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This question already has an answer here:

Take this scenario for example:

I am on a rocket ship that will take a trip into space and back at 90% the speed of light. And my friend "Bob" stays on Earth. As soon as I leave Bob and I both start our clocks and stop them when I return. The Theory of Relativity would obviously state that my clock should be behind when I return to Earth because the faster you accelerate towards the speed of light, the slower time passes for you. But how can this be because according to my view inside the speeding rocket, Bob's time is passing exceptionally quickly compared to mine. So relative to me I am passing time normally while Bob is the one whose time is incorrect.

But at the same time, according to Bob's view, my time is passing very slowly while HIS time is the correct time. So relative to him he has the correct time while mine was wrong.

So my question is who's perspective do we know is correct? Now I know most will say both of ours are correct but if that is so wouldn't my slowed perspective and his "quickened" perspective just cancel each other out and NEITHER are correct because the rocket could have only taken one certain amount of time to leave and return?

Maybe I just can't get my mind to wrap around the concept of time dilation but to me it seems like it can't work. So could someone please explain why it can?

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marked as duplicate by Brandon Enright, Prahar, John Rennie, JamalS, ACuriousMind Nov 30 '14 at 11:13

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    $\begingroup$ The difference is that you felt the acceleration and Bob didn't which means it is NOT symmetrical. $\endgroup$ – Brandon Enright Nov 30 '14 at 4:19
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To be exact, time dilation refers to the difference in time between the two. Plus, usually, the dilation is measured in accordance to a reference clock, usually, Earth. But you are correct, both results are correct within their reference frame but they don't cancel out.

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    $\begingroup$ No, both results are not correct, only in an inertial reference frame is it necessarily true that clocks in motion relative to that frame must run slow, but the twin who accelerates to turn around does not have a single inertial rest frame throughout the trip. $\endgroup$ – Hypnosifl Nov 30 '14 at 5:44

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