Recent theories of quantum gravity have suggested that the speed of light could vary slightly with the energy of the photons involved, thus violating the Lorentz invariance that underlies the special theory of relativity. Now observations of photons from a gamma-ray burst by the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM) on the Fermi Gamma-ray Space Telescope have set stringent limits on such effects.
Fermi observed GRB 090510 on 10 May 2009, when it triggered both the LAT and the GBM, and photons detected by the LAT included one with an energy as high as 31 GeV. Given the large distance to GRB 090510 (around 7000 million light-years), this made it possible to search for small variations in the photons' speed by comparing arrival times over a wide energy range starting at around 100 MeV.
A A Abdo and colleagues used the observations to show that linear variations in the speed of light with energy are ruled out at 1.2 times the Planck scale (EPlanck = 1019 GeV). On the negative side this is a remarkable constraint on quantum gravity theories, while on the positive side it suggests that quadratic effects could be testable in similar observations.