But how would we know if all spatial dimensions were moving along a constant vector? We can't rely on redshit/blueshift if the medium of transmission itself is

But how would we know if all spatial dimensions were moving along a constant vector? We can't rely on redshit/blueshift if the medium of transmission itself is moving like everything else. The next question might be "moving in what, then?" but then why not abstractly ask the same about time.