Is Doubly Special Relativity a Viable Alternative to String Theory?

[QEDick1918]

Physics noob here. I've recently been looking up alternatives to string theory (sorry, I'm just not that much of a fan of it) and found "Doubly Special Relativity" - which seems to be a possible waste of time. Is it? And are there any other testable theories out there (besides the Standard Model) that are currently being worked on?

 

[marcus]

DSR has to a considerable extent been ruled out.

A bunch of people have been deriving prediction of detail that can be looked for in the CMB (cosmic microwave background) based on LQG (loop quantum gravity) applied to cosmology.

One of the lead authors is Aurelien Barrau. I can get some links to papers of his.

LQG does not imply DSR's "energy dependent speed of light"---Smolin had some intuitive or handwaving arguments that it might, but others in the LQG community did not buy. And no rigorous connection was ever established, although some people tried for a couple of years.

So attention has shifted away from DSR and more in the direction of cosmology. What effects in the CMB can one look for, as a possible test.

Here are Barrau's papers---you will see some about testing LQG.
http://arxiv.org/find/grp_physics/1/au:+Barrau_A/0/1/0/all/0/1

 

[atyy]

LQG does not imply DSR's "energy dependent speed of light

Unknown, isn't it - could still turn out to be the case?

 

[marcus]

Unknown, isn't it - could still turn out to be the case?

I think that is right. No proof is known. One cannot say that LQG implies DSR. But I guess in the future somebody might find a proof either way, that the two were either logically compatible or not compatible. I think it is unlikely that implication will be proven given how much effort was expended trying to show that during 2005-2007, to no avail. But can't be sure.

 

[atyy]

This is pretty close http://arxiv.org/abs/1001.2919

Edit: Ooops, wrong reference, I meant http://arxiv.org/abs/0903.3475 though now I see the previous reference also has some discussion of 4D.

 

[marcus]

I guess you realize that some versions of DSR have "energy-dependent speed of light" and others do not. You pointed to a 2009 paper by Oriti, Livine, Girelli. Did you check to see if they found dispersion in what they arrived at? I don't have time to hunt but if you see any discussion of energy-dependent photon speed in that paper please give us a page reference!

 

[atyy]

No, I didn't know that. Where is that discussed?

 

[marcus]

No, I didn't know that. Where is that discussed?

The point was made in a paper by Kowalski-Glikman some years back. I will scan his list of papers and see if I can come up with it.

This might not be it but has something. See page 11 of http://arxiv.org/pdf/hep-th/0312140
"... the speed of massless particle equals 1. Let me stress here once again that this result is DSR model independent, though, of course, the relation between three velocity of massive particles and energy they carry depends on a particular DSR model one uses. Thus this calculation indicates that GLAST should not see any signal of energy dependent speed of light, at least if it is correct to think of photons as of point massless classical particles, as I have implicitly assumed here. "

Actually here are his papers. He is the reigning DSR expert as far as I know. You may have better luck than I:
http://arxiv.org/find/grp_physics/1/au:+Kowalski_Glikman/0/1/0/all/0/1

Because Jerzy K-G is an (the?) acknowledged world authority on DSR he was asked to write the DSR Chapter for Oriti's book Approaches to Quantum Gravity.
http://arxiv.org/pdf/gr-qc/0603022

==quote from Jerzy K-G's chapter: Doubly Special Relativity: facts and prospects==

The prediction of energy dependence of the speed of light is based on the rather naive observation that since in (some formulations of ) DSR the dispersion relation is being deformed, the formula for velocity v = ∂E/∂p gives, as a rule, the result which differs from this of Special Relativity. It turns out however that this conclusion may not stand if the effects of noncommutative spacetime are taken into account.
In the classical theory the non-commutativity is replaced by the nontrivial structure of the phase space of the particle, and, as in the standard case, one calculates the three velocity of the particle as...

Then it can be generally proved that the effect of this nontrivial phase space structure cancels neatly the effect of the modified dispersion relation (see Daszkiewicz et. al. 2004 for details.) Thus, in the framework of this formulation of DSR, the speed of massless particles is always 1, though there are deviations from the standard Special Relativistic formulas in the case of massive particles. However the leading order corrections are here of order of m/κ, presumably beyond the reach of any feasible experiment.
==endquote==

 

[atyy]

Hmm, very confusing - actually I'm not sure the DSR being talked about is the same in all these cases.

BTW, which DSRs did you think were probably ruled out and on what basis?

 

[marcus]

My personal view is
1. I'm not familiar enough with all the ins and outs of DSR to sort it out.

2. The most respected DSR expert I know of is Jerzy K-G so if you want the straight dope the most efficient thing is to pay careful attention to what he says, like in the DSR survey chapter in Oriti's book (Cambridge U. P. 2009) http://arxiv.org/pdf/gr-qc/0603022

3. The common terminology on this is a complete mess. The terms are not, in general, being used consistently.

4. In the past I've tended, so as to be understood, to say DSR when I meant "energy dependent photon speed without actually having preferred frame". A lot of people erroneously think that LQG predicts that! When they say DSR that is what they mean by it. So I try to make the point that LQG has not predicted that kind of "energy dependent photon speed". The attempt (mostly at Perimeter between 2005-2007) to derive a rigorous prediction appears to have been unsuccessful.

So I've adopted what I think is popular/common terminology and said LQG does not imply DSR, in an attempt to get that across. But this is, I confess, sloppy terminology. There could well an interesting version of DSR which does not predict variation in photon speed! There might be something you'd call DSR which Girelli Livine Oriti can derive from something you'd call LQG!

All that is a bit periferal and speculative, Atyy. Too far from my main interest. I am trying to understand http://arxiv.org/abs/1004.1780

 

[ccdantas]

See Sabine H.'s latest blog entry http://backreaction.blogspot.com/2010/08/update-on-status-of-dsr.html" .

 

[marcus]

See Sabine H.'s latest blog entry http://backreaction.blogspot.com/2010/08/update-on-status-of-dsr.html" .

Lee Smolin just weighed in at that discussion a few minutes ago:
http://backreaction.blogspot.com/20...showComment=1281531108878#c960433742298855009

==quote from Lee's post on Bee's blog==

Dear Peter, Sabine and all



First, to answer Peter's query, there are forms of DSR without an energy dependent speed of light. Those are not subject to Sabine's analysis although as I show in my papers some of these, connected with non-commutative geometry, still have issues with apparent non-locality or coordinate ambiguity very far from the origin of an inertial frame's coordinates.

 But, I argue that these are coordinate ambiguities, arising from a breakdown in the procedure to extend inertial coordinates to distant events, and not physical non-localities.

Second, yes, the discussions involving a number of experts in DSR has been respectful and professional. Even if I think that Sabine's claim to have derived an experimental bound on a parameter in a theory is mistaken, I believe that her paper puts a focus on a key issue involving the interplay of lorentz invariance and non-locality in DSR theories that needs to be better understood. Some very interesting work has been done which has deepened our understanding of the proposal of DSR and its physical interpretation.

I would suggest that interested readers read the papers before making up their minds which is correct or whether more work must be done to resolve the issues. I can mention that the second of my papers on this subject, arXiv:1007.0718, was updated yesterday with a stronger argument for the main claim and a note added to address Sabine's critique posted yesterday. The appendix of that paper also gives a summary of the state of the argument as I understand it...
==endquote==

 

[QEDick1918]

Wow, this turned out to be pretty informative. Thanks, guys!