This slowing of the clocks in a moving … [Consider a clock that repeatedly bounces light off a mirror. The light travels along a rod that is perpendicular to the motion so the distance traveled is the same in both systems.] … [p.15-6] On the other hand, when the external … to mirror, is "really" taking a zigzag path, … If in a given time the rod moves … , the distance the light travels in the … is, it takes a longer time for light … the apparent time between clicks is longer for … [This must be true of all clocks, otherwise could distinguish the two systems.]
Google-1
Google-2
Copyright clearance needed for quotation.