This question is from an IB exam in may, 2003 and i'm having a little trouble understanding how they got their answer. The question is, Person A is in a carriage that is travelling in a straight line with uniform speed relative to person B who is standing on the platform. Person A is halfway between two people, X and Y, who are at either end of the carriage. At the moment person A is directly opposite person B as they pass each other at the station, X and Y both light a match, assume it's instantaneous. According to person A the events are simultaneous. Discuss whether the two events will appear to be simultaneous to person B. The answer is that they will not appear to be simultaneous. And the IB awards marks for thinking along these lines. 1) B sees A move away from the signal from X and since A receives them at the same time; 2) and since c is independent of the motion of the source; 3) B will see the light from X first / light from Y will reach B after light from X The problem i've got is their first point, that A receives them at the same time. This is only his reference frame and they're using this 'fact' to prove a point in another reference frame. I wouldn't have thought it's possible to do that. Also wouldn't person B believe that person A doesn't seem them simultaneously? How would you answer the question without using the IB's first premise? or is there some other mistake?