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
re: Some thoughts by Tristan)
The problem states that B travels far away and back at near the speed of light. So B definitely undergoes significant acceleration, while A does not. Acceleration is not relative, only inertial motion is relative. This is the answer to the problem, as stated.
If we assume that neither twin accelerates, then there's actually no real disagreement. We can actually assert that B is younger from A's point of view, and A is younger from B's point of view. This is because, not only is time relative, as well as space, but the whole notion of simultaneity gets bent by relativity. When A asserts that B's age is less than A's age at some point in time, B interprets this statement as B's age being less than A's age at two different points in time. The greater the distance between the two events, the greater the discrepancy in simultaneity can be.
re(2): Some thoughts
