I recently got a copy of Purcell's very excellent Electricity and Magetism book. In the chapter about moving charge, he states without proof that the electric field due to a moving charge points at where the charge is now, as if there is instantaneous action at a distance. This is of course necessary for length contraction to work. After much digging about, I uncovered that this radial electric field, and a flattening of the same field (aka length contraction) is explained by the Lienard-Wiechert potentials, using retarded potentials. The question is, if retarded Coulomb potentials moving in a medium (some preferred frame) explain relativistic effects and also explains the magnetic field (as Purcell shows), why is it preferred to believe there is no medium propagating this information? Of course we cannot detect this absolute reference frame, but such a medium may help us understand the nature of matter. And is a medium not implied by general relativity (a naive question, as I'm not well versed on this). Thanks.