I was asked by a reviewer from outside this forum for a clarification of how the theory could be verified.
In Special Relativity, light travels at c with respect to both the emitter and receiver.聽 (Space and time require redefinition if the mathematics is to work out).聽 However in a single Newtonian-Galilean coordinate system containing both emitter and receiver, if we were to suppose that light is emitted at c with respect to the emitter then the velocity of the electromagnetic wave must be c + v where v is the relative velocity between the particles. The receiving particle would encounter radiation moving at that speed.聽 However the effect of dispersion shifts the wavelength and frequency after the encounter with the receiving particle in such a manner that the frequency and wavelength are equivalent of what is calculated within Special Relativity.
The dispersion process has a bit of time lag due to the mass of the receiving particle. A very tiny bit of the unshifted EM wave slips through unmodified.聽 In other words, the receiving particle only starts to move in reaction to new field fluctuations as the wavefront passes by the particle. The motion of the receiving particle starts to produce new field fluctuations that add to the impinging radiation from the original radiating particle but a small amount of the original wave has already raced out ahead of mixed wave.
Sommerfeld and Brillouin called that very short signal a precursor or forerunner. The problem with detecting such a signal is that any particle with the same mass as the receiving particle that is interacting with the unmodified wave from a vacuum will not be able to register any measurement effects. A measurement of precursors would seem to require a measurement of the movement of particles with less mass where they lie behind the particle which generates the primary dispersive effects.
Steve