  In the past few decades, several models have predicted an energy-dependence
of the speed of light in the context of quantum gravity. For cosmological
sources such as active galaxies, this minuscule effect can add up to measurable
photon-energy dependent time lags. In this paper a search for such time lags
during the H.E.S.S. observations of the exceptional very high energy flare of
the active galaxy PKS 2155-304 on 28 July in 2006 is presented. Since no
significant time lag is found, lower limits on the energy scale of speed of
light modifications are derived.
