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Black hole spin


Moontanman

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2 hours ago, Moontanman said:

Is there a limit to how fast a black hole can spin? If not what if the black hole's spin rate exceeded the speed of light? 

As there is nothing material spinning it is not obvious that the speed of light limit would apply. (it may be bette to think of the angular momentum as being intrinsic, rather like elementary particles.)

But I think there is an upper limit. And, from what I remember, it does mean that the surface velocity (if there were such a thing) would be less than the speed of light. (Just seen your later post that confirms this. Phew!)

1 hour ago, Moontanman said:

Good question but the spin can be measured and even hypothetically increased.

I'm not sure how it could be measured. Frame dragging effect, maybe? Or, more likely, by measuring the oblateness of the even horizon (if you could image it).

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1 hour ago, Strange said:

As there is nothing material spinning it is not obvious that the speed of light limit would apply. (it may be bette to think of the angular momentum as being intrinsic, rather like elementary particles.)

But I think there is an upper limit. And, from what I remember, it does mean that the surface velocity (if there were such a thing) would be less than the speed of light. (Just seen your later post that confirms this. Phew!)

I'm not sure how it could be measured. Frame dragging effect, maybe? Or, more likely, by measuring the oblateness of the even horizon (if you could image it).

Citing this link might have been a bit premature, the author is Fraser Cain a well known science youtuber and while I enjoy his videos I'm not sure he is really an authority on the subject. 

 This quote form the original link might be poignant to the issue. The quote is attributed to Dr. Mark Morris, a professor of astronomy at UCLA

 

Quote

“The speed limit is set by the event horizon, eventually, at a high enough spin, reaches the singularity. You can’t have what’s called a naked singularity. You can’t have a singularity exposed to the rest of the Universe. That would mean that the singularity itself could emit energy or light and somebody outside could actually see it. And that can’t happen. That’s the physical limitation of how fast it can spin. Physicists use units for angular momentum that are cast in terms of mass, which is a curious thing, and the speed limit can be described as the angular momentum equals the mass of the black hole, and that sets the speed limit.”

 The speed limit is less than the speed of light due to the spin eventually revealing the naked singularity. 

Quote

One black hole, at the heart of galaxy NGC 1365 is turning at 84% the speed of light. It has reached the cosmic speed limit, and can’t spin any faster without revealing its singularity.

Evidently the universe frowns on black holes flashing us their singularity :wacko:

If I understand it correctly adding mass increases the black holes angular velocity...  

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4 minutes ago, Moontanman said:

If I understand it correctly adding mass increases the black holes angular velocity...  

I don't think there is a direct relationship between mass and angular momentum. I assume the angular momentum would be increased if the mass was orbiting in the same direction as the rotation (which in most cases, it would be). But I assume it could also reduce the angular momentum if it was going the other way. 

And as mass increases the radius, I'm not sure what the net effect on angular velocity would be...

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3 minutes ago, Strange said:

I don't think there is a direct relationship between mass and angular momentum. I assume the angular momentum would be increased if the mass was orbiting in the same direction as the rotation (which in most cases, it would be). But I assume it could also reduce the angular momentum if it was going the other way. 

And as mass increases the radius, I'm not sure what the net effect on angular velocity would be...

Did you see this quote?

Quote

Physicists use units for angular momentum that are cast in terms of mass, which is a curious thing, and the speed limit can be described as the angular momentum equals the mass of the black hole, and that sets the speed limit.

 

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4 minutes ago, Moontanman said:

Did you see this quote?

 

I didn't quite understand that. Angular momentum is proportional to mass (and radius), so maybe that is all he is referring to. But it almost sounds like he is saying that they can be directly equated

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4 hours ago, Moontanman said:

 Good question but the spin can be measured and even hypothetically increased. I may have short circuited the question with this...  

https://www.universetoday.com/109308/how-fast-do-black-holes-spin/

From Kip Thorne's book, Black Holes and Time Warps, and from memory, as a BH spins, its outer horizon is elongated until a point is reached where the outer horizon region, will coincide with the static BH EH at the polar regions. This is what limits the angular momentum, again from memory. Also speaking theoretically, we or some advanced civilisation could possibly extract energy from the region between the outer horizon and the static horizon, which is called the "ergosphere"

Edited by beecee
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