Einstein considered (a) an initially stationary, closed, hollow cylinder free-floating in space, of mass M and length L , (b) with some sort of arrangement for sending a quantity of radiative energy (a burst of photons) E from the left to the right. The radiation has momentum E / c . Since the total momentum of the system is zero, the cylinder recoils with a speed v = − − E / ( M c ) . (c) The radiation hits the other end of the cylinder in time Δ Δ t = L / c , (assuming v << c ), bringing the cylinder to a stop after it has moved through a distance