I’m extracting here a side discussion on the application of Preferred Reference Frames from a thread on the twins paradox, so that thread does not seem to get “hijacked” by this discussion since the connection may seem thin to some. IMO the preferred frame interpretations as used in Astrophysics in just as accurate with a universally uniform observed physics in all reference frames for solving the twins paradox as is SR alone. The fact that SR defines a Preferred Frame as indefinable only applies to within the use of SR. Astrophysics is much more than SR and more than just the addition of GR with a wealth of observations to apply to the theories to establish the modern science of Astrophysics. But the astrophysical community would never expect two simultaneous events would appear simultaneous in other inertial frames. The point of an absolute (local) frame of reference is to resolve the difference between reference frames as to which one is correct, or at least nearest to correct, as having the fastest relative rate of time and longest relative length of distance, while all other reference frames are slower and shorter in those respects. The way they maintain a uniform physics between all reference frames is from the ability of each reference frame to establish just how far off the local preferred their own frame of reference is (while maintaining a local physics that does behave and look same from as in any other frame). Astrophysics does this with our own frame of reference by identifying our Earth and Solar system frame as being in motion against the local preferred frame as measured by CMBR observations. Likewise even with our motion against the local preferred frame, through careful observation of light from distant space we to know that light from a Mega Parsec away was generated simultaneous with specific time in our past. That info combined with being able to see that the local preferred frame at a Mega Parsec distance is in motion wrt our own local preferred frame by about 70 km/sec is how Hubble established the Hubble Expansion of space. Any scientists that claims no theories can establish or use a preferred reference frame must by definition reject the Hubble expansion of space conclusions made by Astrophysics. And you don’t find many physicists of any type anywhere that reject Hubble Expansion. It is just that SR standing alone cannot establish preferred frames.