the equivalence of gravitational and inertial mass, and Albert Einstein’s observation that the gravitational “force” as experienced locally while standing on a massive body (such as the Earth) is the same as the pseudo-force experienced by an observer in a non-inertial (accelerated) frame of reference.
I was thinking of the Equivalence Principle:
okay, but that would be an accelerated frame of reference, not equivalent to one that is “gravity free”