<p>It's best to distinguish black holes and the singularity at the center of a black hole. From the misnamed <a href="https://www.quantamagazine.org" rel="nofollow noreferrer">Quanta magazine</a> article <a href="https://www.quantamagazine.org/black-hole-singularities-are-as-inescapable-as-expected-20191202/" rel="nofollow noreferrer">Black Hole Singularities Are as Inescapable as Expected</a>,</p>
<blockquote>
<p>A singularity is not a place where quantities really become infinite, but “a place where general relativity breaks down,” Chesler explained.</p>
</blockquote>
<p>I wrote &quot;misnamed&quot; because the person of interest in that article, <a href="https://scholar.google.com/citations?user=GtrUu8AAAAAJ&amp;hl=en" rel="nofollow noreferrer">Paul Chesler</a>, specifically told Quanta magazine that singularities are places where general relativity breaks down. The view that the predicted singularity at the center of a black hole indicates a place where general relativity breaks down is widespread amongst theoretical physicists. Nobody yet knows what will supersede general relativity to describe what happens will inside a black hole.</p>
<p>On the other hand, the view that black holes do exist is almost universal amongst physicists and astronomers. (The only holdouts are generally viewed as crackpots.) Black holes are about the only thing that can explain objects such as <a href="https://en.wikipedia.org/wiki/Cygnus_X-1" rel="nofollow noreferrer">Cygnus X-1</a>, the <a href="https://en.wikipedia.org/wiki/Sagittarius_A*" rel="nofollow noreferrer">supermassive object at the center of our galaxy</a>, (or for that matter, the supermassive objects that appear to reside at the center of almost every galaxy), <a href="https://en.wikipedia.org/wiki/Quasar" rel="nofollow noreferrer">quasars</a>, the collisions detected by the <a href="https://www.ligo.caltech.edu" rel="nofollow noreferrer">Laser Interferometer Gravitational-Wave Observatory (LIGO)</a>, and many other observations that black holes are real. The singularity at the center of a black hole might or might not be real.</p>
<blockquote>
<p>Why do black holes grow?</p>
</blockquote>
<p>Except for Hawking radiation (which has not yet been observed, except in <a href="https://phys.org/news/2021-02-stationary-hawking-analog-black-hole.html" rel="nofollow noreferrer">analogs using sound</a>), black holes are inescapable. An object that crosses the event horizon makes a black hole gain mass, maybe change angular momentum, and maybe change charge. Per the <a href="https://en.wikipedia.org/wiki/No-hair_theorem" rel="nofollow noreferrer">no hair conjecture</a>, mass, angular momentum, and charge are the only three characteristics of a black hole that can be observed from outside the black hole.</p>

CC BY-SA 4.0
2022-12-18T12:54:02.020Z
2022-12-18T12:54:02.020Z
David Hammen
<p>Don't think of the singularity as a &quot;what&quot;, it is more like a &quot;when&quot;. If you fall into a black hole you will end up at the singularity. Every direction in spacetime leads towards it.  In this way it is like a &quot;when&quot; because just as we can't avoid the future, we can't avoid the singularity in a black hole.</p>
<p>Rather than think of a black hole as a singularity that is pulling things towards it, don't think of the singularity as a thing with density. Think of a black hole as a region of spacetime with very strong gravity. This region has three properties: mass, charge and angular momentum. You don't need to send information about the mass, charge and angular momentum, because those are properties of the spacetime, not properties of the singularity (remember the singularity isn't a &quot;what&quot;).</p>
<p>The singularity doesn't gain mass, the black hole gains mass, when things fall into it.</p>
<p>The singularity does have a definable position in spacetime (that is about all it has). It is at r=0 in spherical coordinates. This justifies drawing it as a dot in the middle of the black hole. But that picture can be confusing, since it doesn't account for the bending of spacetime. You couldn't see the singularity because it is always in your future. That is why it is better to think of the singularity as a &quot;when&quot; not a &quot;what&quot;.</p>
<p>Quantum gravity might show that singularities don't exist. But the General Relativity model of black holes is sufficient to describe how they have mass and how the mass can increase.</p>

CC BY-SA 4.0
2022-12-17T07:57:17.593Z
2022-12-17T08:11:40.090Z
2022-12-17T08:11:40.090Z
James K
<p>I'll use the term 'understand' here. By that, I mean that I understand in laymen's terms. From my questions, it should be obvious that I don't understand mathematically.</p>
<p>I understand how black holes form, and I understand how matter is accreted. I even understand how they will eventually evaporate. But, there's a lot that I don't grasp.</p>
<p>At the centre of a black hole is a singularity, and by definition that cannot grow any bigger (or smaller). It is infinitely dense and therefore can't gain any density. It doesn't have a definable position in space-time, so how can something 'hit' it? How does a singularity get bigger in some way -- how does it gain mass?</p>
<p>Then there's the idea that information cannot escape a black hole, but, surely, if adding matter increases its mass and that has an effect on the space-time surrounding the black hole, then some information is indeed escaping -- information about how much it has eaten.</p>
<p>And, how can something outside of space-time affect space-time?</p>
<p>Or, do I need to wait for quantum-gravity to be solved to get any answers?</p>

