Alberto:
You say (If I understood ) :
Nope B is also 40 years old because also the acceleration and deceleration must be also taken in consideration
In other words , in your opinion, there is a sort of compensation
... Uh, no, I didn't. Never did, I'm afraid!
I've tried to explain to you what the problem was, and then posted the solution to it. So with your new post, we've both settled on the same result. No more conflict. Isn't that nice?
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Joey:it IS a paradox if you only look at special relativity.
No, it isn't.
It's just a lot harder to derive than simply using t' = t/\gamma and x' = \gamma*x .
But it still ISN'T a paradox. Special relativity says: In inertial frames, THESE formulas work. But Twin B isn't moving in an inertial frame, and thus, those formulas won't work.
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Spike:because you didnt read and he said that both are 20 when they start.
thus one is 40 the other ne is 28 something.
and why is that? because time is relative to the gravitational force of its suroundings.
Have you read my first post in this topic? You should. It might interest you.
If it's too long to your liking, here's the relevant part quoted for your convenience.
Relativity predicts TWO effects that change the "flow of time", if you will. One is described by special relativity - what is usually referred to as time dilation. It can be summarized as "Moving clocks go slower".
The second effect is due to general relativity. Masses curve spacetime, in other words, it changes the geometry of it. The time you're measuring (the "proper time") is nothing but the length of your path through spacetime (disregarding units). Now, obviously, if masses curve spacetime, it only makes sense that the lengths of your paths change, too (and thus your measured proper time).
In other words: The twin paradox is
not because of masses. it works teh same way if you remove earth from the equation and have A stay somewhere in vacuum, while B travels away and comes back (all in vacuum, with no masses anywhere). Their age difference is due to special relativity and NOT because of masses.
Masses DO curve spacetime and thus can make clocks go faster/slower as well, but that is an effect described by general relativity, and one that is not relevant here. It could be added to this scenario, but you don't have to to get the twin-paradox.
