<p>As far as current physics knows, nothing. This is the reason why it is commonly thought that a singularity  exists in the middle of a black hole. </p>

<p>However, singularities are also thought to be non-physical, so there is most likely something else inside a black hole — we just don’t have the science to describe it just yet. </p>

CC BY-SA 4.0
2019-07-19T08:04:45.980Z
It's not really "commonly thought" that there's a singularity in the middle of the black hole (except in sci-fi, though that also gives you things like the event horizon being a physical barrier you can shoot your way through, so...). It's just the most straighforward result you get from applying general relativity to the problem, and assuming there's nothing beyond neutron degenerancy pressure to prevent further collapse. Not to mention the assumption that the matter actually had time to reach the "singularity". AFAICT, to a physicist, singularity is *not* a thing that exists - it's a mistake
Luaan
CC BY-SA 4.0
2019-07-19T08:07:56.047Z
Exactly, I agree 100%, my wording is inaccurate as this was the exact meaning I wanted to convey
tuomas
CC BY-SA 4.0
2019-07-20T21:20:14.507Z
@Luaan Isn't *"nothing beyond Neutron degeneracy pressure"* a bit misleading? My understanding is, that according to plain GR, the singularity is in the future of anything inside the event horizon, and no force (measurable in Newtons, like exerted by the degeneracy pressure) can prevent that. AIUI, this is why having new strange forms of dense matter is basically irrelevant inside a black hole in the GR, the space-time doesn't care.
hyde
CC BY-SA 4.0
2019-07-21T13:56:44.920Z
@hyde No, there's no magic involved - the spacetime within a black hole is exactly the same as outside of the black hole (as far as plain GR is concerned). The only thing that might be different is the part we now mark as a "singularity". There's certainly nothing special happening as you cross the event horizon. Of course, you still can't go back "outside", but that's mainly because there's no path leading outside. Mind, there is truth to saying "the singularity is in the future of anything inside the event horizon"; but that's analogous to us moving through time - as is leaving the hole.
Luaan
CC BY-SA 4.0
2019-07-21T13:57:16.240Z
@hyde It would definitely make a great question in its own right, though :)
Luaan
CC BY-SA 4.0
2019-07-18T14:19:39.360Z
2019-07-18T14:19:39.360Z
tuomas
<p>In classical General Relativity, there <em>is</em> no limit to the compression in a black hole, hence you get a singularity. However, many astrophysicists feel that's unphysical, and that a theory which unites General Relativity and Quantum Mechanics will impose some kind of limit, perhaps something connected with the quantization of spacetime itself. </p>

<p>We don't have a working theory of quantum gravity, so at this stage we don't exactly know what happens in the core of a black hole. OTOH, we're fairly confident that the core has to be very small, since quantum gravity effects probably don't kick in until a scale much smaller than the size of an atom, and probably smaller than a proton, somewhere around the scale of the <a href="https://en.wikipedia.org/wiki/Planck_length" rel="noreferrer">Planck length</a>.</p>

CC BY-SA 4.0
2019-07-18T14:28:21.000Z
2019-07-18T14:28:21.000Z
PM 2Ring
<p>If a white dwarf compresses to the limit of electron degeneracy, and a neutron star compresses to the limit of neutron degeneracy, what does a black hole compress to the limit of?</p>

CC BY-SA 4.0
2019-07-18T20:33:14.577Z
We don't know...
Mithoron
CC BY-SA 4.0
2019-07-18T23:08:12.917Z
Neither of the above statements is really true. Degeneracy is a continuous parameter and does not have a "limit" except at infinite density. The reasons that white dwarfs and neutron stars do not exist towards infinite density is not because of some limit on the degeneracy is reached, but for other physical reasons - neutronisation in one case and GR in the other.
ProfRob
CC BY-SA 4.0
2019-07-19T16:12:42.870Z
This answer may help you: https://physics.stackexchange.com/a/141876/232868
SurpriseDog
<black-hole>
Pauli exclusion principle - black holes
scorpdaddy
# Pauli exclusion principle - black holes

(Asked by: username_0 on 2019-07-18)

If a white dwarf compresses to the limit of electron degeneracy, and a neutron star compresses to the limit of neutron degeneracy, what does a black hole compress to the limit of?

**Tags:** black-hole
> **Comments:**
> **username_1**: We don't know...  
> **username_2**: Neither of the above statements is really true. Degeneracy is a continuous parameter and does not have a "limit" except at infinite density. The reasons that white dwarfs and neutron stars do not exist towards infinite density is not because of some limit on the degeneracy is reached, but for other physical reasons - neutronisation in one case and GR in the other.  
> **username_3**: This answer may help you: https://physics.stackexchange.com/a/141876/232868  

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## 2 answers

### Answer by username_7 on 2019-07-18. Score: 20 | Accepted answer

In classical General Relativity, there *is* no limit to the compression in a black hole, hence you get a singularity. However, many astrophysicists feel that's unphysical, and that a theory which unites General Relativity and Quantum Mechanics will impose some kind of limit, perhaps something connected with the quantization of spacetime itself.

We don't have a working theory of quantum gravity, so at this stage we don't exactly know what happens in the core of a black hole. OTOH, we're fairly confident that the core has to be very small, since quantum gravity effects probably don't kick in until a scale much smaller than the size of an atom, and probably smaller than a proton, somewhere around the scale of the [Planck length](https://en.wikipedia.org/wiki/Planck_length).


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### Answer by username_4 on 2019-07-18. Score: 7

As far as current physics knows, nothing. This is the reason why it is commonly thought that a singularity exists in the middle of a black hole.

However, singularities are also thought to be non-physical, so there is most likely something else inside a black hole — we just don’t have the science to describe it just yet.

> **Comments:**
> **username_5**: It's not really "commonly thought" that there's a singularity in the middle of the black hole (except in sci-fi, though that also gives you things like the event horizon being a physical barrier you can shoot your way through, so...). It's just the most straighforward result you get from applying general relativity to the problem, and assuming there's nothing beyond neutron degenerancy pressure to prevent further collapse. Not to mention the assumption that the matter actually had time to reach the "singularity". AFAICT, to a physicist, singularity is *not* a thing that exists - it's a mistake  
> **username_4**: Exactly, I agree 100%, my wording is inaccurate as this was the exact meaning I wanted to convey  
> **username_6**: @Luaan Isn't *"nothing beyond Neutron degeneracy pressure"* a bit misleading? My understanding is, that according to plain GR, the singularity is in the future of anything inside the event horizon, and no force (measurable in Newtons, like exerted by the degeneracy pressure) can prevent that. AIUI, this is why having new strange forms of dense matter is basically irrelevant inside a black hole in the GR, the space-time doesn't care.  
> **username_5**: @hyde No, there's no magic involved - the spacetime within a black hole is exactly the same as outside of the black hole (as far as plain GR is concerned). The only thing that might be different is the part we now mark as a "singularity". There's certainly nothing special happening as you cross the event horizon. Of course, you still can't go back "outside", but that's mainly because there's no path leading outside. Mind, there is truth to saying "the singularity is in the future of anything inside the event horizon"; but that's analogous to us moving through time - as is leaving the hole.  
> **username_5**: @hyde It would definitely make a great question in its own right, though :)  

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