Twin Paradox, Lorentz Transformation, Relativity.
at time t is in motion at speed v(t) while, for the frame S_1, the frame S at time t_1 is in motion at speed v_1(t_1). We thus obtain that v(t) = v_1(t_1) (in modulo and for t_1 which corresponds to the travel time measured in S_1 when for the frame S the measured travel time is t) and the two