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First of all, I just want to say that I don't know much about general relativity. But my question concerns an example often found in textbooks about special relativity.

On to my question. I have heard several times that one can travel forward in time (earth-time) by leaving earth, traveling close to c in a spaceship and then come back. But I don't understand this. I know the textbook reasoning behind it, but I'm not satisfied by the textbook explainations.

The textbook explainations go like this:

Say we have a mirror in the ceiling of the traveling spaceship, and send a light pulse straight up from the spaceship floor, let it reflect in the mirror, and come back. We measure the time of that, and call it t.

From earth, the light appears to travels not straight up, but in a longer path since the spaceship moves relative to earth. But since c is the same for all reference frames, the time it takes will appear longer from earth. Let's call it t2. So t2 > t.

And so time moves slower in the spaceship than on earth, so when the spaceship gets back, more time has elapsed on earth than in the spaceship.

My question: What if the mirror experiment had been carried out on earth? Then the exact same reasoning could be made, and the conclusion would have been the opposite: that more time had elapsed in the spaceship than on earth. And without that particular explaination: Why would the time in one reference frame go faster than time in the other, given that the laws of physics are supposed to be the same in all inertial reference frames?

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# Time travel by going fast in a spaceship: questions

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