I am confused about Einstein's thinking. I understand when he formulated his general theory of relativity, he wanted to incorporate 3 foundations for his theory: The relativity principle, the equivalence principle, and Mach's Principle. He believed that inertia and weight were essentially the same phenomenon, that inertia was a function of the surrounding mass of the universe. Spin a rock tied to a string, he believed the centrifugal force tugging on your fingers was essentially a gravitational effect caused by the rotation of the rock relative to the cosmic mass of the univers. Here is my confusion. All of his professional life he insisted that nothing-- light, physical causality,etc. could ever exceed the speed of light. So why would he even attempt to implement Mach's principle, knowing that no effect, wave, field,etc. could travel faster than c?