1. The problem statement, all variables and given/known data In a diagram where I have two observers (one still (A) and one moving with a "v" velocity (B)), where the two parts disagre in the simultaneity of events, how can I prove that the angles of the B person axis that are put in the A person axis are equal. (/alpha=/beta , in the image U'.) 2. Relevant equations --- 3. The attempt at a solution I tried to show that the angles /alpha and /beta follow the same rate of change because the velocity of the B person is constant. Is it because the speed of light goes in a 45° angle? Do I need to calculate something?