  The phenomenon of parametric down conversion from the vacuum may be
understood as a process in classical electrodynamics, in which a nonlinear
crystal couples the modes of the pumping field with those of the zeropoint, or
"vacuum" field. This is an entirely local theory of the phenomenon, in contrast
with the presently accepted nonlocal theory. The new theory predicts a hitherto
unsuspected phenomenon - parametric up conversion from the vacuum.
