This is a great video! However it is also true that the GROUP velocity (not the phase velocity) can be faster than c. This phenomenon is common in nonlinear media or media with gain, where the front of a wavepacket can get amplified and the back can get attenuated, producing an effective group velocity > c. I am not sure whether there is a conservation law there, like when vp > c, vg< c, but I don’t see why there would be.
In any case, in optics we actually define a third velocity, signal velocity, which REALLY can’t be faster than c. I’ve never actually seen someone calculate this in practice though.
This is a great video! However it is also true that the GROUP velocity (not the phase velocity) can be faster than c. This phenomenon is common in nonlinear media or media with gain, where the front of a wavepacket can get amplified and the back can get attenuated, producing an effective group velocity > c. I am not sure whether there is a conservation law there, like when vp > c, vg< c, but I don’t see why there would be.
In any case, in optics we actually define a third velocity, signal velocity, which REALLY can’t be faster than c. I’ve never actually seen someone calculate this in practice though.