Why moving clocks run slow
The short answer
Special relativity in four steps: light’s fixed speed and a clock made of light show why a moving clock ticks more slowly.
0:28ScienceIntermediaterelativitytime dilationspeed of light
Chapters and transcript
- 0:00
Light always moves at the same speed
Everyone measures light at about 300,000 km per second, whether they are standing still or rushing past.
- 0:07
Build a clock out of light
Light bounces between two mirrors. Each round trip, up and back, is one tick.
- 0:14
Now let the clock move past you
Seen from the ground, the moving clock’s light travels a longer, slanted path between the mirrors.
- 0:21
Same speed, longer path: slower ticks
Light’s speed doesn’t change, so each tick of the moving clock takes longer. The effect is tiny at everyday speeds, yet GPS satellites must correct for it.