SUPERLUMINAL DELAYS OF COHERENT PULSES IN NONDISSIPATIVE MEDIA - A UNIVERSAL MECHANISM

We identify the universal mechanism that is responsible for superlumin al (faster-than-light) traversal times as well as the narrowing of wav e packets transmitted through various nondissipative media. This mecha nism is shown to be predominantly destructive interference between suc cessive wave-packet components traversing all accessible causally reta rded paths. It strongly depends on wave-packet coherence and width, an d can cause superluminal traversal not only in evanescent-wave ''tunne ling'' but also in allowed propagation.