How does time on the spaceship compare to time on Earth?

Once your spaceship is in motion, the elapsed time measured by your shipboard clock will be less than the elapsed time measured by a clock on Earth.

The difference between the clock in your spaceship and the clock on Earth depends on the speed at which the ship is traveling. The faster the spaceship moves, the slower the clock runs. To understand why this happens, you need to think of time and space as two parts of a whole known as “spacetime.”

You are always traveling through spacetime. Even when you’re standing still, you’re traveling through time—at a rate of 24 hours a day. When you also travel through space, you travel through time a little bit slower. It’s a tradeoff: In order to travel through space, you have to give up a bit of your travel through time.

 The curve shows how the ship’s speed affects the rate at which time passes on the ship. For example, a clock traveling at 0.866 times light speed measures time at half the speed of an Earthbound clock: one hour passes on the ship for every two hours on Earth. As the ship approaches the speed of light, the passage of time onboard approaches zero.