I have a little bit confuse on the Simultaneity I know if there are 2 coordinates. One coordinate S' is moving V relative to S. We all in inertial reference frame. like the following graph. https://lh3.googleusercontent.com/2rZ1oHZCa3cmBBdc5050sFIuKvIC8rCcuBtCllSUrX87oVtToLhZgwzaketL6MGqd4nLrzNo8z4=w1816-h795 According to special relativity, we know that the person in S' agrees the light which hits A, B in same time. The person in S disagrees with S' because the light does not hit A B in same time. But, If I say, A and B are some kinds of the light sensors. Once A and B detect the light in same time, the car will explode. The guy in S' will die. In S coordinate, the light does not touch the A B in same time because the train is moving. So the car is not going to explode since the light does not reach A B in same time. I cannot understand how can someone be alive in S but is dead in S'