What causes the surface of water in a rotating bucket to curve? According to Newton, nothing happens as long as only the bucket is spinning. Not until the water in the bucket is also spinning does its surface curve. Ernst Mach sounded a note of caution: “You can view it differently. Would the water surface also curve if the bucket were standing still, but the world around it were spinning, although the bucket is standing still in absolute space?” According to this view, the curvature of the water surface is a result of the masses of distant fixed stars and not of absolute space. It was this idea that played a major role for Einstein in his development of the general theory of relativity.