If you were to spin a ball, for example, the texture of the ball’s surface would make it easy to see the ball is rotating. But what if the ball were perfectly smooth?
A Matter of Principle
Imagine taking a bucket of water and spinning it. As you rotate the bucket, the water would fling outward a bit, so that the surface of the water is concave. Compare that to a bucket of water that isn’t spinning, so that the water is flat. If both buckets are perfectly smooth and symmetrical, and the water is perfectly calm, the only difference you would see is that one bucket has concave water and the other has flat. But why are they different?