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CERN press release regarding neutrino experiment (cern.ch)
213 points by AndreiVajnaII on Sept 23, 2011 | hide | past | web | favorite | 80 comments

This press release is a perfect example of how to write a press release for a high-profile, possibly groundbreaking discovery. Understated, but still properly summarizes the importance of the result, and above all doesn't make any hysterical claims. Instead, it just presents the discovery, cites the paper, gives some background, grabs quotes from the scientists to show the reasoning behind their actions, and leaves it at that. Beautiful.

All true. But to be fair, you're praising the easy stuff. This is one of those one-in-a-career, truly groundbreaking results. They don't have to sell it. They just write it up in plain language and it sells itself.

That's the thing - they resisted the urge to sell it. All of the other headlines are screaming about "time travel possible?" and "modern physics all wrong!", while this is just the plain language.

Most importantly, they're not presenting the finding as fact. They're really asking for anyone that can prove them wrong, because they don't believe it, and are almost positive that there's something wrong with either their methodology or measurements.

Yes, this is one of the most painful things to do as an experimentalist. You just know that there's some kind of error in the data. Because -- in the memorable words of some teacher of mine [1] -- "you can't really believe an experiment until it's been confirmed by theory."

And, yet, you've spent lots and lots of energy and you can't find the problem yourself. And what if there is something there? There are many famous cases of experiments that weren't confirmed by theory until well after the fact; theorists often don't bother to think about a particular corner of the unthinkable until an experiment calls attention to it. So you've just got to publish your data, but also be prepared to laugh it off when the mistake gets identified, or the data proves irreproducible. And then to spend the rest of your career laughing it off, because this spurious result may be the most famous thing you ever publish.

You've got to have a good sense of humor to survive as an experimentalist.


[1] I wish I remembered which teacher said this.

I wonder how the Cold Fusion fiasco was presented back in the 90's? Not that they are at all similar, but those were certainly bold claims as well.

There was an interesting (and entertaining) book written about the Cold Fusion Hype, http://en.wikipedia.org/wiki/Bad_Science:_The_Short_Life_and...

From Gary Taubes' (the author) website: Bad Science: The Short Life and Weird Times of Cold Fusion is book of science history by Gary Taubes about the early years (1989–1991) of the cold fusion controversy. It is not a scholarly work, but a popular retelling of the events, based on interviews with over 260 people. The book presents a timeline of the events, making the case that the cold fusion field has many examples of poorly-performed science. The actions of Martin Fleischmann, Stanley Pons, and Steven E. Jones, the scientists who made the dramatic first claims of fusion, are described in rich detail. The book then shows the worldwide reaction and later disrepute of the cold fusion field, with Taubes placing himself in the side of "good science". Taubes says at the end that cold fusion had only demonstrated that research can continue even if the phenomena doesn't actually exist, as long as there is funding available. Taubes had previously written an article for Science in which he insinuates that the cold fusion work of A&M University was fraudulent.

The things that are advertised with bold claims are the things people want to believe. If cold fusion worked, it would mean free energy. Free energy would mean that we could all drive tanks around and eat animals without feeling bad, which would make us feel good. Hence, "cold fusion discovered! all of the world's problems are solved!!!!"

The speed of neutrinos is less interesting, because I don't get anything out of it right now. A "huh" and some new physics textbooks and all that, at best.

(I'm personally excited, but that's because I am more excited by unlocking the secrets of the universe than by being able to drive an SUV for free. Because I already have a vehicle whose energy source is practically free.)

Late 1980s — early 90s, it turns out: http://en.wikipedia.org/wiki/Cold_fusion#Fleischmann.E2.80.9...

> This is one of those one-in-a-career, truly groundbreaking results.

And if it's wrong and you overstate it, it's again one of those one-in-a-career results, but merely because that's where your career as a scientist ends.

This result is from a legitimate experimental group. It's much more likely to be experimental error than new physics, though: previous observations of neutrinos coincident with the light emitted by supernova 1987a (much, much farther away) indicate that neutrinos travel at light speed to 1 part in 10^8 [1,2]

[1] http://adsabs.harvard.edu/abs/1989ARA%26A..27..629A [2] http://blogs.discovermagazine.com/badastronomy/2011/09/22/fa...

