Via SciAm online: http://blog.sciam.com/index.php?titl...&c=1&tb=1&pb=1

The original paper is here: http://arxiv.org/abs/0708.2889

I'm not clear on why these results imply the effects of quantum gravity. The blog entry says that the authors ruled out difference in timing of release of the higher energy gamma-rays vs. lower-energy gamma-rays as a cause of the time delay.

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Yeah, that doesn't sound like it has anything at all to do with quantum gravity, the energies are nowhere close to Planck scales. It instead sounds like a challenge to basic relativity, the only one I've heard of in a long time-- but my money is still on error in observational interpretation.

In the paper, they state that while one might expect the energies required to be near the Planck mass, some theories suggest the threshold might be smaller. I have no clue what to think about it.

If these observations are accurate, then we do have a breakdown of the Equivalence Principle, something that many expect to happen with quantum gravity.

-Richard

But why is the energy uncertain? The gamma rays have to be detected, and the highest energy gamma ray I've ever heard of being detected is 3 times 10 to the 20 eV. That's nowhere close to the Planck scale (10 to the 28 eV)-- such high energy photons are not even thought to be able to propagate through the CMB without interacting.

Now, I know as much about this as I do brain surgery, but according to that paper the idea is one gets a vacuum "refractive index" due to quantum space-time fluctuations of:

r ~ 1 - (E/M_QGn)^n, where n looks like some quantum number thingy. Why that is 1 minus and not 1 plus I don't know, but that's how they put it, and the M_QGn's are the mass threshold.

The say their results constrain M_QG1 > 0.4 *10^18 GeV, and the
M_QG2 > 0.4 *10^11 GeV. That's still pretty high. I don't understand the 'n' progression, and why it would get lower.

It says string theories and some extra dimensional theories will let the M_QG's be less than the Planck mass.

-Richard

I guess the idea is that even though our observations are at least a billion times below the Planck scale, the propagation times are very long and the difference in arrival times could be measurable even if there is just a hint of Planck-scale influence. It's provocative, that's for sure. What bothers me about it is that if you are a billion Bohr radii from a proton that's like a centimeter, so this is like trying to probe the nature of a hydrogen atom by watching electrons that pass a centimeter away from the proton! How do we know how much physics we're missing there? Still, I realize that any confirmed depature from the equivalence principle is probably a Nobel prize.

KenG. Although flare gamma ray emission is not identical to classical GRB's, there's a history there. Most GRB's begin as "hard"...>300 kev...bursts, and then attenuate to "softer"..<300 kev fluences. But it is not always the case. (see section 9d in the link).
There are at least some bursts that begin with soft (the authors will call it NHE...non hard energy) fluence, and then switch to hard (HE, their designation)..fluence. They also find that single sources are capable of both types of fluences within a single burst, or that repeat bursts may be only of a single mode (HE or NHE) without contamination of the other. So, if a physical mechanism exists for inverted mode emission in classical GRB's, can it be ruled out entirely in a Markarian flare emission, too? Can a pair of isolated soft (NHE) followed by hard (HE) bursts occur within a four minute time frame?...seems like thin statistics to claim a failing of relativity here, based on this sole event. pete.

see:http://www.journals.uchicago.edu/ApJ...35715.html#fg9

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Yeah, it sounds like the kind of result you'd have to already be expecting in order to believe that interpretation. Not that I could say one way or the other.

Those are UHECRs (ultra-high energy cosmic rays), mostly likely protons, but perhaps nuclei (up to) iron - it's kinda difficult to determine the composition of these very rare beasts.

The highest gammas that I can recall being reported are ~20 TeV, from an innovative CR 'telescope' (whose name escapes me just now), which uses water instead of air.

When (if?) LOFAR is up and running, it should provide some nice independent checks on this.

However, I think the second part of the last sentence of the (pre-print) abstract is the most important: "but we cannot exclude the importance of some other source effect" - the source is a physical regime so far from anything probed so far in any Earthly lab (and likely to remain unprobed for possibly centuries to come) that constraining 'source effects' will likely be a headache for many decades yet.

Good point, that's ten million times lower than even 10^20 eV, so farther still from the Planck limit. But even more puzzling-- now they are talking about probing the Planck limit with photons that whose energies are 15 orders of magnitude smaller than where the real action is. Hardly an ideal way to test a theory!
Yes, observational interpretation will probably always be the main stumbling block, except for those already predisposed to favor some particular quantum gravity theory (who will cite this result ad nauseum, no doubt!).

This preview of a proprietary New Scientist article gives an interesting hint about something that may be important.

The idea as I understand it is that very high energy gamma rays may travel more slowly than lower energy ones due to the non-linear paths they take through the quantum foam (or some other such explanation).

Up till now, this has never been observed, but MAGIC looks at a *very* high range in the energy spectrum. A flare was observed in Markarian 501, and the super-high energy photons arrived four minutes late.

Does anyone have a link to an arXiv paper on this, or MAGIC press release?

If this is true, this is strong evidence for some flavor of String theory to be on the right track.

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Paracelsus posted about this in Q&A:

http://www.bautforum.com/questions-a...elativity.html

The link to the arxiv paper is there. If this is true, then we have indeed observered a breakdown of the Equivalence Principle, and are seeing a quantum gravity effect.

This is something that is consistent with a quantum gravity effect, but isn't a smoking gun. We're talking needle in a haystick type of stuff, a difference in time of minutes compared to total times of millions of years (or whatever it is, pretty big). There could be some other mechanism there. It's darned interesting, but let's don't get too excited yet.

