A Qualitative Outline Of General Relativity And Space-Time Curvature
...is more an exception than a rule. Technically, any change in speed or direction is called acceleration (or deceleration), which can thus mean slowing down as well as speeding up, or simply a redirection. Ordinarily, an observer in an accelerating frame of reference can perceive its motion. Passengers in a car, for example, fell themselves pressed backward if the car starts suddenly from a dead stop. Their awareness seems to imply that acceleration is absolute, not relative; they need not refer to anything outside their frame of reference to detect their own motion. But if accelerated motion is absolute, it would have to be subject to a different set of natural laws from those that apply to uniform motion a proposition that Einstein found highly objectionable. He thus set out to conjure a more general theory that would apply to motion of all sorts. In the process, he developed a new theory of gravity.
The starting point was Galileo's finding that falling objects accelerate at the same rate despite differences in their mass: if dropped from the same height in a vacuum, a cannonball and a feather would hit the ground at the same time, due to lack of air resistance. Einstein was sceptical of Newton's explanation that the force of gravitational attraction precisely equalled an object's inertial mass. Einstein rejected the notion that this uncanny coincidence was merely an accident of nature.
The Principle of Equivalence and the Weak Equivalence Principle (WEP)
The exact Minkowski space-time of special relativity is incompatible with the existence of gravity, because of gravity's physical quality of warping' space-time. A frame chosen to be inertial for a particle far from the Earth where the gravitational field is negligible will not be inertial for a particle near the Earth. An approximate compatibility between the two, however, can be achieved through a...
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