  One of the cornerstones of special relativity is the postulate that all
observers measure exactly the same photon speeds independently on the photon
energies. However, a hypothesized structure of spacetime may alter this
conclusion at ultra-small length scales. Even a tiny energy-dependent variation
in the speed of light may be revealed, when accumulated over cosmological
light-travel times, by high temporal-resolution measurements of sharp features
in Gamma-Ray Burst lightcurves. We report the results of a study of the
emission from GRB 090510 as detected by Fermi's LAT and GBM instruments that
set unprecedented limits on the dependence of the speed of light on its energy.
