Relativity, Twin Paradox, Time Dilation, Lenghts Contraction, Lorentz Transformation.
that the frame of the spaceship S_1 is empty. (there is only the spaceship in the origin O_1) The astronaut twin can’t think that the terrestrial twin is younger, it can’t be t < t_1. The astronaut twin knows that in the frame of the Earth S the spaceship is heading in the opposite direction to the