1. The problem statement, all variables and given/known data One event is seperated from another by a lightlike interval, can the first event have caused the other. 2. Relevant equations The equations are not important. 3. The attempt at a solution It all seems to boil down to the definition of an event, can you call the light arriving at that point in space an event? Then you could say that the first event caused the other. However if the photon is supposed to "cause" an event that implies that time has to go on before the event occurs, even if an infinitessimaly small amount of time floats by, the first event could not have caused the other since it then would be a timelike interval instead.