:rofl: Discussing with a friend, I was told that using in a derivation the time dilation formula I implicitly use the Lorentz-Einstein transformations.. I mentioned that many Authors derive time dilation without using the LET considering that the transformation equations obscure the physics behind the studied problem. Others derive the addition law of velocities without using LET based on time dilation and length contraction. My oppinion is that as long as we do not perform the transformation of the space-time coordinates of the same event we do not use LET. Your opinion is highly appreciated. Thanks At leasts in physics who is right should be right!