  Early Wheeler-Feynman absorber theories invoke both retarded and advanced
electromagnetic waves for photon emission and absorption in order to remove
problems involving lack of radiative damping during electron acceleration.
Subsequent inquiries have suggested that only certain cosmologies would allow
such a retarded-advanced wave mechanism to exist. These include quasi-steady
state cosmologies and exclude flat, expanding Friedman-type cosmologies. Key to
the exclusion process is a diminishing density of future absorbers in an
ever-expanding universe. Such absorbers would be expected to be real
electromagnetically interacting particles. However future virtual absorber
sites, if they exist, would not be so diminished. Such sites would be plentiful
on the future light horizon, receding from the source at the speed of light.
The present treatment proposes that virtual absorption sites are present at
every point in spacetime, and are characterized by the Fresnel-Kirchhoff
diffraction integral. On the future light horizon, they can remove
electromagnetic energy from the local causal domain and provide advance wave
signals as Wheeler-Feynman absorbers.
