CERN team claims measurement of neutrino speed >c

vanesch

Well, I don't want to know how the GPS system actually works, what counts is what is the result of it. If it gives you the synchronised reference time in a stationary frame, then you assume that they have build in all necessary corrections to do so.

What I wanted to say was that if you "synchronize" in a stationary reference frame Oxyzt, which means that at events "Emission" and "Reception" you measure "t" (the t of the reference frame Oxyzt), but you measure the distance between "Emission" and "Reception" in a frame Ox'y'z't' using worldlines of stationary points (that is, with 0 velocity in frame Ox'y'z't') so that it is easy to measure that distance in that frame, then you cannot combine this distance measured in Ox'y'z't' with a time measured on Oxyzt.

My question was what kind of time coordinate (in what kind of frame) is used in the GPS system (no matter how they actually do it, assuming they do it right), and I thought that it was only possible in an intertial frame. However, I stand corrected, this can also be a time on a rotating geode which also contains another "universal time" as I forgot about the GR correction.

But it DOES matter what reference frame one uses to define "synchronised time", because mixing a time coordinate from one frame and a distance from another is at the origin of all "paradoxes" in introductory SR, such as the pole-barn paradox and the like.

Which should then according to DH be annihilated by the geode effect.

That's if you're working in an inertial frame ! If you work in the rotating frame that is not the case. This is why defining the correct reference frame is so important, and rather tricky in this case.

The point is not that I think I'm smarter than those guys, it is just that nothing of all this was mentioned in the paper.

vanesch

I was talking about beta and thought about a series development in beta, but now you come to say it, for most relativistic corrections the first non-zero term term is beta-squared. So this pushes the effects indeed in the 10^-12 range or so.

I guess this closes the discussion about a relativistic effect due to earth's gravity or rotation...

dan_b

One pulse does not have good enough signal to noise ratio to get a time of flight precision of a few ns. The proton pulse actually doesn't have much current in terms of everyday lab measurements, although it's a huge current in terms of teravolt particles. So they made a model by using many emitter pulses, and comparing many receive pulses to it. Several people pointed out that there can be hidden assumptions when that is done. For example, one hidden assumption might be that the emitter pulse is invariant shape, except for band-limited Gaussian noise. If that's wrong, then the mathematical processing used to put together the 'average' of many pulses goes a little wrong and might make a bias which could be unaccounted for.

All my tentative "might' and "could' words are because, they're smart guys and maybe they already did it just right, but a paper with that level of total detail in it would be unreadable! There's deep exam questions here about experimental technique, just as should be. It's a lot of work for them to answer even a few of the most carefully considered issues of the critics. This will take time. There is no way around it, and they understand that.

Vanadium 50

That's the point - who is using something more complicated than something you buy at Fry's for this particular application? The bigger the market for this, the less likely something is odd in the firmware.

xeryx35

Sorry, didn't know that. But another problem arises with the use of GPS. The satellites which are making these measurements may slip a bit in their orbits - they are not in absolutely perfect geostationary orbits. Even a deviation of [itex]\pm[/itex]1 meter could have an enormous effect on the accuracy of the neutrino reading.

D H

GPS satellites are not in geosynchronous orbits.

Whatever mistake was made, if a mistake was made, was quite subtle. That group has been building up this data for a few years. They looked for obvious explanations, not so obvious explanations, asked outside groups for help, and still couldn't find anything that explained their results.

I'm guessing that they did do something wrong. I'm also guessing that we at PhysicsForums will not be the ones to ferret that mistake out.

xeryx35

It does not matter, there may be +/- a few meters of orbital deviation.

True.

Lord Crc

As I mentioned earlier in this thread, they said in the presentation that they corrected for GR due to the height difference, and that the correction was on the order of 10^-13.

D H

No. Try centimeters.

Furthermore, the errors in the orbit estimations are irrelevant here. Those experimenters used common view mode, which reduces errors in both relative time and relative position by orders of magnitude. Common view mode, relative GPS, and differential GPS have been around for quite some time. The basic concept is thirty years old, but not the 10 nanosecond accuracy claimed by the experimenters.

