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robross
2009-May-11, 10:38 PM
Just before a black hole forms, I imagine in most cases that object is rotating quite rapidly, as the original object shrinks down, conserving angular momentum. Then it reaches critical density, and the even horizon forms.

At this point, what does it mean for the black hole to be rotating? Obviously whatever is inside it is now beyond our horizon and we can't see whatever it's doing. But the event horizon is just a region of space. Is the vacuum around the black hole rotating around it? I'm confused by this concept.

Note I'm not talking about the matter in the accretion disk, which obviously can rotate around the EH. Let's say we have a black hole trillions of years in the future with absolutely no particles left anywhere in its vicinity. Can this black hole be considered to rotate, and what does that mean?

Thanks,

Rob

novaderrik
2009-May-11, 10:58 PM
a black hole is just really, really, really dense "stuff". stuff has momentum.

nauthiz
2009-May-11, 11:03 PM
There are two main features that distinguish a rotating black hole from a non-rotating one. The first is that the event horizon of a rotating black hole is not spherical.

The second is frame dragging, an effect where the spacetime around the object is dragged in circles by its rotation. This effect can be so strong that for an object in a region just outside the event horizon called the ergosphere, it is impossible to be stationary - an object would have to effectively be moving faster than the speed of light in order to stay in one place. It is possible for an object in the ergosphere to escape or orbit the black hole, it just can't stay still.

neilzero
2009-May-11, 11:03 PM
You are correct: We likely will never be able to confirm that the portion inside the event horizon continues to rotate. It is a fairly logical assumption = why should it stop rotating because the event horizon formed? Neutron stars which we think will become black holes when they accreate some more matter seem to rotate very rapidly.
Nauthiz: I don't think we have yet observed a non-spherical event horizon, frame dragging nor the ergosphere of a black hole, but all are logical assumptions that are mathematically sound, if gravity waves are the means by which we detect the the gravity of a black hole..
A slowly rotating black hole interior may be possible, but at some very slow rotational rate, we have to ask: Rotating with respect to what, as all things are relative? Neil

cbacba
2009-May-11, 11:12 PM
good question and what is involved may be too involved for a satisfactory answer. For the gen. relativity crowd, I believe there's a frame dragging involved but I'm not ready to conceptualize space time spinning in a a circle.

For a spinning bh, there may not even be an event horizon. That is something associated with the static state or classical bh.

However, dense objects do exist like millisecond pulsars which have spun up from much slower rotation rates due to infalling matterwhich is coming in at an angle and conveying rotational energy to the body, one particle at a time. This is facilitated by the acretion disk creating a uniform direction of motion for particles that will be falling in.

At an event horizon time stops according to the view from afar - the distant observer. In principle, it would seem that to the distant observer, a bh would never form in the life time of the universe. Considering that the distant observer is a specific distance from the eh and the observer sees the falling clock slowing down, redshifting, and dimming out as it asymptotically approaches the event horizon and since the speed of light is constant as is the distance to the eh, then that means the delays we see as the distant observer are not some long lost echo received at an increasing longer time as the clock was emitting them as it was falling through. That means the object is still in the process of approach and not after the object has fallen through the eh. That is once a photon is emitted from the clock, it travels a defined finite distance to reach the distant observer and so is delayed only by that travel time, effectively the same time as a previous photon took that was emitted almost simultaneously at the clock yet perhaps reached the distant observer years earlier.

cbacba
2009-May-11, 11:16 PM
You are correct: We likely will never be able to confirm that the portion inside the event horrizon continues to rotate. It is a fairly logical assumption = why should it stop rotating because the event horizon formed? Nuetron stars which we think will become black holes when they accreate some more matter seem to rotate very rapidly. Neil

While I tend to remain skeptical of anything within an event horizon being recognizable, in the classical case, everything is pulled into the singularity which has zero diameter.

Hornblower
2009-May-11, 11:30 PM
If I am not mistaken, GR predicts a singularity in the form of a ring, rather than a point, in the center of a rotating black hole. I will now yield to those who are better informed on this topic.

Cougar
2009-May-12, 03:37 AM
For a spinning bh, there may not even be an event horizon. That is something associated with the static state or classical bh.

Do you have a cite for this? Do you have something against Kerr black holes (http://en.wikipedia.org/wiki/Rotating_black_hole)?


At an event horizon time stops according to the view from afar - the distant observer.

Correct, but this is essentially an illusion, since time does not stop from the point of view of the (very hearty) observer falling through the event horizon.


