> the cosmic microwave background, which debatably provides an absolute rest frame It does no such thing. An "absolute rest frame" would be a rest frame that i

> the cosmic microwave background, which debatably provides an absolute rest frame It does no such thing. An "absolute rest frame" would be a rest frame that is inherent in the laws of physics, independent of the distribution of matter and energy. The CMBR only provides a "rest frame" in terms of its distribution of energy, and any piece of matter or energy will do that. For example, you, as a piece of matter, define a "rest frame"--does that mean your rest frame is absolute? Of course not.