The Quest for Quantum Gravity: Testing Times for Theories?
  I discuss some theoretical ideas concerning the representation of quantum gravity as a Lorentz-symmetry-violating `medium' with non-trivial optical properties, which include a refractive index in `vacuo' and stochastic effects associated with a spread in the arrival times of photons, growing linearly with the photon energy. Some of these properties may be experimentally detectable in future satellite facilities (e.g. GLAST or AMS), using as probes light from distant astrophysical sources such as gamma ray bursters. I also argue that such linear violations of Lorentz symmetry may not always be constrained by ultra-high-energy cosmic-ray data, as seems to be the case with a specific (stringy) model of space-time foam. 
N.E. Mavromatos