Lorentz violation for photons and ultrahigh-energy cosmic rays.

Lorentz symmetry breaking at very high energies may lead to photon dispersion relations of the form omega2=k2+xink2(k/MPl)n with new terms suppressed by a power n of the Planck mass MPl. We show that first and second order terms of size |xi1|>orsimilar10(-14) and xi2<orsimilar-10(-6), respectively, would lead to a photon component in cosmic rays above 10(19) eV that should already have been detected, if corresponding terms for e+/- are significantly smaller. This suggests that LI breaking suppressed up to second order in the Planck scale is unlikely to be phenomenologically viable for photons.