Arnold Sommerfeld

“ If I think of the number of waves as more and more enlarged, then I get a finite wave train that I can consider as a signal. An actually infinite wave is traveling with superluminal velocity (Sommerfeld: only the phase, not the energy) . If I have a very long wave train that is limited, then it should travel only with the speed of light and that is not clear to me. I want to ask after the propagation velocity of the wave that is in the middle. If the train is actually infinite in length, then it travels with superluminal velocity ”

“ In electrodynamics, which was founded by Einstein on the principle of relativity, superluminal velocities are impossible under any circumstances, both as convection velocity of a charge or as propagation velocity of an electrodynamic disturbance. ”

“ Now we shall see that it is exactly equal to c, if we think of the observer as equipped with an ideal, i.e. infinitely sensitive detector, no matter whether the medium is dispersive in a normal or anomalous way, whether it is isotropic or anisotropic, whether it contains conduction electrons or not. ”