ELI5: Explain Like I'm 5

Stone–Čech compactification

Okay kiddo, today we're going to talk about something called the Stone-Čech Compactification. It’s a big and fancy name for a way to make a space smaller and ‘complete’ in a special way.

Imagine you have a big playground where many kids play. Each kid is like a point on this playground. But, sometimes you need to call all the kids together and put them in one place. Then you can have a complete idea of who all the kids are and what they do. So, what you do is draw a big circle around the entire playground, so that all the points/kids lie within this circle. This is your compactification - a small circle that contains everything else.

Similarly, Stone-Čech Compactification is a way to make any kind of space into a compact space. We use a special method to do this.

We take our space (which could be anything, like a line, plane or even a separate space) and add more points to it to make it ‘complete.’ We make sure that these points have special properties that we call ‘ultrafilters’. These ultrafilter points help us keep track of all parts of the space in a nice and tidy way.

To make it even simpler, let's think about a line. It is an infinite set of points with no beginning or end. We can imagine that we add some more points to the ends to make it a circle. Now it's a full circle, and so when we move around the circle, we come back to the same point. This is like a compactification, and it helps us understand the full picture of the space.

The Stone-Čech compactification does the same for any space, making it easier to understand and work with. It helps us summarize all the points of space so that they are easier to work with, and allows us to see the entire picture in a way that is compact and simple.