Q'izing GR retains local Lorentz invariance after all?

[marcus]

maybe someone else can clarify;
these recent papers suggest a surprising turnaround in the quantization of General Relativity, contrary to some earlier papers by other people, they predict no quantum gravity dispersion in longrange transmission of light:

On low energy quantum gravity induced effects on the propagation of light---Gleiser/Kozameh/Parisi

http://arxiv.org/gr-qc/0304048

Lorentz Invariance and the semiclassical approximation of loop quantum gravity---Kozameh/Parisi

http://arxiv.org/gr-qc/0310014

the first of these two papers is also available in the journal "Classical and Quantum Gravity" vol. 20 pp. 4375-4385
the second has just been posted as pre-print this month

on page 12, in the conclusions, the first paper says
"In Section 3 we show that a very natural assumption leads to Lorentz invariant field equations. Our conclusion is that we have no reason to believe that a quantum theory of gravity would change the invariance..."

on page 2, in the introduction, the second paper says
"In recent years there as been hope of observing quantum gravity effects via the propagation of light through cosmological distances. This hope is based in some models describing the interaction of quantum Maxwell and gravitational fields that predict a breakdown of Lorentz invariance at a linearized level in the semiclassical approximation. The common feature in these models is a non standard dispersion relation which shows that spacetime behaves as a medium with a frequency dependent index of refraction..." Here they cite papers by Ellis/Amelino-Camelia etal., Gambini/Pullin, Alfaro/Morales-Tecotl/Urrutia, and Sahlmann/Thiemann.

their analysis appears to contradict results by Ellis and also by Sahlmann/Thiemann. It concludes that their supposed predictions are wrong and a dispersion relation (at least of the kind earlier discussed) is not to be expected

my take on it: theory is supposed to develop guided by observation and experiment. LQG is close enough to conventional GR that people can try out details of the theory and various modifications and crank out predictions and see if they are right. Apparently Sahlmann/Thiemann analysed photon/graviton interaction in some fashion and got some numbers and they didnt match observation and these people in Argentina are setting it up slightly differently and saying that they, not Sahlmann/Thiemann and not John Ellis etal., are doing the analysis right and are in agreement with the observations.

I guess I'll have to wait and see if other people think the work of these people at the University of Cordoba is sound, and whether they cite it and so forth. As a general thing it seems like its good its happening.

 

[selfAdjoint]

I looked at these papers a bit. The first one assumes the homogeneous Maxwell equations are valid in the semiclassical approximation of quantum gravity and base their derivation on that. I don't know if this is a meaningful way to procede or not. Maybe what both of us are lacking is a clear understanding of the physics of the semiclassical approximation.

The second one, which I only scanned very briefly, basis its discussion on an experimental result. That is certainly encouraging.

 

[marcus]

Originally posted by selfAdjoint
I looked at these papers a bit...<snip>...basis its discussion on an experimental result. That is certainly encouraging.

thanks for having a look! was hoping you would. I, for one, will just have to wait and see if anyone pays attention and treats the work as valid.
I agree as a general thing it is encouraging to see a branch of theory beginning to encounter data and adapt

 

[Dors_Venabili]

F = ma + mvxw

where: F=force, m=mass, a=static gravitational field, v=velocity of mass 'current', x=cross product, w=angular velocity of rotating mass.

To confirm:

Follow Maxwell's derivation for electromagnetic equations, but rather than initiating with F=qE as the static manefestation of the force, exchange charge for mass by using F=ma. Continue using the classical tensor derivation using mass in the place of charge and the above force equation will become evident.

Of course you can also generate gravitational equations similar to Maxwell's electromagnetic ones, but the above equation is the truly revolutionary one as it replaces the blatently inaccurate F=ma, while, pehaps more importantly, incorporating the elements of so-called 'fictitious forces' (i.e. angular velocity G-fields). This is precisely the 'semi-classical' derivation to which you refer selfAdjoint. And yes, it does indeed work as a complete force equation capable of being unified with its electromagnetic twin.

Feel free to email me if you would like further details.

Zarko

 

[Stingray]

It is actually not that surprising. None of these papers rigorously derives anything from basic principles. There are various assumptions made, and then conclusions drawn. The original people writing these papers are more trying to use experiment to discover analytical results (those assumptions I mentioned), rather than to confirm a framework. Unfortunately, many people quote the results without realizing that.