What causes the superluminal propagation of light pulses

摘要

In this paper, we discuss what causes the superluminal propagation of a pulse through dispersion by solving Maxwell's equations without any approximation. The coherence of the pulse plays an important role for superluminal propagation. When the pulse becomes partially coherent, the propagation changes from superluminal to subluiniual. The energy velocity is always less than the vacuum velocity. The shape of the pulse is changed during the propagation.