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Sounds like after the Interstellar movie, ppl are talking much more about time slowing down near a black hole, instead of traveling close to the speed of light.

My personal hobbyist experience was always that in order to slowdown a clock one would have to travel close to the speed of light, nothing to do with super massive gravity pulls. But recently I have been hearing a lot more about gravity slowing down time.

So it sounds like if one was to travel near the speed of light in a environment of micro gravity, or if one could just stand still near a super massive black whole, it would have the same affect.

Which one is right or better, why are both of these hypothesis correct?

Fabio S.
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  • These are two different effects of time dilatation: One described/predicted by Special Relativity (super speed), the other by General Relativity (super gravity). – Photon Jul 10 '17 at 13:58
  • Both hypotheses are correct. However, finding a black hole and standing next to it would be an impressive achievement. – J. Manuel Jul 10 '17 at 14:14
  • Special and General theories of relativity describe the fact of time dilation with respect to velocity and gravity respectively. The observations are a fact and the theories describe them perfectly so far. – Brad S Jul 10 '17 at 14:21

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The gravity one is correct; the speed one is incorrect.

The basic reason that the speed one is incorrect is: suppose that Alice is moving very fast relative to Bob. You want to say that Alice's clock is moving slowly, because that is how Bob sees it. Unfortunately for you, Alice also sees Bob's clock moving slowly.

Relativity is all about this strange observation: when we accelerate, stationary clocks in front of us appear to tick slightly faster, and clocks behind us appear to tick slightly slower, in proportion both to the magnitude of the acceleration and the distance that they are away from us. Everything else -- length contraction, time dilation -- can be derived from this with some intermediate matrix mathematics. The exact formula can be written this way: when you stop accelerating and keep at a constant velocity $v\ll c$, then your times $t'$ will relate to the stationary times $t$ by $t' \approx t - v x/c^2$: stationary clocks which used to appear in-sync are now desynchronized. Note that you can now scatter some clocks which you think are synchronized and the stationary frame will think that those are desynchronized, $t \approx t' + vc/c^2.$ Once you stop accelerating you both have equivalent perspectives related by mathematical formulas.

Going faster and faster one exponentiates a certain matrix and derives $t' = (t - vx/c^2)/\sqrt{1 - (v/c)^2}$ implying that the one frame sees your frame ticking slow; but the inverse to this is actually $t = (t' - vx'/c^2)/\sqrt{1-(v/c)^2}$ for your coordinate $x'\ne x$, and therefore you see their clocks ticking slow, too. It's a very symmetric relationship and the effects are only seen one way or the other depending on how you accelerate in order to meet again.

Let me explain the twin paradox in this way. One twin, Alice, accelerates in the vicinity of Earth, then travels at constant speed out to the stars, then accelerates twice as hard towards Earth to turn her ship around, then traverses the space again, then decelerates back to rest at Earth. Her twin, Bob, stays on Earth and can imagine that he sees Alice's clock moving in slow motion for the entire journey due to the $1/\sqrt{1 - (v/c)^2}$ factor. As we've said, Alice sees Bob's clock move slowly as well: however in this entire sharp acceleration at the other star, she is accelerating towards Earth and sees Earth's clocks tick faster, in proportion both to her acceleration and this distance that Earth is away from her. That effect overwhelms her sense that Bob's clock has moved slowly, and it overwhelms it considerably because of this very long distance, which is negligible at the beginning and end. Even though Bob's clock moves slowly the rest of the journey back, they both agree on the aggregate effect, that Bob's clock has ticked faster overall and Alice's has ticked slower. What's objective is really the accelerations involved, Bob has not accelerated while Alice has.

If you understand the twin paradox now, you have everything you need to understand gravity.

When you are stationary in a gravitational field you are secretly accelerating upwards; physics is meant to be done in free-falling reference frames and yours is not free-falling. Consequently you see a more-permanent fast-ticking of clocks that are above you, and they see a more-permanent slow-ticking of your clocks. You see, gravity gives you the option of accelerating in order to remain in-place, and that changes everything. As you pass near the black hole you accelerate away from it simply to avoid falling in; but as you accelerate away from it the world outside the black hole seems to be ticking very fast relative to you.

CR Drost
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  • to clarify, in the case where Alice was traveling near light speed, when she comes back to earth, will less time have passed for her relative to Bob? – Fabio S. Jul 11 '17 at 02:09
  • @FabioS. Yes. However what Bob sees as "Alice's clock was ticking slow the whole time," Alice instead sees as "Bob's clock was ticking really slow, then as I turned my ship around it was ticking really really fast, then Bob's clock was ticking really slow as I got back, but still it had out-ticked mine due to the really-really-fast period." From that perspective the entire discrepancy happened during the acceleration and in fact the time dilation explanation is true-but-misleading. – CR Drost Jul 11 '17 at 13:46