Have theories been formed yet on how to stop a black hole?

[Siddharth Menon]

Have theories been formed yet on how to stop a black hole?

 

[Drakkith]

What do you mean by "stop a black hole"?

 

[Siddharth Menon]

If Humans ever came under the need of stopping a black hole i.e. reduce its intense gravity by a huge enough amount.

 

[Drakkith]

No, there is no known or theorized way to do such a thing to a black hole.

 

[.Scott]

If a stellar black hole was known to be orbiting our Milky Way galaxy on a collision course with us, it would be easier to move our Solar System out of the way than to alter the path of the black hole.

 

[Siddharth Menon]

Does the black hole increase in gravitational force for all the matter it sucks in?

 

[phinds]

Does the black hole increase in gravitational force for all the matter it sucks in?

Yes

 

[mathman]

Unless you are very close, the gravity due to a black hole is the same as due to a star of the same mass.

 

[Siddharth Menon]

Why doesn't the black hole collapse on itself?
How does it remain stable with all the mass it compresses into nothing?

 

[phinds]

Why doesn't the black hole collapse on itself?
How does it remain stable with all the mass it compresses into nothing?

Uh ... it DOES collapse. That's what it MEANS to be a black hole ... all the matter has collapsed into the singularity at the center. Now "singularity" is just a word meaning "we don't really know WHAT is going on in there", but collapse is the standard description.

 

[Chronos]

I would prefer to say a collapsing star can form an event horizon. We currently have no accepted model for physics inside the event horizon.

 

[martinbn]

We currently have no accepted model for physics inside the event horizon.

What do you mean? Are you talking about the singularities or simply about the inside of the black hole? Surely physics (general relativity) describes the inside as well.

 

[mathman]

General relativity gives a description. Unfortunately quantum theory says it can't happen that way. That is why no one knows what is really going on.

 

[martinbn]

General relativity gives a description. Unfortunately quantum theory says it can't happen that way. That is why no one knows what is really going on.

Still not sure I understand. If the black hole is huge and you are just within the horizon quantum theory will play no role in the description, not more than anywhere else where spacetime curvature is equally small.

 

[Nugatory]

I would prefer to say a collapsing star can form an event horizon. We currently have no accepted model for physics inside the event horizon.

There are three regions of interest here, not two, are there not? We have the region outside the event horizon, which is fairly well understood because we live there. Then there's the region inside the event horizon but still far from the central singularity, and there's the region near the central singularity. Are you uncomfortable accepting the GR predictions for the inside-but-far region?

(This is a question not an argument).

 

[phinds]

There are three regions of interest here, not two, are there not? We have the region outside the event horizon, which is fairly well understood because we live there. Then there's the region inside the event horizon but still far from the central singularity, and there's the region near the central singularity. Are you uncomfortable accepting the GR predictions for the inside-but-far region?

(This is a question not an argument).

I think it's a great question. I've often wondered about exactly that. It seems logical that just inside the EH of a supermassive BH there would be no significant change to the way things work, BUT ... I've learned that my "it seems logical" isn't much respected by the universe, so I'm curious if there's any theoretical basis for such a conclusion. The main reason I wonder is the statement I see a lot (and don't really understand) that once inside the EH, world lines become time-like, not space-like.

 

[mathman]

The problem with just inside the event horizon is that general relativity describes this as a transient where all matter collapses to a singularity, so just inside could simply be empty except for something just falling in.

 

[phinds]

The problem with just inside the event horizon is that general relativity describes this as a transient where all matter collapses to a singularity, so just inside could simply be empty except for something just falling in.

But that would seem to imply that the transmission of matter from just inside the EH to the singularity at the center happens instantaneous, which seems unlikely. Like Nugatory, I'm not being argumentative, just don't understand.

 

[zonde]

The problem is that you can't study in a scientific way interior of event horizon. That makes inside of event horizon (including event horizon itself) an unscientific speculation.

 

[Shailesh Pincha]

Still not sure I understand. If the black hole is huge and you are just within the horizon quantum theory will play no role in the description, not more than anywhere else where spacetime curvature is equally small.

That's where you had a mistake. Black Hole can't be huge. For an immense density which could also attract light by its gravitational force, you need a radius as small as possible and that's why the largest one has found to be of I guess diameter 64 km. If its huge then you can't be alive, coping with its immense gravity. To escape from it and live a peaceful life, we need to have an escape velocity more than light but we know nothing can be faster than light. Also black hole can collapse only if its density is such greater that it gets attracted by its own gravity and collapse and reaches to a point whereon he couldn't get any smaller and thus a big bang kind of stuff could occur.

But we will never be able to see it( before the big bang and after the black hole) because in between comes the event horizon...

 

[phinds]

That's where you had a mistake. Black Hole can't be huge. For an immense density which could also attract light by its gravitational force, you need a radius as small as possible and that's why the largest one has found to be of I guess diameter 64 km.

This is utter nonsense. Please read up on black holes before making such statements.