Also, https://www.xkcd.com/955/

There are a few interesting explanations in a Guardian article[1] published today:

Heinrich Paes at Dortmund University and colleagues believe it might be possible for neutrinos to move through hidden, extra dimensions of space and effectively take shortcuts through space-time. "The extra dimension is warped in a way that particles moving through it can travel faster than particles that go through the known three dimensions of space. It's like a shortcut through this extra dimension. So it looks like particles are going faster than light, but actually they don't."

Another potential explanation for the observation was given by Alan Kostelecky at Indiana University, who has devoted his career to violations of the limiting speed of light. He proposed in 1985 that an energy field that lies unseen in the vacuum might explain the finding. The field allows neutrinos to move faster through space than photons, the particles that make up light.

"It may very well be that neutrinos travel faster than light does in that medium. It is not at all unreasonable that that would be the case."


Neither sound very good to me, nor are they really explanations. "Hidden" is just as bad as "magic" here. And isn't the second basically the standard aether theory that was soundly defeated at least 100 years ago? http://en.wikipedia.org/wiki/Michelson-Morley_experiment

I don't understand how warping the extra dimension will make particles appear to move faster.

Is it possible that it is everything else that is moving linearly along a fifth dimension, thus slowing us down, and these neutrinos were moving cleanly through space-time?

My guess is there's something wrong with the statistical modeling, but I realize that's not a very useful statement. I missed the press conference, but I did read the paper and I even understood a few things. :-) It was good to see they even thought about seismic activity (centimeters). One thing I noticed in both the article and the media is that the effect is never expressed in terms of distance (about 15 meters), but only in terms of time and speed. It surprises me that they're able to determine the point of creation and detection of neutrinos in such huge instruments, but of course they know their stuff. I did get the impression it took a lot of modelling and advanced statistics to achieve that, hence my earlier "gut feeling" that there lies the problem.

A few other possible explanation that crossed my mind and I'm sure are wrong and already thought about:

* the distance between both sides was measured very accurately, above ground. The earth is not flat, so the distance underground is shorter. I'm too tired to calculate how much shorter.

* relatistic effect of the beam going deeper underground on its way over; I read elsewhere that they already considered the effect of altitude difference between the two stations and that it was orders of magnitude smaller.

* some other mistake in distance measurement; have they tried sending other, easier to measure, signals over to figure out the distance? Or some other independent way to measure that distance?

At this point I'm 99.9% certain that we're looking at some kind of experimental error, but being unfamiliar with the details I'm not about to start speculating on particular things that might be wrong. Any possible error that I could think of would probably, if presented to the experimentalists, result in a rolling of eyes and a "Don't you think we already checked that?"

To put it in terms that people here can understand, suppose you're three days into tracking down some incredibly obscure bug and your mother asks "Did you try turning it off and turning it on again?"

The crazy thing is that when people say that to me, I kinda roll my eyes but try it so that I'm not dismissive... and sometimes it works!

D of earth = 12756km, and assuming a sphere (my physics prof would be proud), I'm computing that 730km covers a ~6.6 degree arc. Also assuming a 100m underground tunnel, the distance would be 21m shorter than its above-ground measurement. Someone needs to recheck my math, though.

According Arethuza's comment below, at least one of the labs is 1400 meters below ground. Also you need to calculate the straight-line distance between the two points, not the distance over the surface. Although for calculating the difference that's probably a good enough approximation. So if both stations are 1400 deep and your calculation is correct, the difference should be 14x21=300 meters. Which is a lot more than the 15m effect they found, which suggests they thought about this.

They claim the geodesic measurements are done to within 0.6m, and the majority of that uncertainty is introduced by the need to triangulate down the tunnel to the experiment.

I watched the news conf. this morning.

The neutrino production "curve" looks like a a square wave pulse as a function of time

____| |_____

and the detection points look like this

___________________________________| |_____

with ∆t ~= 700km/c time distance between the two pulses.

The claim is that the "most likely guess" of the received pulse shape (obtained from many measurements) is too far right to be consistent with speed of light, but the ∆t measured where?

between the onset of the pulses? between the place where the pulses go down?

What they did some mega calculation (maximum likelihood stuff), to predict the best approximation to the shape of the pulse at the receiving end -- so somehow they take on the approximation of the whole pulse (which is much wider than the claimed discrepancy).

