Would enough inert rock clumped together form a black hole ?

[phinds]

When stars that have enough mass lose their internal energy (or most of it) they condense to a black hole. Would a large enough mass of cold, inert rock collapse into a black hole as soon as the mass became big enough as the rocks came together?

 

[narrator]

From what I understand, there usually has to be an event which triggers the reaction (like a star collapsing), but I guess under the Gib Gnab scenario, if you get a huge amount together, it will eventually pull its own trigger. (Just some lay musings...)

 

[Dmitry67]

BH formation does not require high density of matter.
You can make black hole of water, air, donuts, and even from interstellar gas - even without compressing it.

Of course, such material (water, donuts) won't be able to support it's own weight and it would collapse, becoming denser and denser, accelerating the formation of the balck hole. But this is just a side effect.

 

[Chalnoth]

When stars that have enough mass lose their internal energy (or most of it) they condense to a black hole. Would a large enough mass of cold, inert rock collapse into a black hole as soon as the mass became big enough as the rocks came together?

Sure, if enough of it came together, it'd eventually collapse in on itself. However, I should mention that it would more likely collapse into a neutron star, not a black hole. To collapse into a black hole directly, you need many times the mass required to collapse into a neutron star. So the question is: how are you going to bring all those cold rocks together, at the same time, to make a black hole without first making a neutron star?

 

[phinds]

Sure, if enough of it came together, it'd eventually collapse in on itself. However, I should mention that it would more likely collapse into a neutron star, not a black hole. To collapse into a black hole directly, you need many times the mass required to collapse into a neutron star. So the question is: how are you going to bring all those cold rocks together, at the same time, to make a black hole without first making a neutron star?

Thanks for that helpful description, Chalnoth. As you can likely tell, I'm an amatuer at all this. What's the process that makes the difference between a mass becoming a neutron star vs becoming a black hole?

 

[Chalnoth]

Thanks for that helpful description, Chalnoth. As you can likely tell, I'm an amatuer at all this. What's the process that makes the difference between a mass becoming a neutron star vs becoming a black hole?

It's all about pressure. Normal matter, made of atoms, can only hold up so much pressure, and the amount it can hold up depends upon temperature. So once the nuclear reaction at the center of a star halts, the temperature goes down, and, if it is massive enough, the atoms simply can't push hard enough to stop the atoms from collapsing in on themselves. So the electrons combine with the protons, producing neutrons, and we end up with a star comprised primarily of neutrons (expelling a tremendous amount of energy in the process in a supernova). This star is basically one big atomic nucleus.

But even neutrons themselves can only support so much pressure. When the mass of a neutron star gets large enough, it is just not possible for the neutrons to support the massive weight of the matter above them, and so the star collapses again. But this time there isn't anything that can stop the collapse, and so it forms a black hole.

 

[phinds]

Again very helpful. So IF a neutron star managed somehow to get enough additional mass it WOULD collapse further into a black hole. This really answers my underlying question which I now realize I should have posted in this fashion: is it JUST the amount of mass that creates a black hole or is it some sort of process that could not occur in inert rock.

I do understand that the collapse of a star to a neutron star is NOT just a gravitational collapse because energy is released, but since a mass that is not a black hole (a neutron star) CAN be made to further collapse into a black hole, it IS, from the standpoint of my question, the mass that counts.

This question came to me as a result of this thread:

https://www.physicsforums.com/showthread.php?t=494036

Thanks again.

Paul

 

[PRDan4th]

Is it not a combination of mass and distance from the center to the surface (radius) that create a black hole? If we use the Newtonian equation of acceleration due to gravity and set the escape velocity equal to the speed of light, we could calculate a mass/radius relationship that would be required for a black hole? In this case, a very small sub-atomic particle could be a black hole, as long as it had some mass and very very small radius.

 

[phinds]

Is it not a combination of mass and distance from the center to the surface (radius) that create a black hole? If we use the Newtonian equation of acceleration due to gravity and set the escape velocity equal to the speed of light, we could calculate a mass/radius relationship that would be required for a black hole? In this case, a very small sub-atomic particle could be a black hole, as long as it had some mass and very very small radius.

