I seem to be missing something fundamental about length contraction in SR. I would be grateful if someone can point out the error in the following logic. Suppose I have a method to measure lengths of objects in the following manner: I place a mirror at the right end of the object, send a light pulse from the left end (assume it's a one dimensional rod that i am measuring) calculate the time taken for the round trip, length is of course simply given by: L = cT/2 Now, I see someone do the same measurement in another frame of reference that is moving at speed v wrt to my frame and this person calculates the time as T'. Since T' = Tγ shouldn't i conclude that the length as measured by the other person is L' = cTγ/2 = Lγ ? In other words, the length should increase and not contract as we now understand it? Where did I go wrong? I'm pretty stupid so any help is great help. Thanks!