Part 2/9:
Newton posited that absolute unobservable space underpins what we know as inertial frames. In contrast, thinkers like Mach have suggested that it is the physical universe — namely, the mass distribution throughout — that defines this inertial motion. According to Mach’s principle, inertial motion results not from an absence of forces, but from interactions within the gravitational field created by all masses in the universe.