# Twin paradox explained for laymen

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• Tony Wright
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See my corresponding post in the other thread in which you made an almost identical post for the details behind the above responses.
Yes. Somehow I switched to that other thread without me noticing it. I have deleted that post from there and put it here. Thanks for your answers.

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I have deleted that post from there and put it here.

I just undeleted it, since the Post #17 reference you gave is actually in that other thread, not this one. I agree that duplicate posts are normally not a good idea, but in this case I think it's reasonable to have both since similar issues have been discussed in both threads.

• FactChecker
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So one can not use his non-inertial reference frame and SR to calculate the correct answer.
One can use a valid non inertial reference frame and SR to calculate the correct answer. But you have to derive the correct formula anew. You cannot simply use the standard formula derived for an inertial reference frame and just directly apply it in the non-inertial reference frame.

• FactChecker
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One can use a valid non inertial reference frame and SR to calculate the correct answer. But you have to derive the correct formula anew. You cannot simply use the standard formula derived for an inertial reference frame and just directly apply it in the non-inertial reference frame.
When you are doing SR in a non-inertial reference frame, I wonder where you would say that SR ends and GR begins. I just think of GR as allowing non-inertial reference frames. From what you, @vanhees71, and @PeterDonis are saying, that is not right. Does it have to do with the curvature of the space?

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When you are doing SR in a non-inertial reference frame, I wonder where you would say that SR ends and GR begins. I just think of GR as allowing non-inertial reference frames. From what you, @vanhees71, and @PeterDonis are saying, that is not right. Does it have to do with the curvature of the space?
Flat spacetime = SR
Curved spacetime = GR

I think there was some argument in the early days about which label to apply to non-inertial frames in flat spacetime. But ultimately there's no physics in non-inertial frames that isn't in inertial frames - just the maths is harder. There is new physics in curved spacetime. So it makes sense to draw the dividing line there.

• martinbn, vanhees71, Dale and 2 others
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Flat spacetime = SR
Curved spacetime = GR

I think there was some argument in the early days about which label to apply to non-inertial frames in flat spacetime. But ultimately there's no physics in non-inertial frames that isn't in inertial frames - just the maths is harder. There is new physics in curved spacetime. So it makes sense to draw the dividing line there.
I think that I am finally getting to the bottom reason for my stubbornness on this issue. I had the very definition of SR and GR wrong. Sorry. I think I owe an apology to many people.

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I had the very definition of SR and GR wrong.

As I commented earlier, so does the Gron paper that was referenced. There are other sources in the literature that also get this wrong; a big reason for that is that it took a fair amount of time after relativity was first discovered for physicists to get clear about this, because of the issue @Ibix mentioned with regard to non-inertial frames. Even Einstein wasn't entirely clear about it in all of his writings.

• vanhees71 and FactChecker
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Does it have to do with the curvature of the space?
Yes. Flat spacetime is SR and curved spacetime is GR

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