Mach's principle

Summary

Mach's principle is the idea that the inertia of a body is determined by all the matter in the universe, not by an absolute space. The same should hold for the distinction between rotating and non-rotating frames. It is named after Ernst Mach, who criticised Newton's notion of absolute space, and the name was given by Einstein. It guided Einstein towards general relativity, but it has no single precise formulation, and general relativity satisfies it only in part. Frame dragging, where a rotating mass makes nearby inertial frames rotate, is often seen as a Machian effect of general relativity.