Lord Crc

In the presentation they said that this precision was common place, just not in the field of particle physics. Did I misunderstand?

xeryx35

The software could have been buggy, it may be like that for something which is not commonplace like that. There are a thousand other factors which could affect the results. No single factor was responsible for this.

dan_b

Hi Michel,

Likelihood function = probability density function. Just a different name maybe with different normalization. I apologize in advance because I don't think you're going to like this link very much. I don't. It has an approach which obscures the intuition if you not comfortable with the math. It also has links which may be useful. Keep following links, use Google search on the technical terms, and eventually you'll find something you're happy with. Try starting here:

http://en.wikipedia.org/wiki/Probabi...nsity_function

Borg

I have been reading about the accuracy of the GPS timestamps. I’m not sure what to think about two pieces of information. I’m highlighting my concerns below.

Page 9 of the OPERA paper (http://arxiv.org/pdf/1109.4897v1) states :

The Cs4000 oscillator provides the reference frequency to the PolaRx2e receiver, which is able to time-tag its “One Pulse Per Second” output (1PPS) with respect to the individual GPS satellite observations. The latter are processed offline by using the CGGTTS format [19]. The two systems feature a technology commonly used for high-accuracy time transfer applications [20]. They were calibrated by the Swiss Metrology Institute (METAS) [21] and established a permanent time link between two reference points (tCERN and tLNGS) of the timing chains of CERN and OPERA at the nanosecond level.

Reference [19] led me to this paper (ftp://ftp2.bipm.org/pub/tai/data/cggtts_format_v1.pdf) on CGGTTS formats. The conclusion on page 3 states:

The implementation of these directives, however, will unify GPS time receiver software and avoid any misunderstandings concerning the content of GPS data files. Immediate consequences will be an improvement in the accuracy and precision of GPS time links computed through strict common views, as used by the BIPM for the computation of TAI, and improvement in the short-term stability of reference time scales like UTC.

I didn't see any references to the calibration of the PolaRx2e receivers other than the 2006 calibration. It looks to me like they used a calibration that was good for short-term stability and used it over the course of four years. Am I misreading this?

Astronuc

The author of the article, Jon Butterworth makes some good points:

• What would it mean if true? (certainly worth considering, but without being overly speculative)
• Isn't this all a bit premature? (a point that is made numerous times in this thread)
• What might be wrong? (again - a point that is made numerous times in this thread)

and as a postscript to the article.
Butterworth is a frequent contributor to the Guardian - http://www.guardian.co.uk/profile/jon-butterworth

dan_b

Regarding 8.3 km of fiber optic, I did some looking. Admittedly we don't know what kind of cable it is, and they do vary in temperature coefficient of delay (TCD) from one type to another. A good quality cable may have TCD = 0.5e-6/C. The cable delay is roughly 30 us. So 0.5e-6/C makes about 0.015 ns/C of temperature dependent delay. That's too small to worry about.

Back to assumptions about the proton pulse shape consistency. How much might the shape change as a function of anything slow which might subsequently mess up the ability to model and average? Temperature? CERN grid voltage? Other effects?

D H

Assumed? Do you really think that physicists would let such a critical assumption go untested?

A very brief history of physics in the latter half of the 19th century: The development of electrodynamics threw a huge wrench into the physics of that time. Electrodynamics was incompatible with Newtonian mechanics. It was Maxwell's equations (1861), not Einstein's special relativity (1905), that first said that c, the speed of electromagnetic radiation, was the same for all observers. People, including Maxwell, tried to rectify this incompatibility be saying that Maxwell's equations described the speed of light relative to some luminiferous aether. The Michelson–Morley experiment pretty much put an end to that line of thinking. Various other lines of thinking, now abandoned, gave ad hoc explanations to somehow rectify electrodynamics and Newtonian mechanics.

Einstein's insight wasn't to magically pull the speed of light as constant out of some magician's hat. His insight was to tell us to take at face value what 40 years of physics had already been telling us: The speed of light truly is the same to all observers. Refinements of the Michelson-Morley experiment has born this out to ever higher degrees of precision.

The modern view is that there will be some speed c that must be the same to all observers. In Newtonian mechanics, this was an infinite speed. A finite speed is also possible, but this implies a rather different geometry of spacetime than that implied by Newtonian mechanics. Massless particles such as photons will necessarily travel at this speed. Massive particles such as neutrinos can never travel at this speed. Photons are massless particles not only per theory but also per many, many experiments. That neutrinos do indeed have non-zero mass is a more recent development, but once again verified by multiple experiments.

Vanadium 50

minos
t2k.