CERN team claims measurement of neutrino speed >c

Shovel

V50's posts have convinced me there has to be an error somewhere. The numbers just do not match up with previous experiments and the supernova data. Massive photons could be possible, but this experiment would exceed the upper bound on their mass supported by so many other, more accurate trials. The interesting questions are what the error is, how it could be so subtle as to trick so many scientists and engineers, and whether or not it affects other experiments or equipment.

As far as further experiments, would using the same setup/equipment over a longer distance quickly reveal systemic error? It seems to me that an error in the experiment's timing would not scale with the distance the neutrinos travel. So if we move the detector twice as far away and the neutrinos still arrive 60ns early instead of 120ns, would we have very strong evidence to support error? The earth's diameter is over 17x the distance these neutrinos traveled, building another emitter or detector on the far side of the planet would yield better timing allowances. Am I right?

eiyaz

How are they insuring that the neutrinos in Gran Sasso are the same neutrinos from CERN? There is no way to tag these objects. If there are billions of neutrinos passing through our eyes every second, is it possible that this could be neutrinos from another source?

This answer just seems too obvious, but how are they confirming that the neutrinos from CERN are the same as the ones at Gran Sasso.

Shovel

I imagine there is some background-level of neutrinos in the detector, and spikes in activity correspond with CERN's emission timing. Over thousands of bursts, you can be certain that it's coming from CERN.

As an example, billions of photons pass through your cell phone every second, this doesn't stop it from being able to discriminate a signal from the cell tower.

xts

No. Neutrinos of so small energies could not be measured. All the observed neutrinos are highly relativistic. We could take them as massless - except we observe their oscillations, which prove they must have small, but non-zero mass.

Gran Sasso detector is tuned to detect high energy (~17GeV) neutrinos, incoming from precisely defined direction (pointing to CERN).
The background of neutrinos is mostly in thousand times lower energies (solar neutrinos), and the flux of high energy ones is pretty small, when compared to the beam coming from CERN.
Guys at OOPERA estimate the cosmic background events as about 0.5% of the events they used for the analysis.

Haelfix

The cross section of cosmic rays that you can measure is many times smaller than the flux of neutrinos prepared by the experiment. So this is just another example where you need to proceed statistically by taking many events in a short enough time frame..

More generally, it is true that this is not a direct experiment. There is no direct measurement of when and where the actual neutrinos are created, and so there is an uncertainty in the actual positions and timings of departure. Now, there is a statistical procedure that is utilized whereby those quantities can be recreated on average, and they seem to have done a pretty thorough job imo, but it is still a source of systematic uncertainty and the details are technical.

slam7211

Did they correct the distance between the 2 labs for SR effects?

PeterDonis

I had the same thought, but no. See post #177.

diggy

Although the SN1978A results do challenge OPERA's, I don't think there are enough energy-data-points for the two results to rule one another out (i.e. who knows whats happening in between, these territories just aren't measured to the necessary precision yet).

It would be nice to see if MINOS could tighten up their measurement, which I'm sure they will be looking into since it is such a hot topic now.

If the results are "correct", I'm still a little baffled to understand why neutrino's would be special, compared to every other particle we've measured that seem to happily observe the speed of light as law. I will need to go review the FTL neutrino models out there.

Exciting times!

Parlyne

Cherenkov radiation is a strictly electromagnetic effect. Neutrinos have no electric charge; so, they can't radiate in that manner.

harrylin

Good question! I can imagine they did not (or wrongly!) account for it. Although people there routine use SR for their experiments, the frame transformation between the ECI frame and the lab frame is often glossed over, overlooked or misunderstood. Of course also the GR effect must be taken in account (which would affect to the opposite I guess; and according to post 177 it's negligible).

I vaguely recall that there has been a similar issue in astronomy in the past, with claims that the speed of light was in fact c+v, followed by silence on that issue and then no problem at all.

PS here's the link to their Arxiv paper (if someone already gave it, then here it is again):
http://arxiv.org/abs/1109.4897

FlexGunship

(1) Based on current understanding it means that the neutrino has negative mass. It could possibly account for the expansion of the universe if it's found that each neutrino has a very slightly repulsive effect on all matter in the universe.

(2) If it has positive mass and still travels faster than the speed of light, then it'll overturn... um... every equation with the number "c" in it, not the least of which will be E=mc².

(3) If it's a measurement error that has been confirmed to this degree, then it will tell us something about our current understanding of metrology and measurement sciences. Remember that this result was first reported by MINOS in 2007, this is a confirmation experiment.

Keep in mind that these folks compensated for CONTINENTAL FREAKIN' DRIFT! I hardly think they forgot time dilation and special relativity effects.

