I've been wondering about black holes

[Astro Noodles]

I've been wondering about the nature of Black Holes. Perhaps my time would be better spent concentrating on my job or something, but there you are. 😄

Anyway Is it absolutely necessary for a Black Hole to be a singularity? Is it possible to have an intermediate stage between a massive neutron star, for example, and a singularity? Something that is massive enough to prevent light escaping but is not a singularity.

[Ags]

If escape velocity at the surface of the hypothetical body is the speed of light, then the object will become a singularity pretty quick. Unless of course it is made of pure adamantium 😀

[andrew s]

I think you can. Firstly, the event horizon forms round a collapsing massive star before the singularity forms and secondly if you were outside a massive enough spherically distributed mass that was not dense enough to form a singularity (maybe as a thin shell of material).

Regards Andrew 

[saac]

I'm pretty sure that the singularity is a mathematical expression only and not a physical entity .  A singularity would have infinite density so I'm not sure that can be a real physical manifestation , it points more to the mathematics or physics being incomplete.  So if this is correct then the answer would be yes  - there must exist black holes without true singularities  - perhaps all physical black holes do.   A relationship between mass , volume and escape velocity which in turn would generate black holes without singularities would be useful.   Ok here comes the rider - I'm no expert on this so happy to be corrected.  

I suppose a starting point would be the general expression for escape velocity    v_escape= (2GM/R)^0.5^0.5

Edit  - I guess setting v as the speed of light and rearranging the equation to give ratio of M/R would give the mass/volume threshold for black holes ? Although I doubt it could be that simple  

Jim 

Edited July 18, 2021 by saac

[andrew s]

Yes it is it is the same equation for the Schwarzschild radius  https://en.m.wikipedia.org/wiki/Schwarzschild_radius which defines the event horizon for a non rotating spherical mass.

Regards Andrew 

Edited July 19, 2021 by andrew s

[Zermelo]

18 hours ago, Astro Noodles said:

Is it possible to have an intermediate stage

As has been pointed out above, the singularity appearing in the mathematical model is usually interpreted as "here there be dragons", i.e. general relativity predicts its own inadequacy to describe such an extreme condition.

This is all speculation, but it has been proposed that the quantum "degeneracy pressure" in a neutron star that resists further collapse (up to a point) might be succeeded by subsequent stages that could stabilize even denser bodies. See "quark star".

Edited July 19, 2021 by Zermelo
"more massive" > "denser"

[saac]

I was reading an article the other day on the topic of a "naked black hole " also known as a "naked singularity" - it may have been in last month's BBC Sky At Night magazine, can't remember for sure.  I had never heard of a "naked black hole" but it was intriguing reading about these theoretical objects.  In simple terms (needs to be for my understanding)  a naked black hole is a black hole without an event horizon and this is what makes these objects important albeit theorised.  The event horizon of a generic black hole prevents any prospect of information about the singularity being observed  but this may not be the case with a naked black hole.  The singularity may be exposed for observation - but what would that even mean.  If generic black holes are not exotic enough then the prospect of seeing into the heart of a black hole must surely be a theme for a sci fi movie

The thing that I don't understand is that without an event horizon would it still be a black hole - light could escape surely !   

https://en.wikipedia.org/wiki/Naked_singularity

Jim 

 

[Zermelo]

7 minutes ago, saac said:

The singularity may be exposed for observation - but what would that even mean

Possibly not good. Check out the onward link to Penrose's cosmic censorship hypothesis in that wiki article.

10 minutes ago, saac said:

without an event horizon would it still be a black hole

The term is used a little ambiguously, sometimes referring specifically to the singularity. Wikipedia goes for "a region of spacetime where gravity is so strong that nothing—no particles or even electromagnetic radiation such as light—can escape from it". I would say that a naked singularity is not a "region" of spacetime.

If you really want to fry your brain, have a read of the black hole information paradox and the potential role of the AdS/CFT correspondence in its resolution.