They should downgrade the claim from: "speed of neutrinos is...." to: "speed of pulses of neutrinos ..... on average, as predicted by the maximum likely shape of the pulse", which sounds much less profound.

The conf was good though, the speaker stood up to a lot of serious scrutiny. My guess is the problem is with the ML curve shape calculation.

They should downgrade the claim from: "speed of neutrinos is...." to: "speed of pulses of neutrinos ..... on average, as predicted by the maximum likely shape of the pulse", which sounds much less profound.

This would apply to every paper in existence. If you want to know the methodology, you read that section of the paper. Putting it in the title only obscures the message.

It's true that it makes the analysis a lot harder, and one of the questions after the talk was about making shorter bunches of neutrinos. The duration of the pulses are probably just set by the width of the proton bunches in the ring, in which case it would be nontrivial to make it much shorter.

I've read the article[1] and what they do is compare the proton flux as measured in Geneva with the neutrino signal measured in Gran Sasso. The proton signal is timeshifted a small amount until the curves overlap maximally.

Where the leading and trailing edge are is irrelevant here, as the complete shape of the pulses are compared.

1: http://static.arxiv.org/pdf/1109.4897.pdf

If the pulse is wider than the error, this opens up the phase velocity / group velocity avenue to explain the paradox.

Although it is CERN that is getting most of the attention the lab receiving the neutrinos is pretty impressive - 1400m underground:

"three large experimental halls, each about 100 m long, 20 m wide and 18 m high and service tunnels, for a total volume of about 180,000 cubic metres"


Is this needed? Or could the reception be done in the surface?

I mean, the interesting question is now: how much time will it take to repeat the experiment, either by others or by the CERN but with other reception point?

To put that in perspective: a 20 part per million difference in speed over 730 km distance is about 15 metres. So details about the detector are relevant, but they obviously know that.

From the release, there's a live webcast going on right now (16:00 CEST, Sept 23) at: http://webcast.cern.ch/

I so LOL'd when someone in the webinar pointed out that their measurement wasn't a vector so their paper should be titled neutrino speed not velocity. http://arxiv.org/abs/1109.4897

They know where the neutrinos were produced and where they were detected 730km away. That seems like enough for a pretty good guess at their direction of travel. But I guess they didn't really present that aspect of things.

Just watched the tail part of it. I have no clue what they were saying 99% of the time.... but they sure seem thorough as fuck

Guys, I'm sorry I missed this recent news but I've a question that I hope some people can answer for me. So does this mean that Einstein's theory might not hold? Do we see any well-established theories break as result of this? I'm totally naive about this but curious so any answers will be appreciated.

I'm not the best to answer this, but I'll briefly say that if Einstein's theories don't hold, it will be in the same sense that Newton's theories didn't hold post-Einstein. Newton's theories still apply, for all practical purposes, to all every-day life situations, and Einstein's theories will still apply, for all practical purposes, to every fringe case seen so far ("every-day" nuclear physics, etc) until this experiment. Einstein's theories will just break down in even more fringe situations, which we would have to start to understand the nature of now.

We sort of already know that "Einstein is wrong" in this strong sense, since GR is incompatible with quantum field theory. It still works just as well within its realm of applicability, though.

Exactly. Even if the Opera experiment turns out correct, your GPS will not cease to function all of a sudden.

The half-century after Einstein's pioneering work saw the world's basic conceptions of energy resources and of armed conflict transformed. One can only imagine the prospect of living in a post-FTL world.

To be honest, it's most likely someone making a mistake or equipment failure or something. We'll just have to wait and see if someone else can replicate it.

equipment miscalibration, not failure. unless i'm misreading it, the results of these experiments show consistent FTL speeds across 3 years.

Right, my bad.

Actually, this may be a good thing if the experiments turn out to be correct. There has been a lot of difficulty integrating gravity into the current theories to get a Grand Unified Theory/TOE. Maybe this will make physicists think about other novel ideas.

Possibly, but it's best to not get excited over results are this surprising. At least until someone else can duplicate them :-P


No matter what they discovered: Einstein's theories of relativity will hold. They will hold forever, just like Newtons theory of gravity will hold forever. It's just that its range of applicability may become smaller and smaller.

There have been numerous experiments that perfectly validate both the special and general theory of relativity. The outcomes of the same experiments will not suddenly change. The theories predict the outcomes of these experiments to within experimental accuracy. GPS wouldn't work if special relativity wasn't a sufficiently accurate description of reality.

