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Author Topic: Rotation  (Read 4006 times)

Online syhprum

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Rotation
« on: 25/10/2006 17:08:38 »
I find the concept of a rotating black hole difficult to visualize, what is rotating, the event horizon? does this produce a flattening similar to that of Jupiter ?, and does this lead to the radiation of gravitational waves.
Is it the singularity that is rotating, I find this meaningless ????? 


 

Offline daveshorts

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Re: Rotation
« Reply #1 on: 25/10/2006 17:52:35 »
I think the idea is that angular momentum is conserved so if you start off with a load of stuff rotating then when it gets into a black hole it still must be rotating.

I am no expert on general relativity, but I think that the point of a black hole is that because space is so stretched inside one that they are a lot bigger on the inside than the outside. A singularity means that space is infinitely curved at that point, which means that if you were in the black hole there would be space, where outside it looks like a point. I have heard that the singularity in a rotating black hole could form a loop, which would make sense and allow it to have angular momentum in a sane way.
 

Offline Soul Surfer

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Re: Rotation
« Reply #2 on: 26/10/2006 09:03:03 »
The more important thing is that at the limit it must have a loop in all three (or maybe 4) dimensions and the simplest shape that I can think of is a toroidal membrane  this is because as well as rotation around an axis there will always be residual angular momentum reperesenting motion above and below the "equatorial" plane of the axis even if it is very small
 

Online syhprum

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Re: Rotation
« Reply #3 on: 26/10/2006 13:23:31 »
I am puzzled as to what effect the rotation of a mass that one is orbiting will have let us assume it is neutron star and is perfectly homogeneous if I was orbiting the equator I cannot see that its rotation can have any affect unless it somehow drags space-time around with it.
If I was in polar orbit and it was somewhat flattened by its rotation (now no longer homogeneous) I can understand that my orbit would be disturbed but not when I am in an equatorial orbit.

PS it is wonderful to have a built in spell checker it makes me feel educated when I read it back
« Last Edit: 26/10/2006 13:26:36 by syhprum »
 

another_someone

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Re: Rotation
« Reply #4 on: 26/10/2006 13:37:23 »
I am puzzled as to what effect the rotation of a mass that one is orbiting will have let us assume it is neutron star and is perfectly homogeneous if I was orbiting the equator I cannot see that its rotation can have any affect unless it somehow drags space-time around with it.

As I see it, if you look at a single neutron within the surface of the neutron star, assuming the tangential velocity is at relativistic speeds, then on one side of the orbit, that single neutron will be approaching you at relativistic speed, and thus distorting the distance between you and it, then its velocity velocity will change as it comes across the front of the neutron star to moving across your field of view, and finally will change to moving away from you and it progresses along its path on the surface of the neutron star.

The overall number of neutrons on the surface of the star will not change, as when one neutron moves away, another takes its place; but each individual neutron is still moving.

Ofcourse, as the radius of the star collapses, so the tangential velocity on its surface will increase; and as the star progresses towards a singularity, so the tangential velocity would approach infinity if it were able to do so.

That is my rather naÔve view of things, but no doubt Ian will have a more rigorous perspective on it.
 

Online syhprum

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Re: Rotation
« Reply #5 on: 26/10/2006 15:14:21 »
I think I can understand your reasoning although the rotating body about which I am orbiting is homogeneous on a macroscopic scale it must be regarded as having a rough surface rather like the pimples on a golf ball and drag space-time around with it like the golf ball does the air through which it travels.
This I can understand when I orbit a neutron star, but when I orbit the event horizon of a black hole this does not consist of individual particles what is dragging space-time around ?
 

another_someone

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Re: Rotation
« Reply #6 on: 26/10/2006 16:05:43 »
I think I can understand your reasoning although the rotating body about which I am orbiting is homogeneous on a macroscopic scale it must be regarded as having a rough surface rather like the pimples on a golf ball and drag space-time around with it like the golf ball does the air through which it travels.
This I can understand when I orbit a neutron star, but when I orbit the event horizon of a black hole this does not consist of individual particles what is dragging space-time around ?

But I would imagine that the gravitational field you experience comes not from the event horizon, but from what is contained therein.

That having been said, it seems to me to lead to another paradox.  If an event horizon is such that nothing, not even something that has the velocity of light, can leave the event horizon; and yet gravity has the velocity of light; then how does gravity leave an event horizon?
 

Online syhprum

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Re: Rotation
« Reply #7 on: 26/10/2006 17:16:49 »
The same thought occured to me ???
 

Offline Soul Surfer

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Re: Rotation
« Reply #8 on: 27/10/2006 09:55:05 »
Full relativity theory states that there is a gravito magnetic effect analogous to the relationship between electricity and magnetism  (That is a magnetic field is the result of a moving electrical field)  a moving gravitational field does drag space time around with it but it's even more difficult to understand than magnetic fields (which very few people understand properly including me!)

As far as I understand it it makes it more energetic for something to orbit a rotating gravitating mass against the direction of rotation than with it.

The gravitational field of a large mass is essentially static or varying extremely slowly as it accretes matter. It does not have to move from whatever is in the middle of the event horizon to the edge.  It is a property of it so there is no problem.
 

Online syhprum

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Re: Rotation
« Reply #9 on: 28/10/2006 13:35:17 »
I vaguely recall the results of and experiment being published where this space-time dragging effect was measured but I am at a loss to understand it.
I suppose we must imagine lines of gravitational influence extending out and rotating with the rotating mass as magnetic lines of force would extend from a magnetic unipole.
This would of course result in the radiation of energy!
« Last Edit: 28/10/2006 13:44:36 by syhprum »
 

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Re: Rotation
« Reply #9 on: 28/10/2006 13:35:17 »

 

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