that the light speed has been measured incorreclty so far.

Then two hypotesis:

  • the "missing time" is 62.5ns (compatible with 62.1 +/-3.7ns)

  • the electronics involved in the time measurement has some clock domain running at 16MHz.

Of course the conclusion would be to investigate if there is one circuit running on one clock pulse less than expected by design / testing.

user6090
  • 1,436
-2

There was a very reliable report of finding a monopole in 1980s by Caberera(?). There was no other explanation of the glitch in the arrangement of the SQUID, but a capture of one monopole. That never repeated. Never confirmed. Never rejected as being a fake effect. I believe this question needs a couple of years more investigation. A question of physics should be repeatedly be confirmed before a postulate or an inference can be derived. Even after that derivation a sensitive experiment should be perceived to break it through further.

Peter Jones
  • 181
  • This probably should be a comment. Even so, this very experiment was a repeat of a MINOS experiment, which found the same effect at much lower levels of confidence, and this time it involved 15.000+ neutrino detections (which, however, could not be individually labelled faster or slower than light) – MSalters Oct 03 '11 at 14:05
  • 1
    In addition, this paper was signed by a large collaboration. – Carl Brannen Nov 06 '11 at 03:21
  • 1
    @Carl: and this is supposed to make one trust their report more? Large groups are vastly stupider than competent individuals. – Ron Maimon Nov 21 '11 at 16:52
-5

I do not agree with the superluminal neutrinos news for very simple reasons. The difference they found with respect to the speed of light is very small, so some errors in the calulations must have been made. Neutrino is not faster than light. The Special Theory of Relativity (STR) of Einstein, through the principle of the speed limit, makes the magnetic force come from the electric one and the magnetic force is an electric force, as physicists know; an easy demonstration of that can be found in chapter 3 of my file at the following link (also English inside):

http://www.fisicamente.net/FISICA_2/UNIFICAZIONE_GRAVITA_ELETTROMAGNETISMO.pdf

If you get rid of the speed limit principle, the magnetic field cannot exist anymore.

Moreover, as c=1/square root of(epsilon x µ), if you change c with a c'>c, then you have to accept a µ'<µ, so you have to accept different intensities of magnetic fields from a given electric current, so you have to get rid of the electromagnetism, but it's describing so well the currents, the fields, the real world etc. Therefore, there's a mistake in the computation of the speed of neutrinos, in the calculations on the run lenght, in the interaction time calculations, during the generation and also the detection of those evanescent particles!

(another interesting file, also related to this subject):

http://www.mednat.org/new_scienza/strani_legami_numerici_universo.pdf

Sklivvz
  • 13,499
  • 7
  • 64
  • 87
Leonardo Rubino
  • 1