Why do Black Holes Rotate?

[DrNRG]

As a cosmic observation the unique characteristics of galaxies appear to have a correlation to the kinetic energy exhibited by the central black hole.

 

The direction of rotation of the black hole appears to be ascertainable by the shape of the galaxy, particularly the position of the outer arms of stars. Also, the planar nature of galaxies appears to result from the rotational velocity intrinsic to the black hole. -

 

Side question, is it likely that there were bodies outside of the galactic plane affected by the rotational velocity of the black hole that would have collapsed into the black hole thereby increasing it's mass initially?

 

Does the rotation of a black hole allude to some of the mechanics involved with it's initial formation? Since if it is rotating now it is logical to think that it has been rotating since it's inception.

 

Can a black hole without rotation support a galactic formation? - I would guess no as without the kinetic force of rotational velocity there is nothing to counteract the overwhelming force of gravity.

[ajb]

Does the rotation of a black hole allude to some of the mechanics involved with it's initial formation? Since if it is rotating now it is logical to think that it has been rotating since it's inception.

I would think so. It must be due to conservation of angular momentum, the stuff that formed the black hole was rotating about some axes and this meant that the black hole must carry angular momentum.

[derek w]

Is the sun rotating about an axis?

[imatfaal]

Is the sun rotating about an axis?

 

Yes

 

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

[Delbert]

I get a tad confused about black holes. Confused as to how they form in the first place. Like, I understand that we see matter that's approaching a black hole slow down due to time dilation, to the point that it becomes ever fainter and hangs forever close to the even horizon but never actually goes in. In other words as far as we see, matter never actually passes to the other side of the event horizon. Presumably the conclusion being that all the matter that supposedly passed and entered the black hole to form it never did because it's still outside on our side of the event horizon!

 

Doubtless I'm missing something.

Edited April 2, 2014 by Delbert

[Sensei]

Is the sun rotating about an axis?

 

And unlike solid planets, Sun equator rotates at different speed than other regions.

[Endy0816]

I get a tad confused about black holes. Confused as to how they form in the first place. Like, I understand that we see matter that's approaching a black hole slow down due to time dilation, to the point that it becomes ever fainter and hangs forever close to the even horizon but never actually goes in. In other words as far as we see, matter never actually passes to the other side of the event horizon. Presumably the conclusion being that all the matter that supposedly passed and entered the black hole to form it never did because it's still outside on our side of the event horizon!

 

Doubtless I'm missing something.

 

In the matter's own frame of reference it falls in normally.

[Delbert]

 

In the matter's own frame of reference it falls in normally.

But surely that leaves the paradox of: is it inside or outside?

 

Anyway, as for frame of reference and as you appear to infer, time for the falling matter wouldn't slow down, but would be perfectly normal. I seem to recall Prof Brian Cox describing this phenomenon on a TV prog a while ago. Whereby we see the intrepid traveller's time slow down as he or she approached a black hole (ignoring the spaghetti consequence), but as far as the traveller was concerned, his or her time was perfectly normal.

 

What Brian Cox didn't appear to mention (if he did I missed it), was what does the traveller see when he or she looks back from whence he or she came? Presumably the opposite, i.e. he or she sees the time of the Earth bound observer to speed up. And presumably all the other things, like the Earth whizzing round the Sun in its orbit. Not to mention all the other stars if not the whole universe speeding up! Or perhaps it might be safe to say the universe speeds up so much that when the traveller reaches the even horizon he or she sees the end of the universe.

 

I might add in regard to the traveller seeing the Earth's orbit speeding up, presumably to avoid him or her seeing the laws of physics breaking down (a speeded up orbit should see the Earth fly off into space me thinks), the Earth, Sun and whatever will be seen to shrink - to the point whereby the whole universe is seen to shrivel up and disappear, mayhap? In other words one crosses the even horizon at the end of the universe.

 

But perhaps my imagination has run away!

[Strange]

But surely that leaves the paradox of: is it inside or outside?

 

Not really. It is not as if the outside observer sees the matter stop at the surface of the black hole. For the outside observer, the infalling matter disappears from view in a finite amount of time (after all, there are a a finite number of photons emitted by the infalling matter before it passes the event horizon, so we can't see it stuck there forever).

 

Also, the infalling matter will increase the mass and therefore the radius of the black hole. So the location at which we see the matter is soon within the event horizon.

[Delbert]

 

For the outside observer, the infalling matter disappears from view in a finite amount of time (after all, there are a a finite number of photons emitted by the infalling matter before it passes the event horizon, so we can't see it stuck there forever).

A finite amount of time is not what I've read thus far. Unless, that is, the lifetime of the universe one considers to be a finite amount of time.

 

And as for fainter, it's probably also the wavelength that gets ever longer, so the photons per unit time get less as time (as we experience it) progresses. So the finite supply of photons that you suggest are effectively rationed and can last forever or the lifetime of the universe - as we experience or perceive the lifetime of the universe!

[davidivad]

due to the fact that a gas cloud is full of particles moving not all in the same direction, gas closest to the orbital trajectory of center of mass sticks around. molecules that are not beyond escape velocity end up in orbits. eventually gravity wins by pulling the gas together. the result is like an ice skater pulling her or his arms inward and speeding up. the denser the object becomes,the higher the spin velocity.

 

as far as the schwartzchild radius is concerned, this is not where things hang around in a frozen state. that is closer inside the black hole and hidden from our view. consider the concept of an orbit. the schwartzchild radius is the orbit with an escape velocity of the speed of light.

with a massive black hole, you will definitely pass the event horizon. consider that the orbit is a result of ANGULAR MOMENTUM meaning that light moving in opposition to the black hole actually can reach past the event horizon but is severely stretched. this can only happen if photons are emitted far enough away from the singularity and the black hole would most likely have to be feeding. my question is whether or not the energy from these emissions heats up in-falling gas in addition to this gas already being heated. could this be where the information goes (outward by thermal entropy)? or am i just explaining the already understood black body radiation.

[Schneibster]

The two easy answers are,

 

1. Because they're made from stars that were rotating and angular momentum is conserved;

and

2. Because they can

which seems like a smartass answer but is actually quite interesting and provocative if followed up upon.

[ajb]

2. Because they can which seems like a smartass answer but is actually quite interesting and provocative if followed up upon.

It is interesting in light of the no hair theorem; black holes are characterised by their mass, electric charge and angular momentum, nothing else. So, they can rotate, that is carry angular momentum.

[Schneibster]

They get mass and angular momentum from their formation. Electric charge, however, is another matter.

[Endercreeper01]

They get mass and angular momentum from their formation. Electric charge, however, is another matter.

No black holes with significant electrical charge have never been found. This is because the repulsive force is greater than the attractive force of gravitation when matter with significant charge is compressed.

[Schneibster]

Considering the electron jets from the quasar machine (which everyone pretty much figures is a young, active black hole) I'd be careful about making any blanket statements myself.

 

It's not like we can go measure charge. A) we're too far away and B) seems like all the quasars have shut off.

 

OTOH, I do have to agree that forming a black hole that has a charge could be impossible due to the point you've made. I think though that one getting a charge by some separation of charged moieties in the disk and/or jet processes after it already exists might be possible.

 

As a speculation I wonder if it's possible to collect enough electric charge to make an electric singularity. Not on this thread though; let's stick to angular momentum hair.

Edited April 6, 2014 by Schneibster