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Various experiments have shown superluminal group and signal velocitiesrecently. Experiments were essentials carried out with microwave tunnelling,with frustrated total internal reflection, and with gain-assisted anomalousdispersion. According to text books a superluminal signal velocity violatesEinstein causality implying that cause and effect can be changed and timemachines known from science fiction could be constructed. This naive analysis,however, assumes a signal to be a point in the time dimension neglecting itsfinite duration. A signal is not presented by a point nor by its front, but byits total length. On the other hand a signal energy is finite thus itsfrequency band is limited, the latter is a fundamental physical property inconsequence of field quantization with quantum $h \nu$. All superluminalexperiments have been carried out with rather narrow frequency bands. Thenarrow band width is a condition sine qua non to avoid pulse reshaping of thesignal due to the dispersion relation of the tunnelling barrier or of theexcited gas, respectively. In consequence of the narrow frequency band widththe time duration of the signal is long so that causality is preserved.However, superluminal signal velocity shortens the otherwise luminal time spanbetween cause and effect.Comment: 5 pages, 3 figure

From superluminal velocity to time machines?