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The article says that group velocity is almost always what determines the signal velocity. While that is true, I think it would be better to clearly distinguish the three velocities phase, group and signal. After all, there are recent experiments which speed up the group velocity above c (without transmitting useful signals), and these will haunt us forever if we don't clarify the different speeds from the beginning. In fact, those experiments have a lot to do with refractive indices and their dependence on frequency, so maybe they should be mentioned on the main page. --AxelBoldt |
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