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Even if there were no external matter in the vicinity of a black hole, its brightness would still not be zero thanks to Hawking radiation.

Nobody's observed Hawking radiation, so we don't know for sure.

Anyway, it's not obvious to me that the edge of a black hole is the event horizon. The event horizon is mostly empty space too. Maybe it's the ergosphere, which can be further out. Since a black hole is just a region of space, the boundary is sort of arbitrary.

Hawking radiation has been observed in multiple experiments. https://arxiv.org/abs/1009.4634 https://www.nature.com/articles/s41586-019-1241-0

Neither of those observations are from black holes.

That is not the claim I addressed. He said Hawking radiation has not been observed. I provided citations that it had.

It's been observed in the same way that magnetic monopoles have been observed: physical analogs have been built, but nobody has observed an actual magnetic monopole.

"Nobody has observed a magnetic monopole" is still true, even if systems that behave like quasi-monopoles have been built.

My conclusion (that we can't know for sure that black holes produce Hawking radiation) only makes sense if I was talking about observing actual Hawking radiation rather than analogs, which I was.

The claim that black holes are perfectly black is subject to those same constraints.

No, it hasn't. Not in the real world. They have observed Hawking radiation in simulation (in an analog computer with some quantum properties).

There won't be any radiation from a supermassive black hole, or a stellar mass black hole. The bigger a black hole is, the colder it is, and any black hole bigger than a thimble is colder than the Cosmic Microwave Background. Its brightness isn't technically zero, but it's still darker than the background, and it's absorbing heat (and getting slightly bigger) rather than giving it off.

Whether or not it is absorbing radiation faster than it is emitting is not relevant to whether or not it is emitting at all. Black holes are pretty dang black but they are not "0 by definition".

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