system
Given the following post, respond as if you're commenting on Reddit:
human
How do we define what is rotating in the universe?

In relativity, we find that if a moving observer would look at the distant stars and it would seem as if the he is stationary and the stars are moving about him/her, it does not matter whether it is the observer moving or the universe is moving past him, they would experience the same relativistic effect, however, if there was an object spinning in space, there are 2 ways of looking at it. Either the object is spinning, or the observer and the entire universe is rotating around the object. So which inertial frame of reference is the right one or does it not matter? also since rotating bodies experience centripetal force, would the spinning body experience the centripetal force or would the observer experience it?
gpt
You kind of answered your own question: pseudo forces appear in accelerated systems.  


Rotation is an accelerated movement and acceleration is Lorentz invariant. That means, all observers in every inertial (un-accelerated) frame of reference measure the same acceleration of a given system.

Thus, there will always be an experiment that tells you whether or not you are being accelerated at a time.