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Comments on Ultra-High-Energy Photons and D-Foam models

We revisit the constraints that the non-observation of ultra-high-energy photons due to the GZK cutoff can impose on models of Lorentz violation in photon propagation, following recent work by Maccione, Liberati and Sigl [arXiv:1003.5468] that carries further an earlier analysis by the present authors (Phys. Rev. D 63,12402 (2001), [hep-th/0012216]). We argue that the GZK cutoff constraint is naturally evaded in the D-brane model of space-time foam presented recently by the present authors (Phys. Lett. B 665, 412 (2008) [arXiv:0804.3566]), in which Lorentz-violating effects on photon propagation are independent of possible effects during interactions. We also note a novel absorption mechanism that could provide a GZK-like cutoff for photons in low-scale string models.