CC BY-SA 4.0
2022-12-17T08:07:20.813Z
From  https://physics.stackexchange.com/a/144458/123208 "A singularity in GR is like a piece that has been cut out of the manifold. It's not a point or point-set at all."
PM 2Ring
CC BY-SA 4.0
2022-12-17T08:08:50.800Z
My answer [here](https://astronomy.stackexchange.com/a/32677/16685) may be helpful. "The gravitational field of a black hole is sometimes described as a "fossil field". All matter & energy falling into the black hole modifies the spacetime curvature as it approaches the event horizon. And once it crosses the event horizon it can no longer change the spacetime curvature outside the horizon, so those curvature changes are preserved (until something else comes along to add its own curvature changes)."
PM 2Ring
CC BY-SA 4.0
2022-12-17T09:24:55.513Z
@PM2Ring - Oh, I see. That's really interesting and clears it up nicely. Of course, it leads to other questions, but I suppose these things always do. Thanks.
Software Engineer
CC BY-SA 4.0
2022-12-17T11:16:56.507Z
To be a bit mathematically pedantic, this: "It is infinitely dense and therefore can't gain any density" doesn't mean the *mass* can't grow. The supposed "infinite density" comes from taking a finite mass and dividing it by zero volume. As long as the volume stays zero, you can increase the mass as much as you like.
Peter Erwin
CC BY-SA 4.0
2022-12-17T18:56:31.337Z
On "infinity cannot get bigger" note that infinity plus anything is still infinity.  And infinity plus infinity is still infinity.  It's not a number in the everyday sense and you cannot think of it that way.
StephenG - Help Ukraine
CC BY-SA 4.0
2022-12-18T09:20:34.800Z
@StephenG-HelpUkraine - to get really pedantic, 'infinity + anything' isn't actually infinity -- it's undefined because the normal arithmetic operators are undefined when dealing with infinity: as you say, it's not a number. That's why I say that something that is infinitely dense can't gain any density, because you can't add anything to it once it is infinite.
Software Engineer
CC BY-SA 4.0
2022-12-18T13:04:54.290Z
@SoftwareEngineer Per [What Every Computer Scientist Should Know About Floating-Point Arithmetic](https://docs.oracle.com/cd/E19957-01/800-7895/800-7895.pdf), you as a self-proclaimed software engineer should know that 1+infinity (or any finite number plus infinity) is infinity rather than a NaN (not a number). The IEEE floating point standard was intentionally designed to emulate the [extended real number line](https://en.wikipedia.org/wiki/Extended_real_number_line).
David Hammen
CC BY-SA 4.0
2022-12-18T13:13:59.067Z
@SoftwareEngineer: Here's a simple python3 script (python typically uses IEEE floating point): `import math; print(math.inf+1);` The result is `inf` rather than `nan`.
David Hammen
CC BY-SA 4.0
2022-12-18T19:09:25.673Z
@PMRing: Does the crossing of the event horizon occur for an outside observer though, in finite time? I think it does not. Still, the gravitational force of the material close to the EH is added on top of the pre-existing curvature outside of the EH. I remember discussions of 'massive rings' just outside the EH, is that what essentially gives Sgr A* its mass, or is that concept incorrect?
AtmosphericPrisonEscape
<black-hole>
Why do black holes grow?
Software Engineer
# Why do black holes grow?

(Asked by: username_0 on 2022-12-17)

I'll use the term 'understand' here. By that, I mean that I understand in laymen's terms. From my questions, it should be obvious that I don't understand mathematically.

I understand how black holes form, and I understand how matter is accreted. I even understand how they will eventually evaporate. But, there's a lot that I don't grasp.

At the centre of a black hole is a singularity, and by definition that cannot grow any bigger (or smaller). It is infinitely dense and therefore can't gain any density. It doesn't have a definable position in space-time, so how can something 'hit' it? How does a singularity get bigger in some way -- how does it gain mass?

Then there's the idea that information cannot escape a black hole, but, surely, if adding matter increases its mass and that has an effect on the space-time surrounding the black hole, then some information is indeed escaping -- information about how much it has eaten.

And, how can something outside of space-time affect space-time?

Or, do I need to wait for quantum-gravity to be solved to get any answers?

