Current constraints on photon velocity variability are summarized and displayed in terms of an energy-dependent vacuum refraction index. It is shown that the energy-momentum balance of high energy Compton scattering is very sensitive to the outgoing photon speed. A missing energy observation in HERA Compton polarimeter data indicates that photons with 12.7 GeV energy are moving faster than light by 5.1(1.4) mm/s. An asymmetry spectrum measured by the SLC longitudinal polarimeter implies however an effect which is 42 times smaller, although the interpretation of the data is less clear here. (c) 2005 Published by Elsevier B.V.
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