<p>Ignoring quantum gravity, which we don't know much about, current theory (GR) states that an event horizon forms when there is enough matter/energy density to create it, and logically, a singularity I guess forms a tiny fraction of a second later.</p>
<p>I know from other reading that in GR:</p>
<ul>
<li>the event horizon forms when enough matter/energy density occurs in a region of space that all futures point inward (this is the point we can say a BH comes into existence)</li>
<li>that infalling matter at a black hole reaches the singularity after finite time as it perceives time, but can't see it in advance, as its never in their past light cone.  I'm not sure what the terminology/adjective is, for their time in this context, but finite time seems an intuitive term</li>
<li>that the notion of time and really any spacetime coordinates is complex and need choosing with care,and an easy source of confusion</li>
<li>that the apparent slowing/dimming/redshifting of an infalling object is effectively as perceived by outside observers, rather than the &quot;experience&quot; of the infalling matter, and can probably be ignored in this issue <em>(mentioned for clarity, rather than because relevant per se)</em></li>
<li>that the singularity isn't as much a &quot;point&quot;, as a gap or absent part of spacetime geometry.</li>
</ul>
<p>Logically, imagining a spherical collapse, matter density and EH are achieved before the originating material reaches the &quot;centre&quot;, so the EH presumably predates the creation of the singularity by some tiny amount of time.</p>
<p>So I'm curious about what GR has to say, about after an event horizon comes into existence as matter collapses inward (say), and up to/including the singularity coming into existence (if other effects don't prevent it).</p>
<p>Beyond the rather trite &quot;it happens&quot;, that is.</p>

CC BY-SA 4.0
2021-04-21T12:39:22.347Z
This could belong on Physics SE too.
WarpPrime
CC BY-SA 4.0
2021-04-21T21:05:14.260Z
Here's an existing Physics.SE question that (kind of) answers your question. https://physics.stackexchange.com/q/433712/123208
PM 2Ring
CC BY-SA 4.0
2021-04-21T22:26:52.107Z
Relevant but not really an answer? It does raise the question whether an EH first forms as infinitesimal (for all collapses regardless, and can then grow after), or first forms when sufficient matter has collapsed within any sufficiently small region of space, with initial size = that of the region of space. But not the sequence leading to existence of the singularity, so to speak.
Stilez
CC BY-SA 4.0
2021-04-21T22:56:58.350Z
Well, it says that in the Oppenheimer-Snyder collapse, the EH starts forming at the centre, and grows outwards. O-S is not a very realistic model but at least it's dynamic and it has some matter in it (the original Schwarzschild solution is an eternal vacuum solution).
PM 2Ring
CC BY-SA 4.0
2021-04-22T09:42:46.743Z
I would better see this on Physics SE.
Alchimista
<black-hole><general-relativity><singularity><event-horizon>
Does General Relativity predict anything about the creating of a singularity, once enough matter collapses inward to form an event horizon?
Stilez
# Does General Relativity predict anything about the creating of a singularity, once enough matter collapses inward to form an event horizon?

(Asked by: username_0 on 2021-04-21)

Ignoring quantum gravity, which we don't know much about, current theory (GR) states that an event horizon forms when there is enough matter/energy density to create it, and logically, a singularity I guess forms a tiny fraction of a second later.

I know from other reading that in GR:

* the event horizon forms when enough matter/energy density occurs in a region of space that all futures point inward (this is the point we can say a BH comes into existence)
* that infalling matter at a black hole reaches the singularity after finite time as it perceives time, but can't see it in advance, as its never in their past light cone. I'm not sure what the terminology/adjective is, for their time in this context, but finite time seems an intuitive term
* that the notion of time and really any spacetime coordinates is complex and need choosing with care,and an easy source of confusion
* that the apparent slowing/dimming/redshifting of an infalling object is effectively as perceived by outside observers, rather than the "experience" of the infalling matter, and can probably be ignored in this issue *(mentioned for clarity, rather than because relevant per se)*
* that the singularity isn't as much a "point", as a gap or absent part of spacetime geometry.

Logically, imagining a spherical collapse, matter density and EH are achieved before the originating material reaches the "centre", so the EH presumably predates the creation of the singularity by some tiny amount of time.

So I'm curious about what GR has to say, about after an event horizon comes into existence as matter collapses inward (say), and up to/including the singularity coming into existence (if other effects don't prevent it).

Beyond the rather trite "it happens", that is.

**Tags:** black-hole, general-relativity, singularity, event-horizon

> **Comments:**
> **username_1**: This could belong on Physics SE too.  
> **username_2**: Here's an existing Physics.SE question that (kind of) answers your question. https://physics.stackexchange.com/q/433712/123208  
> **username_0**: Relevant but not really an answer? It does raise the question whether an EH first forms as infinitesimal (for all collapses regardless, and can then grow after), or first forms when sufficient matter has collapsed within any sufficiently small region of space, with initial size = that of the region of space. But not the sequence leading to existence of the singularity, so to speak.  
> **username_2**: Well, it says that in the Oppenheimer-Snyder collapse, the EH starts forming at the centre, and grows outwards. O-S is not a very realistic model but at least it's dynamic and it has some matter in it (the original Schwarzschild solution is an eternal vacuum solution).  
> **username_3**: I would better see this on Physics SE.  

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## 0 answers

