Me and my twin develop a rocket that can fly through space near the speed of light... He gets in and takes off and his rocket approaches C. From my perspective time slows down for him, he ages slower than me on earth, and when he returns more time has passed for me. But from his perspective earth has taken off at near C away from him, so shouldn't he see time slow for me. And from his perspective, when earth returns shouldn't he have aged more rapidly than me? If its all relative, what is the mechanism that decides who ages, when both brothers see the exact same scenario from different points of view?
