The following is an attempt to explain that time dilation in relativity is an apparent phenomenon only, i.e., when one frame moves relative to another at a constant speed, it only appears that its clock runs slower than the other. In the first (simple) case, the box remains stationary. In the second, it moves horizontally at speed = 0.5c. By having lights flash simultaneously at the ends of the box, the “photos” that reach the observers (at positions = 0 in each frame) record simultaneous positions for comparison to determine the “true” box length because both photos are taken at the same time, even though they do not reach the observers simultaneously. Each photo records the light flash and the corresponding positions and times in both frames when the flash occurred. The conclusion drawn from this analysis is that, whether or not reference frames are moving relative to one another, time does not vary – any such variation is apparent only.
