The light clock

A clock made of light

Imagine a clock where light bounces between two mirrors, one above the other. To the person on the spaceship, the light simply goes straight up and down. But seen from outside, the ship moves forward while the light travels, so the light takes a longer, slanted path. Light always has the same speed, so each round trip takes longer. Seen from outside, the ship’s clock runs slow.

Why?At 60% of light speed, the ship moves 3 steps sideways while the light goes 4 steps up. So the slanted path the light really travels is 5 steps long, like a right triangle with sides 3, 4 and 5. Each tick takes 5 steps instead of 4, so the clock runs at 4 ÷ 5 = 0.80 of the normal rate.

Learn moreThe person on the ship notices nothing. Their heartbeat, their thoughts and their clock all slow down together. The slowdown only shows up when you compare it with another clock. This idea is called special relativity.

Clock we are watching

A clock on a ship moving at 60% of light speed

How much slower

12 min slower per hour

Clock rate

0.80

seconds this clock ticks while a clock far away in empty space ticks one second · calculated with the formulas of relativity