When we talk about a particular problem in Physics. For instance, let's say that light is coming from somewhere to hit the earth. We often say that the light is coming from "infinity." Let's say that we're tackling a black hole and we have a person somewhere as an example and we say that let's say that the person is at "infinity." My question: why "infinity"? Can't we say that it's at a distance `x` that we don't know, or some place far away? Why "infinity"?