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What is a black hole, and what would happen if you fell in? (for a curious adult who has not studied physics). Cover, in this order: What is a black hole?; How can gravity be strong enough to trap light?; How does a black hole form?; What is the event horizon?; What happens if you fall into a black hole?; If nothing escapes, how do we see black holes?; Do black holes really suck everything in?; Can a black hole die? — page 4: The event horizon
Remember escape speed. The closer you get to the center, the faster you'd need to go to leave. It keeps climbing as you approach.
Find the distance where escape speed equals light speed. Nothing can outrun light, so nothing escapes from inside that boundary. This sphere is the event horizon.
Outside the circle, a light beam can climb away. Inside, even a beam aimed straight outward gets pulled back toward the center.
Nothing is physically there. No wall, no skin, no crust. It's empty space where the rules change: you can cross going in, never out.
Size depends only on mass. Squeeze the Sun into a black hole and its horizon is about six kilometers wide. Double the mass, double the size.
That's why it looks black. Not because something dark sits there, but because no light from inside ever reaches us. The horizon hides everything behind it.
So a black hole's edge is not a surface at all, just the invisible line past which escape ends, and next we cross it.