Are Black Holes Necessarily Spherical?

[~enigma~]

A neutron star is the less massive cousin of a black hole and is composed almost entirely of neutrons. In a neutron star, protons and electrons are squeezed so tightly, they form neutrons and thus lose their charge. I imagine the same thing must happen in a black hole so how could a significant charge exist?

Reread what I wrote. A significant charge can't exist. Not because of any degenerate matter but because a BH would attract oppositly charged particles thus neutralizing any significant charge.

 

[Monketi Ghost]

Yes, the are necesserily sphrical. Always.

Sweet Fanny Moses, you've got to get away from
1) Making such sweeping statements (especially with regard to your bailiwick, physics; 'always' often applies, and often does not.)

2) Being so confidently wrong.

 

[KingMerv00]

Reread what I wrote. A significant charge can't exist. Not because of any degenerate matter but because a BH would attract oppositly charged particles thus neutralizing any significant charge.

I understood what you wrote. I was agreeing with you.

 

[~enigma~]

I understood what you wrote. I was agreeing with you.

Oh...ok

 

[Dorfl]

Hmm what does charge do to a black hole's shape?

Not much, I think. At least, Wiki's Charged Black Hole article does not mention any change. On the other hand, it does somehow add another event horizon inside the normal one. It's explained in this article, but I don't claim to understand any of it.

 

[sol invictus]

Not much, I think. At least, Wiki's Charged Black Hole article does not mention any change. On the other hand, it does somehow add another event horizon inside the normal one. It's explained in this article, but I don't claim to understand any of it.

Charged, non-rotating black holes are spherical - they have to be, since there's nothing like an angular momentum vector to pick out a special direction - but their spacetime curvature is different, and (like rotating holes) they at least naively seem to have two horizons (I think any real hole actually only has one, but that hasn't been definitely settled either way).

 

[KingMerv00]

Charged, non-rotating black holes are spherical - they have to be, since there's nothing like an angular momentum vector to pick out a special direction - but their spacetime curvature is different, and (like rotating holes) they at least naively seem to have two horizons (I think any real hole actually only has one, but that hasn't been definitely settled either way).

Two questions Sol, both are born from random curiousity:

1. What exactly do you do for a living? You always come out as the most knowledgable authority in threads like this.

2. What do you think of the theoretical possibility of a naked singularity (ie a singularity without an event horizon)? I've heard they could possibly form from a rapidly spinning black hole.

 

[Dorfl]

Charged, non-rotating black holes are spherical - they have to be, since there's nothing like an angular momentum vector to pick out a special direction - but their spacetime curvature is different, and (like rotating holes) they at least naively seem to have two horizons (I think any real hole actually only has one, but that hasn't been definitely settled either way).

Hmm... Do their radius change any if you change the charge?

 

[Lensman]

Would it be possible to have a toroidal black hole, ie. the event horizon would be a toroid?

I read somewhere (possibly in an Isaac Asimov popular science book) that it would be possible, if it could spin fast enough. I know The Good Doctor was a genius & an excellent researcher, but black hole physics were a little out of his bailiwick.

 

[KingMerv00]

Would it be possible to have a toroidal black hole, ie. the event horizon would be a toroid?

I don't know the answer but for some reason I think a toroidal black hole would not create an event horizon.

(WARNING WARNING! I may be full of ***** ! Don't take my word for it!)

Edit: From this link:

From concepts drawn of rotating black holes, it is shown that a singularity, spinning rapidly, can become a ring-shaped object. This results in two event horizons, as well as an ergosphere, which draw closer together as the spin of the singularity increases. When the outer and inner event horizons merge, they shrink toward the rotating singularity and eventually expose it to the rest of the universe.

 

[~enigma~]

Would it be possible to have a toroidal black hole, ie. the event horizon would be a toroid?

The singularity of a rotating BH is a toroid.

 

[KingMerv00]

The singularity of a rotating BH is a toroid.

So is my link above flat out wrong or does the naked singularity situation only apply to specific toroidal black holes?

 

[~enigma~]

So is my link above flat out wrong or does the naked singularity situation only apply to specific toroidal black holes?

Your link says rotating BH have toroidal singularities. I was not speaking of naked singularities. But then again, I would never get too comfortable with wikipedia as a scientific source.

 

[KingMerv00]

Your link says rotating BH have toroidal singularities.

Yes and that the event horizons could cancel out leaving neither one.

But then again, I would never get too comfortable with wikipedia as a scientific source.

I usually check the references and besides, I recall reading about it in "Scientific American" or some similar magazine. I'm not saying it is true, only that it isn't yanked from the ether.

 

[sol invictus]

1. What exactly do you do for a living? You always come out as the most knowledgable authority in threads like this.

I plead the 5th, as they say in the States.

2. What do you think of the theoretical possibility of a naked singularity (ie a singularity without an event horizon)? I've heard they could possibly form from a rapidly spinning black hole.

