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I was on the worldbuilding SE and saw this question

So I tried a quick google on what it was but there seemed to be a lot of people asking the same question, without answer.

So what is a dormant black hole?

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    $\begingroup$ As far as I know, "dormant black hole" is not a technical term. The user could potentially be trying to refer to a supermassive black hole in what was once an active galactic nucleus which is now quiet (i.e. the black hole is not accreting matter), but he or she hasn't specified. $\endgroup$ – HDE 226868 Oct 2 '16 at 18:38
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A black hole "feeds" on its environment, usually. So matter falls into the black hole. Due to angular momentum conservation the matter will start to spiral. At some point it will hit the "centrifugal barrier" and keeps orbiting the black hole. When more matter is accumulated, the black hole will create a massive ring of matter.

Tidal forces will crush the matter, friction will heat the matter. The heat will become so large that the thermal radiation pushes matter away again. Other parts of the matter lose energy and fall beyond the event horizon.

When a black hole has consumed all the matter in its vicinity, it can no longer generate radiation from its belt. At this point you could call it "dormant" since nothing falls into it.

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  • $\begingroup$ Hmm, so it becomes "dormant" because there is nothing around it left to suck in... $\endgroup$ – Nelson Oct 3 '16 at 3:39
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    $\begingroup$ @Nelson Black holes don't suck. There are still orbits around the black hole, including stable orbits. If our Sun magically turned into a black hole, our orbit around it would stay pretty much the same. $\endgroup$ – Luaan Oct 3 '16 at 7:15
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    $\begingroup$ @Luaan Assuming that transformation wouldn't be preceded by, say, a supernova, in which case Earth's orbit would become a minor problem for its inhabitants... $\endgroup$ – Tobias Kienzler Oct 3 '16 at 10:49
  • $\begingroup$ @Nelson There is also an effect whereby black holes also fling stuff out into interstellar space at high speed. Look up the notion of minimum radius stable circular orbit - which is thrice the Schwarzschild radius. When things get nearer than this, small perturbations to their orbits grow exponentially with time, with the result that they either fall into the hole or are flung out. In the latter case, the effect is somewhat like the transfer of Earth's rotational kinetic energy to the Moon, but instead of billions of years needed for the Moon to reach 400 000km radius, the effect is ... $\endgroup$ – WetSavannaAnimal Oct 3 '16 at 12:27
  • $\begingroup$ @Nelson ... is complete in a few decades or years owing to the much stronger gravity. $\endgroup$ – WetSavannaAnimal Oct 3 '16 at 12:30
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A dormant black hole, as the name suggests is a sleeping black hole, i.e. one that is not actively swallowing matter around it. Black holes can't turn on and off, so a dormant black hole would not be in contact with any large sources of matter.

In contrast, black holes which are not dormant are actively pulling surrounding matter closer to its horizon, thereby creating a glowing hot accretion disk around it.

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