G. Barton, K. Scharnhorst: QED between parallel mirrors: Light signals faster than c, or amplified by the vacuum. Journal of Physics A: Mathematical and General 26(1993)2037-2046 (DOI: 10.1088/0305-4470/26/8/024). An abstract of the article is also printed in: D. Robaschik (Ed.): Proceedings of the Second Workshop on "Quantum Field Theory under the Influence of External Conditions" held at University of Leipzig, September 14 to September 20, 1992. University of Leipzig, Naturwissenschaftlich-Theoretisches Zentrum, Leipzig, 1992, p. 113. [SPIRES record]
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). 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(∞) < 1, in which case the signal velocity c/n(∞) 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(∞) = n(0) up to corrections of order e6 at most.