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We analyze the propagation of a pulse across a vacuum gap separating oppositeflat parallel faces of two transparent dielectrics by means of an explicitlycausal and retarded propagator constructed directly from the free-space waveequation. Nevertheless, our approach yields apparently superluminal propagationfor the case of frustrated total internal reflection (FTIR), that is, atransmitted wave packet appears on the far side of the gap at the same timethat the corresponding incident packet crosses the front one. Thus, in thisexample superluminality is just an illusion, being consistent with bothcasuality and classical electrodynamics. We study the origin of the apparentsuperluminality in this case, which is inherent to light pulse propagation infree space and does not depend on the particulars of light-matter interaction,and find that it is due to propagation from the lateral wings of the incidentpulse to the central part of the transmitted pulse. Thus, notwithstanding theirsimilarities, FTIR is not equivalent to 1D tunneling. We propose experiments totest our explanation of superluminality using opaque screens to block part ofthe wavefront, although we demonstrate that the propagation of smooth finitepulses constrained to be made up completely of evanescent Fourier components isindistinguishable from truly superluminal propagation, i.e., it may becompletely accounted for using an explicitely superluminal and acausalpropagator as well as the causal subluminal one.

Frustrated total internal reflection and the illusion of superluminal propagation