Quote by jtbell One way to re-phrase this is: without "looking" at the CMB itself, it is (as far as I know) impossible to decide whether we are moving with respect to the CMB. This is similar to the (hopefully well-known) fact that if we are in a train moving with constant speed along a perfectly straight, level, smooth track, with silent engines, and with shuttered windows, we cannot tell (without being able to "look outside") whether we are moving or not.
Naturally I know about the train, and the nature of inertial motion.

The difference here is that there is no "outside" to look at, is there? There's no outside the CMB.

What is bothering me is that if you go out to interstellar (or better intergalactic) space, it sounds like you can find an inertial trajectory (v0) in which there is no red/blue shifting of the CMB. Why is this not "at rest relative to the universe"?

If it is, then, using inertial gyros, you should be able to track the changes made to match the inertial motion any other body and thereby provide a speed relative to v0, which is therefore a speed relative to the universe. A problem here is that you can accelerate forever, which would give you ridiculous speeds (but sensible rapidities), but since you are only manoeuvering to match other inertial motions, I doubt it would be necessary to accelerate forever.

Once you have matched the inertial motion of another body, since you know what changes you made (by means of inertial gyros/accelerometers), should you not then be able to say what sort of red/blue shift that other body should experience when viewing the CMB? That should be testable, shouldn't it?

Or, if one observer went to intergalactic space on the other side of the Milky Way and one to intergalactic space on this side, would they both find different speeds at which the CMB has no red/blue shift (ie after finding themselves at rest with respect to the CMB, they would then find that they are not at rest with respect to each other)? Alternatively, if one observer went out to intergalactic space found and maintained a (safe) trajectory which was at rest with respect to the CMB and went into cryonic suspension for a billion years, would that observer on revival still find that the trajectory is at rest with respect to the CMB?

What I am getting at is there is on one hand the statement "Relativity principles forbid the existence of (absolute) frames" from xepma and most of those interested in relativity, and on the other hand "in some sense, the cosmic microwave backgrond (CMB) does the provide an absolute cosmological frame" from George Jones (who gets extra points for being an expert in the area).

Is this reconciled by considering jtbell's comment "When we talk about "absolute motion" it's usually in the context of assuming a special (inertial) reference frame in which the laws of physics take a special, particularly simple form" - in other words, are we permitted to have an "absolute" rest frame (meaning at rest with respect to the universe) so long as that is not meant to imply that there are special forms of the laws of physics applicable in that frame? Or, is the "absolute" rest frame absolutely forbidden?

(I think that if the intergalactic observers found that they could both be at rest with respect to CMB while not being at rest with respect to each other or at rest at one time and later not at rest even while maintaining the same inertial trajectory, then that would imply that an "absolute" rest frame is forbidden not just by scholars, but also the universe.)

cheers,

neopolitan