Impossibility of measuring faster–than–c signaling by the Scharnhorst effect

It is argued that the Scharnhorst effect cannot in principle give rise to measured signal velocities larger than that of light in vacuum. Scharnhorst [1] and subsequently Barton [2] (see also [3]) have shown that the refractive index of the vacuum between parallel plates can be less than unity for all frequencies ω < mc/h̄, where m is the electron mass [4]. This arises from the decreased polarizability of the vacuum between the plates via virtual electron–positron pair production, suggesting the possibility in principle of faster–than–c signal propagation. Along these lines one could also conceive of a decreased polarizability in the presence of nuclei with charges above the critical charge (Z ≥ Zcr ≈ 164–172), associated with the charged vacuum as ground state [5]. In the ∗Permanent address: Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, U.S.A.