On apparent faster-than-light behavior of moving electric fields

Abstract

For every observer, however distant, the electric field of a uniformly moving charge is always directed away from, or points towards, the instantaneous present position of the charge and not away from, or towards, the retarded position at which the observer sees it (due to the finite speed of light). This fact is a well-established consequence of, among others, the application of the Liénard-Wiechert potentials, and its significance for fundamental physics is probably not fully appreciated. Here we show how and why this property has non-negligible consequences for what we take for granted about the relativity of simultaneity and faster-than-light communication. In particular, if we consider two opposite electric charges whose distance shrinks to zero at a constant velocity (shrinking electric dipole), then the cancellation of the total field seems to be instantaneous everywhere in space and in every inertial reference frame. A simple variant of the shrinking electric dipole setup appears to allow a sort of faster-than-light communication of information. Our results provide simple theoretical support to the conclusions of recent experiments on the propagation speed of Coulomb and magnetic fields. It would also be interesting to explore any possible connection between our findings and quantum non-locality.