(Katie Mack) There's this thing that physicists do they try to find ways to say that things that look different are actually aspects of the same thing viewed from different angles.

It's this, like, really satisfying thing when you can say, “oh it looks like, you know, that's two faces and a vase,” but actually you're just looking at in different ways and actually they're the same, they’re the same picture, right? (John Green) Yes, novelists also find this very fulfilling work. (Katie Mack) Right, exactly. So this is something that physicists are always trying to do.

We're always trying to make things simpler. So we don't like it when there are lots and lots of different particles. We want there to be, you know, one kind of particle doing different things, for example.

So when it was discovered that, you know, all these different elements are actually different arrangements of the same fundamental particles, That was amazing. That was mind blowing. You know, the fact that hydrogen and carbon are actually just different numbers of protons and neutrons and electrons, like, awesome, right?

That's great. So, we want to be able to do that with other things, too, right? So the idea that like, oh, we have all these different forces.

No, no, no, they're really the same force. They just look different because we're in this weird, low energy perspective. (John Green) Oh! (Katie Mack) That's what we really want to do. And so that's what Grand Unified Theories are all about, is trying to say, no, no, no, we don't have all these different forces.

Actually, we have one force and something happened to make it look like there are different forces. Something changed in the early universe that broke the symmetry that was all set up. Everything was all super, like, nicely symmetric and beautiful and perfect.

And it broke at some point in the early universe. And that's why now we have all these different things. (John Green) That’s why we’re here - because something broke. (Katie Mack) Exactly. Exactly.

And so we've got good theories to do that for the fundamental forces of particle physics: strong, weak, and electromagnetic. We haven't yet been able to do that with gravity. We're still trying.

So the reason I'm putting gravity aside is because, okay, so this is where we get into weird namings again. If you can unify the strong, weak, and electromagnetic forces, you have a “Grand Unified Theory.” But if you can unify those with gravity, you have a “Theory of Everything.” (John Green) Oh! (Katie Mack) So that’s even better. That’s even cooler. (John Green) Because that would explain literally everything. (Katie Mack) Yeah.

If you can get gravity to work together with the other forces, then you've solved it, right? And that's the “Theory of Everything.” (John Green) That was a clip from “The Universe,” a new limited series podcast from Crash Course, where Dr. Katie Mack, a theoretical astrophysicist, walks me through the entire history of the universe, including the parts that haven't been written yet.

It's available now both on the Crash Course YouTube channel and wherever you get your podcasts.