Twin paradox facts for kids

The twin paradox is a cool idea in physics. It's like a thought experiment where you imagine a situation to help understand a scientific rule.

Here's the idea: Imagine two identical twins. One twin stays on Earth. The other twin travels into space in a super-fast rocket. When the space-traveling twin comes back home, they find that the twin who stayed on Earth has gotten older. But the space twin has aged less! It's called a "paradox" because it seems to go against what we expect.

A Space Trip Example

Let's look at an example to understand this better. Imagine a spaceship traveling from Earth to the closest star system. This star system is about 4 light-years away. The spaceship travels really fast, at 80% of the speed of light.

To make the math easy, we'll pretend the ship instantly reaches full speed. We'll also pretend it can instantly turn around at the star.

People on Earth and the travelers on the ship will see the trip differently.

What Earth Sees

From Earth, mission control calculates the trip like this: The round trip (there and back) will take 10 years. So, everyone who stays on Earth will be 10 years older when the ship returns.

But clocks on the spaceship, and the aging of the travelers, will slow down. This is because of something called time dilation in special relativity. For this trip, the time for the travelers will be 0.6 times slower.

So, when the travelers return, they will have aged only 6 years (0.6 × 10 years).

What the Travelers See

The crew members on the ship also figure out their trip from their point of view. They know that Earth and the distant star system are moving past their ship at high speed.

Because of their fast speed, the distance between Earth and the star system seems shorter to them. This is called length contraction. To the travelers, the distance is only 2.4 light-years (0.6 × 4 light-years). This shorter distance applies to both the trip out and the trip back.

Each part of the journey (out or back) takes 3 years (2.4 light-years / 0.8 speed of light). So, the whole round trip takes 6 years (3 years + 3 years).

Their calculations show they will arrive home having aged 6 years. This matches what the people on Earth calculated for them!

What Happens in the End

No matter how they calculate it, everyone agrees on the final clock readings.

So, if two twins are born on the day the ship leaves, and one goes on the journey while the other stays on Earth, they will meet again. The twin who traveled will be 6 years old. The twin who stayed on Earth will be 10 years old.

Related pages

• Special relativity

See also