How can events that are simultaneous not always be simultaneous?
This is a consequence of the fact that light has speed in all frames. The thought experiment with the red and green flashes demonstrates this. Jackie knows she’s at the center of her ship, and you know this too. At exactly the moment she passes you, green and red lights flash at the two ends of the spaceship, simultaneous in your frame. You then see them at the same time. Because you know they originated at points equidistant from you (and you know the speed of light is ), you know the flashes were simultaneous in your frame. Now consider Jackie: During the time the light is traveling from the ends, the spaceship has moved a little bit forward. So Jackie will hit the green light (from the end she is moving towards) first. Then, later, the red light will catch up to her. Because she knows that the speed of light is in her frame too, and she knows that the flashes came from equidistant ends of her ship, she concludes that the two flashes of light were not simultaneous in her frame! For h