However, it may turn out that there are also experiments that are not sufficiently described by the theories of relativity, even thought that was previously expected.

The theory of relativity is based on the fact that the speed of light is the same for any observer, no matter if that observer is standing still or moving. From this supposition, the rest of the theory is just mathematics. So if there is something than can move faster than that speed, then the theory will still hold, as long as everybody observes the same speed, in any state of movement they are.

The problem is that we already have previous measurements of the speed of light (in a vacuum) to high precision and they have all been roughly the same. Now I haven't read the paper but the surprise of this latest experiment is that they are measuring the speed of a particle as faster than previous measurements of the speed of light. If the latest measurements are accurate then that implies that it is possible for particles to move faster than the speed of light, or the speed of light has changed, or previous measurements were worse than expected. The first or the second possibilities would both violate special relativity.

Also, if I recall my college physics correctly, special relativity not only predicts that the speed of light will be the same for all observers but that it will also have a particular value, based on some physical constants related to electromagnetism.

No, like I said, the special theory of relativity is based on the speed of light being the same for all observers, not on its exact value. That is a supposition on which the theory is based, not one of its predictions.

If, for example we now find out that neutrinos move with 1.0002% of the speed of light, but that their speed is the same, measured by any observer, we will just replace the speed of light constant in the special relativity formulas with the speed of neutrinos constant.

But that all depends what the interpretations of the results are going to be.

My background is in physics and one of my first programming jobs was writing code for a lab of international importance. At the time, I would engage the other members of the lab in an ongoing debate about Einstein, where I'd take the position that Einstein was wrong (About anything, or everything, whatever the handy topic was.) I was, and am, an Einsteinian skeptic.

I learned a lot in that, and one thing that actually surprised me is that in many cases, I was able to make a good argument that Einstein was wrong, and there was no empirical evidence to support his view on the particular point we were discussing.

The point of that is not that I believe Einstein was wrong. It is that this is not a situation where any single experimental result can show that the theory doesn't hold, or does hold. There are many experiments where his theories do seem to hold (though I enjoyed poking holes in them). If this result is repeatable and turns out to be correct, it will cause many physicists to re-evaluate many theories, and have huge implications.

Einsteins work resulted in many theories, and of course a grand set of them called relativity (and special relativity)... the media simplifies this to "nothing can move faster than the speed of light".

I can see the situation where this causes an adjustment in the specific interpretations of his theories. Or it could turn out that these results are both true, and consistent with his theories. For instance, prior to the understanding of matter there were many theories about mass that are essentially true on the macroscopic scale, though once you understand that matter is made up of atoms you see where they don't hold on the microscopic scale.

This result could reveal a level of reality beyond what Einstein understood, such that he's right from our macroscopic scale, but there's a whole other branch of physics in there.

So, I'm not declaring victory. I think this is good news, though, because it might be the beginning of the revelation of an error in understanding that, when resolved, results in a big jump forward in physics.

PS- I'm not interested in getting into a physics debate. Its been too many years, and I've spoken vaguely because the specifics are not what I'm addressing.

You say:

> I learned a lot in that, and one thing that actually surprised me is that in many cases, I was able to make a good argument that Einstein was wrong, and there was no empirical evidence to support his view on the particular point we were discussing.


> I'm not interested in getting into a physics debate. Its been too many years, and I've spoken vaguely because the specifics are not what I'm addressing.

You can't go making a claim like "there was no empirical evidence to support his view" about nearly any of Einstein's physics papers and then not want to debate it.

That's like saying "I have proof that bigfoot exists, but it's been too many years and I don't want to actually present that proof, so just believe me because my background is in being a bigfoot expert".

In any case, your assertion is false - there is a great deal of empirical evidence for all of Einstein's important theories.

Sure none of this evidence is proof, but it is good evidence. We know that if Einstein's theories of relativity are not true they are at least very good approximations over a very wide range of scales. It would take truly extraordinary evidence to justify a conclusion about the falsehood of relativistic theories at large scales. Some of this has been done (physics, like all science, is an ongoing debate), but many of those original theories are still believed to hold.

> So, I'm not declaring victory. I think this is good news, though, because it might be the beginning of the revelation of an error in understanding that, when resolved, results in a big jump forward in physics.

