Hope this is a valid question. I can't quite understand this very simple thing. Prof Shankar's introduction to relativity on Youtube starting at about 50 minutes has a quick calculation that comes up with the Lorentz transform. I follow the maths, no problem. But he confuses me a bit when he's setting the stage. He says two trains, mine and yours, cross at the origin. Yours is moving. As they cross a beam of light is sent forward to the detector, distance x from the origin for me. distance x prime for you. And he says there is a distance u for you. So that x prime = x - u. That's what confuses me. Where is the distance u from? Is that the distance you will travel during the time it takes for the light beam to reach the detector? I am thinking it must be but I'm so much a novice at all this I'd love confirmation or correction if I'm wrong.