Hi CharlesEGrant

The rules of relativity allow any reference frame to describe the same physics, ie they are invariant. But this “invariance” idea is a concept that is carried too far, typically is used to show that there is no “absolute” reference frame.

However, absolute reference frames are indicated in nature. I would not call the cosmic microwave background an arbitrary reference frame (or what Mach or others call the reference frame of the background of stars).

Similarly consider a variation of the twin “paradox” , but this time all we see are two space ships passing by each other. It is only by the passage of time that it is revealed that one of the twins in one of the ships is aging slower than the other. But from what reference frame are we determining who ages faster? Lurking in the background of relativity is the idea of an ‘absolute’ or “preferred” reference frame.

John