A century after Ernst Mach and Albert Einstein cast doubt on absolute space, we still don’t know how a gyroscope stays pointed in a fixed direction.
As physicist Erwin Schrödinger wrote, however, in a striking 1925 article on Mach’s Principle, “…every naïve person has to ask: With respect to what, according to the theory, does the orbital ellipse perform this precession, which according to experience takes place with respect to the average system of the fixed stars?” In calculating the effects of curved space around the Sun, Einstein needed to assume that at large distances from the Sun, spacetime becomes flat—and absolute. In other words, he had to impose by fiat “boundary conditions” at infinity to complete his solution. General relativity, in and of itself, did not entirely determine the precession of Mercury’s orbit.
Why does a gyroscope point in a direction fixed relative to distant quasars?