What Travels Faster Than What?

[Moontanman]

Don't you see as a limitation the fact that it is impossible to predict if an individual neutrino sent at one point will be detected at the other? I mean they needed 10^20 neutrinos to obtain 10^4 detection events after a few years. I would say this limits sending any information faster than light that differentiates itself from random noise no matter how strong the emitter is (and there's also energy limitations to that).

 

Doesn't this assume that neutrinos are some how fundamentally impossible to ever measure accurately?

[quantumtopology]

Doesn't this assume that neutrinos are some how fundamentally impossible to ever measure accurately?

Hmm, yes, somewhat like a virtual quantum particle, let's keep in mind we actually don't detect the neutrinos proper but their effects on electrons, muons etc. You might argue that something similar can be said of photons but they have the advantage that they interact electromagnetically and can be detected in a one photon-one hit basis.

[CraigD]

What the OPERA group is reporting is a phenomenon that could be used for genuine faster-than c signaling – in principle, a strong, very brief neutrino emitter could send information to a receiver before a light signal could, for a sufficiently large emitter-detector distance d, in less time than [imath]t=\frac{d}{c}[/imath]. Non-local quantum effect such as the measurement of complementary states of entangled particles at great distances from one another can’t be used for signaling, because it’s not possible to force either measurement to be a given state. In other words, such setups can “communicate” instantly, but can their signal must be “100% noise, 0% signal”, incapable of sending information. The recent OPERA result, and Nimtz Stahlhofen’s FTIR experiment’s result, don’t have this limitation. They could, in principle, send information.

Don't you see as a limitation the fact that it is impossible to predict if an individual neutrino sent at one point will be detected at the other? I mean they needed 10^20 neutrinos to obtain 10^4 detection events after a few years. I would say this limits sending any information faster than light that differentiates itself from random noise no matter how strong the emitter is (and there's also energy limitations to that).

No.

 

All that matters in signaling is that a state created at will at the sender be reliably detected at the receiver – in short, a “switchable signal”. It doesn’t matter, in principle, if the sending device requires a huge machine and lots of energy to produce a huge number of particles of which only a tiny fraction are detected by the receiver, another huge machine, as is the case with the OPERA experiment, though it’s a huge practical engineering problem if you’re attempting to build a machine to realize a practical application of causality violation, such as sending lottery ticket numbers back in time to yourself so you can pick them to win a lottery.

 

Keep in mind that a superluminal signal is only one of the necessities for building a causality-violating backward-in-time signaling machine. You need to have two sender-receiver pairs with high (appreciable fractions of c) relative speeds times to be near one another in space at precise instants. The amount of time into the past the signal is a function of both the speed of the pieces of the machine, and the distance between them, so this should be as great as possible, say on a many lightyear scale. As recent results hint that such signaling requires a dense medium, this mean the machine might need dense physical structures many light years long.

 

Though prognosticating about engineering so far beyond our present-day capabilities is wildly uncertain, my guess is that the power requirement and signal loss of the sender are among the easiest of challenges involved in building such a machine.

 

I'm not sure but I don't even think for instance Nimtz claims that his experiments defy causality, only that they show Lorentz violation, actually here http://arxiv.org/abs/physics/0009043 Nimtz claims causality is preserved. Certainly the closely related Scharnhorst effect in the words of Scharnhorst himself according to wikipedia argues that the effect can't be used to create causal paradoxes.

Nimtz’s effect involve not signal speeds, but wave group and phase speeds in excess of c. That such speeds can exceed c without conflicting with relativity is roughly analogous to the speed with which a point of light produced by a beam swept across a surface can exceed c – the point the speed of which is being measured doesn’t correspond to a physical particle, and can’t be used for signaling.

 

The Scharnhorst effect, as best I’m able to tell with some hurried reading (your post introduced it to me, quantumtopology – thanks :thumbs_up) could, like OPERA’s neutrinos or FTIRed photons, be used to violate causality in the intuitive sense I’ve been using, like using information from the future, such as a winning lottery number to purchase a winning ticket. The only “Scharnhorst effect can’t violate causality” explanation I’ve yet read, this well-written 2001-2002 paper by Liberati, Sonego, and Visser, appear to argue two main points:

that the intuitive definition of “causality violation” I’ve used isn’t a good, formal definition, so the strange things like cheating at the lottery I’m describing aren’t really causality violations;

and the same point I’ve been making, that actually doing such strange things is so practically difficult as to be practically impossible.

