Searching for Quantum Gravity in Gamma-Ray Bursts

[marcus]

http://arxiv.org/abs/astro-ph/0610571
On the Problem of Detecting Quantum-Gravity Based Photon Dispersion in Gamma-Ray Bursts
Jeffrey D. Scargle, Jay P. Norris, Jerry T. Bonnell
63 pages, 19 figures, to be submitted to the Astrophysical Journal

"Gamma-ray bursts at cosmological distances offer a time-varying signal that can be used to search for energy-dependent photon dispersion effects. In particular, we argue that short bursts with narrow pulse structures at high energies will offer the least ambiguous tests for energy-dependent dispersion effects. We discuss an array of quantitative methods to search for such effects in time-tagged photon data. Utilizing observed gamma-ray burst profiles extrapolated to GeV energies, as may expected to be observed by GLAST, we also demonstrate the extent to which these methods can be used as an empirical exploration of quantum gravity formalisms."

 

[marcus]

Scargle is at
Space Science Division
NASA/Ames Research Center, Moffett Field, CA 94035-1000

Norris is at
Denver Research Institute
University of Denver, Denver CO 80208

Bonnell is at
Astroparticle Physics Laboratory
NASA/Goddard Space Flight Center, Greenbelt, MD 20771.

It is really nice to see these people gearing up.

It is really nice to check out their figure 2 on page 41

"Fig. 2: The intense portion of GRB 051221a, binned at 1-ms timescale
(15–350 keV)."

 

[ccdantas]

Marcus,

Thanks for this reference; it's nice to see some astrophysicists working on this problem, and submitting their paper to ApJ.

Cheers,

Christine

 

[marcus]

the ApJ will accept it gratefully, if they know what is good for them

 

[marcus]

a kind of shorthand indicating the current situation in QG, in essentials, was a bit from Smolin's book which I'll type in when I have time (from pages 236-237), and echoed in this L.S. quote of 21 October at Woit's blog:

http://www.math.columbia.edu/~woit/wordpress/?p=475#comment-17990

L.S.: "...As to consistent quantum theories of gravity which show that general principles like Lorentz invariance must be modified, let me recommend the recent Freidel-Livine work on 2+1 gravity with matter, which shows that Poincare invariance is necessarily quantum deformed as a result of including quantum gravity. (btw, not for the first time, I have no idea what the ambiguity is in 2+1 quantum gravity some people keep referring to. Carlip wrote about different approaches to 2+1 gravity but my understanding is that with the same matter content they all give the same theory.)..."

I think this means that it still has not been worked out whether background indep QG in 3+1 D requires dispersion (more energetic photons must go a Planck hair faster----or else the theory is falsified)

it is a reasonable conjecture that the 3+1D theory does require dispersion because it has been shown that the 2+1D theory requires it----so one hopes that researchers will be able to extend the Freidel-Livine work to higher dimension.

 

[marcus]

double suspense

there is a kind of double suspense now, as GLAST nears launch.

1. will the prediction of dispersion be extended to 3+1D? which would make the most prominent (or some of the most prominent) background independent models of QG-and-matter falsifiable.

2. and if that happens, will these models then be SHOT DOWN? by GLAST not seeing gamma-ray dispersion.
------------------------

L.S summarizes this situation in some paragraphs around pages 236-237
at the end of chapter 14. I will try to copy some of that in.

he concludes with an apparently serene observation which I like:
"Either way, we would be doing real science."

In fact, I like that quote so much I think I'll make a sig of it.

 

[hellfire]

After reading some parts of the paper I get the impression that the modified dispersion relation is something that would only show that the current low energy physics is incorrect, but, currently, it would not give any hint about the correct theory of quantum gravity:

The enthusiasm for the empirical side of this topic seems to stem mostly from the assumption that QG shows itself in the form of higher order terms in a Taylor series expansion of the classical dispersion relation. However, this idea lacks a basis in any specific physical theory of QG. It might be argued that this lack is compensated for by the fact that the idea is fairly generic, since it is independent of the theoretical approach.

Seams to be a bit discouraging.

 

[marcus]

from page 237 of Smolin's book:

The question at issue in the GLAST and Auger observations is the symmetry of space and time. In a background-dependent theory, this is decided by the choice of a background. As long as a theory allows it, you may get any answer you need to get by choosing an appropriate background. This is very different from making a prediction.

What about other approaches to quantum gravity? Have any predicted a breakdown of special relativity? In a background-independent theory, the situation is very different, because the geometry of spacetime is not specified by choosing the background. That geometry must emerge as a consquence of solving the theory. A background-independent approach to quantum gravity must make a genuine prediction about the symmetry of space and time.

As I discussed earlier, if the world had two dimensions of space, we know the answer. There is no freedom, the calculations show that particles behave according to DSR. Might the same be true in the real world, with three dimensions of space? My intuition is that it would, and we have results in loop quantum gravity that provide evidence, but not yet proof, for this idea.

My fondest hope is that this question can be settled quickly, before the observations tell us what is true. It would be wonderful to get a real prediction out of a quantum theory of gravity and then have it shown to be false by an unambiguous observation. The only thing better would be if experiment confirmed the prediction. Either way, we would be doing real science.