Illustration by Randall Munroe

There once was a doctor with cool white hair. He was well known because he came up with some important ideas. He didn’t grow the cool hair until after he was done figuring that stuff out, but by the time everyone realized how good his ideas were, he had grown the hair, so that’s how everyone pictures him. He was so good at coming up with ideas that we use his name to mean “someone who’s good at thinking.”

Two of his biggest ideas were about how space and time work. This thing you’re reading right now explains those ideas using only the ten hundred words people use the most often.1 The doctor figured out the first idea while he was working in an office, and he figured out the second one ten years later, while he was working at a school. That second idea was a hundred years ago this year. (He also had a few other ideas that were just as important. People have spent a lot of time trying to figure out how he was so good at thinking.)

The first idea is called the special idea, because it covers only a few special parts of space and time. The other one—the big idea—covers all the stuff that is left out by the special idea. The big idea is a lot harder to understand than the special one. People who are good at numbers can use the special idea to answer questions pretty easily, but you have to know a lot about numbers to do anything with the big idea. To understand the big idea—the hard one—it helps to understand the special idea first.

Illustration by Randall Munroe

People have known for a long time that you can’t say how fast something is moving until you’ve said what it’s moving past. Right now, you might not be moving over the ground at all, but you (and the ground) are moving very fast around the sun. If you say that the ground is the thing sitting still, you’re not moving, but if you say that the sun is, you are. Both of these are right: it’s just a question of what you say is sitting still.

Some people think that this idea about moving was the space doctor’s big idea, but it wasn’t. This idea had been around for hundreds of years before him. The space doctor’s idea came up because there was a problem with the old idea of moving.

The problem was light. A few dozen years before the space doctor’s time, someone explained with numbers how waves of light and radio move through space. Everyone checked those numbers every way they could, and they seemed to be right. But there was trouble. The numbers said that the wave moved through space a certain distance every second. (The distance is about seven times around Earth.) They didn’t say what was sitting still. They just said a certain distance every second.

It took people a while to realize what a huge problem this was. The numbers said that everyone will see light going that same distance every second, but what happens if you go really fast in the same direction as the light? If someone drove next to a light wave in a really fast car, wouldn’t they see the light going past them slowly? The numbers said no—they would see the light going past them just as fast as if they were standing still.

Illustration by Randall Munroe

The more people thought about that, the more it seemed like something must be wrong with their numbers. But every time they looked at light waves in the real world, the waves followed what the numbers said. And the numbers said that no matter how fast you move, light moves past you at a certain distance every second.

It was the space doctor who figured out the answer. He said that if our ideas about light were right, then our ideas about distance and seconds must be wrong. He said that time doesn’t pass the same for everyone. When you go fast, he said, the world around you changes shape, and time outside starts moving slower.

The doctor came up with some numbers for how time and space must change to make the numbers for light work. With his idea, everyone would see light moving the right distance every second. This idea is what we call his special idea.

The special idea is really, really strange, and understanding it can take a lot of work. Lots of people thought it must be wrong because it’s so strange, but it turned out to be right. We know because we’ve tried it out. If you go really fast, time goes slower. If you’re in a car, you see watches outside the car go slower. They only go a little slower, so you wouldn’t notice it in your normal life; it takes the best watches in the world to even tell that it’s happening. But it really does happen.