The gravitational force is equivalent to being in an accelerated frame of reference.
- In official terms: “The effects of a gravitational field are in principle locally indistinguishable from being in an accelerated frame”
- So, in Newtonian gravitational theory, the very fact that inertial mass and gravitational mass are equal was unexplained. So entered Einstein with his “Elevator” thought experiment.
- If you’re in a windowless cabin, apparently feeling some universal force, there’s no way for you to tell if you’re in an accelerating reference frame (say, a rocket ship in deep space) or in a gravitational field (sitting in a lab).
- Gravitation is just an effect of being in a non-inertial frame. Bodies that feel gravity just move freely, i.e., are in inertial motion.
- BUT! The motion of say a planet looks very different from the usual ideas of inertial motion.
- Hence came up a new concept of inertial motion, by changing the geometry of spacetime. (Straight lines now become Geodesics - Straightest possible lines in that geometry)