If its huge then you can't be alive, coping with its immense gravity. To escape from it and live a peaceful life, we need to have an escape velocity more than light but we know nothing can be faster than light. Also black hole can collapse only if its density is such greater that it gets attracted by its own gravity and collapse and reaches to a point whereon he couldn't get any smaller and thus a big bang kind of stuff could occur.

But we will never be able to see it( before the big bang and after the black hole) because in between comes the event horizon...

This is blather. It's too garbled to even try to correct. Please read some basic cosmology.

 

[Drakkith]

Black Hole can't be huge. For an immense density which could also attract light by its gravitational force, you need a radius as small as possible and that's why the largest one has found to be of I guess diameter 64 km.

Not true. Supermassive black holes exist, having radii of millions of kilometers and densities less than water.

 

[Tejaslion]

Why doesn't the black hole collapse on itself?
How does it remain stable with all the mass it compresses into nothing?

it is a single point. what we call a black hole, is just a point, it is called singularity.

 

[phinds]

it is a single point. what we call a black hole, is just a point, it is called singularity.

No, a black hole is everything inside the Event Horizon, not just at the center. The "singularity" is just the place where our current math model breaks down. The current belief (but no evidence yet) is that it is most likely NOT a point but until there is a good theory of quantum gravity, it can't be modeled in any other way than what we currently have, which gives a most-likely-non-physical ("singularity") result.

 

[RodB]

But that would seem to imply that the transmission of matter from just inside the EH to the singularity at the center happens instantaneous, which seems unlikely. Like Nugatory, I'm not being argumentative, just don't understand.

I'm not knowledgeable enough to vouch for it but one of the references describes it as the proper time of an in falling object crossing the Event Horizon is zero. Like one outside the EH can stop space movement but not time movement, one inside the EH stops proper time movement but can not stop space movement and (I assume) invariable heads for the singularity.

 

[RodB]

No, a black hole is everything inside the Event Horizon, not just at the center. The "singularity" is just the place where our current math model breaks down. The current belief (but no evidence yet) is that it is most likely NOT a point but until there is a good theory of quantum gravity, it can't be modeled in any other way than what we currently have, which gives a most-likely-non-physical ("singularity") result.

I read in one reference that the "singularity" is actually a Planck length radius, 10-35 m, though this strikes me as a distinction without a difference.

 

[Nugatory]

Black Hole can't be huge.

What exactly do you mean by "huge" here? If you mean the size of the central singularity, that's not a meaningful concept.

If you mean the volume inside the event horizon (strictly speaking, the volume of a sphere with radius equal to the Schwarzschild radius) then it can be arbitrarily large if there's enough mass in the black hole. For example, the black hole at the center of our galaxy is probably about 250 million kilometers across. Whether that's "huge" or not depends on your perspective, but if you were in close orbit around it, you'd probably be thinking "huge".

 

[RodB]

Not true. Supermassive black holes exist, having radii of millions of kilometers and densities less than water.

This doesn't sound correct as escape velocity is as much related to density as mass.

 

[Bandersnatch]

This doesn't sound correct as escape velocity is as much related to density as mass.

Take the escape velocity equation
$V=\sqrt{\frac{2GM}{r}}$
set $V=c$ and rearrange with $r$ on one side:
$r=\frac{2GM}{c^2}$
This is the Schwartzschild radius equation. As $M$ goes up, the radius increases linearly.
At the same time, assuming uniform density distribution of material inside, a sphere of density $\rho$ and radius $r$ will have mass:
$M =\frac{4}{3}\pi r^3\rho$
For $M$ and $r$ to change at the same rate, $\rho$ must change as $\frac{1}{r^2}$
That is, it has to fall as the black hole grows bigger.

 

[RodB]

Take the escape velocity equation
$V=\sqrt{\frac{2GM}{r}}$
set $V=c$ and rearrange with $r$ on one side:
$r=\frac{2GM}{c^2}$
This is the Schwartzschild radius equation. As $M$ goes up, the radius increases linearly.
At the same time, assuming uniform density distribution of material inside, a sphere of density $\rho$ and radius $r$ will have mass:
$M =\frac{4}{3}\pi r^3\rho$
For $M$ and $r$ to change at the same rate, $\rho$ must change as $\frac{1}{r^2}$
That is, it has to fall as the black hole grows bigger.

If one looks at only “normal” physics escape velocity is directly related to the square root of the density and to the radius of the body. If you hold the radius constant (like for the Earth) then escape velocity is determined only by the square root of the density times a constant. But then, as always, funny things happen as one approaches the speed of light and black holes. In either case you can have densities of 1.0 though black hole dimensions get slightly grotesque even for cosmological things – Schwartzschild radii of 13 billion Km with 4 trillion solar masses (assuming my math is correct). My only point is that it sometimes is convenient to think of escape velocity in terms of the density of the base body. But this gets really screwy with black holes. For example, what is the density of a black hole with a R-schwartzschild of a million Km and any mass but all concentrated in a singularity? Density is a helpful parameter with normal things but as you point out not much help with black holes.