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Old Jun 11th 2017, 03:22 PM   #1
wad
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Shape of Black Holes

Most depictions of black holes on the web or magazines depict black holes as a vortex - looking into one as though like looking at the top of a tornado.

However, Hawking in his "A Brief History of Time" (p 100) uses the description of a black hole as have north and south poles, i.e., suggesting a spherical shape.

Is there any consensus of the probable physical 'shape' of a black hole?
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Old Jun 11th 2017, 03:59 PM   #2
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Originally Posted by wad View Post
Most depictions of black holes on the web or magazines depict black holes as a vortex - looking into one as though like looking at the top of a tornado.

However, Hawking in his "A Brief History of Time" (p 100) uses the description of a black hole as have north and south poles, i.e., suggesting a spherical shape.

Is there any consensus of the probable physical 'shape' of a black hole?
What you've seen in magazines come in three forms:

1) an embedding diagram which illustrates the relationship between radial distances and the Schwarzschild radius.

2) a diagram which shows the accretion disk of a black hole and not the black hole itself.

3) the false imagines created by sci-fi writers who don't understand the physics of black holes.

Hawking was referring to a specific kind of black hole, i.e. either a rotating one or a magnetized one. Otherwise a pole of a black hole doesn't have a real meaning.

A non-charged, non-magnet, non-rotating black hole is indeed spherical.
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Old Jun 11th 2017, 05:10 PM   #3
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And what then is the meaning of the event horizon for a sphere?
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Old Jun 11th 2017, 06:05 PM   #4
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Originally Posted by wad View Post
And what then is the meaning of the event horizon for a sphere?
I assumed that you were speaking of the geometrical shape of the event horizon. The event horizon has the geometry of a sphere. That means that all points of the event have the same Schwarzschild radius.
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Old Jun 11th 2017, 10:28 PM   #5
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Pmb is correct.

As I recall, the event horizon is simply the point of no return, i.e. a curved 2-dimensional plane made of all points where the escape velocity happens to equal the speed of light. Because gravity acts equally in all directions, that surface tends to be roughly spherical. The ubiquitous funnel shape might originate from diagrams that merely attempt to show the curvature of 4D spacetime on 2D paper and screens, but I hate when sci-fi gets it wrong. Stars and everything else likewise curve spacetime, though to a lesser magnitude. They have similar gravity wells that would be drawn similarly in such diagrams, yet we don't depict stars as smaller funnels -- because they are not funnels. There are many common misconceptions. A black hole isn't black, either, as you will learn from Hawking.
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Old Jun 11th 2017, 11:22 PM   #6
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Originally Posted by RonSpain View Post
Pmb is correct.

As I recall, the event horizon is simply the point of no return, i.e. a curved 2-dimensional plane ...
A plane isn't curved, its flat.
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Old Jun 12th 2017, 12:50 AM   #7
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That is quite correct. A plane is generally not curved by definition. However, I am referring to a curved surface, which is in essence a curved or deformed plane, at least by my standards. I dislike the word surface here, as it incorrectly implies the surface of a solid object like a planet.

But I'd rather debate whether there's really a singularity at the heart of a black hole.
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Old Jun 12th 2017, 08:12 AM   #8
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While on the topic of the shape of black holes, I have to mention the fascinating hypothetical possibility of time travel or travel to another universe via the effects on spacetime by the Kerr ring of a spinning black hole. If I recall correctly, the black hole would need to be huge (i.e. heavy) for you to avoid spaghettification.

But according to Wikipedia: "While it is expected that the exterior region of the Kerr solution is stable, and that all rotating black holes will eventually approach a Kerr metric, the interior region of the solution appears to be unstable, much like a pencil balanced on its point. This is related to the idea of cosmic censorship." Darn, foiled again. Still, a very interesting idea.
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Old Jun 12th 2017, 02:22 PM   #9
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Originally Posted by RonSpain View Post
While on the topic of the shape of black holes, I have to mention the fascinating hypothetical possibility of time travel or travel to another universe via the effects on spacetime by the Kerr ring of a spinning black hole. If I recall correctly, the black hole would need to be huge (i.e. heavy) for you to avoid spaghettification.

But according to Wikipedia: "While it is expected that the exterior region of the Kerr solution is stable, and that all rotating black holes will eventually approach a Kerr metric, the interior region of the solution appears to be unstable, much like a pencil balanced on its point. This is related to the idea of cosmic censorship." Darn, foiled again. Still, a very interesting idea.
The Kerr black hole is indeed only acceptable for outside the horizon. As far as time travel or going to another universe or something I'm afraid I have to point out that you aren't going to be doing anything that can be recorded in this Universe. You can't exit a black hole and you wouldn't be able to come out a "white hole" on the other side because the solution (so far as I know) is not stable. (And probably can't exist at all.)

And think of it this way, If you actually could get inside a black hole the surface of the star that exploded is right under the horizon. If you go in you will be burned to death pretty quickly.

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Old Jun 13th 2017, 07:18 AM   #10
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The term Black Hole is often used to refer to the Event-Horizon,
however the Black Hole proper lives inside the Event-Horizon.

If one takes the equations at face value, the Black Hole is a "singularity"
It is a point with NO length breadth or depth,
What is the shape of a point?

I suspect that there is some fundamental limit which will stop the ultimate collapse into a singularity, in which case the shape adopted will almost certainly be a sphere because that is the shape within which the forces pulling inward will most naturally balance the forces pushing outward.

However it is probably moot, since the actual black-hole will be forever hidden from the rest of the universe by the event-horizon.
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