[Ended]

>Rotation is inherently about different particles moving at different velocities. This is why there is an absolute reference frame for it: it is defined by these differences, so by reducing them to zero and making every particle in the ball have the same velocity, we can reach the "absolute".

Imagine the ball is floating in space, and you are watching things through a camera fixed to the ball. To you, the two dots will always appear stationary with zero relative velocity. So there is no way to determine your absolute reference frame.

Now suppose you are watching through an external camera, and suppose you observe the dots having a relative velocity. Is the ball spinning, or is the camera orbiting around the ball? Again, there is no way to tell.

[lmm]

But if you were the camera you'd feel a force, or not, right? It's not like being in an elevator in free-fall, where you can't tell whether you're accelerating downwards or sitting still in flat space.

[Ended]

True, but the GP suggested that you can find the absolute reference frame by looking at the relative velocities of the dots (independent of any force measurements).

[philipov]

The elevator analogy breaks down if you're larger than a point-particle in a point-elevator. Gravity's force varies with distance, causing tidal forces on your body, allowing something large/sensitive enough to feel the difference. Gravity stretches you whereas uniform acceleration does not, and non-uniform acceleration compresses you.

In other words, you can determine that you're in a gravitational field by measuring the difference in force at different locations in the elevator.

[JoBrad]

How would you tell whether that force is gravitational or centrifugal?

[lmm]

If it were gravitational there would have to be a mass in the right place to cause it. Maybe you could look at how the force changed as you moved around?