QED between parallel mirrors: light signals faster than c, or amplified by the vacuum

Abstract

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 ). An earlier low-frequency result that n(0)4, i.e. that n( infinity )=n(0) up to corrections of order e⁶ at most.