Friday, June 26, 2009

Constraining Modified Dispersion Relations with Gamma Ray Bursts


    Prospects for constraining quantum gravity dispersion with near term observations
    arXiv:0906.3731v3 [astro-ph.HE]
    By Giovanni Amelino-Camelia, Lee Smolin












nige said...

Thanks for this post. I would expect higher energy photons to travel more slowly than low energy7 photons!

The higher the energy of a photon, the more momentum it carries p = mc, where m is the effective mass of the photon by E = mc^2. (Yes, of course everyone knows that photons have no rest mass, but photons are never at rest; thus the fact they have no rest mass is totally irrelevant to physics!)

The effective mass is the quantum gravity charge of the electron. Thus, the more energy it has, the more effective gravitational charge it has.

The vacuum contains gravitational fields from the 3 × 10^52 kg of observable stars (page 5 of ), plus other mass contributions from dark matter like neutrinos.

These gravitational fields interact with the photon's gravitational charge (mass) in much the same way that the photon's electromagnetic field interacts with the electromagnetic fields of molecules in a block of glass, thus slowing down photons.

nige said...

In the deflection of starlight by gravity, starlight is deflected with twice the Newtonian a = GM/r^2 prediction, i.e. a = 2GM/r^2. The amount of effective transit "mass" (gravitational charge) of the photon of starlight, m = E/c^2, is trivial compared to the sun's mass and thus has no effect on the deflection (similarly, as Galileo found, an apple and a lead cannon ball fall in the same time). It is purely because of (1) the really immense energy of some of these gamma ray burst photons and (2) the really immense travel times of the photons from distant gamma ray bursts to the earth, that there is now a measurable difference in velocity between photons of differing energy.

nige said...

Sorry, my sentence "The effective mass is the quantum gravity charge of the electron" should end "photon" of course! (It's past my bed time!)

Arun said...

It is interesting that the vacuum is effectively scale-invariant to photons over many orders of magnitude of length scale. But I'm wondering if that is built into the way we do QFT - i.e., it is a predetermined result. I mean to say, have we ruled out any purely electrodynamic effects that might confer dispersion on the vacuum? It is unthinkable, of course, but are we precluded from even thinking it simply by our formalism?

Arun said...

I suppose the standard answer to my question is that QED is renormalizable and gauge-invariant (so the photon remains massless and also we don't get terms that would induce E^2 = p^2 + a * p^4 + b * p^6 + ....

Daniel de França MTd2 said...

There is one more hypothesis that they raise at the end, it is that the decrease in the speed of light is not an absolute quantity, distribution that peaks at a value smaller than the speed of light. In this way, high energy photons arriving sooner than what should be expected wouldn't spoil right away a variable speed of light.

Rhys said...


If you say so, nige. Do you know what a Casimir operator is? And more physicallY: how many polarisation states does a massless vector particle have, and how many does a massive one have?

Bee said...

Arun: Your equation isn't Lorentz Invariant. If it was, you wouldn't have a modification on-shell. (Which btw is exactly what happens in my model).

Bee said...

Daniel: Right, there could be a stochastic element to the propagation.

Plato said...

To me Bee, it appears this article is very insightful because of the understanding of "shape being distributed by the telling photons" as if pasting a Lagrangian view of the universe in a most colourful way.

It is describing the vacuum method of discernibility. This does not depart from the idea of expression of local regions of our universe and the local description arising from those cosmological events. To me this supports the understanding of "vacuum in space" as a prime mover of geometrical considerations. It does not lessen Linde's versions either in terms of our own universe.

Again the path of "least resistance" is a clear sign of the value of the distribution of what is held in context of the geometrics of the gravitational field? Photon energy calculations?


Phil Warnell said...

Hi Bee,

A most interesting post and I must try to read the paper before forming any kind of opinion. The one thing it prompts though is to ask when does an entity of energy become one of matter. From Einstein’s perspective is to say that’s when it becomes subluminal in it’s speed or another way to look at it is when time becomes a dimension, which is realized as to be applicable. That would imply that as a photon becomes more energetic and resultantly having its velocity drop, would mean it no longer is solely a entity of energy in such respect.

On the surface this seems to be just a further attempt to find flaw with relativity, such that it might be removed as an encumbrance to unification. I’ve often wondered what has many so convinced that relativity must be altered, rather than quantum mechanics or perhaps both to succeed in furthering the completion to physical law.



Uncle Al said...

What are the electric and magnetic fields associated with 10^14 - 10^17 eV photons? The vacuum sparks for atomic nuclei somewhere beyond Z = (1/fine structure constant). The vacuum goes dichroic for magnetic fields around a teratesla.

