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I did some research and found that an earth-mass black hole, has theoretical mean density inside the Schwarzschild radius about 4•10^17 kg/m³. This is well within density of neutron stars which are almost entirely made of neutrons. So what kind of matter makes them? Is it the elementary particles or heavier elements?

PS: I found one very complicated answer on Reddit but I'm looking for a more elementary one, if possible at all. Thanks

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  • $\begingroup$ The "density" of a black hole (usually defined as the mass divided by the volume enclosed by the event horizon) is not a constant but depends on it's mass (and gets smaller for more massive holes). You've calculated the density for a particular mass, not a general density. $\endgroup$
    – dmckee --- ex-moderator kitten
    Oct 7 '19 at 15:12
  • $\begingroup$ Oh okay, I'll edit it in. $\endgroup$
    – Lagaash
    Oct 7 '19 at 15:17
  • $\begingroup$ I'm not sure you understood the implication. The supermassive black holes at the centers of large galaxies have a density lower than water at STP. Density doesn't provide any handle to use in probing the internals. $\endgroup$
    – dmckee --- ex-moderator kitten
    Oct 7 '19 at 15:21
  • $\begingroup$ I agree, the density just refers to distribution of mass so it can be very low for very large black holes. But I don't have any more actual data to strengthen the question. Most of answers I've read so far talk about stuff I really don't know, so I asked as in relation to density. Thanks for insight dmckee. $\endgroup$
    – Lagaash
    Oct 7 '19 at 15:26
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    $\begingroup$ Possible duplicate of What kind of matter are black holes made of? $\endgroup$
    – Kyle Kanos
    Oct 7 '19 at 15:33
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A black hole is made out of whatever it consisted of before it collapsed, plus everything that subsequently fell into it. The manner in which that mass happens to be distributed inside the event horizon is dealt with in the work cited by Safesphere in his comments below.

Viewed from outside the event horizon, the black hole appears to retain all the mass, charge and spin that its constituents possessed right up to the point when they vanished into the event horizon. But because to us, time slows down and stops at the event horizon, it appears from the outside as if all that stuff is squeezed down into an infinitely thin shell at the event horizon.

This means that the "density" of the black hole depends on how you are looking at it, which in turn means that its density is not a useful way to describe it. This means that the black hole has mass (which we can measure by its gravitational field) and a size (which looks to us as the diameter of its event horizon) but it doesn't possess density in the same way a baseball or a planet does.

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  • $\begingroup$ @safesphere, Thorne's book on black holes does describe them as point objects subject to random quantum positional uncertainty. I promise to find another copy (having given my first one away) and re-read it on this point. $\endgroup$
    – niels nielsen
    Oct 9 '19 at 5:46
  • $\begingroup$ This paper is a very good read despite some math: arxiv.org/abs/0804.3619 - See page 5 where it explains the geometry inside, "The 'space' of that universe [...] is a homogeneous cylinder", "the Schwarzschild space as a whole is non-static [...] The radius of the cylinders S falls and it is its vanishing at η = 0 that is referred to as the Schwarzschild singularity", "Evidently, for any observer in M+ the singularity is in the future and, in particular, nobody (on whichever side of the horizon) can ever observe it." [Continued] $\endgroup$
    – safesphere
    Oct 9 '19 at 16:30
  • $\begingroup$ One might naively expect that since the horizon is a sphere [...] then what it bounds is a [...] punctured ball, with a singularity at the center. That would perfectly fit the idea that the Schwarzschild solution “describes the field of a point mass (located at the center, the singular point of the metric)” [5]. The said idea [...] is amazingly widespread even today. It should be stressed therefore that the just drawn picture [...] is grossly misleading. We shall see, in particular, that the term “central” is applicable to Schwarzschild’s singularity no more than, say, to Friedmann’s. $\endgroup$
    – safesphere
    Oct 9 '19 at 16:39
  • $\begingroup$ answer is edited. I invite you to post your own answer, in which case I will delete mine -NN $\endgroup$
    – niels nielsen
    Oct 9 '19 at 18:22

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