 Finally, let's wrap up with length contraction. So Alice is on her train of length LA, traveling at a speed V. The tricky thing is to frame this in terms of a Lorentz transformation. It seems like delta XA should be LA, but what about delta TA? How does a time interval make sense? So Lorentz transformations transform between events. So we need to pick two points in space time with position and time coordinates that'll give us the length. So Alice's idea of length comes from, in her frame, simultaneously measuring the position on each end of her train. So if each event is Alice measuring the position of one of the train walls, and these events happen simultaneously at either end of the train, what is delta XA and what is delta TA?