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SESK: Stuff Everyone Should Know · sesk.info/astro-event-horizon

What is a black hole's event horizon?

The boundary of no return: nothing inside, not even light, can affect the outside.

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  1. The boundary from which nothing can escape ✔
  2. The hot disk of gas around it
  3. The point-like center

Why: The event horizon is the surface where escape speed reaches the speed of light. Anything inside can never again interact with the rest of the universe. The hot gas outside is an accretion disk.

Huh, didn't know that: The name black hole was popularized by physicist John Wheeler starting in the late 1960s.

The dad joke
Why don't black holes have friends? They're too clingy.
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References

Answer: The boundary from which nothing can escape. The event horizon is the surface where escape speed reaches the speed of light. Anything inside can never again interact with the rest of the universe. The hot gas outside is an accretion disk.

  1. Astronomy 2e, 24.5 Black Holes openstax.org/books/astronomy-2e/pages/24-5-black-holes
    Events inside the event horizon can never again affect events outside it.
    Checked 2026-10-10.

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