What Is a Cone?

The reason it's often called a light cone is that it's hard to draw four dimensions on a sheet of paper. So in two dimensions, the sphere we were just talking about, becomes a circle. If you take those set of circles and you extend them, like vertically, above the surface in time, make time i that vertical direction, then what you get is a cone. And so it's a cone only in a universe that has two spatial dimensions and one time dimension, right? It's coing.