From Superluminal Velocity To Time Machines?
  Various experiments have shown superluminal group and signal velocities recently. Experiments were essentials carried out with microwave tunnelling, with frustrated total internal reflection, and with gain-assisted anomalous dispersion. According to text books a superluminal signal velocity violates Einstein causality implying that cause and effect can be changed and time machines known from science fiction could be constructed. This naive analysis, however, assumes a signal to be a point in the time dimension neglecting its finite duration. A signal is not presented by a point nor by its front, but by its total length. On the other hand a signal energy is finite thus its frequency band is limited, the latter is a fundamental physical property in consequence of field quantization with quantum $h \nu$. All superluminal experiments have been carried out with rather narrow frequency bands. The narrow band width is a condition sine qua non to avoid pulse reshaping of the signal due to the dispersion relation of the tunnelling barrier or of the excited gas, respectively. In consequence of the narrow frequency band width the time duration of the signal is long so that causality is preserved. However, superluminal signal velocity shortens the otherwise luminal time span between cause and effect. 
G. Nimtz, A.A. Stahlhofen, A. Haibel