  Unruh radiation is the thermal flux seen by an accelerated observer moving
through Minkowski spacetime. In this article we study Unruh radiation as
tunneling through a barrier. We use a WKB-like method to obtain the tunneling
rate and the temperature of the Unruh radiation. This derivation brings
together many topics into a single problem - classical mechanics, relativity,
relativistic field theory, quantum mechanics, thermodynamics and mathematical
physics. Moreover, this gravitational WKB method helps to highlight the
following subtle points: (i) the tunneling rate strictly should be written as
the closed path integral of the canonical momentum; (ii) for the case of the
gravitational WKB problem, there is a time-like contribution to the tunneling
rate arising from an imaginary change of the time coordinate upon crossing the
horizon. This temporal contribution to the tunneling rate has no analog in the
ordinary quantum mechanical WKB calculation.
