Sorry for eventual factual errors, I am just hobbyist.

A spaceship flies with nearly speed of light away from earth and back. Time on board were going slower than on earth during flight, so back on earth the pilot is younger than his twin on earth, also the pilots clock show that less time got by for him than for his friends on earth.

So far so good, but how does time "know" that the spaceship moves away from earth and not the other way around? I mean motion is relative so it could be also seen that earth (or the whole universe) moves away from the spaceship? To distinguish those case an absolute universal coordinate system would be needed which I don't believe exist. What am I missing here?

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    $\begingroup$ My answer here is one of many on this site that address your question: physics.stackexchange.com/a/486122/4993 $\endgroup$
    – WillO
    Commented Nov 21, 2020 at 21:36
  • $\begingroup$ You are basically asking about the "twin paradox" of which there are many explanations available online. You will find that there is a way to distinguish the two cases. $\endgroup$ Commented Nov 21, 2020 at 21:53
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    $\begingroup$ Welcome to twin (apparent) paradox. if you search for it in this site, it is well explained. For example physics.stackexchange.com/q/242043/195949 $\endgroup$ Commented Nov 21, 2020 at 21:56
  • $\begingroup$ We already have lots of questions & answers about the twin paradox, so please look at the linked questions (& their links), and if you're still not satisfied please edit your question so we know what bits you don't understand. $\endgroup$
    – PM 2Ring
    Commented Nov 22, 2020 at 5:38
  • $\begingroup$ The key factor is that the traveller occupies 2 (or more) inertial frames. As I said in this answer: It's not so much that the acceleration causes time dilation, it's merely the mechanism whereby the reference frame is changed. $\endgroup$
    – PM 2Ring
    Commented Nov 22, 2020 at 5:41

1 Answer 1


You aren't missing anything here, the problem describes the spaceship to be moving away from Earth and that means that in the reference frame of the spaceship the Earth is moving away-and then when he returns, he sees the Earth coming towards him. The point to be noted here is that time, like space isn't absolute, so both Earth and the Space Ship have their own time coordinates.

This is the origins of Twins Paradox, and the end goal is to realise why the twin on the spaceship is younger.

If you're asking fo why the twin in the space ship is younger, then here's the solution to the Twins paradox.

The twin in the spaceship switches his direction at a point, and for that small amount of time, that particular twin is in a non inertial reference frame, and it is that switching of reference frame that makes him younger.

If you look at this space time diagram, the green lines represent every second experienced by the twin in the space ship, while the y axis represents the time passed on Earth. Clearly, the switching of reference frame has caused the age difference between the two twins

  • $\begingroup$ Physics doesn't depend on what's written in the question. The solution to the paradox is that one twin is not in an inertial frame during acceleration, so those two frames are distinguishable. $\endgroup$ Commented Nov 22, 2020 at 0:28
  • $\begingroup$ @infinitezero well the question doesn't ask for the solution, rather it is asked if the the time coordinates of both frames are the same or different, I've answered only what was asked $\endgroup$
    – SK Dash
    Commented Nov 22, 2020 at 2:10

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