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This question already has an answer here:

I have heard that time-travel is possible...relative to some observer.

So, as I understand it, the following example would be accurate:

There are two twins- TwinA and TwinB. Both have very accurate clocks, that are exactly synchronised. TwinA leaves earth on a rocket, which travels at approaching the speed of light. After he has counted 24 hours, he comes back to Earth, only to find that more time has passed there, and his clock is now behind TwinB's.

However what confuses me is that speed is relative. So who is to say that TwinA was the one who was moving fast?

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marked as duplicate by Jim, Kyle Kanos, Danu, Emilio Pisanty, BMS Aug 26 '14 at 14:20

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  • $\begingroup$ @Jim Related, but not a duplicate. I am referring to the same "Twin Paradox", but a different aspect of it. I'm interested in how you can tell which twin is the one moving faster if speed is relative. $\endgroup$ – Urbycoz Aug 26 '14 at 14:10
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    $\begingroup$ The short answer is that one of the two has to undergo some kind of acceleration, breaking the symmetry (this is also the resolution to the standard twin paradox problem). $\endgroup$ – Danu Aug 26 '14 at 14:18
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The twin paradox is generally considered to be an illustration of time dilation, not time travel.

To answer your question, the difference between the twins is that Twin A was accelerated several times. Speed is relative; if an object is moving at a constant speed, whether the object is considered to be moving or not depends on which inertial frame of reference you choose to measure the object's speed in. The same is not true of acceleration; an accelerating object's acceleration will be measured to be nonzero no matter which inertial frame of reference is used to measure the acceleration in. So all observers will agree that it was Twin A instead of Twin B who accelerated between different inertial frames of reference.

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  • $\begingroup$ Thanks. Such a good answer I will even forgive you shattering my dreams that time travel is possible. $\endgroup$ – Urbycoz Aug 26 '14 at 15:08

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