  An explanation for superluminal phenomena based on wave-particle duality of
photons is suggested. A single photon may be regarded as a wave packet, whose
spatial extension is its coherence volume. As a photon propagates as a wave
train in vacuum, its velocity is just the speed of light. When it tunnels
through a barrier as a particle, its wave function collapses and it will travel
faster than light. Superluminal motion can occur only within the coherence
length and the time constrained by uncertainty principle. A massive particle
cannot be superluminal during the tunneling process. So superluminality does
not violate causality. As for the superluminal and negative group velocities in
anomalously dispersive medium, they are merely reshaping effect of the pulse,
and they will become subluminal at large distances. A couple of experiments are
proposed to test the superluminal phenomena.
