Mach, Newton and others observed that centrifugal forces appear in a object when it rotates in relation to the stars. Einstein was convinced by this and tried, unsuccessfully as far as I understand, to incorporate what he called Mach’s Principle into General Relativity. From Wikipedia -”Mach’s principle” “Einstein was convinced that a valid theory of gravity would necessarily have to include the relativity of inertia: So strongly did Einstein believe at that time in the relativity of inertia that in 1918 he stated as being on an equal footing three principles on which a satisfactory theory of gravitation should rest: The principle of relativity as expressed by general covariance. The principle of equivalence. Mach's principle (the first time this term entered the literature): … that the gµν are completely determined by the mass of bodies, more generally by Tµν. In 1922, Einstein noted that others were satisfied to proceed without this [third] criterion and added, "This contentedness will appear incomprehensible to a later generation however." It must be said that, as far as I can see, to this day Mach's principle has not brought physics decisively farther. It must also be said that the origin of inertia is and remains the most obscure subject in the theory of particles and fields. Mach's principle may therefore have a future – but not without the quantum theory. Abraham Pais, in Subtle is the Lord: the Science and the Life of Albert Einstein (Oxford University Press, 2005), pp. 287–288.” To illustrate it from my point of view: A little thought experiment: A scientist is in a closed space capsule with no windows. The capsule is set spinning. The scientist has a gyroscope and can use his thrusters to stop the spacecraft from spinning. Another closed spacecraft at a distance (the other side of the solar system or the other side of the galaxy) is doing the same thing. Both scientists do their work and declare themselves not rotating, open the door, lean out and wave to each other. They notice that the capsules are not rotating in relation to each other. How did they do this? They used no external clues, all they used was the gyroscope to determine their state of rotation. They both locked onto the same thing to stop the rotation - inertia. They also notice that the stars aren’t moving. So what do you have: Inertial forces are locked to the stars. This means that inertial forces cannot reside within an object but must be an interaction between mass and the distant stars. In fact it is not possible to rotate an object in relation to the stars without developing inertial of centrifugal forces. Also a spinning gyroscope’s axis will always point to a fixed place in the sky unless there is pressure on the axis to make it precess Michaelson and Morley, and others proved that there is no such thing as absolute movement. Acceleration, however is absolute. This is why the axis of the gyroscope points to a fixed point in the firmament. Linear acceleration is also absolute. If an object develops inertial forces it will be found that it is accelerating in relation to the stars and the forces developed are not in relation to local objects or a reference frame that we have arbitrarily defined. (let’s leave out acceleration due to gravity for now) As was stated in the quote at the top “It must be said that, as far as I can see, to this day Mach's principle has not brought physics decisively farther.” This may be correct but it doesn’t mean that Mach’s principle isn’t true. Inertial forces are tied to the stars, hence rotation is absolute and we must figure this out. It is not enough to ignore facts because they don’t fit our favourite theory. I even saw somebody say that Mach’s principle was passè. How can a fact of physics be passè.