black holes?

[jajrussel]

Does a black hole have to be spherical?

[insane_alien]

yes, this is because gravity is spherically symmetric(pulls equally in all directions.)

 

although in saying that it is possible to distort the event horizon from a sphere with another gravitational field but your not going to to see significant distortion unless you have another blackhole passing close by.

[jajrussel]

The reason I asked is because if the black hole could flatten out due to its own spin and or its spin and the gravitational pull of its surounding galaxy then the event horizon would have to change shape as well. Possibly reaching a point where its poles would no longer fit the discription of a black hole allowing energy and matter to escape.

[Klaynos]

The event horizon of a single black hole is spherically symmetric. There is a high chance it and it's accretion disk are spinning though. What do you mean about it's poles?

 

A black hole is just an object with all of it's mass within it's schwartzchild radius.

[J.C.MacSwell]

The event horizon of a single black hole is spherically symmetric. There is a high chance it and it's accretion disk are spinning though. What do you mean about it's poles?

 

A black hole is just an object with all of it's mass within it's schwartzchild radius.

 

Using the event horizon as the definition, it could be non-spherical while absorbing mass.

[jajrussel]

By poles I meant perpendicular to its spin. I guess it was a wayward thought. Thanks.

[iNow]

It's possible I'm being a bit too pedantic, but, since it spins, I'd presume it bulges at the center, hence... not a sphere. Is that not the case? Wouldn't it look more like a soccer ball with someone sitting on top than an actual sphere?

[J.C.MacSwell]

It's possible I'm being a bit too pedantic, but, since it spins, I'd presume it bulges at the center, hence... not a sphere. Is that not the case? Wouldn't it look more like a soccer ball with someone sitting on top than an actual sphere?

 

A spinning mass inside the event horizon would certainly have an event horizon that was non-spherical while collapsing. So at least temporarily that would be correct (I think?)

[Martin]

It's possible I'm being a bit too pedantic, but, since it spins, I'd presume it bulges at the center, hence... not a sphere. Is that not the case? Wouldn't it look more like a soccer ball with someone sitting on top than an actual sphere?

 

 

 

What's it called? The Kerr solution? The Schwarzschild solution is only for nonrotating.

There are two horizons, the outer one is oblate---like the slightly squashed soccer ball. But inside that there is another spherical surface which is the real event horizon.

I dug up these sources, if anybody else has some good ones please share:

http://www.astrophysicsspectator.com/topics/generalrelativity/BlackHoleKerr.html

http://en.wikipedia.org/wiki/Kerr_metric

The Wikipedia article looks very informative. I just read the first part of it.

Edited July 7, 2009 by Martin

[Daecon]

A black hole can only be spherical for the same reason that a star like our Sun can only ever be spherical too.

[ajb]

I expect that it is possible to have non-spherically symmetric horizons in 4d. (But what about non-topologically spherical?) However, only solutions to the field equations with high amount of symmetries can be described analytically. It is this I think to be the main reason people really only discuss spherical black holes.

 

There are also black strings and rings in higher dimensions.

 

See Roberto Emparan and Harvey S. Reall, Black Rings, Classical and Quantum Gravity, 23, 2006.

[jajrussel]

I hadn't thought of the sun. The sun is an in - out system. Things go in and things come out. Hawking radiation sugest the same for black holes. I am thinking about physical ways this could happen. I have accepted that a singularity with infinite density should resist changing shape, but once it begins to spin wouldn't the surrounding space begin to change shape, including the space between the singularity and it's event horizon?

[jajrussel]

I think I am beginning to understand. I have been thinking of the event horizon as being something of substance when all it is is a reference point related to a central point. It has no choice but to be spherical. When something crosses the event horizen the mass in the center will overwhelm the mass of what has been added by shear size, so the central point will remain unchanged. The only change in the event horizon would be it's reference to the center point, which would occure spherically.

After thinking about it I can't see anything crossing the event horizon then getting back out, except maybe space. I have wondered if space can be squeezed out of matter like water being squeezed out of a sponge.

[Sisyphus]

You're right not to think of it as "substance" (all the "stuff" is at the singularity itself) but I think calling it a "reference point" understates its significance. It's a boundary with very real physical significance. It is the point beyond which it is impossible that anything can escape. As I understand it (which might be a flawed understanding), because of the way space has been warped, no straight line originating within the event horizon leads anywhere but elsewhere inside the event horizon.

[occam]

Actually there are two solutions.

 

I can't seem to embed diagrams in the posts so you'll have to have an attachment.

 

occam

[jajrussel]

It wasn't my intent to lend insignificance to the event horizon. I think black holes have a very significant role to play in the universe as a whole. Thoughts that might be borderline crazy actually, (crazy but not magical). Some of the stuff I have read about time, space, and matter do seem more magic than physics, but then again things do seem magical until you understand the physics. The answer is how valid is the physics, and most of the time how many people accept the physics. (Occam, I want to say thank you for the attachment.)

[lakmilis]

sigh. Schwarzchild holes do not form naturally.

 

Yes, Kerr holes or even Kerr-Newman holes are the naturally occurring celestial ones. People always* hypothesize over these SR things.. double sigh.

 

IF something could cross an event horizon and disregarding the *actual* grav. tide effects etc. then sure, you could send a non-angualr momentum and non charged object and consider effects on that possibly Schw.

 

Artificial BHs if we ever create them certainly could be SR. not natural ones. big no no.