Shape of a black hole?

[Vay]

I am confused about what is the shape of a black hole. I hear that it is a sphere and then it is a doughnut with a hollow middle, or are these two shapes the process of a black hole, with the black hole taking the shape of a sphere and then a doughnut? Can anyone clarify this?

Edited April 4, 2011 by Vay

[Klaynos]

The event horizon is spherical.

[iAmSchrodinger'sCAT]

To tell you the truth I don't think there is an answer to this. For one we were never near a black hole, so we probably don't know its shape. And its called a black hole for a reason (I honestly don't know why its called a black hole)...

But again I could be wrong

[Spyman]

I am confused about what is the shape of a black hole. I hear that it is a sphere and then it is a doughnut with a hollow middle, or are these two shapes the process of a black hole, with the black hole taking the shape of a sphere and then a doughnut? Can anyone clarify this?

A Black Hole consists of a spherical Event Horizon surrounding a hypothetical singularity which is thought to be ring shaped in the plane of rotation.

[Klaynos]

A Black Hole consists of a spherical Event Horizon surrounding a hypothetical singularity which is thought to be ring shaped in the plane of rotation.

 

Do you have a reference for the singularity shape?

[Spyman]

Do you have a reference for the singularity shape?

The underlined parts are links.

 

Description of a ring-singularity

When a spherical non-rotating body of a critical radius collapses under its own gravitation under general relativity, theory suggests it will collapse to a single point. This is not the case with a rotating black hole (a Kerr black hole). With a fluid rotating body, its distribution of mass is not spherical (it shows an equatorial bulge), and it has angular momentum. Since a point cannot support rotation or angular momentum in classical physics (general relativity being a classical theory), the minimal shape of the singularity that can support these properties is instead a ring with zero thickness but non-zero radius, and this is referred to as a ring singularity or Kerr singularity.

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

[Klaynos]

The underlined parts are links.

 

Description of a ring-singularity

When a spherical non-rotating body of a critical radius collapses under its own gravitation under general relativity, theory suggests it will collapse to a single point. This is not the case with a rotating black hole (a Kerr black hole). With a fluid rotating body, its distribution of mass is not spherical (it shows an equatorial bulge), and it has angular momentum. Since a point cannot support rotation or angular momentum in classical physics (general relativity being a classical theory), the minimal shape of the singularity that can support these properties is instead a ring with zero thickness but non-zero radius, and this is referred to as a ring singularity or Kerr singularity.

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

 

Thanks.

[Airbrush]

A Black Hole consists of a spherical Event Horizon surrounding a hypothetical singularity which is thought to be ring shaped in the plane of rotation.

 

Doesn't a black hole need to be rotating at relativistic speed in order for the singularity to be flattened? The highest rotation possible is under 1,000 revolutions per second. Why a ring shape and not a flattened disc shape? How can a black hole have a ring shaped singularity? This defies reason it seems to me because the center of a ring is empty space. How can the very center of a black hole be empty space?

 

A black hole that is not feeding would appear like a flat black sphere. If the Earth could be crushed down into a black hole it would appear like a flat black marble only an INCH in diameter!

Edited April 5, 2011 by Airbrush

[Riot]

Doesn't a black hole need to be rotating at relativistic speed in order for the singularity to be flattened? The highest rotation possible is under 1,000 revolutions per second. Why a ring shape and not a flattened disc shape? How can a black hole have a ring shaped singularity? This defies reason it seems to me because the center of a ring is empty space. How can the very center of a black hole be empty space?

 

A black hole that is not feeding would appear like a flat black sphere. If the Earth could be crushed down into a black hole it would appear like a flat black marble only an INCH in diameter!

Not true. A black hole has no volume. Therefore a black hole is not a sphere. The event horizon only appears to be sphere to our eyes.

 

 

A black hole takes up no space. Therefore any celestial bodies absorbed by the black hole appear flat but actually are non-existent because they are not in our space time in addition to the fact that the celestial body now takes no space also.

 

 

[steevey]

The underlined parts are links.

 

Description of a ring-singularity

When a spherical non-rotating body of a critical radius collapses under its own gravitation under general relativity, theory suggests it will collapse to a single point. This is not the case with a rotating black hole (a Kerr black hole). With a fluid rotating body, its distribution of mass is not spherical (it shows an equatorial bulge), and it has angular momentum. Since a point cannot support rotation or angular momentum in classical physics (general relativity being a classical theory), the minimal shape of the singularity that can support these properties is instead a ring with zero thickness but non-zero radius, and this is referred to as a ring singularity or Kerr singularity.

http://en.wikipedia....ing_singularity

 

Doesn't that imply that a singularity isn't an object so small it doesn't actually exist? Or in other words, that in some level no matter how small, its 3D?

Edited April 7, 2011 by steevey

[csmyth3025]

Is there a reason why the singularity of a Kerr black hole is thought to be a ring with zero thickness but non-zero radius rather than a disk with zero thickness but non-zero radius?

 

Chris

[Airbrush]

Is there a reason why the singularity of a Kerr black hole is thought to be a ring with zero thickness but non-zero radius rather than a disk with zero thickness but non-zero radius?

