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waterfall is offline
Feb12-12, 12:31 AM
P: 381
Quote Quote by marcus View Post
Remember you are dealing with a mathematical science. Back in 2002 Rovelli published a paper showing that LQG was compatiible with Lorentz invariance, not with Lor. violation. No rigorous proof or derivation has ever shown that LQG implies Lorentz violation. Various people thought that it MIGHT and tried to prove it, but failed. By now the opposite has been shown, in fact!

Here's a 2010 paper about this:
C. Rovelli and S. Speziale, “Lorentz covariance of loop
quantum gravity,” arXiv:1012.1739.

GLAST or FERMI results in no sense came near falsifying Loop. By now, at this stage, that is just ignorant talk. Or perhaps in some cases malicious.
The main Loop spokesperson, the one they usually invite to write a review articles and give overview talks etc, is Rovelli, and if not him then it's Asktekar. Neither ever suggested that the theory predicted Lor. violation. Or energy-dependent speed of light etc.
So here is what one of them had to say in 2010: page 18
Recent Planck-scale observations appear to support local Lorentz invariance [144]. This has been er- roneously presented by some authors as evidence against loop gravity. I want to stress the fact that loop gravity does not imply a violation of Lorentz invariance. In particular, the naive argument, often heard, that a minimal length is incompatible with Lorentz invariance is wrong, because it disregards quantum theory. The same argument would imply that a minimum value for a component of the angular momentum would be incompatible with rotation invariance, a conclusion manifestly contradicted by the quantum mechanics of angular momentum. For a complete discussion of this point, see [145]. The Lorentz invariance of the loop and spinfoam formalism can be made manifest: see [18] and references therein. For the moment, existing theoretical evidence is against Lorentz violations [149], and in accord with observations. So, for now we have no useful information from this direction.
The most likely window of opportunity at present seems to come from early cosmology. Quantum effects could be significative shortly before the onset of inflation and could affect, for instance, the CMB at multiple moments somewhat higher than the ones presently measured. The hope that the theory could provide an input to early cosmology sufficient for predicting observable quantum effects, and interesting attempts in this direction exist [150– 155]. But for the moment, I see no definite prediction that could be used to falsify the theory. To make loop (or any other) quantum theory of gravity, physically credible, this must change.
The reference [18] is to the paper I mentioned: proving covariance.
Btw.. I have a question. It is said that "the discrete nature of space causes higher-energy gamma rays to travel slightly faster than lower-energy ones". Why is that? I saw this claimed in 2004. How did Rovelli banish the problem? I read the following in Smolin article in Scientific American in January of 2004 (I know it's old news but I can't understand the above paper):

"RADIATION from distant cosmic explosions called gamma-ray bursts might provide a way to test whether the theory of loop quantum gravity is correct. Gamma-ray bursts occur billions of light-years away and emit a huge amount of gamma rays within a short span. According to loop quantum gravity, each photon occupies a region of lines at each instant as it moves through the spin network that is space (in reality a very large number of lines, not just the five depicted here). The discrete nature of space causes higher-energy gamma rays to travel slightly faster than lower-energy ones. The difference is tiny, but its effect steadily accumulates during the rays’ billion-year voyage. If a burst’s gamma rays arrive at Earth at slightly different times according to their energy, that would be evidence for loop quantum gravity. The GLAST satellite, which is scheduled to be launched in 2006, will have the required sensitivity for this experiment."

Again, what is the reason for the statement "The discrete nature of space causes higher-energy gamma rays to travel slightly faster than lower-energy ones" and how did Rovelli make the problem go away??