If the force of gravity is really acceleration, what is accelerating, and in which direction?

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Offline thedoc

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David Cheshire asked the Naked Scientists:
   1) If the force of gravity is really acceleration, what is accelerating, and in which direction?  For example, if a person steps into an empty liftshaft, and the reducing distance between his feet and the bottom of the liftshaft is the result of acceleration, what is accelerating – the person or the bottom of the liftshaft? 2) If Dr Chris Smith in Cambridge, and Dr Kat Arney in Sydney, were to step into an empty liftshaft at precisely the same moment in time, would they both touch bottom at exactly the same moment? If it is the planet that is accelerating, does it choose to accelerate to Dr Smith first, or to Dr Arney? Or does it accelerate in both directions simultaneously? If so, how does it manage that?

What do you think?
« Last Edit: 21/07/2016 18:23:02 by _system »


Offline Atomic-S

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Probably the best answer to this is that it is the liftshaft that is accelerating, and it is accelerating in both Cambridge and Sydney, each liftshaft accelerating in a different direction while, paradoxically, remaining the same distance apart. And the thing that makes that possible is the distortion of space and time that is different in Cambridge than in Sydney. In uniform spacetime, the acceleration of Cambridge and of Sydney would be restrained in significant ways by the constant distance between them, but if spacetime is different in the two locations, this no longer applies.  The entire surface of the Earth is accelerating away from the center as measured in each local reference frame, but because these measurements are based on time passing nonuniformly in different places, the situation reconciles with the diameter of the Earth remaining constant.