I doubt it - I suspect it's impossible. There's a conjecture to that effect, but it hasn't been proven.

Hmm... Do their radius change any if you change the charge?

"Radius" is a slightly tricky concept in a curved space. But if you define it (as is usually done) as the square root of the area of the horizon divided by 4 pi, then yes - it depends on the charge (with mass held fixed).

Would it be possible to have a toroidal black hole, ie. the event horizon would be a toroid?

I read somewhere (possibly in an Isaac Asimov popular science book) that it would be possible, if it could spin fast enough. I know The Good Doctor was a genius & an excellent researcher, but black hole physics were a little out of his bailiwick.

Not in three spatial dimensions. But it was recently discovered that it's possible in more than three - as are arrangements like "black Saturns" (a ring around a sphere).

The wiki referenced above is talking about the singularity, which is something else.

 

[Dorfl]

"Radius" is a slightly tricky concept in a curved space. But if you define it (as is usually done) as the square root of the area of the horizon divided by 4 pi, then yes - it depends on the charge (with mass held fixed).

Hmm... It seems like I will need to wait a few years before I know enough maths to understand what that answer really means.

Not in three spatial dimensions. But it was recently discovered that it's possible in more than three - as are arrangements like "black Saturns" (a ring around a sphere).

That sounds bizarrely awesome. Do you have some link where we could read more?

 

[sol invictus]

Hmm... It seems like I will need to wait a few years before I know enough maths to understand what that answer really means.

It's not so complicated. Imagine drawing a circle on a sphere - a latitude line, say. That latitude line has a length - the circumference of the circle. If you follow a longitude line from the north pole down to the latitude line, you might call that length the radius (you might also call the length from the south pole the radius). But as you'll see if you check, the circumference C does not equal 2 pi R if you define the radius that way. So instead, you might define radius as C/(2 pi).

In general in curved space "radius" can mean different things to different people. A good way to define it for black holes is the square root of the area divided by 4 pi, because that would be the radius in flat space.

That sounds bizarrely awesome. Do you have some link where we could read more?

Enjoy.

 

[geni]

So is my link above flat out wrong or does the naked singularity situation only apply to specific toroidal black holes?

My understanding is that it is posible to generate them briefly with highly asymetrical collapses.

 

[Ziggurat]

Hmm... It seems like I will need to wait a few years before I know enough maths to understand what that answer really means.

It's not actually that hard to understand, but it isn't intuitive to newcomers and it's not taught in highschool geometry.

Suppose I have a flat but stretchable sheet of rubber. I draw a circle on that sheet, and pit a dot at the center. If I ask you what the radius of the circle is, well, you can answer that pretty easily and unambiguously.

But what if I hold the circle down on my table top, grap the spot at the center, and stretch it up? Now what's the radius of the circle? Is it the distance from the circle to the spot I've stretched up, or is it the circumference divided by 2 pi? Either definition is sensible, but they are no longer the same, so one must be careful to specify which it is one is talking about.

In the case of black holes, something a little similar happens in terms of the distortion of space in and around a black hole, though there are additional complications which we don't need to get into here. But the basic idea remains the same: we need to specify what it is we mean by radius since we're not in Euclidean space, and since we can't probe what's happening inside the black hole, a definition based on the surface area is an eminently sensible (though not the only possible) choice.

 

[Dorfl]

It's not so complicated. Imagine drawing a circle on a sphere - a latitude line, say. That latitude line has a length - the circumference of the circle. If you follow a longitude line from the north pole down to the latitude line, you might call that length the radius (you might also call the length from the south pole the radius). But as you'll see if you check, the circumference C does not equal 2 pi R if you define the radius that way. So instead, you might define radius as C/(2 pi).

In general in curved space "radius" can mean different things to different people. A good way to define it for black holes is the square root of the area divided by 4 pi, because that would be the radius in flat space.

It's not actually that hard to understand, but it isn't intuitive to newcomers and it's not taught in highschool geometry.

Suppose I have a flat but stretchable sheet of rubber. I draw a circle on that sheet, and pit a dot at the center. If I ask you what the radius of the circle is, well, you can answer that pretty easily and unambiguously.

But what if I hold the circle down on my table top, grap the spot at the center, and stretch it up? Now what's the radius of the circle? Is it the distance from the circle to the spot I've stretched up, or is it the circumference divided by 2 pi? Either definition is sensible, but they are no longer the same, so one must be careful to specify which it is one is talking about.

In the case of black holes, something a little similar happens in terms of the distortion of space in and around a black hole, though there are additional complications which we don't need to get into here. But the basic idea remains the same: we need to specify what it is we mean by radius since we're not in Euclidean space, and since we can't probe what's happening inside the black hole, a definition based on the surface area is an eminently sensible (though not the only possible) choice.

Ok. I think I sort of get that. I have some kind of intuitive picture of what it means now.

Enjoy.

Thanks.