  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.
