In physics, the concept of absolute rotation—rotation independent of any external reference—is a topic of debate about relativity, cosmology, and the nature of physical laws. For the concept of absolute rotation to be scientifically meaningful, it must be measurable. In other words, can an observer distinguish between the rotation of an observed object and their own rotation? Newton suggested two experiments to resolve this problem. One is the effects of centrifugal force upon the shape of the surface of water rotating in a bucket, equivalent to the phenomenon of rotational gravity used in proposals for human spaceflight.The second is the effect of centrifugal force upon the tension in a string joining two spheres rotating about their center of mass.