20-3 Consequences of Einstein' s postulates 1.The relativity of time We consider two observers: S is at rest on the ground, and S is in a train moving on a long straight track at constant speed u relative to s The observers carry identical timing devices, illustrated in Fig 20-4, consisting of a flashing light bulb F attached to a detector d and separated by a distance Lo from a mirror m The bulb emits a flash of light that travels to the mirror when the reflected light returns to d, the clock ticks and another flash is triggered20-3 Consequences of Einstein’s postulates 1.The relativity of time We consider two observers: S is at rest on the ground, and S’ is in a train moving on a long straight track at constant speed u relative to S. The observers carry identical timing devices, illustrated in Fig 20-4, consisting of a flashing light bulb F attached to a detector D and separated by a distance from a mirror M. The bulb emits a flash of light that travels to the mirror, when the reflected light returns to D, the clock ticks and another flash is triggered. L0