Through what medium does EM propagate in "empty" space? Einstein stated in his Leyden address (1926, I think) that an EM ether was mandatory for the transmission of EM waves through "empty" space. He was unable to reconcile this with the dynamical gravitational ether that had to exist to make GR work, so he proposed an EM ether that had NO sensible properties. In other words, it could not possibly be subject to polarization, densification, or any other measurable variation, unlike all the other fields known to exist. This concept seems a little silly, since fields are known to exhibit variations and the variations must be explainable via physical laws. Is there anybody here that is willing to contemplate that the quantum vacuum might be this EM field - the palette upon which the Universe is written?