Mach's Principle

CORE DEFINITION

In theoretical physics, particularly in discussions of gravitational theory, Mach's Principle (or Mach's conjecture) is the name Albert Einstein gave to an imprecise hypothesis often attributed to the physicist and philosopher Ernst Mach. The hypothesis attempts to explain how rotating objects, such as gyroscopes and rotating celestial bodies, maintain a frame of reference. The proposition holds that the existence of absolute rotation (the distinction between local inertial frames and rotating reference frames) is determined by the large-scale distribution of matter, as illustrated by the following example: You stand in a field looking up at the starry sky. Your arms hang naturally at your sides, and you see the distant stars motionless. Now you begin to spin. The stars whirl around you, and your arms move away from your body. Why do your arms pull away when the stars rotate? Why do your arms hang freely when the stars are still? [a] Mach's Principle states that this is not a coincidence—there is a physical law that links the motion of distant stars to local inertial frames. If you see all the stars rotating around you, Mach's Principle suggests that there is some physical law that causes you to feel centrifugal force.