Acuasal Behavior in Quantum Electrodynamics
  Acausal features of quantum electrodynamic processes are discussed. While these processes are not present for the classical electrodynamic theory, in the quantum electrodynamic theory, acausal processes are well known to exist. For example, any Feynman diagram with a ``loop'' in space-time describes a ``particle'' which may move forward in time or backward in time or in space-like directions. The engineering problems involved in experimentally testing such causality violations on a macroscopic scale are explored. 
A. Widom, Y. N. Srivastava, E. Sassaroli