Comments on: Gamma-ray burst restricts ways to beat Einstein’s relativity

By: Stephen

Stephen — Fri, 13 Nov 2009 03:32:44 +0000

What would happen if QFT does not hold up at the Planck scale? The idea of a high energy cut-off seems plausible…

Has it been experimentally proven that the virtual particle pairs in vacuum fluctuations actually “feel” gravity at all? (We know that they “feel” Elecro-magnetic forces…)

By: sean

sean — Thu, 29 Oct 2009 23:29:38 +0000

The evidence clearly refutes a linear relationship between photon energy and speed of light as was suggested by earlier MAGIC results, and this is being conservative. It looks like LI extends beyond planck scale, if these results are combined with those of Auger and others. My hope is that we grow humble in realizing just how beautifully constraining the symmetries of nature truly are. The universe may be composed of strings, but they certainly weren’t stitched!

By: johnjohn

johnjohn — Thu, 29 Oct 2009 17:15:01 +0000

If we assume ordinary QFT holds up to the Planck scale, then any theory that predicts Lorentz violation at that scale would already seem to be ruled out, since the RG flow back to low energies suppresses Lorentz violation logarithmically at best. To avoid being ruled out, the Lorentz violation would need to be extremely fine tuned at the Planck scale, and it is not clear at all why such a tuning would be allowed in a fundamental theory, unless there is a limit in which it is exactly Lorentz invariant. It would be interesting to know the exact amount of fine tuning required.

It should be noted that string theory is believed to be exactly Lorentz invariant (in a fashion similar to the way GR is), and so such considerations wouldn’t apply.

Personally, the Lorentz-violating nature of many of these alternative theories is one of the main points I’d need to clarify before taking them seriously. The other even more basic question is whether they reproduce GR at long distances in the first place, which I profess I am ignorant about.

By: Bee

Bee — Thu, 29 Oct 2009 07:51:43 +0000

Thanks for the link We also had a post two months ago about That Photon from GRB090510, which stirred up some discussion.

About the question at “what scale should quantum gravity become important?” You write “the Planck energy and Planck length might be the reasonable scales.” This raises for me the question why the energy should be the relevant quantity and not the energy density. We know that effects of quantum gravity become strong in the strong curvature regime. The total energy of a matter distribution (burst, photon, whatever) over the Planck mass is to me not a limit in which to expect effects of quantum gravity.

By: lololololololol

lololololololol — Wed, 28 Oct 2009 19:31:40 +0000

this is awesome but how does gamma ray burst effect earth when it hit earth think about that