The earth’s mass curves what was flat space toward the point N. That means that Alice, though she thinks she’s moving forward in time and not moving in space, is in fact following the curved line from point A to point N. And though Bob, like Alice, thinks he’s moving forward in time and not moving in space, he, too, is following a curved line, from point B to N. So, Alice and Bob move inexorably closer to each other and to point N not because they’re being pulled by a gravitational force, but because that’s the path they must follow in curved space if they remain still. So the key idea to
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