L, S, and V summarize their arguments nicely, I think, at the end of the paper’s "a paradox with multiple pairs of moving plates?" section:

The present arguments do not guarantee the total absence of causality violations, but they do demonstrate that the most naive estimates of the causality violating regime are likely to be grossly misleading.

:) At the risk of engaging in precisely the naïve and grossly misleading estimation LS&V describe, I’d really enjoy working out some actual specifications for the lottery cheating machine I describe, accepting the OPERA experiment results as unambiguously correct. It’d take me quite a few hours of study and work, though, so likely won’t happen for a while. :(

[Moontanman]

If I had a communicator here on the Earth and one on a planet orbiting Alpha Centari, and that communicator can transmit some sort of signal to Alpha Centari in one year and then transmit it back in one year could I transmit the lottery numbers to me in the future by sending them to Alpha Centari and then sending them back to me, would I receive them before they were sent or would I receive them two years after I sent them and just beat the light waves but not communicate with the future?

[quantumtopology]

All that matters in signaling is that a state created at will at the sender be reliably detected at the receiver – in short, a “switchable signal”.

 

That reliability is precisely what you lack in a experiment with neutrinos by definition, due to the weak interaction being ... weak, basically.

 

Keep in mind that a superluminal signal is only one of the necessities for building a causality-violating backward-in-time signaling machine...

Exactly, so why do you assume the other necessities are met?

Surely you are aware of the distiction between FTL signals and sending useful information FTL. The former doesn't guarantee the latter.

I'm afraid it's gonna be a tough one winning the lottery :angry: ;)

 

 

Nimtz’s effect involve not signal speeds, but wave group and phase speeds in excess of c. That such speeds can exceed c without conflicting with relativity is roughly analogous to the speed with which a point of light produced by a beam swept across a surface can exceed c – the point the speed of which is being measured doesn’t correspond to a physical particle, and can’t be used for signaling.

 

The Scharnhorst effect, as best I’m able to tell with some hurried reading (your post introduced it to me, quantumtopology – thanks :thumbs_up) could, like OPERA’s neutrinos or FTIRed photons, be used to violate causality in the intuitive sense I’ve been using, like using information from the future, such as a winning lottery number to purchase a winning ticket. The only “Scharnhorst effect can’t violate causality” explanation I’ve yet read, this well-written 2001-2002 paper by Liberati,

Actually both exploit the quantum tunneling effect (according to wikipedia), virtual particles effects and negative or at least less than unity index of refraction materials. So to me conceptually they are quite similar. This paper by Scharnhorst himself is very clear about his effect not violating causality: http://arxiv.org/PS_cache/hep-th/pdf/9810/9810221v2.pdf

[Jay-qu]

While this is a potentially exciting result, I am highly sceptical. Fitting the neutrino beam "profile", a mere 16,000 counts over the life of the experiment, to the proton beam is tricky to say the least. Before systematic effects are subtracted there is discrepancy of 1050ns, they account for -990ns, leaving the neutrinos 60ns faster than light over the ~720km. Have a look at figure 11/12 and ask yourself how much difference it makes to the fit if you shift the red line by just 50ns (one division/bin in fig 12) to the right. This would bring the result within error of c. And you don't have to worry about upsetting the fit for the trailing edge, no errors for the possible change in beam profile have been included.

 

A further sobering note worth pointing out is that 30 physicists (of the 200) in the collaboration asked to have their names removed from this result.

[coldcreation]

OPERA apparently made an error. At least that's what CNN just reported.

 

I think this article explains the problem quite well, for the laymen:

Faster-than-Light Neutrino Puzzle Claimed Solved by Special Relativity, The relativistic motion of clocks on board GPS satellites exactly accounts for the superluminal effect, says physicist.

 

[...] So from the point of view of a clock on board a GPS satellite, the positions of the neutrino source and detector are changing. "From the perspective of the clock, the detector is moving towards the source and consequently the distance travelled by the particles as observed from the clock is shorter," says van Elburg.

 

By this he means shorter than the distance measured in the reference frame on the ground.

