system
Given the following post, respond as if you're commenting on Reddit:
human
Are black holes really holes? 

From what I've gathered about these phenomena, they seem to me to be incredibly dense regions of space, like giant, planet-sized magnets. Are they really *spheres* of such great density and gravitational-pull that we simply cannot see their "surface?" I know they are of such uniqueness that terminology can be hard to accurately apply, but if they truly were holes, wouldn't they have to have "empty" somewhere?
gpt
They're "holes" in a metaphorical but important sense. A black hole is an object of such great density that, near it, gravity is so strong that, quite literally, there are no trajectories that move away from it. (This is commonly said as "even light cannot escape", but IMO that doesn't capture the really fundamental character of the geometric constraint well.) Thus, if you think of spacetime in terms of paths through it, a black hole really *is* a 'hole": paths can go in, but they can't come out.