Gamma Ray Bursts, and Quantum Gravity

[Royston]

As some of you probably know, quantum gravity theory depends on background independant geometry and the idea that space is discrete, as opposed to continuous. So for observation purposes this means probing scales and energys of the planck order, which in particle accelerators is impossible. Even creating a mini black hole, AFAIK is nowhere near the energies required to witness space around the planck epoch of the universe.

 

I listened to the interview Martin provided with Lee Smolin, and the subject was brought up in 'Three Roads to Quantum Gravity' in that by studying gamma ray bursts, it's possible to probe such energies. This makes sense to a point, but surely all we can study / observe are the after effects of a gamma ray burst. How is it possible to probe into the initial energies of such phenomena...is it just studying the behaviour of photons released that paint a picture of incredibly high energy ?

 

Please correct anything that I may have got wrong with the above, or if it's a misguided question.

 

Also, is it just me or does Lee Smolin sound a little bit like Ed Witten when he talks...is this some sort of prerequisite for cleverness.

[Martin]

 

...by studying gamma ray bursts, it's possible to probe such energies.

...

 

Several non-string QG theories are exposed to a big risk of being proven wrong by the fact that they predict a slight energy dependence of the speed of light.

 

this is not something the researchers ASKED for or put in by hand but rather something forced on them, that came out of the equations as they were building the model.

 

this is not a feature of every post-string QG model that is under construction, but it is a feature of several of the most promising ones.

 

One that comes to mind is what Laurent Freidel is working on.

http://arxiv.org/find/grp_physics/1/au:+Freidel/0/1/0/all/0/1

e.g. the December 2005 paper called "3D quantum gravity and effective...."

 

Freidel's version of QG will be seriously discredited, if this energy-dependence is not observed. He will have to abandon his line of investigation and change research direction. (but this is what science is about, a theory to have predictive value must run the risk of being wrong---a theory that can accomodate any future outcome of any future experiment is useless mush and doesnt tell us anything)

 

In principle the energy-dependent speed of light is very simple to observe using GRB.

 

A GRB is essentially a SPIKE of high energy photons, of all different energies, which should arrive all together if they are all traveling the same speed. But if the more energetic ones have a slight speed-advantage, they will arrive sooner!

 

The GLAST satellite, scheduled for launch in 2007, will be able to see and record a GRB, millisecond by millisecond, and to distinguish the higher energy gamma photons----so it will see whether or not they arrive a fraction of a millisecond earlier.

 

IIRC this was discussed earlier by Smolin in a 2003 paper called "How far are we from the quantum theory of gravity?" and also in a 2005 paper called "Falsifiable predicitons from semiclassical quantum gravity."

 

But when Smolin wrote, Freidel had not yet gotten the hard results and it was more of a possibility or a conjecture that QG theories would insist on energy-dependent speed of light. Smolin has some foresight and when he and others wrote about this, e.g. in 2003-2005, they were applying general reasoning----it was not yet nailed down.

 

http://arxiv.org/find/hep-th/1/au:+Smolin_L/0/1/0/all/0/1

 

 

2. hep-th/0605052 [abs, ps, pdf, other] :

Title: Generic predictions of quantum theories of gravity

Authors: Lee Smolin

Comments: For inclusion in "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", edited by D. Oriti, to be published by Cambridge University Press

 

6. hep-th/0501091 [abs, ps, pdf, other] :

Title: Falsifiable predictions from semiclassical quantum gravity

Authors: Lee Smolin

Comments: 9 pages

Journal-ref: Nucl.Phys. B742 (2006) 142-157

 

20. hep-th/0303185 [abs, ps, pdf, other] :

Title: How far are we from the quantum theory of gravity?

Authors: Lee Smolin

 

these technical papers will not necessarily be readable, Snail

but I have to give some sources in case some other person wants to check and follow up on what I'm saying.

 

the basic thing is very simple

 

when you make a quantum model of spacetime it can turn out from the model that there is a speed advantage (measured as a fraction of the speed of light) which is proportional to energy advantage (measured as a fraction of the Planck energy)

 

I will look up some numbers so I can say this in more detail, but I think you may already have gotten the idea.

[Spyman]

A GRB is essentially a SPIKE of high energy photons, of all different energies, which should arrive all together if they are all traveling the same speed. But if the more energetic ones have a slight speed-advantage, they will arrive sooner!

 

The GLAST satellite, scheduled for launch in 2007, will be able to see and record a GRB, millisecond by millisecond, and to distinguish the higher energy gamma photons----so it will see whether or not they arrive a fraction of a millisecond earlier.

 

Is the GRBs known enough to be sure that the "energetic ones" didn't have a head start ?

[Severian]

The most interesting thing about gamma ray bursts is that some of them have absolutely huge energies - far far higher than we could ever acheive in a particle collider. Since a photon can turn into a particle-antiparticle pair (+ another photon) it is possible that decaying gamma rays (which are photons) could turn into very high mass exotic particles. These particle would live a very short time (because they are so heavy) so we wouldn't have noticed them without looking closer. But if we can figure out a way of seeing them it would give us some insight into the very high energy physics applicable to quantum gravity.