 

The OPERA team overlooks this because it thinks of the clocks as on the ground not in orbit.

 

How big is this effect? Van Elburg calculates that it should cause the neutrinos to arrive 32 nanoseconds early. But this must be doubled because the same error occurs at each end of the experiment. So the total correction is 64 nanoseconds, almost exactly what the OPERA team observes.

 

 

For a more detailed explanation of the OPERA error see: Time-of-flight between a Source and a Detector observed from a Satellite, Ronald A.J. van Elburg, 12 October 2011.

 

Michelson and Morley showed that an interference pattern is reference-frame independent. However, the distance between a particle’s production and detection site is reference-frame dependent due to Lorentz contraction and detector movement. For the OPERA experiment detector movement in the satellite reference frame leads to corrections which can account for most of the ±60 ns discrepancy between expected and observed time of flight.

 

 

Well, it looks like Einstein was right after all. :P

 

 

CC

[CraigD]

OPERA apparently made an error. At least that's what CNN just reported.

 

I think this article explains the problem quite well, for the laymen:

Faster-than-Light Neutrino Puzzle Claimed Solved by Special Relativity

...

Well, it looks like Einstein was right after all. :P

While I’m pretty sure the OPERA neutrinos aren’t exceeding c, I’m not confident in van Elberg’s claim to have found their error. Elberg appears to be a Computer Science instructor, not a physicist, and is the only author of his preprint. The paper doesn’t site where in OPERA’s paper its authors “seem to include a correction for the Lorentz transformations, but do not explicitly correct for detector movement in the satellite refrence frame”, and if my attempt at reading the OPERA paper is any indication, Elberg may have been in over his head trying to understand it in detail.

 

I’ll say this for him, though: unlike me, he appears at least to have given the OPERA paper some study, and found fault in it using basic – though not easy – SR.

 

Hopefully, given its appearance to millions on the front pages of many news services, the OPERA team will respond to van Elberg’s paper, but given the small army of physicists that scrutinized their work before publishing, I’d be very surprised if their response is anything like “doh! We missed that! Thanks, Ronald Elberg.”

Edited October 19, 2011 by CraigD
Fixed misspelled name

[coldcreation]

While I’m pretty sure the OPERA neutrinos aren’t exceeding c, [...]

 

 

The OPERA results appear to be refuted in another paper, this time by Andrew G. Cohen, Sheldon L. Glashow (two who shouldn't be over their heads with the topic)

 

New Constraints on Neutrino Velocities

 

Abstract:

The OPERA collaboration has claimed that muon neutrinos with mean energy of 17.5 GeV

travel 730 km from CERN to the Gran Sasso at a speed exceeding that of light by about 7.5 km/s

or 25 ppm. However, we show that such superluminal neutrinos would lose energy rapidly via

the bremsstrahlung of electron-positron pairs ( ! + e− + e+). For the claimed superluminal

neutrino velocity and at the stated mean neutrino energy, we find that most of the neutrinos would

have suffered several pair emissions en route, causing the beam to be depleted of higher energy

neutrinos. Thus we refute the superluminal interpretation of the OPERA result. Furthermore, we

appeal to Super-Kamiokande and IceCube data to establish strong new limits on the superluminal

propagation of high-energy neutrinos..

 

 

 

 

CC

[coldcreation]

The ICARUS team tested the OPERA result based on the article recently published by Cohen and Glashow (see link above).

 

A search for the analogue to Cherenkov radiation by high energy neutrinos at superluminal speeds in ICARUS

 

 

Abstract:

The OPERA collaboration [1] has claimed evidence of superluminal propagation between CERN and the LNGS with . We find that the neutrino energy distribution of the ICARUS events in LAr agrees with the expectations from the Monte Carlo predictions from an unaffected energy distribution of beam from CERN. Our results therefore refute a superluminal interpretation of the OPERA result according to the Cohen and Glashow prediction [2] for a weak currents analog to Cherenkov radiation. In particular no events with a superluminal Cherenkov like e+e- pair or gamma emission have been directly observed inside the fiducial volume of the "bubble chamber like" ICARUS TPC-LAr detector, setting much stricter limits to the value of delta comparable with the one due to the observations from the SN1987A.

 

 

I'm not sure to which paper CNN, yesterday, had referred to as a refutation of the OPERA results. I'll see if I can find it.

