Mach's principle

Mach's principle (mäks) [for E. Mach], assertion that the inertial effects of mass are not innate in a body, but arise from its relation to the totality of all other masses, i.e., to the universe as a whole. Thus, the inertial forces experienced by a body in accelerated motion have the same physical origin as the gravitational forces it experiences near mass concentrations, namely the mass-energy field described by the general theory of relativity. Inertial forces have a much longer range than gravitational forces, so the role of very distant matter becomes preponderant. According to Mach's principle, a body experiences no inertial forces when it is at rest or in uniform motion with respect to the center of mass of the entire universe. When its motion is nonuniform (accelerated) with respect to the total mass of the universe, it experiences forces such as centrifugal force (see centripetal force and centrifugal force) and the Coriolis effect. Hence, the "local" behavior of matter is influenced by the "global" properties of the universe, i.e., those properties that describe the universe as a whole, which are studied in cosmology.

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Mach's principle

Hypothesis that the inertial forces acting on a body in accelerated motion are determined by the quantity and distribution of matter in the universe. Albert Einstein found its suggested connection between geometry and matter helpful in formulating his theory of general relativity; unaware that George Berkeley had proposed similar views in the 18th century, he attributed the idea to Ernst Mach. He abandoned the principle when he realized that inertia is assumed in the geodesic equation of motion (see geodesy) and need not depend on the existence of matter elsewhere in the universe.