Do black holes rotate?

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Do black holes rotate?
« on: 09/05/2011 05:30:03 »
melvin perry  asked the Naked Scientists:
I am helping my son with his science project. A lot goes over my hear but with black holes I have this question. If the gravity of a Black Hole is as strong as it is, does it rotate?  And if it rotates does it create a magnetic field like the one Earth has, creating those jets of gas that looks like a supped up aurora borealis?

Melvin Perry

What do you think?
« Last Edit: 09/05/2011 05:30:03 by _system »


Offline yor_on

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Do black holes rotate?
« Reply #1 on: 09/05/2011 06:22:38 »
Let us make a disclaimer first. nobody has really measured a black hole as far as I know. but we see evidence for them existing everywhere. When it comes to what we think about them, how we define them today, we seem to split them into three categories.

1. The Schwarzschild Black Hole is a non rotating black hole without a magnetic field.

2. The Reissner-NordstrÝm Black Hole is non rotating black hole with a magnetic field.

3. The Kerr Black Hole has both a spin and a magnetic field.

"In rotating black holes and/or ones with a magnetic field, the matter forms a disk (accretion disk) due to the mechanical forces present. In a Schwarzschild black hole, the matter would be drawn in equally from all directions, and thus would form an omni-directional accretion cloud rather than disk.

The matter in accretion disks is gradually pulled into the black hole. As it gets closer, its speed increases, and it also gains energy. Accretion disks can be heated due to internal friction to temperatures as high as 3 billion K, and emit energetic radiation such as gamma rays. This radiation can be used to "weigh" the black hole. By using the doppler effect, astronomers can determine how fast the material is revolving around the black hole, and thus can infer its mass.

Jets form in Kerr black holes that have an accretion disk. The matter is funneled into a disk-shaped torus by the hole's spin and magnetic fields, but in the very narrow regions over the black hole's poles, matter can be energized to extremely high temperatures and speeds, escaping the black hole in the form of high-speed jets."

From Black Holes.

And some of those Black Holes spinning have been measured, well indirectly but still measured, to be spinning near to the speed of lights. And when something that massive spins that fast, the space around it get dragged along with its motion, also called 'frame dragging'.

Think of frame dragging this way. Everything moves to gravity, a black hole is a infinite amount of gravity as you come near its event horizon. You could also say that it punch a gravitational hole in space. As it rotate that 'hole' follows it as well as getting 'smeared out' as it winds itself around the black hole. alternatively you can think of it as a sort of friction where space somehow through its gravity communicates a 'friction' to the space around it like a rotating ball inside a lose jello. But I like mine better as space is 'empty' to matter. But gravity is in a way space as I expect gravity to be its boundary, outside that 'SpaceTime' won't exist as i think.

Gravity can be directly translated into motion too. You can as Einstein defined it do the exact same experiments constantly accelerating at one Gravity as you can do on Earth, also having a one Gravity, now ignoring any other gravitational effects mixing, as earth spinning around creating a frame dragging effect.

So you can say that Earth is constantly uniformly accelerating, without as far as we can see going in any specific direction due to its gravity, at one constant Gravity, nowhere :)

And that is true, the universe is not what we think, but what we learn.
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Do black holes rotate?
« Reply #2 on: 09/05/2011 08:23:34 »
The simple truth is that it is extremely unlikely that any non rotating black holes exist or have ever existed. 

Any structure collapsing from large to small under the effect of gravity is almost certain to contain some residual angular momentum and this will be speeded up as the structure gets smaller.  Remember a black hole with the mass of the sun is only about one mile across.  This means that most black holes will probably be rotating at the absolute maximum rate they can rotate (there is one). 

When any bodies collapse to form stars and black holes the usually have to get rid of excess angular momentum energy  this is seen in the jets shooting out from the poles of these objects.  The reason the material does not fly off the edge (from the equator), like water in a bowl or a bucket would if you spin it too fast, is that the shear of the rotating material as it approaches the gravitating object prevents it from escaping that way.
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