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In the past few decades, several models have predicted an energy-dependenceof the speed of light in the context of quantum gravity. For cosmologicalsources such as active galaxies, this minuscule effect can add up to measurablephoton-energy dependent time lags. In this paper a search for such time lagsduring the H.E.S.S. observations of the exceptional very high energy flare ofthe active galaxy PKS 2155-304 on 28 July in 2006 is presented. Since nosignificant time lag is found, lower limits on the energy scale of speed oflight modifications are derived.

Limits on an Energy Dependence of the Speed of Light from a Flare of the Active Galaxy PKS 2155-304