  The interactions of different particle species with the foamy space-time
fluctuations expected in quantum gravity theories may not be universal, in
which case different types of energetic particles may violate Lorentz
invariance by varying amounts, violating the equivalence principle. We
illustrate this possibility in two different models of space-time foam based on
D-particle fluctuations in either flat Minkowski space or a stack of
intersecting D-branes. Both models suggest that Lorentz invariance could be
violated for energetic particles that do not carry conserved charges, such as
photons, whereas charged particles such electrons would propagate in a
Lorentz-inavariant way. The D-brane model further suggests that gluon
propagation might violate Lorentz invariance, but not neutrinos. We argue that
these conclusions hold at both the tree (lowest-genus) and loop (higher-genus)
levels, and discuss their implications for the phenomenology of quantum
gravity.
