Mach didn’t believe in “absolute space”, that space had an independent existence from the objects in it. He believed that space was merely a relational quantity between objects. However this was contradicted in classical physics, as newton showed, because for example when you have a situation like the stars spinning around you and you are standing on the ground stationary relative to those stars, there is no known physical mechanism that could make feel a centrifugal force. In the situation where you are spinning in a circle and the stars are stationary relative to you, your arms will be pulled out to your side. Clearly in classical mechanics, the two situations are not physically equivalent. Newton used this to prove that absolute space existed, which things accelerate relative to. Mach disagreed with newtons conclusion of the thought experiment, instead suggesting that the inertia of one object is determined by every other object in the universe (i.e, the stars in this example), and that provided some sort of vague physical explanation for the existence of an additional centrifugal force on you if the stars were spinning and you were stationary. Einstein interpreted Mach’s principle to be that the Stars induce a “force” on you, in relation to the mass-energy content of the stars, which affected inertial motion. This lead to Einstein’s theory of general relativity. However, General Relativity doesn’t exactly conform to Mach’s Principle as far as i’m aware. And in a modern sense, Mach’s principle can be seen as a stunted classical version of the holographic principle.
