Recently I have been trying to understand the Special Relativity and Time Dilation. From, my understanding, if a person is moving with a constant velocity "v" with respect to a person not moving then both are moving relative to each other and experience each of their own clocks faster than the other. So, there arises the Twin Paradox problem.

But, from what I have understood, the solution lies in acceleration, whenever the person who is moving with constant velocity, "accelerates" either upon return journey to Earth (Twin paradox problem) or during at any point, acceleration changes the frame of reference of the moving person. Hence, the extra time elapsed to person staying on Earth

So my question is Whether somebody moving without any acceleration,in a constant velocity (closer to light speed) age less than the person staying still OR they will age simultaneously in the same rate, since both of their clocks run faster, looking from each others perspectives,because of Symmetry

PS : I don't have much of a background in Physics, forgive any misunderstanding from my part.

  • 2
    $\begingroup$ I've had these arguments extensively with physicists. The time dilation effect is proportional to the amount of time spent moving and the (relative) speed at which that movement occurs, it is not due to acceleration (that is, it is not due to the forces of acceleration - the amount of dilation is a function of the length of time spent moving at a steady speed). $\endgroup$
    – Steve
    Commented Apr 8, 2020 at 11:36

1 Answer 1


In their own reference frames, both observers age at the same rate, on second per second. But because they have different notions of simultaneity, they think that the other ages more slowly. This is illustrated in this diagram

enter image description here

  • $\begingroup$ So, what will happen when it is time to get back home and sync up the clocks? Who is younger? Nobody or "both"? $\endgroup$ Commented Sep 1, 2021 at 15:35
  • $\begingroup$ The one who travelled is younger. This is because the one on the spaceship changed their frame of reference away from the one from earth and then back towards the one from earth. The earth person never changed their frame of reference. So the travelled one is younger. $\endgroup$
    – Molten Ice
    Commented Jan 2, 2022 at 17:52

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