Taking the position that a piece of science will be eventually proven wrong and gloating when it is proven wrong isn't big or clever. Every theory of the past has gone the way of Phlogiston, and we can reasonably expect every theory of today to go the same way. However, saying "I think this is wrong" isn't contributing until you say "and here is my evidence".

By the way, "I'm not declaring victory, but" is the same as "I'm not a racist, but".

"You can't go making a claim like "there was no empirical evidence to support his view" about nearly any of Einstein's physics papers and then not want to debate it."

You're taking a statement I made about a particular view in a set of topics that were chosen for the purposes of debating where he might be wrong, and applying it to the entirety of his "papers".

"In any case, your assertion is false - there is a great deal of empirical evidence for all of Einstein's important theories."

I never asserted there was no empirical evidence for Einstein's theories.

The fact that you didn't address the point I was trying to make, and instead are giving me what seems to be an unsophisticated laymen view of "einstein couldn't have been wrong about anything!!!" is exactly why I'm not interested in debating physics here. Plus, its irrelevant to the actual topic.

You do realize that proving "Einstein was wrong" is a favourite activity of the many physics cranks who hang around sending letters to physics departments year in year out?

And that of all the thousands of folks who have claimed to have a "proof" that relativity is wrong, the they break down as about seventy percent simple misconceptions and thirty percent complete nonsense, with the remainder being zero?

At the time, I would engage the other members of the lab in an ongoing debate about Einstein, where I'd take the position that Einstein was wrong (About anything, or everything, whatever the handy topic was.) I was, and am, an Einsteinian skeptic.

It's a good thing you left physics for programming, because it doesn't sound like you have a particularly strong commitment to scientific principles. To get a Nobel Prize it would be perfectly sufficient to show convincingly that Einstein was wrong about one thing [insert caveats here, obviously I don't mean trivial things]. If you believe Einstein was wrong about "anything and everything" then you're just committing the cardinal sin of believing things because you want them to be true, rather than because there's sufficiently convincing evidence that they're true.

believing in something to be the truth, no matter how much 'convincing evidence' there is to support that conclusion, isn't really a scientific princple. skepticism, on the other hand, most certainly is.

science isn't about believing in anything to be the truth; it's about constructing analytical models that make accurate predictions while consistently exercising skepticism about the ability of any model to do so.

Yes, the English language really lacks the vast suite of words we need in order to express our degrees of certainty about things. Words such as "know", "believe", "think" and "suspect" don't express these things properly, but we have to make do with what we have.

I don't think that "believe" is a particularly bad word for the relationship of a rational person to a fairly well established fact, though. I believe, for instance, that the Earth has an iron-nickel core. I fully acknowledge the possibility that it might not, and am fully ready to change that belief based on new evidence, but I think it's fairly well established and I am willing to act as if it were true.

It is very apparent that Einstein was wrong, only because his theory does not account for quantum effects. But it works for human sized stuff and up to galactic supercluster motion.

It is also apparent that Quantun Chromodynamics is also wrong because it does not account for gravity. But it works for super small stuff including CPUs and subatomic theory.

But they both work tremendously well for their scope. Hence the seeking for a Theory of Everything, because each is incompatible with the other without that mystery glue. But they both are "Wrong" - because they themselves say their theories are incomplete.

Look, I know there's some disagreement with this comment, but why has this received so many downvotes? It isn't wildly unhelpful, is rather reasonably phrased, and doesn't attempt to pounce on the situation and make sensationalist claims...

Again, I'm not arguing this shouldn't be addressed if you disagree with some or all of it, but it isn't like this is some kind of trollbait that usually receives this kind of aggressive downvoting...

lotsa basic FTL stuff here if people wanna get a broad picture http://www.weburbia.com/physics/FTL.html

That's a pretty good summary. It progresses from "things that might seem to be FTL but don't really count" through "things that are kinda sorta FTL in some sense but still don't carry any information" and into "highly speculative things which could potentially be properly FTL but haven't been shown to be possible".

A great article on various disagreements between Tesla's and Einstein's view of the physical world.


Will Tesla have the last laugh?

Usually I hate people who comment only with "I stopped reading at...", but in this particular case I'll make an exception. For myself.