Considering that the distant observer is a specific distance from the eh and the observer sees the falling clock slowing down, redshifting, and dimming out as it asymptotically approaches the event horizon and since the speed of light is constant as is the distance to the eh, then that means the delays we see as the distant observer are not some long lost echo received at an increasing longer time as the clock was emitting them as it was falling through.

Uh, no, not an echo....


That means the object is still in the process of approach and not after the object has fallen through the eh.

Only from the distant observer's viewpoint. And on what does the distant observer have to base her science other than her direct observations? Apparently the answer is Einstein's general relativity, and it seems Susskind's black hole complementarity also fits in there pretty well. Depending on the observer, two totally different things are observed. Both are correct, like light is a wave OR a particle....


...once a photon is emitted from the clock, it travels a defined finite distance to reach the distant observer and so is delayed only by that travel time, effectively the same time as a previous photon took that was emitted almost simultaneously at the clock yet perhaps reached the distant observer years earlier.

I guess I get your point, but I'm not sure what you're driving at. I think a response to this is going to take a bit more time than I've got this evening...

astromark
2009-May-12, 10:11 AM
Its pedantic and petty to pursue this question to its obvious depth... When the fact is , there is some things that we do not know with absolute certainty...
I am trusting that those at work on the Higgs Boson Particle project will reveal more of this to us. Its going to take a while...

As mater collesses into the stellar disk rotational forces dictate rotation. Strong gravity forces can change the rate of rotation. Just as friction does to my flat white when stirred... The image is useful for understanding this action. Its not the same thing however. The gravity of a Black Hole is very powerful. The spin rate of some Black holes must be very fast. This can be established as the rotoinal velocity can be tested.
A great deal of discussion has been given to this subject and its apparent there will be more. Understanding the words used is not some times enough. We want to know the answers to many questions. Finding the understanding is the difficult part. The scale of these things is hard to grasp. I have seen this else ware; Think of the ice skater. In the pirawhet he spins faster as he pulls his arms in... just as the Black Hole rotates faster as more mater is drawn in. It makes sense does it not ?
I can only offer as advice ( worth nothing ) that being patient will help.

WaxRubiks
2009-May-12, 11:02 AM
nothing crosses the event horizon, problem solved.

cbacba
2009-May-12, 12:25 PM
Do you have a cite for this? Do you have something against Kerr black holes (http://en.wikipedia.org/wiki/Rotating_black_hole)?



Correct, but this is essentially an illusion, since time does not stop from the point of view of the (very hearty) observer falling through the event horizon.



Uh, no, not an echo....



Only from the distant observer's viewpoint. And on what does the distant observer have to base her science other than her direct observations? Apparently the answer is Einstein's general relativity, and it seems Susskind's black hole complementarity also fits in there pretty well. Depending on the observer, two totally different things are observed. Both are correct, like light is a wave OR a particle....



I guess I get your point, but I'm not sure what you're driving at. I think a response to this is going to take a bit more time than I've got this evening...

A cite? I guess the one you gave will do. Note their comment about the inner sphere.

Do I have a problem with the Kerr metric? I don't know. It doesn't really seem to have that event horizon characterized by the classic dust collapse solution. As such, that inner part does not seem to be isolated from the rest of the universe and that might be problematic.

BHs are somewhat of a catch all. Observations tell us with great certainty that there are objects that are quite small and unbelievably massive yet are effectively invisible. However, whether they are the theoretically described BH or something stable whose collapse has been limited by that which we do not know remains to be determined. It might sound irrelevent or like nitpicking but it isn't as the ramifications are likely capable of being detected.

I use the term 'echo' not to mean a reflection but to try to convey the concept of the detection of an emission from something that is no longer there. Also, I stated the emission was not an echo. That is, in the time frame of the rest of the universe, given by the distant observer, decades will pass and the clock falling in will still occaisionally emit yet another photon because it is still present at the edge of the eh and showing T- 0.00000......0000001 seconds till eh crossing. Once emitted, that photon travels the finite distance to the distant observer in approximately the same time as the photons took before the clock reached the point where the general relativity effects became substantial. That is to say, even years later for the distant observer, the clock hasn't crossed the boundary yet and will continue to emit photons in the future that have a relatively short travel time once emitted.

