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## Homework Statement

It is not a homework question.I am trying to understand the concept in the attached diagram but I stuck.Hope someone can help

## Homework Equations

## The Attempt at a Solution

Please see the lower part of the note,stating that Δt' = Δt/γ.However,imagine you are in the moving frame S'(the stationary train).Yes just a stationary train.If you shine a beam towards the head and back of the train,they should arrive at the same time(in your frame).Hence Δt' = 0,but this also implies Δt = 0.What's wrong?

Thanks!