  We present a microscopic model for light-cone fluctuations ``in vacuo'',
which incorporates a treatment of quantum-gravitational recoil effects induced
by energetic particles. Treating defects in space-time as solitons in string
theory, we derive an energy-dependent refractive index and a stochastic spread
in the arrival times of mono-energetic photons due to quantum diffusion through
space-time foam, as found previously using an effective Born-Infeld action.
Distant astrophysical sources provide sensitive tests of these possible
quantum-gravitational phenomena.
