- Testing Einstein's special relativity with Fermi's short hard gamma-ray burst GRB090510
By Fermi GBM/LAT Collaborations
which reports the detection of an high energetic photon of 31 GeV within the first second of onset of the Gamma Ray Burst (GRB) 090510. As I explained in earlier posts (here, here and here), some scenarios with Lorentz Invariance Violation (LIV) motivated by quantum gravity do predict an energy dependence in the travel time of photons. Over such long distances as ours to the origin of the GRB, tiny delays add up and can upon arrival of the photon in our detectors result in an energy-dependent modification of the signal. The case that has been favored in the last years is that photons with high energies would arrive later than the lower energetic ones even when emitted simultaneously.
Previously reported cases indeed indicated high energetic photons arrive with a measurable delay. The status of the constraints on LIV derived from these events was nicely summarized in Lee and Giovanni's recent paper. One has to keep in mind though, without a precise knowledge of the emitting source it is hard to tell whether a measured effect happened during propagation. Besides this, the LIV modification in the propagation of photons is only measurable if it is a first order effect (of order Energy/Planck mass). If it is quadratic or higher, the effect would be too small to affect the spectrum of GRBs.
To let you know what my stakes are on the matter, I don't find it plausible quantum gravity would affect the propagation of photons from GRBs. I wrote a paper on that some while ago that explains very clearly why. Without going to much into the details, whether or not quantum gravitational effects are relevant depends on the energy density, not the energy. The energy density even of the GRB itself, and certainly that of the traveling photons, is many orders of magnitude too small to cause any measurable effect. It adds to this that models with violations of Lorentz invariance do either break Lorentz invariance, on which there are strong constraints from many experiments already, or they "deform" Lorentz invariance (known as "Deformed Special Relativity"), which causes conceptual difficulties that are so far unresolved. For these reasons, I am not very convinced such a quantum gravitationally induced energy-dependence of the speed of light would be observable in GRBs. It is however an interesting and testable scenario.
In any case, the measurements of GRB090510 reported in the above mentioned paper do clearly not support the energy-dependence of the speed of light. In the paper, they derive a bound on the Planck scale from their measurements that is some orders of magnitude higher than we expect the Planck scale to be. This means if there was an effect it is much smaller than it should have been. However, it gives me the creeps if people draw conclusions from single photons. As also Lee and Giovanni pointed out explicitly in their paper, the propagation could have a stochastic component since it's a modification caused by a quantum gravitational effects of the background. In that case, only more statistic could allow conclusions. It also remains to be explained what caused the delay in the other measured gamma ray bursts.
Thus, stay tuned...
[Via Stefan via Lubos].