The Constancy of the Speed of Light

Picture yourself in the back seat of a moving bus, tossing a ball to a friend in the front seat. To your friend on the bus, the ball just flies at an ordinary speed. But someone watching from outside the window sees something different.

From outside, the ball carries the bus's speed along with it. The faster the bus goes, the faster the ball looks. In the world we live in, speeds add and subtract just fine.

It works the same way when you bike after a friend. If you ride right next to them at the same speed, they look like they're standing still. The same object can look like it's moving at different speeds, depending on who is watching and how fast that person is moving.

That's why, when we measure speed, we always ask who is doing the watching. This addition rule works for balls and cars alike. But there's one thing that refuses to follow it.

This time, on the same bus, let's swap the ball for a flashlight and shine it forward. By the rule above, the light should get a speed boost equal to the bus's speed. But when we actually measure it, that doesn't happen.

Whether you measure from inside the bus or standing on the sidewalk, the speed of that light comes out the same. Run toward the light or run away from it, and the result doesn't change. The addition that worked for the ball simply stops working for light.

The speed of light in a vacuum is about 300,000 kilometers per second (roughly 186,000 miles per second). That's fast enough to circle the Earth about seven and a half times in one second. And this number stays the same no matter how fast the person measuring is moving.

At first, many people assumed the measuring equipment must be faulty. But no matter how many times they changed their methods and measured again, the result stayed the same. In the end, people had to accept that the speed of light really is special.

Here's where things get strange. Speed is the distance traveled divided by the time it took. If speed can't change, then something else has to change instead.

The answer is time and length, the two things we always assumed were fixed. A clock on the bus and a clock on the sidewalk tick at slightly different rates. To keep the speed of light fixed, time and length stretch and shrink in its place.

Keep in mind, though, that this principle is about light traveling through a vacuum. Light moves more slowly through water or glass. And at bus speeds, the change in time is far too tiny for us to notice.

So in everyday life, it's perfectly fine to add and subtract speeds like we do with a thrown ball. It's only when you get close to the speed of light that this story starts to show itself.