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?

(In reply to Some thoughts by tomarken)

If there is acceleration, a complete explanation would require, as has been mentioned, not special relativity, but general relativity.

But this paradox does not require acceleration.  Perhaps twin B is already in motion.  As twin B and A pass each other at t=0, they are the same age.  Similarly, every other point of acceleration can be alternatively explained without acceleration, but I will say no more.

Edited on May 29, 2006, 2:45 pm

Posted by Tristan on 2006-05-26 16:37:07