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So how DOES a black hole form?

HansMustermann said:
So, from the point of view of an external observer, does that collapse ever finish? Does that collapsing neutron star ever actually gets all its mass inside its Schwarzschild radius?
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You don't ever see it finish collapsing, no. It just red-shifts until the intensity drops below your detection threshold. Which is the same thing that would happen if you watched something fall into a black hole that had been infinitely old: you wouldn't see it reach the event horizon, you'd just see it red-shift as it approached, until it was too red-shifted to see anymore.
 
HansMustermann said:
Before I start, let me state that I'm not pushing any explanation of my own, because I don't even have one. Which is unsurprising, I guess, since I'm not a physicist. I genuinely get a brainfart just trying to think about how it swallows matter, and I'm hoping someone more knowledgeable can help me understand it.

Now I'm not asking about the whole thing about how many solar masses you need and all that, because that part I know already.

I'm also not asking about matter falling below the event horizon, and the Schwarzschild radius, 'cause that part is clear too.

But, here's the thing I don't get: how it gets an event horizon in the first place, and how does it get more mass.

And I'll start by stating how I understand it. Which is probably wrong, since I can't get anywhere with it. But maybe it will help someone spot and point out to me the point where I'm going into gaga land with it.

Schwarzschild calculated that limit for an eternal black hole. It has always existed and it always will. Well, that one is easy, 'cause you don't have to deal with it forming in the first place. And I know physics simplifies the model to what's relevant for the problem at hand, and I have no problem with it. (Not that it would matter to anyone else if a layman did have a problem with it, mind you.) Just it doesn't answer MY problem.

And my problem stems from the fact that we don't have anything that always existed, since the universe ain't that old. So it has to have formed at some point.

So let's say some chunk of matter swirls down the drain... err, accretion disk, and falls down into the black hole. From its point of view, of course, that happens in a finite time. From OUR frame of reference, though, time dilates increasingly the closer to the event horizon it gets, and it goes asymptotically towards the actual event horizon. So essentially it takes an infinite time for us to see it fall in. It only gets there at +infinity on the time axis.

And it doesn't help if I put it on a pinrose diagram, 'cause that line is still at infinity.

Essentially if I don't start with a pre-existing black hole, it sems to me like I can only get a FUTURE black hole, infinitely into the future. The matter never actually gets inside it, information never disappears into it, it just gets stuck at an apparent horizon, in infinitely slow motion.

So... how did it form in the first place? How can I end up with a present black hole, instead of a future one? Where am I thinking all wrong?
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Black holes do not exist. Collapsing stars become "planets". Strong x-ray sources are not infinitely collapsing stars, they are just strong electromagnetic phenomenon that we do not understand.

https://1.bp.blogspot.com/-Fa-snj_YZxc/WMGzhbALqBI/AAAAAAAAAYg/ks__VdicI0M1UWM1bCJz_jCehk_fv6_DwCLcB/s1600/snip.JPG
 
jeffreyw said:
Black holes do not exist. Collapsing stars become "planets". Strong x-ray sources are not infinitely collapsing stars, they are just strong electromagnetic phenomenon that we do not understand.

https://1.bp.blogspot.com/-Fa-snj_YZxc/WMGzhbALqBI/AAAAAAAAAYg/ks__VdicI0M1UWM1bCJz_jCehk_fv6_DwCLcB/s1600/snip.JPG
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I get the idea that dead stars turn into planets. I mean, it's completely wrong, but if you don't understand physics, I can see why someone might conclude that.

But what would make anyone think that planets like earth turn into a small moon over time? How's that supposed to work? What kind of delusion would lead someone to think something that crazy stupid?
 
I don't get the whole "time stops at the event horizon so how does matter fall in" argument. The uncertainty principle means that matter frozen in time must have an uncertain position, right? It seems like uncertainty in position plus the extreme gravity means nothing would stay at the horizon, it should quantum tunnel past it almost immediately, no?
 
phunk said:
I don't get the whole "time stops at the event horizon so how does matter fall in" argument.
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It's based on confusing a coordinate singularity with a real singularity.

The center is a real singularity. The event horizon is a coordinate singularity, if you use Schwarzchild coordinates. But it's not a singularity if you use other coordinates. However, since those other coordinates look "weird" away from the horizon, that's not what most people want to deal with, or can wrap their heads around.
 
Ziggurat said:
It's based on confusing a coordinate singularity with a real singularity.

The center is a real singularity. The event horizon is a coordinate singularity, if you use Schwarzchild coordinates. But it's not a singularity if you use other coordinates. However, since those other coordinates look "weird" away from the horizon, that's not what most people want to deal with, or can wrap their heads around.
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If I stand between an observer and a black hole, I can wrap my head around it.
 
Ziggurat said:
You don't ever see it finish collapsing, no. It just red-shifts until the intensity drops below your detection threshold. Which is the same thing that would happen if you watched something fall into a black hole that had been infinitely old: you wouldn't see it reach the event horizon, you'd just see it red-shift as it approached, until it was too red-shifted to see anymore.
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So does that mean I'm right that we can't actually "see" any real black holes, just stuff where most of the mass is frozen in time just outside it?