**Tags:** black-hole
> **Comments:**
> **username_1**: From  https://physics.stackexchange.com/a/144458/123208 "A singularity in GR is like a piece that has been cut out of the manifold. It's not a point or point-set at all."  
> **username_1**: My answer [here](https://astronomy.stackexchange.com/a/32677/16685) may be helpful. "The gravitational field of a black hole is sometimes described as a "fossil field". All matter & energy falling into the black hole modifies the spacetime curvature as it approaches the event horizon. And once it crosses the event horizon it can no longer change the spacetime curvature outside the horizon, so those curvature changes are preserved (until something else comes along to add its own curvature changes)."  
> **username_0**: @PM2Ring - Oh, I see. That's really interesting and clears it up nicely. Of course, it leads to other questions, but I suppose these things always do. Thanks.  
> **username_2**: To be a bit mathematically pedantic, this: "It is infinitely dense and therefore can't gain any density" doesn't mean the *mass* can't grow. The supposed "infinite density" comes from taking a finite mass and dividing it by zero volume. As long as the volume stays zero, you can increase the mass as much as you like.  
> **username_3**: On "infinity cannot get bigger" note that infinity plus anything is still infinity.  And infinity plus infinity is still infinity.  It's not a number in the everyday sense and you cannot think of it that way.  
> **username_0**: @StephenG-HelpUkraine - to get really pedantic, 'infinity + anything' isn't actually infinity -- it's undefined because the normal arithmetic operators are undefined when dealing with infinity: as you say, it's not a number. That's why I say that something that is infinitely dense can't gain any density, because you can't add anything to it once it is infinite.  
> **username_4**: @SoftwareEngineer Per [What Every Computer Scientist Should Know About Floating-Point Arithmetic](https://docs.oracle.com/cd/E19957-01/800-7895/800-7895.pdf), you as a self-proclaimed software engineer should know that 1+infinity (or any finite number plus infinity) is infinity rather than a NaN (not a number). The IEEE floating point standard was intentionally designed to emulate the [extended real number line](https://en.wikipedia.org/wiki/Extended_real_number_line).  
> **username_4**: @SoftwareEngineer: Here's a simple python3 script (python typically uses IEEE floating point): `import math; print(math.inf+1);` The result is `inf` rather than `nan`.  
> **username_5**: @PMRing: Does the crossing of the event horizon occur for an outside observer though, in finite time? I think it does not. Still, the gravitational force of the material close to the EH is added on top of the pre-existing curvature outside of the EH. I remember discussions of 'massive rings' just outside the EH, is that what essentially gives Sgr A* its mass, or is that concept incorrect?  

---

## 2 answers

### Answer by username_4 on 2022-12-18. Score: 3

It's best to distinguish black holes and the singularity at the center of a black hole. From the misnamed [Quanta magazine](https://www.quantamagazine.org) article [Black Hole Singularities Are as Inescapable as Expected](https://www.quantamagazine.org/black-hole-singularities-are-as-inescapable-as-expected-20191202/),

> A singularity is not a place where quantities really become infinite, but “a place where general relativity breaks down,” Chesler explained.

I wrote "misnamed" because the person of interest in that article, [Paul Chesler](https://scholar.google.com/citations?user=GtrUu8AAAAAJ&hl=en), specifically told Quanta magazine that singularities are places where general relativity breaks down. The view that the predicted singularity at the center of a black hole indicates a place where general relativity breaks down is widespread amongst theoretical physicists. Nobody yet knows what will supersede general relativity to describe what happens will inside a black hole.

On the other hand, the view that black holes do exist is almost universal amongst physicists and astronomers. (The only holdouts are generally viewed as crackpots.) Black holes are about the only thing that can explain objects such as [Cygnus X-1](https://en.wikipedia.org/wiki/Cygnus_X-1), the [supermassive object at the center of our galaxy](https://en.wikipedia.org/wiki/Sagittarius_A*), (or for that matter, the supermassive objects that appear to reside at the center of almost every galaxy), [quasars](https://en.wikipedia.org/wiki/Quasar), the collisions detected by the [Laser Interferometer Gravitational-Wave Observatory (LIGO)](https://www.ligo.caltech.edu), and many other observations that black holes are real. The singularity at the center of a black hole might or might not be real.

> Why do black holes grow?

Except for Hawking radiation (which has not yet been observed, except in [analogs using sound](https://phys.org/news/2021-02-stationary-hawking-analog-black-hole.html)), black holes are inescapable. An object that crosses the event horizon makes a black hole gain mass, maybe change angular momentum, and maybe change charge. Per the [no hair conjecture](https://en.wikipedia.org/wiki/No-hair_theorem), mass, angular momentum, and charge are the only three characteristics of a black hole that can be observed from outside the black hole.


---

### Answer by username_6 on 2022-12-17. Score: 2

Don't think of the singularity as a "what", it is more like a "when". If you fall into a black hole you will end up at the singularity. Every direction in spacetime leads towards it. In this way it is like a "when" because just as we can't avoid the future, we can't avoid the singularity in a black hole.

Rather than think of a black hole as a singularity that is pulling things towards it, don't think of the singularity as a thing with density. Think of a black hole as a region of spacetime with very strong gravity. This region has three properties: mass, charge and angular momentum. You don't need to send information about the mass, charge and angular momentum, because those are properties of the spacetime, not properties of the singularity (remember the singularity isn't a "what").

The singularity doesn't gain mass, the black hole gains mass, when things fall into it.

The singularity does have a definable position in spacetime (that is about all it has). It is at r=0 in spherical coordinates. This justifies drawing it as a dot in the middle of the black hole. But that picture can be confusing, since it doesn't account for the bending of spacetime. You couldn't see the singularity because it is always in your future. That is why it is better to think of the singularity as a "when" not a "what".

Quantum gravity might show that singularities don't exist. But the General Relativity model of black holes is sufficient to describe how they have mass and how the mass can increase.


---

