

Faster-than-light neutrinos face time trial - llambda
http://www.nature.com/news/2011/111005/full/news.2011.575.html

======
Groxx
Shouldn't this be easily testable? Just run the experiments while not re-
synchronizing the clocks. If the too-fast measurement increases as time goes
on, you now know the exact dilation amount and can account for it
(retroactively even, if they recorded the times they resynchronized and the
times they measured. I'd _hope_ they did).

Whether it pans out or not, that's a nice eye for details. Props to the people
who found it, it's at least _something_.

~~~
VladRussian
napkin calculation i did back then shown that the gravitational time dilation
effects (according to GR as we know it) are several orders of magnitude less
than required. The more probable explanation would be this (as they seems to
measure distance and time in flight thus it is perfectly possible to come up
with speed higher than "c" (there is a Russian saying for such a measurement
style - "average patients' temperature across the hospital")) :

<http://news.ycombinator.com/item?id=3027409>

------
yread
> Because the CERN site lies closer to the centre of the Earth than Gran
> Sasso, and consequently feels a smaller gravitational pull

Isn't it the other way around? _Gran Sasso_ site lies closer to the center of
Earth and that's why it has _bigger_ gravitational pull

~~~
jules
Nope. It's not immediately obvious though.

Let the radius of the Earth be R. Suppose that you are somewhere between the
center of the Earth and the Earth's surface, say at x meters from the center.
Then you can divide the Earth into two pieces: one smaller ball of radius x
below you, and a shell with inner radius x and outer radius R "above you".

If you are inside or at the inner surface of a spherical shell, then the
gravity from all the atoms in the shell is pulling at you in all directions.
Now it so happens that all these little pulls exactly cancel each other out.
This can be shown with Gauss' law. [Does anybody know an intuitive proof of
this?]

So effectively only the inner ball of radius x counts. Now the question is
whether this ball has bigger or smaller gravity than standing on the Earth's
surface. It's not completely clear that the gravity is smaller because the
density of the smaller ball could be different. If we assume that the density
of the Earth is roughly uniform however, then it is clear that the gravity
inside the Earth is lower than on its surface.

The precise condition for the gravity being lower at height x is that x/R <
a/b where a is the average density of the of the Earth and b is the average
density of the inner ball.

~~~
iwwr
For reference, Earth's interior gravitational acceleration:
<http://en.wikipedia.org/wiki/File:EarthGravityPREM.jpg>

As you see, gravity gets stronger up to a maximum in the outer core, because
the core is much more dense.

But the issue here is not about spherical Earths at all. Earth deviates
significantly from a spheroid and even an ellipsoid shape-wise and gravity-
wise. Just saying "location X lies closer to the center of the earth and
therefore experiences less gravity" is somewhat irrelevant here.

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redcap
A question for those who are up to date with their particle physics: how does
one know that the neutrinos detected were the ones that they actually sent? Is
it based on the direction they were sent from and the amount of neutrinos
received?

I asked because the one thing that the media repeats is that there's a lot of
neutrinos hitting us right now, so how do we know they're the ones we're
looking for?

Also, is there a normal speed that neutrinos travel at? Can you catch one at
rest?

~~~
lutorm
There may be a lot of neutrinos around, but they are extremely hard to detect.
The fact that you see a bunch of them at a time and a place where you expect
based on how they were created makes it very unlikely that they are not
associated.

There are craploads of neutrinos left over from the Big Bang (see
<http://en.wikipedia.org/wiki/Cosmic_neutrino_background>) but they are very
low energy and are impossible to detect. They are likely the closest you can
find neutrinos at rest. How fast they go depends on their rest mass, which we
only know to be small but nonzero.

------
pbhjpbhj
Here's something interesting in the OPERA paper -

>" _The high-accuracy time-transfer GPS receiver allows to continuously
monitor tiny movements of the Earth’s crust, such as continental drift that
shows up as a smooth variation of less than 1 cm/year, and the detection of
slightly larger effects due to earthquakes. The April 2009 earthquake in the
region of LNGS, in particular, produced a sudden displacement of about 7 cm,
as seen in Fig. 7. All mentioned effects are within the accuracy of the
baseline determination. Tidal effects are negligible as well._ " //

Now that's accuracy, allowing for movement of the Earth's crust in your
experiment!

Wonder if this can be used for Earthquake prediction.

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lutorm
My impression from the webcast was that because they resynchronize the clocks
using GPS very frequently, the different running speeds would amount to a
negligible cumulative offset in the times. But I didn't read the paper.

------
zipstudio
So if it's the gravitational pull that makes the clocks different, it seems
obvious that doing the experiment in the other direction is the next step.
Measure the time in both directions to average out this effect.

~~~
QuestionWriter
If synchronized clocks are a problem, I wonder why they don't race the
neutrino against a photon. Obviously the photon would have to take a different
path - maybe bouncing off a satellite.

I wonder how accurate the GPS synchronization is too. I'm wondering if it
takes into account different atmospheric conditions and the index of
refraction of radio waves in air.

~~~
lutorm
Time measurements are interestingly difficult when you start to care about
high accuracy or long times. There's a kind of hard-to-read but interesting
running log on the state of the leap second at
<http://www.ucolick.org/~sla/leapsecs/onlinebib.html>. (What, you didn't know
there were leap seconds? See <http://en.wikipedia.org/wiki/Leap_second>)

------
nknight
"Dario Autiero [...] counters that"

"Contaldi admits"

"Contaldi already has some company"

I wish they'd stop this. It's not a fight between the OPERA team and the
world. They're not trying to get people to accept their result. This is
everybody collectively trying to figure out why the data came out this way,
with the expected result that there's a mistake somewhere.

Contaldi pointed out what he thought it might be, OPERA is saying "we don't
think that's it, here's why", not "you're wrong, our results are correct".

~~~
pbhjpbhj
> _Contaldi pointed out what he thought it might be, OPERA is saying "we don't
> think that's it, here's why", not "you're wrong, our results are correct"._
> //

I don't think that's it! [see last line].

Surely they're saying "you're wrong, that's no why our results are incorrect"?

"We don't think that's it" is just the polite version of "you're wrong" is it
not. Of course one can (and should where possible) counter with "no you're
wrong, here's why".

~~~
nknight
The relative politeness is the entire _point_. The tone the article sets
reflects what the media wants (conflict! drama!) instead of the reality
(global scholarly investigation into a big WTF).

