The Lorentz Transformation Sign Ambiguity and Its Relation to Measured   Faster-than-c Photon Speeds
  It is shown that time intervals Delta t' measured for photons moving with speed v > c can be of the same sign for all observers according to special relativity, thereby avoiding any violation of Einstein causality. Previous assertions to the contrary have led to unnecessarily complicated interpretations of experiments which indicate that single photons do travel with faster-than-c speeds in regions of anomalous dispersion 
Robert J. Buenker