I saw in an other topic the following thought experiment: In the middle of a train there is a bom with two lightdetectors. At both ends of the train there are two switchable lightsources. The two lightdetectors measure the respective light from both sources. We switch on both switches simulateously as seen from within the train, and the lightsources start sending a photon in the direction of the bomb. Now, if the bomb detects both photons simultaneously, the bomb explodes. Suppose that seen from within the train, the photons arrive simulaneously at the bomb and the bomb sets off. Now, the obvious question is, how does this look from the platform? At relativistic speed, in this particular event, the photons don't arrive simultaneously! However, the bomb still explodes. So, how is this not contradictory? The mechanism in the bomb requires simultanuity. Nevertheless, the bomb sets off... when? When the first photon seems to arrive? When the second photon seems to arrive? It is a mystery to me. Anyone an answer? I hope I was clear.