This is a classic thought experiment relating to the Special Theory of Relativity.

Two twins of the same age start on planet Earth. Twin A stays on Earth while twin B travels far, far away and back at near the speed of light.

Because of Special Relativity, A looks at B's clock, and it moves slower than his own. So at the end of B's journey, A is much older than B.

However, B's frame of reference is equally valid. According to B, A's clock is moving slower than his own. So at the end of the journey, B is older than A.

What faulty reasoning leads to this apparent contradiction?

I don't remember this exactly, but I think it has something to do with the fact that Twin B has to accelerate at certain points during her voyage, once to get up near the speed of light and once again to slow down upon her return (not to mention when she is turning around). 

Relativity applies to inertial reference frames, and the relationship between A's reference frame and B's reference frame breaks down when B is accelerating with respect to A.  Therefore B will age slower than A despite the apparent paradox.  Hopefully someone who knows more about this can make sense of what I'm saying. :)

Posted by tomarken on 2006-05-24 17:15:40