My understanding is that tiny black holes like you suggest can be created in accelerators, and they evaporate very quickly, so yes I do believe you are correct, BUT an uncollapsed mass of inert rock would, I think, have an EH, if such a thing exists for it, that is far inside the surface of the mass. That is, the radius you are talking about would be less than the radius of the ball of matter. Somehow, I don't think that qualifies as an EH

 

[Chalnoth]

Again very helpful. So IF a neutron star managed somehow to get enough additional mass it WOULD collapse further into a black hole. This really answers my underlying question which I now realize I should have posted in this fashion: is it JUST the amount of mass that creates a black hole or is it some sort of process that could not occur in inert rock.

Well, sort of. Basically, once matter is dense enough compared to its surroundings, it necessarily forms a black hole. In principle, there is nothing that prevents any amount of matter, no matter how small, from achieving this density and becoming a black hole. So in principle it is possible to make a black hole out of a golf ball.

However, in practice, the only way we know to achieve the densities required is through gravitational collapse of a very massive body. And there it is a combination of the total mass and the amount of pressure it is capable of sustaining. For example, you can make a star a couple hundred times the mass of our sun, and it will still not form a black hole as long as the core remains hot enough to support that outward pressure.

Once the core cools, it can't support the pressure any more, and it collapses to densities sufficient to produce either a neutron star or a black hole.

I do understand that the collapse of a star to a neutron star is NOT just a gravitational collapse because energy is released, but since a mass that is not a black hole (a neutron star) CAN be made to further collapse into a black hole, it IS, from the standpoint of my question, the mass that counts.

Well, actually, it is primarily gravitational collapse. The energy release is comes from gravitational potential energy. If the Earth were to be collapsed to be as dense as a neutron star, for example, it would go from having a radius of about 6,400,000 meters to a radius of about 60 meters. But with the same mass, that results in a tremendous drop in gravitational potential energy, which, in turn, ends up meaning a tremendous release of energy.

In practice, when a massive star goes supernova, the core collapses into a neutron star while the outer layers of the star are blown away by the tremendous energy released from the gravitational collapse. The final neutron star is much less massive than the star it collapses from.

For a black hole, the difference is even greater: an Earth-mass black hole would have a radius of only a few centimeters (about the size of a golf ball).

 

[phinds]

Thanks again, Chalnoth. Could you take a look at post #9 and see if you have any comment about that as well.

 

[Chalnoth]

Thanks again, Chalnoth. Could you take a look at post #9 and see if you have any comment about that as well.

This is where the first comment by bcrowell in the other thread you linked is relevant: a black hole with an event horizon is a vacuum solution to General Relativity. This means that it is a solution that is only valid if everything outside the event horizon is a vacuum.

When you're talking about a ball of matter that has a mass with a corresponding Schwarzschild radius smaller than the ball of matter, you aren't talking about something that acts like Schwarzschild black hole at all. There simply isn't any event horizon.

 

[phinds]

I thought that would be the case, which is why I said " ... if such a thing exits for it."

"This means that it is a solution that is only valid if everything outside the event horizon is a vacuum. " That I didn't know/understand.


Thanks.

 

[Chalnoth]

I thought that would be the case, which is why I said " ... if such a thing exits for it."

"This means that it is a solution that is only valid if everything outside the event horizon is a vacuum. " That I didn't know/understand.


Thanks.

I should mention that the Schwarzschild solution is valid for the vacuum outside of any spherical hunk of matter that isn't rotating (you have to use the more complicated Kerr metric for rotating objects). It just isn't valid in the interior of the object.

 

[DaveC426913]

BH formation does not require high density of matter.
You can make black hole of water, air, donuts, and even from interstellar gas - even without compressing it.

Of course, such material (water, donuts) won't be able to support it's own weight and it would collapse, becoming denser and denser, accelerating the formation of the balck hole. But this is just a side effect.

Would you care to explain this? You first say it does not require a high density matter, then you say it will collapse under its weight and become dense enough to form a BH.

I think what youre saying is that the starting matter doesn't have to be dense. If you get enough of it, it will become dense.

What I don't understand is why you needed to state that, since no one said otherwise. The OP was suggesting inert rock, not something particularly dense.