All that being said, I believe we will find that neutrinos travel below the speed of light, but that we will learn something new about our measurement techniques.

seerongo

This morning's Dario Autiero seminar video link was posted on post 187, but it was apparently not available at that time. Here it is again.

http://cdsweb.cern.ch/record/1384486

It should answer a lot of questions asked here concerning methodology, etc,. in addition to the paper.

PAllen

Just want to record here a speculation on error source from Lubos Motl's blog that doesn't seem to have been raised here. I didn't notice any discussion of this in the arxiv paper.

The issue is simply that light travels slightly slower in air than in vaccuum. This would mean that estimates of distance inferred from signal travel times to or from the GPS sattelites would be small by a tiny amount, unless this is corrected for. Calculations of this effect suggest it could scale the 730 km by around the right amount.

I wonder about the plausibility of this: it would suggest that all GPS distances are slightly scaled down, and no one noticed. Presumably, this would have little impact on navigational uses, but I'm not sure ... what about high precision military uses?

PAllen

It looks like the discussion on Motl's blog used the refractive index of air for visible light. I would expect this does not apply to radio waves - it would be some other value. My guess (presumably others here would know better) the refractive index for radio is smaller, maybe small enough that it can be completely ignored even at the precision of these measurements.

[EDIT: I found at least one university source that claims air's refractive index for radio waves is similar to visible light. However, it further explained how this regularly must be accounted for in radio transmission applications, so it now seems preposterous that GPS doesn't account for this.]

xts

Speed of light in air (and in ionosphere - which is more important, as it varies much more) is taken into account and compensated even by simple car navigator GPS's. So it might not be an issue here. Professional GPS's compensate it on several methods, with accuracy of single cm.

Off-topic: we are now in solar activity maximum year, so the ionoshere is densier and thicker than usually. In some situations, when one of the satellites is just above the horizon, older car navigators (unable to receive WAAS corrections) may show your position displaced by 30 meters or so off the road - that happened to me few days ago.

Enthalpy

Read (too quickly) the paper from arXiv.

My first comment is that I'd strongly prefer the propagation time to be measured between two neutrino detectors, one at Cern and the other at Gran Sasso. Presently it's measured between a proton beam current detector at Cern and a neutrino detector at Gran Sasso. As the neutrino beam is 3km*3km wide at arrival, a small detector at the source would provide as many event there for a more direct comparison - err... IF the mu neutrinos can be detected with the same inefficiency as the tau neutrinos are, which I ignore to a high degree of precision.

GPS signals are jammed but many techniques, especially differential GPS, overcome it. From the comments in the paper, scientists there obviously know that better than I do and took care of these clock and position measurements, end of paragraph.

The signal from the proton beam intensity shows a decline instead of a steady plateau. Could it be that a fuzzy signal from the neutrino detector correlates better with the inclined reference if it's shifted forward, just as a result of the waveforms?

Now, things I'm easier with.

The 200MHz modulation of the proton beam brings no certainty at all to the discussed 30ns. If the slower beam current envelope, lasting 2µs, could be measured with 5ns certainty, then the 200MHz modulation would improve the correlation precision to about 10ps, which isn't the case here. The measurement relies only on the 2µs envelope.

I believe to understand that 200MHz is the frequency of the accelerator cavities, and modulate the beam intensity fully, something like 0% to 200% of the mean intensity. Though, the diagrams on page 6 show only +-15% modulation depth at 200MHz, so even though the beam current transformer and supposedly the acquisition device have a broader bandpass than 200MHz, something attenuates the 200MHz component, be it a medium to long cable or something else.

Unfortunately, the thing that attenuates at 200MHz is probably dispersive, that is, it introduces a propagation delay that depends on the frequency. A cable for instance delays precisely at 200MHz by its known speed but gets slower at lower frequency as its series resistance adds to the inductance, and here we're talking about 30ns precision over a 2µs waveform with 500ns transitions - that is, the measurement results from a rather strong statistical interpolation.

Hence I wish this possible dispersion be eliminated. Fortunately, this looks easy, thanks to the 200MHz modulation. It just needs to suppress the DC and LF components of the signals, both at Cern and at Gran Sasso, and compare only the tone-burst envelope. It needs a filter around 200MHz, a broad one like 100-300MHz to minimize its propagation time. Over this favourable and limited frequency band, all cables and transformers will show their normal delay. Maybe these filters can be made by the same piece of software, introducing the same delay. The correlation will oscillate at 5ns, but this is meaningless. The envelope of the correlation will be meaningful and independent of LF dispersion.

Marc Schaefer, aka Enthalpy

KWillets

http://lmgtfy.com/?q=gps+atmospheric+correction

But I suppose it's worth saying there's also a lot of cross-calibration, measurement from known points, etc., so it would be hard for an error to creep in.