Relativity, Twin Paradox, Time Dilation, Lenghts Contraction, Lorentz Transformation.
while the planet is motionless. In this case x_1 = -v*t_1 and we therefore get t_1 = d/v (if x_1 = -d). Always considering the two Lorentz transformations x_1 = γ*(x -v*t) and x = γ*(x_1 + v*t_1), we necessarily obtain x = 0 and t = (1/γ)*t_1. If the planet is in motion, the planet itself reaches