-Richard

This topic came up in the ATM forum as well: New Evidence for Tired Light. I review the paper in that thread (post #5). I think that the New Scientist article, and the blog piece posted in ATM both seriously exaggerate the significance of the observations.

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Having looked at the paper, I'm possibly misreading it as saying that the deviation from c is proportional to the energy divided by the Planck mass, or it is proportional to the square of that same ratio. This is one data point, so you can draw any line you like through it, but the implication is that the deviation is linearly proportional, and at a few TeV, is enough to provide a 240 second delay over a 140 quadrillion second flight (roughly a part in 10¹⁵).

I think that there will come a time in the near future when MAGIC, or a similar installation, will have observed similar flares from a different more distant BL Lac object, and the numbers will be more clear.

As a first observation of this sort, this is very exciting news.

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I agree, it is potentially Nobel prize stuff here. What I find so interesting is that we have, in effect, a way to measure a time difference with 10^-15 precision, simply by having a "fast" process that happened a "real long time" ago! If you have a process that is going on all the time, you want to give it plenty of time to happen, so you look at something very bright and very far away.

I think they are flying blind here. Too much is not known about the sources of Gamma Rays to run very far with this. As Neried observed in the abstract the authors caution, "We cannot exclude the possibility that the delay we find, which is significant beyond the 95% C.L., may be due to some energy-dependent effect at the source."

The authors are looking at one of many possible reasons the energy peaks in different wavelengths at different times. Without a good physical mechanism that explains exactly how theses events originate, it is difficult to weigh-in on how the energy should be expected to be distributed.

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It's a big universe out there...is it really unwinding, really burning out?

But those kinds of issues will emerge more clearly with more data. It sounds like a promising avenue, that could reap real rewards if the delay scales with distance.

If this preprint results in lots more attention being paid to GRBs, to a speed up of the installation of LOFAR (and the building of another half dozen or so, in different locations), of Milagro (ditto), etc ... then it will have been a fantastic paper ... even if (as I think likely) nothing about quantum gravity emerges at the end of the day.

Study of time lags in HETE-2 Gamma-Ray Bursts with redshift: search for astrophysical effects and Quantum Gravity signature:Different - in many ways - from the MAGIC pre-print, so a null result is not necessarily inconsistent ...

Furthermore, their "lower limit" for the quantum gravity scale is at least 4 orders of magnitude below the expected Planck scale, so on the surface that would seem to render their null result pretty irrelevant. I'm not sure how the MAGIC observation manages to probe to such a more precise level.

MAGIC looks at far higher energies than HETE does. When the effect is energy dependent, that translates to higher sensitivity.
Oh, and an experiment that says "we looked at this, and we saw nothing unexpected" is still a good thing. It would be very unwise not to look because we expect to see nothing.

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I was working on space being curved so is it possible that two different wavelengths might arrive at slightly different times?

I was thinking more along the line of sound actually say travelling in a large curved medium like the ocean. If the path taken could be shown to be ever so slightly different based on pitch then would that help?

I was just thinking of wave propagation so if the curvature of the ocean around the earth is too invariant then a crescent of metal such as niobium which has the best resonance of any metal to detect the path of a moving wave through a known curved medium.

This may indicate if there is a curvature associated with being in a galactic structure as such given that the spiral arms are not straight.

Given that time and space are curved and light propagates as a wave function as does sound I wondered if this helped?

OK, that makes sense.I agree, but the issue is not "was the experiment worth doing", it's "does it have anything to say about the interpretation of the MAGIC experiment". Owing to the shortfall in predictive power in regard to the Planck scale, I would say it does not. Indeed, it sounds like a positive result by HETE would actually be a refutation of the MAGIC interpretation as a quantum gravity effect-- on the grounds of it not seing enough of an effect.

I think that GLAST will look for similar results from GRBs.

http://www.astronomytoday.com/cosmol...antumgrav.html

GLAST will look at energies lower than those MAGIC looked at (but higher than those seen by HETE-2), but I think that the GRBs would be much further away than MAGIC's gamma-ray flairs.

That will be interesting to watch for. They're certainly looking for more ways to sell GLAST!

If GLAST can see up to 300 GeV, AND the observations from MAGIC and Markarian 501 were correct, then at the highest energies, GLAST should see a 10-30 second delay (at 450 million light years).

IF the short burst GRBs give off gammas at that energy (not an easy assumption) this effect should be pretty obvious, and clearly defined.

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That would be very exciting indeed-- they could sell it as "the first careful look at nothingness".

"The MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescope found that high-energy photons of gamma radiation from a distant galaxy arrived at Earth four minutes after lower-energy photons, although they were apparently emitted at the same time. If correct, that would contradict Einstein's theory of relativity, which says that all photons (particles of light) must move at the speed of light.

"Everybody's very excited," about this result, said Daniel Ferenc, a physics professor at UC Davis and a member of the MAGIC collaboration. Ferenc cautioned that the results need to be repeated with other gamma-ray sources and that a simpler explanation had not been ruled out. But, "it shows that such measurements are possible," he said.

The researchers propose that the delay could be caused by photons interacting with "quantum foam," a type of structure of space itself. Quantum foam is predicted by quantum gravity theory, an attempt to unite quantum physics and relativity at cosmic scales. "
http://www.physorg.com/news110480559.html


What else could you use besides "quantum Foam"?

New physics.....???? I thought that was the realm of ATM......

If I could provide an explanation that required "no new physics"(for real), would that be more acceptable??

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