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Feb24-12, 03:49 PM
P: 3,967
Quote Quote by pervect View Post
GR is not really about time moving faster, or slower - except insofar as one can model a curved geometry by imagining that rulers change length.
This throwaway line makes me sad and I could not let it go unchallenged. One notable feature of GR is that time dilation is not reciprocal as it is in SR. While in SR, A might think B's clock is ticking slower and B thinks A's clock is ticking slower, in GR both observers agree which clock is ticking slower. If for example A is high up and B is low down close to an event horizon, then A will say B's clock is ticking slower, and B will agree that A's clock is ticking faster. This is not just an optical effect due to signal travel times. Clocks low down in a gravity field really do tick slower.

Let us say we set up an experiment with twins, both initially high up. Let us say B descends over a period of 2 days at a controlled velocity to a location near the event horizon. A waits for 50 years and then descends at the same controlled rate over a period of two days to meet up with his sibling. It is perfectly possible according to the rules of GR that A has aged around 50 years and B has only aged by a couple of weeks. This experiment is designed to cancel out any time dilation due to motion as both move in exactly the same way, just at different times. I am not sure why people in this forum constantly try to dismiss gravitational time dilation and try to make out it is not a real effect.