Perhaps the anomaly is simply the expectation, contributing low probablilty Feynman diagrams no longer being quite so improbable.

Neil' said...

Not everyone believes in doubly-special/deformed relativity etc., but is there general support for the idea that a photon having Planck energy or above is "special" in some sense? Of course, it wouldn't be that much in some other frames which is part of the issues thereof.

Bee said...

Neil: The whole point of DSR is that the Planck energy is the Planck energy in every restframe.

Phil Warnell said...

Hi Bee,

After reading the paper you referred to I can appreciate as to understand its motivation more so then its content. That being the authors are most concerned that not only the theorists pay more attention to the data, yet rather they look to their theories to see if they can be more definitively made to predict the consensus that will be eventually formed by the results. That’s best brought out when in the summation of the paper they say:

“To quantum gravity theorists we suggest urgent attention be given to any possibility of deriving predictions for these observations from theories of quantum gravity, otherwise it may be only a matter of months to a year or two before we theorists are demoted to the role of postdictors of great experimental discoveries.”

I find the authors concern and level thereof a trifle unwarranted, as for instance the unexplained orbit of Mercury didn’t lessen the significance of GR when it was shown to actually be able to account for it, whereas the Newtonian approach had failed. That’s like saying the Copernican model had no advantage over Ptolemy’s without making predictions beyond what was then known.

I thus feel too often science rushes to find consensus before the dust has settled. The most important thing is that the theories be logically consistent in relation to what we observe as to be able to be understood being more important then what they may seem to predict. I am thus reminded of a quote of John Bell’s I recently read in the book I just finished entitled “ The Age of Entanglement by Louisa Gilder” where when addressing John Wheeler in regards to this he said:

“I’d rather be clear and wrong than foggy and right.”

Smolin above all others in relation to his objections about string theory should be most sympathetic to this and such I find this comment a bit out of character in terms of consistency.

Best ,


P.S. I must lend thanks to Doug Hemmick for pointing out this most wonderful book I mentioned.

Neil' said...

Thanks Bee. What I meant was, in conventional relativity the photon energy would not be the same in different frames. Hence if there's something special or "troublesome" about a photon having Planck energy in any frame at all, then wouldn't we need DSR etc. to deal with that? IOW, DSR should be considered a likely or necessary prospect and no. And if so, how does that fit in with quantum gravity schemes etc.

Bee said...

Hi Nige,

As far as I am concerned, from a quantum gravitational perspective there is no reason to expect an effect to set in at a particular distance or energy. The reason is that qg effects become relevant in the high curvature regime, not at a large energy (energy is not a local quantity, it's an integrated one). You will thus look for curvature invariants that reach Planckian size to determine whether qg effects become relevant. By construction, these invariants are independent of the reference frame. In an effective description then you would replace what would actually "really" be quantum gravity effects that we don't know how to deal with by a model, but still the effect should become important when the background would have a high curvature. (You can find more details on this point of view here and here).

So, the short answer is, no, DSR in the form considered by the authors is not necessary, and in my opinion not very well motivated either. But then, they are frequently quoting evidence from the 2+1 dimensional case. It probably depends on your taste how convincing you find that. Either way, the question isn't so much how likely I or anybody else thinks this scenario might be. What I find more important is that it be self-consistent and testable.



Bee said...

Hi Phil,

I think the sentence you are quoting isn't so much about reaching consensus, as about putting the cards on the table before it's clear what cards are the winning ones. That is to say a pre-diction is generally considered more convincing than a post-diction. I would agree with you that one can overstate this though. What will happen if you push people is simply that they will try to cover all possibilities and consequently be right whatever the data says, which isn't helpful either. (You see, the photons could arrive earlier, or later, or it could be stochastic and it could be both. And oh, if the modification is second order, they arrive neither earlier nor later. Thus, our model is consistent with everything.)

Keep in mind however that there have been alternative explanations for the perihelion precession of Mercury. The deciding factor was the prediction for the light deviation (whether or not that wasn't originally as decisive as it was claimed to be). Best,


Giotis said...

What do you mean by that Bee? The gravitational coupling which determines the strength of the gravitational interaction becomes strong at very high energies (at Planck scale) and thus qg effects become important in that regime. Or you want to say something else maybe?

Bee said...

Hi Giotis,

I meant exactly what I said. I wasn't talking about the running of the coupling, I said qg effects become strong not at Planckian energies, but in the strong curvature regime. The Planck mass is about 10^-5 g, some molecules have masses in that range. Needless to say, there's nothing quantum graviational about them (in DSR there is btw). Best,