In some cases, according to Wikipedia, the envelope of a gaussian beam can go faster than speed of light hence leading to superluminal group velocity. However, the signal/energy still propagates at subluminal speed which is seen from the speed of the rising front of the pulse.
Do you know a practical example for which this situation arises? Is it possible to have a interactive picture of the corresponding wave?
I presume the pulse should distort quite significantly.
Answer
Superluminal group velocity can occur in near absorption peak, known as regions of anomalous dispersion. So-called "superluminal tunneling" experiments have bee conducted in thes regions, but when carefully analyzed there is no information transferred faster than light.
Some references are given here: https://www.rp-photonics.com/superluminal_transmission.html
I'm not familiar with any applications, but everything is good for something, certainly strong absorption lines are useful.
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