It's becoming a little more well-known in recent years that Einstein was in fact a strong advocate for a "new ether" from 1916 until his death. He denied the 19th Century version of the ether in his 1905 paper on SR, and spent about 11 years defending his banishment of the ether from physics. But in 1916, in a letter to Lorentz (a lifelong advocate of the ether concept), Einstein admitted that GR entailed a "new ether" that, at the least, imparted acceleration and rotation to ponderable mass. And Einstein remained a defender of the ether concept until his death, attempting to create a unified field theory that would unite GR and quantum mechanics in a relativistic field/ether (for Einstein, fields, ether and space, became somewhat synonymous). This interesting history is detailed in Ludwik Kostro's 2000 book Einstein and the Ether. My question is: if some concept of ether is required to, at the least, provide the basis for distinguishing bodies undergoing acceleration from those at rest - and thus allowing relativistic effects to be felt - isn't this ether also the basis for "absolute rest"? In other words, if the ether allows the distinction between different frames of reference, based on one frame undergoing acceleration, and another not, isn't there necessarily a "resting frame," that is absolute rest? If so, doesn't accepting even this non-physical concept of the ether, as Einstein did, undermine the principle of relativity, with its denial of absolute rest?