 

Chris

 

Exactly. It doesn't make sense. Can someone please explain it? Did someone make a mistake entering that in Wikipedia?

 

Not true. A black hole has no volume. Therefore a black hole is not a sphere. The event horizon only appears to be sphere to our eyes.

 

 

A black hole takes up no space. Therefore any celestial bodies absorbed by the black hole appear flat but actually are non-existent because they are not in our space time in addition to the fact that the celestial body now takes no space also.

 

Thanks for commenting but I think you are not correct. Black holes have event horizons which should be visible as a dark circle over a stary field. The volume inside the event horizon is empty space, but the boundaries of this should be very tangible. Also the singularity at the center of a black hole takes up no space. A black hole is all empty space except for the singularity, if there is no accretion disk remaining. The mass of any body is not annihilated. It's mass adds to the mass of the black hole (not much because most of it flies off in energetic sparks that don't go into the black hole). This additional mass will increase the size of the event horizon sphere slightly.

Edited April 7, 2011 by Airbrush

[IM Egdall]

Is there a reason why the singularity of a Kerr black hole is thought to be a ring with zero thickness but non-zero radius rather than a disk with zero thickness but non-zero radius?

 

Chris

 

Ah, a non-rotating black hole has a point singularity. If you now rotate this point around a non-zero radius, you get a ring with zero thickness, not a disc. Is it that simple?

 

And I think these so-called singularities are suspected to not exist for real black holes. But physicists need a theory of quantum gravity (combining quantum mechanics and general relativity) to make a definitive prediction. No such theory has yet been validated by experiment.

[Arch2008]

A star spins, giving it a property known as angular momentum. When a super-massive star collapses into a singularity, it continues to spin. Angular momentum cannot be destroyed, so the singularity must also spin. A point cannot exhibit spin. A disk is unstable, because the interior would be attracted to the edge by the centrifugal force of the spin. So a ring singularity is stable and exhibits angular momentum. Its also totally flat, so its volume is zero (width times length times zero), making its density infinite (mass divided by zero). This is what Kerr figured out.

Edited April 8, 2011 by Arch2008

[Riot]

Exactly. It doesn't make sense. Can someone please explain it? Did someone make a mistake entering that in Wikipedia?

 

 

 

Thanks for commenting but I think you are not correct. Black holes have event horizons which should be visible as a dark circle over a stary field. The volume inside the event horizon is empty space, but the boundaries of this should be very tangible. Also the singularity at the center of a black hole takes up no space. A black hole is all empty space except for the singularity, if there is no accretion disk remaining. The mass of any body is not annihilated. It's mass adds to the mass of the black hole (not much because most of it flies off in energetic sparks that don't go into the black hole). This additional mass will increase the size of the event horizon sphere slightly.

I appreciate your corrections.

 

 

 

[Spyman]

Doesn't a black hole need to be rotating at relativistic speed in order for the singularity to be flattened? The highest rotation possible is under 1,000 revolutions per second. Why a ring shape and not a flattened disc shape? How can a black hole have a ring shaped singularity? This defies reason it seems to me because the center of a ring is empty space. How can the very center of a black hole be empty space?

I agree with I ME and Arch2008. The core of a spinning Black Hole doesn't flatten due to high spin like Earth would if its spin would increase. According to General Relativity the original body is crushed by the immense gravity and would compress to an singularity even if it was not spinning at all. Instead the spin prevents the core from shrinking to zero radius because it needs to conserve angular momentum, but it would still be incomprehensible tiny and spinning incredible fast.

 

IMHO our knowledge and models are not complete, the theory of General Relativity is not enough so resolve how of the inside of Black Holes truly are like, maybe a new theory of quantum gravity can resolve this, but likely it will also reveal new questions and mysteries to solve.

 

 

What do you think is rotating with a maximum and why do you think it's absolute at 1 000 revolutions per second?

 

The core in a Black Hole is not firmly attached to a solid Event Horizon with rigid spokes but when a Black Hole spins its angular momentum pulls surrounding space around with it, causing severe frame dragging. When astronomers talk about spin rates of Black Holes they speak of how fast space is rotating at the Event Horizon and not how fast its core is spinning.

 

AFAIK the Kerr solution have a limit of maximum angular momentum for rotating Black Holes which sets a maximum spin speed for a certain mass, but different mass affects both the Event Horizon radius and the angular momentum. The Black Hole GRS1915+105 with roughly 14 solar mass have a theoretical rotation limit of 1 150 spins a second but I don't think that is an absolute maximum spin rate regarding all Black Holes independent of their mass.

 

I am not an expert on Relativity and might be wrong but suspect a "lighter" Black Hole would have a much smaller radius for its Event Horizon and should therefore be able to spin faster, while a "heavier" even though able to carry more angular momentum would boast such large Event Horizon radius it would not be able to achieve that high rate.

 

A confirmation of the relation between mass and maximum spin rate would be appreciated.

 

 

----------

 

 

Not true. A black hole has no volume. Therefore a black hole is not a sphere. The event horizon only appears to be sphere to our eyes.

