Visualizing the kinematics of relativistic wave packets
  This article investigates some solutions of the time-dependent free Dirac equation. Visualizations of these solutions immediately reveal strange phenomena that are caused by the interference of positive- and negative-energy waves. The effects discussed here include the Zitterbewegung, the opposite direction of momentum and velocity in negative-energy wave packets, and the superluminal propagation of the wave packet's local maxima. 
Bernd Thaller