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The time delays between gamma-rays of different energies from extragalacticsources have often been used to probe quantum gravity models in which Lorentzsymmetry is violated. It has been claimed that these time delays can beexplained by or at least put the strongest available constraints on quantumgravity scenarios that cannot be cast within an effective field theoryframework, such as the space-time foam, D-brane model. Here we show that thismodel would predict too many photons in the ultra-high energy cosmic ray fluxto be consistent with observations. The resulting constraints on the space-timefoam model are much stronger than limits from time delays and allow for Lorentzviolations effects way too small for explaining the observed time delays.

Ultra high energy photons as probes of Lorentz symmetry violations in stringy space-time foam models