ELI5: Explain Like I'm 5

Sub-Riemannian geometry

Okay, kiddo, let's imagine a little robot named Riri who loves to move around in a special world. This world has many different paths and obstacles, like walls and rocks, that Riri needs to avoid.

Now, imagine that Riri's owner wants to measure how far Riri can move in this world, but they can't just use a normal ruler because Riri needs to avoid those obstacles. So instead, they use something called sub-riemannian geometry.

Sub-riemannian geometry is a way to measure distances in complex spaces that have lots of obstacles. It's like a special ruler that can measure how much distance Riri can cover while avoiding obstacles.

To use this special ruler, we need to imagine drawing a line from where Riri starts to where they want to go. But since there are obstacles in the way, we need to go around them. This special ruler measures the shortest possible distance Riri can travel to reach their destination while avoiding obstacles.

Sub-riemannian geometry is useful in many areas of math and science, like robotics and quantum mechanics, where we need to understand how things move in complicated spaces. With sub-riemannian geometry, we can better understand these spaces and how to move in them, just like Riri can better understand their world and move around it without bumping into things.