Einstein imagined a strip of material that allows propagation of signals at the faster-than-light speed of W (as viewed from the material strip). Imagine two observers, A and B, standing on the x -axis and separated by the distance L . They stand next to the material strip, which is not at rest, but rather is moving in the negative x -direction with speed v . A uses the strip to send a signal to B. From the velocity composition formula, the signal propagates from A to B with speed ( W − − v ) / ( 1 − − ( W v / c 2 ) ) . The time T required for the signal to propagate from A to B is given by