by Niagara » Tue 02 Jan 2007, 23:34:07
$this->bbcode_second_pass_quote('PenultimateManStanding', 'I')'ve got one that always puzzled me: how come if a twin goes up in space and flies at great speed relative to the earth and comes back a long time later, his twin on earth has aged more than him? isn't speed relative? if there is no absolute speed (except c which is the same in all reference frames) then wouldn't the twin on earth be speeding fast relative to the twin in the spaceship? I get the idea of time in a gravity well, but the relative speed thing always stumped me.
You're referring to the so-called "twin paradox". To understand the solution you need to consider the differences between special and general relativity.
The twin on earth is in an inertial references frame; he did not accelerate at any time during the brother's journey. The traveling twin accelerated then decelerated during each leg of the trip. This non-inertial frame is much more complex and changes the time equations.