ELI5: Explain Like I'm 5

Stable homotopy theory

OK, let's imagine you have a bunch of shapes and you want to study how they can change or twist around in a nice way. That's what stable homotopy theory is all about.

First, you need to understand what homotopy means. It's like a fancy word for "continuous deformation." Imagine you have a piece of playdough. You can squish and stretch it any way you want, as long as you don't tear it or poke holes in it. That's what we call a homotopy.

Now, let's say you have two shapes and you want to know if they are the same in some sense. But you don't just want to compare their shapes right now, you want to compare all the shapes they can turn into. That's where stable homotopy theory comes in.

In stable homotopy theory, shapes are not just simple shapes like squares or triangles. They are more like complicated spaces that mathematicians like to study. These spaces can have a lot of weird twists and turns, but we want to understand how they are related to each other.

To do this, we use a special kind of math called algebraic topology. This math lets us translate the shapes into equations and formulas. Then we can manipulate these equations and formulas to study how the shapes can change.

One important concept in stable homotopy theory is called stable homotopy groups. These groups are like big families of shapes that have similar properties. We can compare different stable homotopy groups to see if the shapes are similar or different.

Another important concept is called the stable homotopy category. This is like a big library full of all the shapes we can study. We can organize the shapes into different categories based on their properties, and then study how they relate to each other.

So, in a nutshell, stable homotopy theory is all about studying how shapes can change and twist around in a nice way. It uses math to translate the shapes into equations and formulas, and then compares these equations to understand how the shapes are related. It's like a big puzzle where we try to fit all the shapes together and see how they match up.