  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.
