Speeding up

Drag the slider

Use the slider at the top to change the ship’s speed. On the left is Earth’s clock, on the right the ship’s clock. At half the speed of light, the ship’s clock runs at 0.87 of the normal rate. At 90% it runs at 0.44, and at 99% at 0.14. The closer you get to light speed, the bigger the slowdown, without limit.

Why?Near the speed of light, pushing the ship harder adds only a little more speed. That is why a ship with mass can never reach the speed of light.

Learn moreFast clocks running slow has been measured many times. In 1971, scientists flew precise atomic clocks around the world on airplanes and compared them with clocks left on the ground. Tiny particles called muons, made high in the sky and rushing down very fast, reach the ground before they decay because their time runs slow.

Clock we are watching

The spaceship’s clock

How much slower

Try the speed slider

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