Saturday 22 September 2018

electromagnetism - Front velocity and Superluminal group velocity


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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