Hi, there is something I can't understand: Consider a stationary observer at A. Now consider an observer B in a train that moves with constant velocity v with respect to A. In the train, B tries to measure the speed of light using an empty tube of length L0 (proper length as measured by B). He sends a light signal at extremity E, the signal reaches extremity F (where a mirror exists) and return back to the extremity E. B measure this duration T0 (proper period as measured by B). Now B measure the speed of light as: CB = 2L0/T0 Now according to A the length of the tube is contracted and the time is dilated, so he measure the speed of light as: CA = 2L/T = (2L0/γ)/(γT0) = (L0/T0)/(γ^2) = CB/(γ^2) which is different from the value measure by B (divided by Gamma squared) What did I miss in this analysis? thanks.