Einstein and General Relativity
When constructing his general theory of relativity, Einstein took the result that an accelerated body feels an apparent gravitational field (geeforces), and inverted it: an object in a gravitational field, if it is not accelerated, will not be able to detect the existence of the field by making local measurements ("a falling man feels no gravity"). Einstein was then able to complete his general theory by arguing that the physics of curved spacetime must reduce over small regions to the physics of simple inertial mechanics (in this case special relativity) for small freefalling regions.
Einstein referred to this as "the happiest idea of my life".
Read more about this topic: Local Reference Frame
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