# What the hell is the fourth dimension? If it's time, like a lot of people say, I'm having a hard time wrapping my head around it.

Tonight’s 5th question! This was fun.

The fourth dimension has lots of meanings, depending on what field you apply it to. I don’t have a complete understanding of any of them, but I do have a couple favorites.

In physics, time is unified into “space” to become “spacetime”. That’s to say that the three dimensions that govern everything we can see, touch, etc. also need a fourth dimension of measure to be meaningful. This is where time comes in. The state of the world yesterday was clearly different than today, right? The state of the universe is just different as we go through time. Entropy, yadda yadda yadda. So we need a dimension of measure and description to prove that then is different than now. If that makes sense. This video may help.

But my favorite idea about the fourth dimension has to do with pure scale, thinking about a universe made of strings. Now, string theory goes far beyond the fourth dimension, into maybe eleven or so, which is just not imaginable. You’ll hurt yourself trying. But maybe we can go one beyond the three. Picture yourself as a tightrope walker, balancing on a line. You can move forward, backward, side-to-side or down (if you fall). That’s your three dimensions of reality. Now imagine a flea is walking along the same tightrope, and bumps into your foot. Fleas have things to do, so he needs to get by. The flea can crawl to the bottom side of the tightrope, and walk around you, exploiting a dimension of reality that doesn’t exist to you because of your scale.

Chew on it. This kind of stuff grows wrinkles in your brain.