1. The problem statement, all variables and given/known data It is not a homework question.I am trying to understand the concept in the attached diagram but I stuck.Hope someone can help 2. Relevant equations 3. 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!