Is There Evidence of a Delay in High-Energy Photons from Distant AGN Flares?

[marcus]

In August 2007 the MAGIC team caused controversy by posting a paper on arxiv that
http://arxiv.org/abs/0708.2889
1. reported they thought they might have observed a 4 minute delay of the more energetic gammaray photons, compared with less energetic, from an Active Galactic Nucleus (AGN) flare some 500 million lightyears away.

2. suggesting that the delay, while it might have occurred at the source, might also have a QG explanation----that the very energetic (2 - 10 TeV) photons might conceivably experience a slightly different spacetime geometry and, in effect, have farther to go or travel more slowly than light of lower, more familiar energy.

The MAGIC telescope is a Imaging Atmospheric-Cherenkov Telescope (IACT) on San Juan in the Canary Islands. The MAGIC team got some prominent theorist/phenomenologists such as John Ellis and Nick Mavromatos to co-author with them making the suggestion of a possible QG cause of the delay more visible.

We aren't even sure the delay really happened, might have been an artifact, and if it happened it might have been caused by some other mechanism at the source. But IF it is real and replicable and does NOT originate at the source then there is this nagging idea that it might have a QG explanation.

There was audible irritation in the science community that MAGIC raised this issue, with Ellis and Mavromatos help, in such a noticeable way, back in August.

So now the idea has appeared again, but in a more SUBMERGED WAY.
ON PAGE 16 OF A WONDERFUL 20 PAGE REVIEW PAPER that everybody interested in high energy and very high energy (VHE) astronomy should read. some new types of orbital and groundbase telescopes have recently come into use and a revolution is underway. This paper is a good brief overview with lots of photos, graphs, data, and references to the literature. It is by de Angelis, Mansutti and Persic.

http://arxiv.org/abs/0712.0315

Anyway the discussion of the controversial delay is modestly submerged down at page 16 by the careful authors dA, M, and P. And they give a nice one-page presentation!
What is more, they bring in OTHER SUPPORTING OBSERVATIONS, where according to them the data could be interpreted as containing a proportionately similar delay from one or more other sources.

In particular they mention data from another IACT, called HESS, taken in 2005 and discussed in Aharonian et al
http://arxiv.org/abs/astro-ph/0506593
Aharonian et al did not SAY they had an energy-dependent delay but somebody has delved into their data and dA,M,P assert that there is this possible interpretation supportive of what MAGIC said. They calculate some numbers which are consistent.

 

[staf9]

Are there other telescopes (aside from HESS and MAGIC) that are experiencing similar delays in photons? If these tests are repeatable then we can statistically rule out errors in the computing.

Now using this as a QG explanation, that seems interesting. It looks like if we can take more controlled measurements of high energy particles (LHC perhaps?) we could experimentally confirm or disprove this as a result of QG. I'd like to look into different explanations for the delay in the meantime (keeping QG in mind still, however, just incase)... delving into quantum gravity gets into a whole mess of GR/QM unification and such.

Of course, if we could link this with a current theory it may serve as some experimental evidence, always a good thing.

I'm starting to really get into this - they may be on to something. E8 theory is a really big player these days and is taking up most of peoples' attentions though (at least on these forums).

 

[Chronos]

I may be a little dated, but I thought such delays were predicted by DSR.

 

[marcus]

You are hardly dated about that, but there are several different DSRs and they are merely theory. And I don't know of anyone definite unique DSR version that arises from any of the leading QG.

The whole DSR thing is very frustrating. A DSR is just a modification of Special Relativity and by itself has no QG content. If there were a firm tie-in between some type of DSR and some type of QG, like a spinfoam model (which there IS in the 3D case, but it doesn't carry over to 4D) then you could TEST the DSR and if you falsified it then that would automatically falsify that particular QG model that gave rise to it. But there is not any clear exclusive connection like that, of DSR with some form of spinfoam or any other leading QG.

I don't believe that MAGIC saw a delay, and I don't DISbelieve it. I am completely in Limbo about this. And except for Pereira and Aldrovandi, I don't see any of the nonstring QG guys I watch stepping up to the plate and PREDICTING there should be such and such delay.
So I am temporarily completely frustrated about this.

That is why I was glad when dAMP posted their paper (de Angelis, Mansutti, and Persic). I figure it will shake those guys tree a little, to hear some more about a possible delay observed by that other telescope array, and provoke them into coming up with some definite predictions

 

[Jim Kata]

Ok, If the magic results hold, I think the theoretical explanation is fairly obvious. That is, the speed of light depends on the energy of the photon. I know this messes up lorentz invariance and all that, but it seems like the Ockman's razor solution. Now, why would this be? Something I don't understand is why [tex]\hbar[/tex] and [tex]c[/tex] are treated as these universal constants, but the other constants in the standard model are allowed to have a sliding scale dependent on the energies of the physics involved. I think the simplest solution would be that [tex]\hbar[/tex] and [tex]c[/tex] depend on the energies involved.

