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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.

Ronan Tarik Drevon
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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.

Peter Diehr
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  • That's a good information ! "Propagation of a Gaussian Light Pulse through an Anomalous Dispersion Medium" gives the form of the pulse as a function of time at different points in space. Distorsion is seen as pulse narrowing and broadening depending on super/sub luminal regime. I presume such effects may also arise in the context of non linear optics .. – Ronan Tarik Drevon Feb 29 '16 at 11:43
  • Nonlinear optics is a broad field, of which I use a small part; ask a specific question andd – Peter Diehr Feb 29 '16 at 12:20