  In quantum theory the vacuum is defined as a state of minimum energy that is
devoid of particles but still not completely empty. It is perhaps more
surprising that its definition depends on the geometry of the system and on the
trajectory of an observer through space-time. Along these lines we investigate
the case of an atom flying at constant velocity near a planar surface. Using
general concepts of statistical mechanics it is shown that the motion-modified
interaction with the electromagnetic vacuum is formally equivalent to the
interaction with a thermal field having an effective temperature determined by
the atom's velocity and distance from the surface. This result suggests new
ways to experimentally investigate the properties of the quantum vacuum in
non-equilibrium systems and effects such as quantum friction.
