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I kind of already know the answer to the second part (which must be 'yes'), but I'd still like to know what black holes essentially are. Are they made out of a special material to have density as high as they do? Also could artificial black holes be made(I'd think not, as even if it were possible, the gamma ray bursts would be a problem)?

cst1992
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  • I already know black holes form from explosions of massive stars, what I'd like to know is what they become in terms of material(like hydrogen becomes helium due to fusion). – cst1992 Mar 05 '16 at 13:23
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    In a certain sense, black holes are 'made of' highly curved spacetime. For example, there is no matter anywhere/when in the Schwarzschild spacetime. – Alfred Centauri Mar 05 '16 at 13:27
  • Possible duplicate of Is there a limit as to how fast a black hole can grow? – Mithoron Mar 05 '16 at 13:45
  • Also http://physics.stackexchange.com/q/18981/ – Mithoron Mar 05 '16 at 13:45
  • @Mithoron How is my question a duplicate of 'how fast a BH can grow?'? I'm not asking how fast it'll happen; just what will happen when it eats enough. As for the first part, yeah, I think the second one answers it. – cst1992 Mar 05 '16 at 13:54
  • Och, sorry, but still seems related. See also http://physics.stackexchange.com/q/23118/ – Mithoron Mar 05 '16 at 14:01
  • Why would GRBs be a problem with artificial black holes? – Kyle Kanos Mar 05 '16 at 15:34
  • What do you mean by "eating enough"? Black hole appetite is not limited: it can grow forever, no matter how much stuff you throw in. – mpv Mar 06 '16 at 19:57
  • @mpv Won't the average density of the black hole go on decreasing? And since the radius is increasing from the "stuff", escape velocity will slowly decrease until it's below the speed of light. The formula I calculate comes out to e = $sqrt(8/3 * G * rho * pi * r^2)$ which means as rho decreases, e will as well. – cst1992 Mar 07 '16 at 05:38
  • @KyleKanos What other method do you know of making a black hole other than making a small object more and more massive until it collapses, releases a GRB and becomes a black hole? High energy collisions are there, but none has been observed yet. – cst1992 Mar 07 '16 at 05:53
  • Yes, the density of black hole does decrease as its mass increases. There can be black holes with the density of air. But such "density" is a vague term. The black hole never loses the event horizon. – mpv Mar 07 '16 at 06:03

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An astrophysical stellar-mass black hole is formed through the core-collapse of a massive star. The origins of supermassive black holes is more uncertain.

The answer to your main question to some extent depends on whose point of view you are interested in.

From an external observer's point of view, the matter that made up the black hole is still there. Material that falls towards the event horizon appears to move more and more slowly, the light it emits becomes more and more redshifted. If one had a telescope that worked at long enough wavelengths, the material would still be "visible", nearly frozen above the event horizon. However, to all intents and purposes this would behave like a black hole and would have all the properties associated with one because the material affects the curvature of space in exactly the same way. So from that point of view you could say a black hole is made of what it is eating.

If one were travelling with the material collapsing towards a black hole, the story is different. You would reach (or at least approach) a singularity of negligible size and infinite density within a finite time. There isn't a good physical theory about what happens to that material or what state it is in (it is hidden from us behind the event horizon). So from that point of view you could argue that you know what went into making up the black hole, but you don't know what state it is in. As you are travelling with material falling into the black hole, you would not have time to ponder this before performing your own personal experiment.

Continued accretion of mass onto a black hole simply results in a bigger black hole (both in terms of its mass and its Schwarzschild radius). It is still a black hole.

ProfRob
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