The twins’ trip

A trip to a star 10 light-years away

One of two twins flies at 99% of light speed to a star 10 light-years away (as far as light goes in 10 years) and comes back. On Earth, about 20.2 years pass. But on the ship, only about 2.8 years pass. When they meet again, the traveler is more than 17 years younger.

Why?If moving clocks run slow, then from the ship it is Earth that moves, so shouldn’t Earth’s clock be the slow one? The difference is who turns around. Only the ship turns back at the star. So the two twins are not in the same situation, and when they meet, the traveler has aged less.

Learn moreA light-year is the distance light travels in one year, about 9.5 trillion km. The calculation leaves out the short times spent speeding up, slowing down and turning around. This story is called the twin paradox, but it is not really a contradiction in the theory.

Clock we are watching

A clock on a ship moving at 99% of light speed

How much slower

52 min slower per hour

Clock rate

0.14

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