Ps. I apologize for the babble

 

[aaroman]

Not that obvious.

Is c not a constant just because the light travels through a medium with a different speed than c (and energy dependent, too)?
Is charge not a constant just because a particle with charge might be "dressed"?

We might not have a theory that explains the delay, but that does not necessarily mean that c is not a constant. Making c not a constant is not an Ockham solution, because: "this messes up lorentz invariance and all that". I don't think this would simplify the theories.

Anyway, this is an extraordinary claim, they should wait for extraordinary evidence.

 

[turbo]

Anyway, this is an extraordinary claim, they should wait for extraordinary evidence.

Extraordinary evidence would consist of 1) multiple observations of a delay in the arrival of HE gamma rays and 2) delays that are proportional to the redshift of the source.

There is no harm or foul involved in getting preliminary findings into the literature. If GLAST finally launches next February (those slippery launch schedules), we may get a chance to see if HE gamma rays are routinely slower to arrive than those of lower energy, and observe those arrival times directly (no atmospheric absorption). If such confirmatory observations are made, AND if the delay is proportional to the redshift of the source, then we will have observational evidence (not just theory) that space is a refractive medium, and that dispersion is wave-length dependent. To those of us that are trained opticians, this development would not be much of a surprise.

 

[friend]

Ok, If the magic results hold, I think the theoretical explanation is fairly obvious. That is, the speed of light depends on the energy of the photon. I know this messes up lorentz invariance and all that, but it seems like the Ockman's razor solution. Now, why would this be?

Just guessing here, but wouldn't higher energy photons curve spacetime to a greater extent than low energy photons? And wouldn't this greater local curvature mean that the photon would move slower as viewed from far away? This would be similar to how events appear to slow down near the horizon of a black hole. The high energy photon would represent a higher concentration of energy, would curve spacetime more, and would move through this curved spacetime more slowly. What do you think?

 

[marcus]

Just guessing here, but wouldn't higher energy photons curve spacetime to a greater extent than low energy photons? And wouldn't this greater local curvature mean that the photon would move slower as viewed from far away? This would be similar to how events appear to slow down near the horizon of a black hole. The high energy photon would represent a higher concentration of energy, would curve spacetime more, and would move through this curved spacetime more slowly. What do you think?

I know you asked Jim Kata, and I want to hear his opinion, but also want to interject my own---I concur: I think that if there turns out to be a demonstrable effect, then what you say is a good way to approach it. Another way to look at it is to imagine that (due to a generalized uncertainty principle) the geometry of the continuum is more and more chaotic (foamy, fractalish) as it is probed at smaller and smaller scale. Then as a bumpy road is longer if you measure with a small ruler that measures all the ups and downs, and as a little wheel travels a longer distance than a big wheel (that doesn't see as many humps and pits) in traversing the same road, so a high energy photon could be thought of as traveling a longer distance in going from galaxy A to galaxy B. This is not meant to be a rigorous idea, but just another intuitive provisional way to start looking at it, if it turns out that the energydependent delay is real.

BTW friend, in their most recent paper Aldrovandi and Pereira agree with what you say (and not with what I just said)---they have the higher energy photon affect the geometry it is passing through and, with certain assumptions, they retroactively predict the 4 minute delay observed by MAGIC---a bold or foolhardy gamble depending on how one looks at it. If you have not seen their paper and need a link to it, just ask.

 

[friend]

I know you asked Jim Kata, and I want to hear his opinion, but also want to interject my own---I concur: I think that if there turns out to be a demonstrable effect, then what you say is a good way to approach it. Another way to look at it is to imagine that (due to a generalized uncertainty principle) the geometry of the continuum is more and more chaotic (foamy, fractalish) as it is probed at smaller and smaller scale. Then as a bumpy road is longer if you measure with a small ruler that measures all the ups and downs, and as a little wheel travels a longer distance than a big wheel (that doesn't see as many humps and pits) in traversing the same road, so a high energy photon could be thought of as traveling a longer distance in going from galaxy A to galaxy B. This is not meant to be a rigorous idea, but just another intuitive provisional way to start looking at it, if it turns out that the energydependent delay is real.

BTW friend, in their most recent paper Aldrovandi and Pereira agree with what you say (and not with what I just said)---they have the higher energy photon affect the geometry it is passing through and, with certain assumptions, they retroactively predict the 4 minute delay observed by MAGIC---a bold or foolhardy gamble depending on how one looks at it. If you have not seen their paper and need a link to it, just ask.

Yes, I'm asking for that link. Thank you.

It would seem straight forward that higher energies effects geometry more. The only problem I would have is if the geometry cannot move faster than the photon, then is the photon actually surrounded by the potential well that slows down its time? Or does the geometry lag behind the photon or just effect the back half of the photon? I don't know.

 

[turbo]

Here you go, friend.

http://arxiv.org/abs/0711.2274