It is the invisible “fence” in space where gravity is so strong that the speed needed to escape exceeds the speed of light. Since nothing in the universe can travel faster than light, nothing that crosses this line—not even light itself—can ever get back out. It is a mathematical boundary, not a physical surface. If you fell through it, you wouldn’t “hit” anything. : Information can go in, but none can come out. This is why black holes are “black”—no light can escape to reach our eyes.
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