# What defines stationary vs spinning in space?

To produce artificial gravity on a space station, we simply spin it around some central point, and the acceleration makes objects "fall" outwards. If our space station is so far in deep space that it cannot detect any light from stars whatsoever, we can still tell if we are spinning or not based on the artificial gravity produced.

Why? What are we spinning relative to? If I were to take a stationary space station and spin everything else in the universe around it, would it produce the same effect?

Or to put it another way, suppose I took all of the matter in the Universe and made it into a gigantic disk. Would it be possible to spin the disk? What would it be spinning relative to?

• Read up on Newton's bucket argument, and Mach's principle. – James K Nov 28 '17 at 20:53
• you are spinning relative to the axis of rotation – Kevin Milner Nov 28 '17 at 22:39
• @KevinMilner That's not a good answer because it assumes the axis of rotation is part of some sort of "fixed" frame of reference, but then you can ask what that frame of reference is fixed with respect to. – zephyr Nov 29 '17 at 15:07
• I'm voting to close this question as off-topic because this is basic physics – Carl Witthoft Nov 29 '17 at 18:44

Issac Newton described an experiment in which a bucket containing water is spun. As the water in the bucket starts to rotate, it becomes concave. The reason for this can be understood in terms of rotating frames of reference.

Newton supposed (as an axiom of mechanics) that there are frames of reference (a system of locating particles in space relative to an origin, and in time) in which his 3 laws of motion are true, and momentum is conserved. Such a frame is called "inertial". If one inertial frame exists then any frame with an origin that moves at constant velocity relative to the inertial frame is also inertial.

As I mentioned, the existence of inertial frames is an axiom of Mechanics. The truth of the axiom is verified by observation. It can't be proved. If a frame of reference is rotation with respect to an inertial frame, then the rotating frame is not inertial, and Newton's laws (in their simplest form don't hold). Instead of "F=ma" there are extra terms for the Coriolis and centrifugal force.

In the case of a spinning space station, it is spinning relative to an inertial frame, and that is why there is a centrifugal force.

For Newton, the bucket experiment proves that there is a notion of "absolute space". This interpretation is not accepted by all, and in particular, Ernst Mach rejected the idea of any absolute space. His ideas were influential on Einstein.

You can read more about Newton's bucket experiment at http://www-history.mcs.st-andrews.ac.uk/HistTopics/Newton_bucket.html and you can read about Ernst Mach's ideas regarding absolute space at https://en.wikipedia.org/wiki/Mach%27s_principle

Unlike linear motion, rotation is relative to absolute space. However, near a large rotating body, like a black hole, frame dragging would have a relative effect.