Hey guys can someone explain to me what does an inertial frame of refence mean? From what i know its a frame of reference whereby the observer frame of reference is at constant velocity? But i dont really get what it means. I have been reading some articles on SR and it seems that there is this case about a train moving at a constant speed and 2 bolts of lightning strike both end of the train at the same time. There is one observer in the train and one outside. The article claims that the observer outside the train see that the bolt of lightning strike both ends at the same time but the observer inside sees the lightning strike one end 1st. That does not make sense to me can someone explain this to me?