QED BETWEEN PARALLEL MIRRORS - LIGHT SIGNALS FASTER THAN C, OR AMPLIFIED BY THE VACUUM

Because it is scattered by the zero-point oscillations of the quantized fields, light of frequency omega travelling normally to two parallel mirrors experiences the vacuum between them as a dispersive medium with refractive index n(omega). Our earlier low-frequency result that n(0) < 1 is combined with the Kramers-Kronig dispersion relation for n and with the classic Sommerfeld-Brillouin argument to show (under certain physically reasonable assumptions) that either n(infinity) < 1, in which case the signal velocity c/n(infinity) exceeds c; or that the imaginary part of n is negative at least for some ranges of frequency, in which case the vacuum between the mirrors fails to respond to a light probe like a normal passive medium. Further, the optical theorem suggests that n exhibits no dispersion to order e4, i.e. that n(infinity) = n (0) up to corrections of order e6 at most.