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My question is related to this one, but no one actually answered his question.

The event horizon is where gravity from the singularity is so strong that nothing can escape. Assume you're located directly at the event horizon. Now assume another massive body (not necessarily another black hole) was placed above you. Shouldn't the gravity from that body "help you escape" and lower the event horizon in that spot? I know GR is different from simply adding forces, so what would really happen?

HiddenBabel
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  • That seems unintuitive. How can the event horizon expand? Is it like the body outside is already contributing to the mass of the BH since it will eventually be swallowed? – HiddenBabel Jun 12 '18 at 21:52
  • Yes, the event horizon is due to the total gravitational field, not just the stuff inside the black hole. So putting enough mass close to it makes it expand. – Anders Sandberg Jun 12 '18 at 22:04
  • remember that GR black hole solution is an idealization of real BHs. Surrounding matter contributes to the gravtitational properties of a BH – Kosm Jun 13 '18 at 04:47

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