# Twin paradox explained for laymen

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FactChecker
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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.

PeterDonis
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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
Dale
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2021 Award
Does it have to do with the curvature of the space?
Yes. Flat spacetime is SR and curved spacetime is GR

vanhees71