2003 probably was the bes yer for LQG. It was published a relatively short paper (authored by Rovelli) that presented the basics of the canonical LQG. It stated the result of quantization of the area operator, related it to the idea of a minimal lenthg and, and this is the important point, stated that it implied an experimental prediction: "Vacuum light speed depends on it´s frequency". The paper also contained an introduction to LQC (loop quantum cosmology) and the results on big bounce evitancy and a few other topics. But the mimportant one, at least for a lot of people, was the first.
Also, Nic Mavromatos, an Jonh Ellis, from CERn, published around the same time a paper stating that noncrititical Liouville strings predictes similar results (with the dependence of speed on the frequency going in the opossite direction that in LQG.
Those people expressed the hope taht the launch , around 2005, of the GLAST satelite, could experimentally test tht ideas. GLAST satelite laucnhing got somewhat delayed. In the meantime, in august 2007, an experimental recipe, known as MAGIC, obtained some results that indicated a possible confirmation of those ideas. I told
something about it in this entry. But GLAST satelite was finally launched and at last there are some results. You can read a press release by NASA here.
The NASA paper speaks about gamma ray bursts, not about quantum gravity. Where is the relation?
Well, in the end of the release you can read this:
One curious aspect of the burst is a five-second delay separating the highest-energy emissions from the lowest. Such a time lag has been seen clearly in only one earlier burst.
That´s the point. The idea expresed but that proposals of quantum gravity is that the more energetic fotons travelat different speed than the slower ones. That implies that the fastests one arrive sooner than the slower ones. In ordinary distances that can´t be observed, but for very distant objects the path is long enought to give meausereable results.
So, has LQG been confirmed? Not quite. I´ll soon tell why. But first I´ll make a few clarifications. The way LQG was supposed to predict those effects was by means of it´s prediction of a minimal length. That leads to DSR (double special relativities) that take tha minimal length as a premise and from that they obtain the dispersion relations of vacuum light speed. I have also heard of another argument, supposedly stated by Fotini Markopuolu Kalamara. The idea is that the more energetic fotons , of shorter wave-lengt collide more ofthem with the "atoms of space-time". I did a search in arxiv of the actual paper of fotini but I wasn´t able to find it (if some reader know the link I would knoowledege if he would leave it in the comments).
Let´s return to the question of why the GLAST satelite results don´t favour this LQG (or noncritical string theory) prediction. The trick is that there are models on how the GRB are formed. The GRB are supossed to be originated in matter beeing acreted into a black hole. The models of disk acrretion predict that less energetic photons are produced sonner that more energetic ones. I have´t readed the actual papers about those models (semeengly that calculations are available here ). I simply readed the conclusion, that the variations on light speed are excluded.
You can read more about it in the Lubos motl blog. Also there is a discusion about the topic in physic forums, concretely in this post.
In physics forums it is stated that , in fact, that presumably firm prediction of LQG in facthaas been progresively desestimated. To beguin with it has not been possible to rigurously deduce that behaviiour directly from canonical LQG. In fact the whole subject of canonical LQG is nowadays almost forgoten. Accordin to Marcus actually lQG is focussing in:
There is really only one form of LQG that is getting the bulk of attention. It is designated EPRL (gamma<1) and also FK(gamma < 1). These two turned out to be equivalent as long as the Immirzi gamma is < 1. Two different vertex amplitude formulas which coincide in the important case.
Essentially every new paper I see about the full theory (not the LQ cosmo offspring, but LQG)
as well as every new video-stream seminar talk at PIRSA is about this one version of LQG.
From my perspective it is getting all the attention since sometime in 2007.
Rovelli and his group call it Covariant LQG.
EPRL is Engle Pereia Rovelli Livine, FK is Freidel Krasnov.
There was a revolution and convergence starting in 2006.
It is misleading to say there are a whole lot of LQGs. There HAVE been in the past. But in 2007 the old vertex amplitude formula of BarrettCrane was thrown out and several new ones tried and the two I mentioned turned out to coincide and to give results on hypersurfaces consistent with those of the old pre-2000 canonical LQG. So it amounts to a rederivation of the whole theory.
Rovelli gives a good summary of this in his talk to the string theorists at Strings 2008. Ask if you don't have the link and want to watch the talk.
I am almost not following LQG development nowadays. ALl I can say is that I listened the Rovelli talk at strings 2008 (I folllowed almost all talks) and most of it´s talk looked very similar to the introductory 2003 paper I talked at the beguining. If he actually told about the new developments it was a very brief mention that resulted almos ot perceibed by most participants (at least i I must judge by other blog entries about strings 2008).
Well, if LQG doesnt predict that light speed dispersion, does it predict something. Well, a new thread in PF was created in the subject, this. Seemengly most of that predictions are related to LQC and not to LQG. And the conexion among LQC and LQG is not totallly clear.
Well, LQG comunity clearly has the right to change it´s opinion about what it is or what it is not a prediction of their theories. But certainly, it makes sound the previous claim somehow as a publicitary strategy. It is not as if that publicity trick are not something totally new in physics, but, certainly, are not the bes way to give credibility. Anyway, LQG people will probably keep doing researche in their theories, but probably they have lost a great oportunity to win a clear triumph.