 

 

A black hole takes up no space. Therefore any celestial bodies absorbed by the black hole appear flat but actually are non-existent because they are not in our space time in addition to the fact that the celestial body now takes no space also.

Airbrush is correct, for observers at a safe distance from a Black Hole, the Event Horizon surrounding its singularity, appears as a spherical surface encapsuling an equal large volume of space as a spherical solid body would. However for an observer traversing to the inside its internal volume is not that simple.

 

More on that in this thread: Volume inside a black hole?

Edited April 8, 2011 by Spyman

[Airbrush]

Spyman: "AFAIK the Kerr solution have a limit of maximum angular momentum for rotating Black Holes which sets a maximum spin speed for a certain mass, but different mass affects both the Event Horizon radius and the angular momentum. The Black Hole GRS1915+105 with roughly 14 solar mass have a theoretical rotation limit of 1 150 spins a second but I don't think that is an absolute maximum spin rate regarding all Black Holes independent of their mass."

 

Yes, I just found that also by Googling:

 

"This black hole is about 14 solar masses. At the point of the event horizon, the black hole is spinning at over 950 times per second…. or 50 percent the speed of light.

 

"According to theory, the absolute maximum rate at which this black hole could possibly spin, essentially the speed of light, is 1,150 times per second."

 

http://www.nasa.gov/vision/universe/starsgalaxies/spinning_blackhole.html

[MigL]

Einstein's field equations were first solved by Swartzchild for a 'simple' collapsing mass. They produce an event horizon and a point were the functional equations are not well behaved ( like a function with a discontinuity in its derivative ), but indicate a singularity ( dimensionless point mass/energy ).

Later on a couple of Norwegian guys ( Nordstrum may be one) solved the equations for charged collapsing masses, as charge is conserved. An interesting effect is that there are two event horizons and adding charge beyond a certain amount will force the outer horizon inside the inner one, resulting in a bare singularity ( if they exist ). Now charged black holes will attract opposing charge and neutralise so the chance of finding one are slim.

Later yet, Kerr, an Australian solved the equations for spinning collapsing mass and these produced a flattened event horizon with another one inside it. The singularity ( if it exists ) is indicated to be ring shaped. Adding more angular momentum again brings the outer horizoncloser to the inner one, and at the limiting rotation, it would cross inside and again produce a naked singularity ( if they exist ) This may be the only way to actually determine if singularities exist. You need to create a microscopic black hole, and add to its angular momentum until we can actually determine what's inside.

 

An einstein -Rosen diagram is not very accurate ( and actually wrong in its representation ), but a Penrose diagram actually shows that you can pass through a rotating black hole without meeting the singularity, and pass into another part of our universe, another time of our universe, or another universe altogether ( all purely speculative ).

 

You can call the inside of an event horizon space/time, but its not our space/time as the horizon separates it from ours. The horizon is, in effect, a one way door out of our universe. Even time is not our time as upon crossing the horizon there is only one future for you, the singularity ( if they exist ).

[csmyth3025]

A star spins, giving it a property known as angular momentum. When a super-massive star collapses into a singularity, it continues to spin. Angular momentum cannot be destroyed, so the singularity must also spin. A point cannot exhibit spin. A disk is unstable, because the interior would be attracted to the edge by the centrifugal force of the spin. So a ring singularity is stable and exhibits angular momentum. It's also totally flat, so its volume is zero (width times length times zero), making its density infinite (mass divided by zero). This is what Kerr figured out.

If I understand your reply correctly, a dimensionless point (a singularity) cannot rotate and therefore cannot have any angular momentum. On the other hand. such a point possessing mass cannot rotate around a center unless there is some other point possessing mass to "counterbalance" it. Since two such points would merge almost instantaneously in the black hole's severe gravity environment, the only viable configuration is a one-dimensional line representing a non-zero circumference for the rotating mass. This circumferential line would have length (2*pi*r) but no volume.

 

Is this approximately the description of a ring singularity?

 

Also, Airbrush cited a maximum of 1150 rev/s for a 14 solar mass black hole singularity. What determines this limit and what happens if it's exceeded? Conversely, is there any known mechanism that would prevent this limit from being exceeded?

 

Chris

[Jasper]

I am confused about what is the shape of a black hole. I hear that it is a sphere and then it is a doughnut with a hollow middle, or are these two shapes the process of a black hole, with the black hole taking the shape of a sphere and then a doughnut? Can anyone clarify this?

 

http://www.maranatha...8/blackhole.gif

 

this a is clear picture of one a stellar black hole

Edited April 17, 2011 by Jasper

[Spyman]

Also, Airbrush cited a maximum of 1150 rev/s for a 14 solar mass black hole singularity.

No, Airbrush quoted a maximum spin for the Event Horizon and not the Singularity.

 

 

What determines this limit and what happens if it's exceeded?

Kerr metric determines the circumference of a spinning Black Holes Event Horizon and mass is the only variable.

 

According to Kerr's metric the Event Horizon would vanish leaving the singularity naked.

 

 

Conversely, is there any known mechanism that would prevent this limit from being exceeded?

Frame dragging are limiting objects from entering rotating Black Holes due to angular momentum.

 

Relativistic jets are thought to be extracting energy from spinning Black Holes.