  A test mass, $M$, moving through an ambient medium of light particles with
lower average kinetic energy than itself suffers a deceleration caused by its
scattering of the light particles. The phenomenon is usually referred to as
dynamical friction. The velocity, $\v$, of the test mass decays on a timescale
independent of $\v$ in the non-relativistic case. We derive expressions for
dynamical friction in the case that the test mass and the light particles are
relativistic, and that the scattering is weak (with impact parameter, $b\gg
M$). In the case that the light particles are ultra-relativistic, and isotropic
in the frame in which $M$ moves with velocity $v$, we find an explicit
expression for the dynamical friction. The well known factor of 2 correcting
the Newtonian scattering of photons to give the Einstein angle, $4M/b$, has the
largest effect on the resulting friction, which is modified by a factor of
roughly $16 / 3\gamma_v$ over the simple non-relativistic case. In the
non-relativistic case, the largest contribution to the friction comes from
light particles moving slower than $v$. We find that this is not the case for
ultra-relativistic scattering, essentially because the scattering angle is
independent of $\v$. Some astrophysical implications are discussed. (Accepted
for publication in Monthly Notices.)
