What defines a black hole, as opposed to a neutron star surface?
Answer
Its event horizon.
While a neutron star possesses a tangible, albeit incredibly dense, surface—a point one could hypothetically land upon, despite the crushing gravity—a black hole is fundamentally different because its mass has collapsed entirely beyond this point. A black hole is defined not by a physical surface but by the event horizon. The event horizon is the boundary in spacetime where the gravitational pull becomes so strong that the escape velocity required to leave exceeds the speed of light. Consequently, anything crossing this boundary, including electromagnetic radiation, is irrevocably trapped, making the event horizon the defining feature of a black hole.
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