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So, I read Can a black hole be formed by radiation? and I think I get it. However, I do have a follow-up question. Would it be possible for an interstellar civilization to build huge mirrors around stars to focus enough photons into a small enough space to make a black hole?

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Nathan Ringo
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No, using optical means the radiation flux will always be less that that at the source (the star). Because the flux in a star is not concentrated enough to form a black hole, the same is true from any concentrated radiation that you could get by optical means

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    Isn't it possible to combine the radiation output of multiple stars into a single point, though? – Nathan Ringo Sep 30 '15 at 13:02
  • no, it would be equivalent to having a star with a larger surface. –  Sep 30 '15 at 23:41
  • Sorry, I don't get how that wouldn't work. Wouldn't it be possible with enough stars to focus enough energy into the point to create a black hole? – Nathan Ringo Oct 01 '15 at 00:02
  • you could do it, but not by purely optical means (that is, mirrors and lenses). I'll try to find you a reference to the theorem –  Oct 01 '15 at 01:16
  • This question might help http://physics.stackexchange.com/questions/88821/is-it-possible-to-build-an-optical-system-that-increases-the-perceived-surface-b/189404#189404 and also this https://en.wikipedia.org/wiki/Etendue –  Oct 01 '15 at 01:22
  • A simple explanation: https://what-if.xkcd.com/145/

    You would need to use lasers or some other active means. It's a law of optics that passive mirrors or lenses can never focus something tighter than it was originally focused.

     – ikrase Sep 05 '19 at 03:19