  Real photon-photon scattering is a long-predicted phenomenon that is being
searched for in experiment in the form of a birefringent vacuum at optical and
X-ray frequencies. We present results of calculations and numerical simulations
for a scenario to measure this effect using multi-MeV photons generated in the
collision of electrons with a laser pulse. We find that the birefringence of
the vacuum should be measurable using experimental parameters attainable in the
near future.
