the setting: A train travels at high speed. A runner on the train sprints towards the back of the train with the same speed (wrt to the train). as the train moves forward (wrt the earth). therefore the runner doesnt move wrt to earh. The paradox: the runner's clock runs slower than compared to the train clocks. therefore the runner's clock should run doubly slow wrt to earth's clock cause the trains clocks run slow compared to the earth's clocks. but the runner is not mving wrt to the earth therefore the runner's clock must be run the smae as the earth's clocks. how to resolve it? well im given a hnit to resolve it by relativity of simulatentity, now if we were to send a light beam from the opposite direction of the runner towards him as soon as he starts running in the train, then obviously in the train it would we simultaneous while in the earth it won't, we can measure the time it takes the beam to reach the runner, is this approach correct have i resolved the paradox, i feel that i hvent really.