ELI5: Explain Like I'm 5

Sub-Riemannian manifold

Okay, let's imagine that we're going on a rollercoaster ride. But not just any rollercoaster - this one has a bunch of different ways to move from one point to another, like tunnels you can crawl through, swinging ropes you can grab onto, and even trampolines you can bounce on!

Now, imagine that instead of a rollercoaster, we're actually moving around on a surface, like a big hill or a bumpy soccer ball. The tunnels, ropes, and trampolines represent different ways that we can get from one point to another on this surface. Some of these ways might be really easy to use, like walking along a flat path, while others might be much more difficult, like crawling through a narrow tunnel or balancing on a wobbly rope.

In mathematics, we use the term "manifold" to describe these types of surfaces. And on some manifolds, certain ways of moving from one point to another are easier than others. This is where the idea of a "sub-riemannian manifold" comes in.

Basically, a sub-riemannian manifold is a particular type of surface where some ways of moving from one point to another are "forbidden" or "less efficient" than others. So imagine that on our rollercoaster ride, there are certain tunnels that we're not allowed to crawl through, or ropes that are too wobbly to use safely. These "forbidden" or "less efficient" ways of moving around create a sort of obstacle course on the surface, making it much more difficult to get from one point to another.

But why would we care about studying these types of surfaces? Well, sub-riemannian manifolds show up in a lot of different mathematical problems, from understanding the behavior of molecules in chemistry to studying the way different animals move and navigate in their environments. By understanding the obstacles and challenges that these manifolds present, we can gain insights into a wide variety of different scientific and mathematical phenomena.