Mach's principle

Mach's principle is the proposed idea that local inertial frames are determined by the distribution and motion of matter throughout the universe. Its precise formulation and role in physical theory remain disputed.

Inertial frame: A reference frame in which unforced objects move at constant velocity and Newton's laws take their simplest form. Mach's principle asks what determines these frames rather than treating them as independent background structures.
Ernst Mach: An Austrian physicist and philosopher who criticized absolute space and analyzed the foundations of mechanics. His critique supplied the ideas Einstein later associated with Mach's principle.
Absolute space: Newton's proposed fixed spatial structure against which motion and rotation can be defined. Mach rejected absolute space as the ultimate explanation of inertia.
Gödel metric: An exact solution of Einstein's equations describing a rotating universe with closed timelike curves. Its rotating cosmology challenges simple claims that cosmic matter uniquely fixes inertial frames.
Newton's bucket argument: A thought experiment in which water's curved surface persists when a rotating bucket moves relative to the distant stars. It crystallizes the dispute over whether rotation is defined relative to absolute space or cosmic matter.
Isaac Newton: The English mathematician and physicist who formulated classical mechanics and the concept of absolute space. Newton's account of rotation provided the target of Mach's criticism.
Newtonian mechanics: The classical framework describing motion through forces, mass, and absolute time and space. Its standard formulation permits inertial frames without deriving them from the universe's matter.
Einstein–Cartan theory: A gravity theory extending general relativity by allowing spacetime torsion coupled to intrinsic spin. It offers an alternative geometric setting for investigating links between matter and inertial structure.
Frame-dragging: The relativistic effect in which rotating mass-energy influences nearby inertial frames. It is a measurable effect often associated with Machian ideas, though it does not establish the principle in full.
Albert Einstein: The physicist who developed special and general relativity and connected gravity with spacetime geometry. Einstein drew inspiration from Mach while developing general relativity, though their views later diverged.

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