Reply #15: I think that will be a version of the 'twin paradox' [View All]

[muriel_volestrangler]

in which one twin goes off in a rocket, returns, and both of the twins see that the travelling twin has aged slower. Because one is accelerating (and decelerating), what each experiences, and see happening to the other, is not truly identical, and so it is not a real paradox when though through carefully. Wikipedia attempt to explain it (don't ask me, I just trust people when they say it's been thought through OK): http://en.wikipedia.org/wiki/Twin_paradox