  A very small dispersion in the speed of light may be observable in Fermi
time- and energy-tagged data on variable sources, such as gamma-ray bursts
(GRB) and active galactic nuclei (AGN). We describe a method to compute the
size of this effect by applying the Feynman sum-over-histories formalism for
relativistic quantum electrodynamics to a discrete model of space-time called
\emph{causal set theory}.
