Understanding the Twin Paradox

Ever since seeing Interstellar I’ve garnered an increasing interest in the science of time.

To get oriented I read about the “Twin Paradox,” a classic example of a mind-boggling scenario involving near-light-speed travel and separate inertial frames. It goes as follows:

The Story

Take identical twins. Put the first in a spaceship, one that travels near the speed of light, and have her go to a distant star and back. Let the second remain on Earth. When the twin returns, she will be younger than the twin that remained on earth.

Wait, what?

The Explanation

It all comes down to “time dilation” and light. To make this easier to understand, let’s pick a slower method of transportation, a train, and something slower than light, throwing a ball.

One twin goes on the train, the other stays at the station. The twin on the train throws a ball in the air. The trajectory, from her perspective, is up and then down. For the twin at the station, the ball moves up and to the right, and then down, to the right, an arched path.

The total distance traversed by the twin on the track is much longer than that of the twin on the train.

Let’s go back to the original case. The spaceship is traveling at a significant speed (e.g. 50%+ speed of light), and we’re measuring the traversal of light emitting from a bulb as it goes to the ceiling of the spaceship and back to the bulb. For the twin on Earth the light travels a great distance, dramatically greater than the twin on the spaceship.

Since light travels at a constant speed, it takes longer for the twin on Earth to observe that path than the twin in the spaceship. If we consider everything relative to the twin on Earth, time has effectively slowed down for the twin on the spaceship.

This process is constantly happening around us. But it’s negligible and unobservable. If your friend travels 60mph on the highway, they are traveling only 0.000009% the speed of light.


I’m considering doing more posts like this where I post a breakdown of something I find interesting. It may not always be science related, we’ll see!

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