"In the present paper, we adopt the view that these calculations, taken together, actually imply that the Scharnhorst effect is physical, and not an artifact of some approximation: it appears that quantum-polarization can induce true superluminal velocities, albeit well outside the realm of present day experimental techniques. ...
Between conducting plates, or in a background gravitational field, light does not propagate at the usual speed, simply because the boundaries (or the background) single out a preferred rest frame, which shows up in the property of the quantum vacuum of not being Lorentz-invariant. Thus, light behaves in a non-Lorentz-invariant way only because the ground state of the electromagnetic field is not Lorentz-invariant. The Euler– Heisenberg Lagrangian, from which the existence of the effect can be deduced, as well as all the machinery of QED employed in its derivation, are still fully Lorentz-invariant. For this reason, one often speaks of a “soft breaking” of Lorentz invariance, in order to distinguish from a situation in which also the basic equations, and not just the ground state, are no longer Lorentz-invariant.
1Hereafter, by “superluminal” we always mean “faster than light in unbounded empty space”, i.e., with speed larger than c = 2.99792458×108 m s−1. This terminology may produce some weird-sounding sentences, such as “light travels at a superluminal speed in the Casimir vacuum”, but it cannot lead to any misunderstanding if interpreted in the strictly technical sense described above.
2Following common usage, we identify the “signal velocity” with the phase velocity at infinite frequencies, equivalent to the so-called “front velocity” [4]. See [7] for other possible definitions."
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