It is true that the formulas in special relativity for both time-dilation and length-contraction require to be solved in an inertial frame. However, that does not mean that accelerated paths cannot be solved in there - after all, we were able to calculate the proper time that passed for Twin B -- we only had to do it in an ienrtial frame (Twin A's system).

In that level, the paradox is solved simply by saying that it is not valid to simply assume the same formulas are valid for both for the WHOLE duration, since there is no inertial frame comoving with Twin B.


It may be derived without using Twin A's system [obviously], though of course it has to be an inertial frame, so "Twin B's system" is out of the question [no inertial frame].


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I always wonder why in those science-threads, the tone often gets aggressive?
I do agree with Joey, your last comment can easily come across as arrogant. The fact that we're all speaking in different languages than our native one makes it all the more difficult to judge the intention behind comments.
Not sure how to resolve that, I'm just trying to explain why Joey might have reacted the way he did.

You might be correct about the tone of Joey's replies - for me, it feels different and less hostile (though maybe at times, there is a hint of arrogance), but again, I do believe that language (and maybe customs in those) plays a big part of it [Joey and I both are native germans].


I, for one, stick with my tone of careful idiocy