I'm reading Einstein's book about relativity and having some trouble - two to be specific. 1. You all probably know this experiment already but I'll copy and paste it from the book just in case.There's a moving train and an embankment, with respect to the embankment two flashes of lightning happen at A and B simultaneously. "When we say that the lightning strokes A and B are simultaneous with respect to the embankment, we mean: the rays of light emitted at the places A and B, where the lightning occurs, meet each other at the mid-point M of the length A —→ B of the embankment. But the events A and B also correspond to positions A and B on the train. Let M' be the mid-point of the distance A —→ B on the travelling train. Just when the flashes 1 of lightning occur, this point M' naturally coincides with the point M, but it moves towards the right in the diagram with the velocity v of the train. If an observer sitting in the position M' in the train did not possess this velocity, then he would remain permanently at M, and the light rays emitted by the flashes of lightning A and B would reach him simultaneously, i.e. they would meet just where he is situated. Now in reality (considered with reference to the railway embankment) he is hastening towards the beam of light coming from B, whilst he is riding on ahead of the beam of light coming from A. Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A." As far as my understanding goes, the difference in the judgment of simultaneity is only because the observer is at M' (by the time light reaches M' AM' will have become larger and BM' will have become shorter), but if we place some sort of light detectors at the exact points where the lightning strikes, there shouldn't be any difference. Is that correct? 2. A person on a moving train walks a distance from A to B, why is this distance different when it is judged from the embankment that when it is judged by a passenger?