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This question already has an answer here:

Suppose there are two observers A & B, separated in space and one is moving towards the other. Their clocks were somehow synchronised at the beginning meaning they both started from $0$ or just started noting $\Delta t$. Now their clocks have same reading, now as soon as they both reach the same place in space-time, they note the readings. Now the readings are different. If readings are different, and they write it down on a piece of paper and then show that paper to each other, they will know one of the readings is less, because time is a real number and real numbers have order properties.

So, they will violate the symmetry of both of them being inertial and equal and hence, one will know whose clock was actually running slow because earlier they could both accuse each other of having slower clocks, but now they know one was slower for real.

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marked as duplicate by joshphysics, John Rennie, Kyle Kanos, Brandon Enright, Dan Jan 13 '14 at 21:33

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    $\begingroup$ possible duplicate of How is the classical twin paradox resolved? $\endgroup$ – joshphysics Jan 13 '14 at 9:01
  • $\begingroup$ @joshphysics The example is quite different, here the clocks are being compared without changing the frames of the observers ever that is no one returns, it is just that they synchronise clocks earlier when they were at a different place in space time and then meet to compare clock readings. $\endgroup$ – user37026 Jan 13 '14 at 9:59
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    $\begingroup$ The question is somewhat ambiguous. If they were together at one time, the one who experienced more acceleration, say measured with a spring scale, is the one with the greatest time dilation. This is how the "twin paradox" is always unraveled. There is no paradox. One experiences accelerations and one does not, so it is easy to tell them apart, or for them to tell what should happen. $\endgroup$ – C. Towne Springer Jan 13 '14 at 18:26
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Your mistake lies in assuming they can synchronise clocks when they are spatially separated. Because of the relativity of simultaneity, whether two events are simultaneous or not depends on the frame of reference of the observer. Thus, if their clocks are 'synchronized' according to one of them, they won't be according to the other.

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