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**:8(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.
[INSPIRE 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 e^{4}, i.e. that
n(∞) = n(0) up to corrections of order e^{6} at most.

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