I stopped reading at:

"As far as I know, no book on the history of physics mentions Tesla"

which is a statement approximately as correct as "As far as I know, no book on the history of the United States mentions Woodrow Wilson"

Not based on the linked article, which is horrible. I gave up when it was simultaneously trying to claim that the ether exists but space can have no properties.

The author could have done a better job of explaining this, if he would have said "Einsteinian Space" rather than "space" in that section. That's fairly easy to infer from the context, but it could have been explained better. Einstein viewed "space" as a vacuum. Tesla argues that view is flawed, that nothingness by definition can't have any properties, and as a result can't bend, because there's nothing to bend. He proposes aether/ether as an alternative explanation of "space".

If all that we're talking about is terminology then the entire article is pointless.

claiming what? who?

What a steaming pile of junk.

This article from a couple days ago did a good job of dampening any wild hopes http://io9.com/5843112/faster-than-light-neutrinos-not-so-fa...

A probably nonsensical idea about the super-nova point they make: maybe what it means isn't that neutrinos are traveling faster than light but that they are traveling just ahead of light. In other words, they are some kind of shock wave moving through space at speed c but at a location just slightly ahead of the disturbance. Maybe we are detecting the supernova neutrinos a few nano seconds before when we're supposed to be.

Still, it's probably a measurement error.

http://xkcd.com/812/ - relevant xkcd.

The path from the things people observed to "and neutrinos travel faster than light" travels down a long string of inferences. Inferences about the equipment working properly and inferences about other aspects of physical law.

Most likely this is a problem with the equipment. I'm sort of hoping that some other aspect of our knowledge of physics has been revealed to be off, though, since that is where real progress comes from.

It might even be that the most straightforward but most improbable case is right, and these neutrinos actually are traveling faster than light. That would be awesome, but also terrifying since then its only a matter of technology to get a device that would permit communication with the past.

Does anybody know why they were doing this experiment (measuring speed fo neutrinos) at all? Were they assuming that neutrinos travel faster than speed of light in the first place?

as i understand it, they were doing experiments to watch how neutrinos change between "electron", "muon" and "tau" flavors. the timing of when the neutrino was sent from cern and viewed by the experiment was required to match what the neutrino was before and after.

Do they at any time say how much faster than light are those neutrinos moving? 1% faster? 100 times faster? I couldn't see that info anywhere.

0.002% faster.

+/- 0.002%

"+/- 0.002%"

How did you derive a negative bound? A negative bound suggests that arrival times could be slower than c.

The paper indicates that arrival times are always faster than c.

From the paper:

"An early arrival time of CNGS muon neutrinos with respect to the one computed assuming the speed of light in vacuum of (60.7 ± 6.9 (stat.) ± 7.4 (sys.)) ns was measured."

Perhaps my one-word comment was an insufficient indicator of my sarcasm. 't'was a joke, good sir.

a velocity 20 parts per million above the speed of light

My SWAG: the relativist effect of time being slowed down in Earth's gravity was not taken into account. (Time is slowed down, therefore neutrinos appear to arrive 60ns "early".)

They spent a lot of time triple checking the accuracy of their instruments without seemingly thinking about higher level factors such as the Theory of relativity... http://static.arxiv.org/pdf/1109.4897.pdf Of course there are 99% chances I am wrong, but just throwing this out there :)

You didn't look at the webcast, did you? Someone asked this question and they said "no we took it into account in the sense that the clocks at CERN and Gran Sasso run at different speeds because they are at different elevations. However, that's only a 1e-18 effect and the signal is 1e-5.

I am talking about the presence of the gravitational field, not its difference between the 2 sites. Anyway, I calculated gravitational time dilation on Earth out of curiosity. It has an effect of only about 1e-9.

There is a 100% chance that you are wrong. Do you honestly believe that they wouldn't think of taking that into account, if it could be relevant? That you can immediately think of it, but not an entire group of physicists; let alone a group trained and experienced in performing time-critical experiments? How far removed from reality do you believe physicists to be?

Anyway, relativity cannot possibly be relevant in explaining the time difference. I will not explain why: please actually read what they published. It will be crystal clear why it isn't relevant.

I meant to say they may have made a mistake on just this one relativist effect... even though I am sure they did think of it.

BTW, relativity is relevant in the OPERA experiment. Theoretical physicist Luboš Motl explains why: http://motls.blogspot.com/2011/09/potential-mistakes-in-oper...

inches vs centimetres. damn those units.

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