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## Main Question or Discussion Point

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.

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.