Consider the following scenario:

  • I get in a spaceship, and travel really close to the speed of light for a while, and then come back.

  • A lot of time has passed on the Earth, but since I was traveling so fast, I only experienced a few years passing.

  • So, my friends on Earth are dead, whereas I'm only a few years older.

But what I'm having trouble wrapping my head around, is why is it them that's dead, and not me?

After all, given what I understand about relativity, it's just as fair to say that my spaceship stayed still, and it was actually the Earth that traveled really fast and then came back to my ship.

In that scenario though, the Earth being the fast-moving ship, and my ship being the stationary body, wouldn't it be that I am dead, and everyone on the Earth is just a few years older?

If there really is no preferred frame of reference, then why does the ship-traveler live while the people on the Earth die?


The simple answer is that because you are the traveller and therefore have to slow down, stop, accelerate in the opposite direction and come back again. The situation is not therefore symmetric as the Earthbound observers do not accelerate. You accelerate, so you are NOT in an inertial frame at all times. The easiest calculation is therefore done in the at-all-times inertial (or almost so, neglecting circular motion around the Sun) of your friends: since the frame is inertial, we readily calculate in this frame that you, the traveller, age less than those in that frame. Suppose you accelerate quickly to near the speed of light and then continue for a long time at this constant velocity. At all times you are at this constant velocity, the calculation is indeed valid from both frames and both observers would infer the other's time were dilated: this is a little paradoxical but not contradictory because the two observers are at a nonzero, indeed big, timelike separation, so there is no way they can send signals to one another which would confirm at one place the contradictory result that both observer's clocks were running slower than the other. You can also do this calculation from your own frame, but taking accelerations into account properly will lead you to the same result: you've aged much less than your dead friends.


It's actually in the first paragraph of the wiki page on the Twin Paradox. I see your 'symmetry' argument, but the earth and the space travelers aren't symmetric -- an easy way to see this is that one of them spent a lot of energy (the rocket fuel, say) to make this situation happen, and one of them didn't.


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