I'm not saying it doesn't finish falling in in other frames of reference, mind you. I know it does. I'm just talking about its effects on the rest of the universe. Basically that the rest of the universe hasn't ever "seen" an actual black hole.
 
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HansMustermann said:
So does that mean I'm right that we can't actually "see" any real black holes, just stuff where most of the mass is frozen in time just outside it?
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If most of the mass is frozen in time just outside the boundary, then more mass arrives, the boundary expands. The mass that was frozen in time is now inside the new boundary.
 
HansMustermann said:
So does that mean I'm right that we can't actually "see" any real black holes, just stuff where most of the mass is frozen in time just outside it?
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In one sense, yes. But the light coming out from infalling objects will not only redshift more and more over time, the intensity will also drop off. So the distinction between an object truly vanishing and an object becoming asymptotically "frozen" becomes, as a practical matter, impossible to observe.
 
Modified said:
If most of the mass is frozen in time just outside the boundary, then more mass arrives, the boundary expands. The mass that was frozen in time is now inside the new boundary.
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The mass itself is never actually frozen in time. Rather, the light from that mass falling through gets spread out over infinite time. Even if the event horizon expands because of additional mass, the exiting light is still spread over infinite time. It doesn't get chopped off. Additional mass will simply red shift it faster, it won't make it just vanish.
 
Ziggurat said:
The mass itself is never actually frozen in time. Rather, the light from that mass falling through gets spread out over infinite time. Even if the event horizon expands because of additional mass, the exiting light is still spread over infinite time. It doesn't get chopped off. Additional mass will simply red shift it faster, it won't make it just vanish.
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If the additional mass expands the event horizon, it will certainly make it vanish sooner. Also it isn't infinitely stretched, light is quantized. There will be a last photon emitted before the mass crosses the event horizon and that will escape in finite time.
 
theprestige said:
Obviously it does, since we see black holes and not weirdly time-dilated neutron stars.
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With our ability to image them, what would be the difference that we could detect? One has material inside the horizon, the other has it microscopically outside the horizon, and redshifted into invisibility.
 
Ziggurat said:
In one sense, yes. But the light coming out from infalling objects will not only redshift more and more over time, the intensity will also drop off. So the distinction between an object truly vanishing and an object becoming asymptotically "frozen" becomes, as a practical matter, impossible to observe.
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I know. I wrote the same thing about spreading the energy over more time in my SF gripes thread.

I just wanted to make sure I understood the theory right, rather than any kind of practical concerns. I mean, maybe other than using one as energy storage, like the Romulans in ST, I'm not even sure what practical thing to connect with a black hole. So, yeah, it's just my trying to understand the GR of it.

BTW, thank you very much for taking the time to answer. It is very much appreciated.
 
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BTW,

How fast is the actual collapse from a neutron star to becoming a black hole?

I wonder if there is an actual definite moment when a black hole is "emerging" or irreversible. If it is all dependent from the mass, there must be that one last single atom, that switches the mass into a black hole.
What would happen, if an observer could remove mass during the process of collapse? Would it stop the collapse? Which brings me back to the question about where/when is the irreversible moment happening?
 
Well, I should probably let the actual experts talk, but AFAIK, yeah, it's just a question of mass. It's that limit of about 2 solar masses I was talking about.

If it's below the limit, then neutron degeneracy pressure can stop the collapse. Basically you can imagine that the neutron star has enough compressive strength to bear its own load. At right below the limit of two solar masses, the whole neutron star has an about 12 km radius, which is about twice its Schwarzschild radius of about 6 km. So, yeah, as was said, twice the mass of the sun in a ball that's barely 24km in diameter, it's going to be very dense. And the surface gravity is going to be insane. But the neutron star is just about strong enough to bear the extreme weight of itself.

If it's above the limit, then no known force can stop the collapse, and essentially it goes into free fall towards its own centre. Singularity, here we come.

How fast, well, from outside, infinitely slow by the end.

That limit btw is for what's left of a much bigger star after it blows itself up at the end of its life. If the original star was just two solar masses, then what's left at the end is not nearly enough to form a black hole. So you need a much bigger star, so you have over two solar masses or so left at the end of it all.
 
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Ziggurat said:
The mass itself is never actually frozen in time. Rather, the light from that mass falling through gets spread out over infinite time. Even if the event horizon expands because of additional mass, the exiting light is still spread over infinite time. It doesn't get chopped off. Additional mass will simply red shift it faster, it won't make it just vanish.
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If the event horizon has expanded, then you can't have any more photons coming from inside the event horizon, no?
 
RussDill said:
If the event horizon has expanded, then you can't have any more photons coming from inside the event horizon, no?
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They didn't come from inside the event horizon. These are the photons which were moving outward already and made it out ahead of the advancing event horizon.
 
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