 

(EDIT: that report apparently has not made it to the CNN website)

 

 

CC

[coldcreation]

While I’m pretty sure the OPERA neutrinos aren’t exceeding c, I’m not confident in Elberg’s claim to have found their error. Elberg appears to be a Computer Science instructor, not a physicist, [...]

 

Ronald van Elburg, an AI researcher at the University of Groningen, apparently has a PhD in physics.

 

"1996-2000 PhD-position at the Institute for Theoretical Physics Amsterdam, leading to my PhD-thesis: `Quasi-Particles for Fractional Quantum Hall Systems'." (Source and source)

 

 

CC

[Qfwfq]

For a more detailed explanation of the OPERA error see: Time-of-flight between a Source and a Detector observed from a Satellite, Ronald A.J. van Elburg, 12 October 2011.

I don't think that van Elburg has pointed out the error of OPERA at all. I even wrote him a short email saying what I think and asking him if he had read the relevant part of the OPERA preprint (part 3, entitled "Principle of the neutrino time of flight measurement" and fig. 5). The OPERA folks have already dismissed his point as being the solution to the puzzle.

 

As for Cohen-Glashow, I think it is superfluous, as I don't think the neutrino goes faster than [imath]c[/imath] but maybe instead just closer to it than the photon does. This is what everybody, literally everybody, overlooks (except me and maybe two people that got my point here).

 

Looking through the OPERA preprint, I also got the impression that their analysis is better than Jay-Qu suggests, even though it is a good point at least in appearance.

[CraigD]

First, let’s set the understanding that offering lots of short explanations of why the OPERA measurement of neutrino speed > c must be incorrect is a good thing, and how the scientific process should work – in short, it’s discussion by interested and informed parties, worthwhile regardless of whether it’s ultimately proven correct, not, or simply abandoned.

 

Hopefully, given its appearance to millions on the front pages of many news services, the OPERA team will respond to van Elberg’s paper ...

… The OPERA folks have already dismissed his point as being the solution to the puzzle.

Do you have a link to, or can you excerpt an email with anything written by the OPERA folks in response to van Elberg? I was hoping to read a succinct response, along the lines of “yes, we did properly account for motion and travel time of our clocks and timing signals, as follows”.

[Qfwfq]

Do you have a link to, or can you excerpt an email with anything written by the OPERA folks in response to van Elberg?

No, in the sense that I may have been tricked by grammatical subtlety. In:

 

http://www.pcmag.com/article2/0,2817,2394747,00.asp#fbid=7xp4_Q0GleR

 

it seems they did not actually reply to van Elburg but that their preprint already claims having allowed for such things (as I would well expect of them). I don't know of any reply directly by them to van Elburg but I'll tell you that I see it more as him being misguided about their method than as them having allowed for his exact point. The very title of his paper shows he argues in a manner which does not apply. Try making the comparison yourself instead of waiting to see OPERA's reply to him.

[Qfwfq]

Well, I wasn't quite expecting to get a reply from van Elburg, but I did. Less than 10 hours!

 

He says he is reading up on their methods, as it isn't his usual field. He says their whole methodoly described in the paper is reasonable, except for the outcome of the experiment. :hihi: Then he adds that, all the same, he puts his bets on Einstein. Obviously. So it seems he's in the club of those who find no fault with the result ---- except that it kills SR.

 

Now I'll ask him what he thinks about my take.

[Jay-qu]

I don't think Elburg's point is relevant. The distance measurements are made with the GPS and transformed into the European Terrestrial Reference System (set in 2000 (it changes due to continental drift)), this is routine GPS work requiring special and general relativity. The timing measurements are checked with synchronised Caesium atomic clocks at each lab.

 

I like your idea Qfwfq, but how do you resolve it with supernova neutrino observations?

[Qfwfq]

I like your idea Qfwfq, but how do you resolve it with supernova neutrino observations?

I said a few very vague words about it upstream, like maybe out in intergalactic space (or extragalactic, from here to the Magellanic clouds) there could be much less difference in velocity than around here where it has been measured.

 

Have you ever looked up VSL and PV? The implications could be interesting, as well as that it isn't seeming much like the neutrino has rest energy (I've been thinking for a while that maybe it doesn't).