Absolute rotation

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. The second is the effect of centrifugal force upon the tension in a string joining two spheres rotating about their center of mass.

A related third example, given by Albert Einstein in the development of general relativity, is a rotating elastic sphere. Like a rotating planet bulging at the equator, the sphere deforms into a squashed spheroid depending on its rotation. Explaining this deformation requires external causes in a frame of reference in the spheroid it is not rotating, and these external causes may be taken as "absolute rotation" in classical physics and special relativity. In general relativity no external causes are invoked. The rotation is relative to the local geodesics, and since the local geodesics eventually channel information from the distant stars, there appears to be absolute rotation relative to these stars.

In theoretical physics, particularly in discussions of gravitation theories, Mach's principle is the name given by Einstein to a hypothesis often credited to the physicist and philosopher Ernst Mach.

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