Superluminal Signals: Causal Loop Paradoxes Revisited
  Recent results demonstrating superluminal group velocities and tachyonic dispersion relations reopen the question of superluminal signals and causal loop paradoxes. The sense in which superluminal signals are permitted is explained in terms of pulse reshaping, and the self-consistent behavior which prevents causal loop paradoxes is illustrated by an explicit example. 
J. C. Garrison, M. W. Mitchell, R. Y. Chiao and E. L. Bolda