Speaking of experiments and waves or particles, it seems like there was a flurry of papers two years back concerning a simple young's slit experment variation that showed both - one as a null result and the other as a detection - perhaps violating the copenhagen convention of interpretation.

did you read any of that?

grant hutchison
2009-May-12, 01:07 PM
If I am not mistaken, GR predicts a singularity in the form of a ring, rather than a point, in the center of a rotating black hole. I will now yield to those who are better informed on this topic.Yes. A ring supports angular momentum, whereas a point does not.


A cite? I guess the one you gave will do. Note their comment about the inner sphere.

Do I have a problem with the Kerr metric? I don't know. It doesn't really seem to have that event horizon characterized by the classic dust collapse solution. As such, that inner part does not seem to be isolated from the rest of the universe and that might be problematic.Kerr produces two horizons: an outer horizon that behaves like the classic Schwarzschild, and an inner Cauchy horizon with various exotic properties, like an infinite blueshift. The region inside the Cauchy horizon contains "conventional" spacetime, but it is isolated from the rest of the Universe by the outer event horizon and the region between the two horizons, where the radial direction is timelike.
The only state in which the Kerr event horizon disappears is when the black hole is in such an extreme state of rotation that the two horizons meet, eliminating the timelike radial span between them, and allowing the singularity access to the outside Universe. There do seem to be physical reasons why it might be impossible to spin a black hole up through that extremal state, however.

Grant Hutchison

robross
2009-May-12, 08:04 PM
My original point was that the event horizon of a black hole is not a physical boundary. It can be perceived as that area of space where nothing on the other side is able to communicate with our universe any more.

So that was my main question, how can a black hole rotate when its outer boundary is not a physical substance. What rotates? Also, people speak as though "some black holes may rotate." Well, I cannot think of a black hole creation scenario in modern cosmological times where the progenitor of the black hole was not rotating to begin with. Certainly after starting massive gravitational collapse even the tiniest bit of angular motion would cause it to rotate quickly.

The only thing that makes sense to me in talking about black holes rotating is that the vacuum of space on "our" side of the event horizon rotates around the black hole. Which doesn't make sense to me either :confused:

Space expands, space can curve into higher dimensions, but neither of these imply that space "tears". Rotating space implies a tear. If space is quantized, then the smallest unit of space is a plank length long. Two such areas of space in immediate contact form the ultimate boundary on which space could move relative to to other space. So at the boundary of a rotating black hole, there is some area of space "sitting there" as normal, while next to this boundary, space is moving relative to the "stationary" space. That part is confusing to contemplate.

Rob

nauthiz
2009-May-12, 08:10 PM
What's rotating is the matter that actually makes up the black hole.

I don't think it makes sense to say that the event horizon is rotating, but the event horizon is not really the black hole itself, it's just a region in space where a certain phenomenon occurs because of the presence of a black hole.

Argos
2009-May-12, 08:18 PM
People generally assume Schwarzschild BHs to be the regular, run-of-the-mill ones. In fact, the formation of a static BH would be extremelly unlikely, since every star rotates at some pace. So, Kerr BHs must be the norm, the real physical entities. Static, Schwarzschild ones live only in the abstract realm.

tdvance
2009-May-12, 08:25 PM
Nauthiz: I don't think we have yet observed a non-spherical event horizon, frame dragging nor the ergosphere of a black hole,


We may have observed frame dragging. I once attended a talk by one of the technicians for one of those infrared telescopes that observed the central black hole of the Milky Way. Based on the observations, an animated simulation was produced showing the motions of the stars close to the event horizon (interpolating between observations). Those that approached closely got quite a kick in acceleration. I expect frame dragging could be measured in that.

JohnD
2009-May-12, 08:30 PM
robross,
I defer to all those above, but I don't think anyone has mentioned conservation of energy, beyond the ice dancer analogy.

The kinetic energy of the mass's spin must be conserved, beyond its collapse into a BH. Sure, much energy will be thrown out, taking some 'spin' with it but much will remain. The energy is still there. How else can you describe the resulting BH but by saying it spins?
We don't know what spins, but the spinning energy is still there, causing the effects described above.

My image of a spinning BH is not of "torn" space, but space curved, just as it is around a mass that is not a BH. We can visualise that in the 'rubber sheet' analogy. Imagine that rubber sheet space with a network to represent the shape of space. In a plan view, 2D, the central dip around the mass is invisible - you just see radial lines, with the circumferential lines getting closer and closer together the nearer they are to the mass, as the gravity well gets deeper and its gradient steeper.
Now imagine the mass is a spinning BH. The circumferential lines of the net stay the same, but the radial lines become spirals. The faster the BH spins, the tighter the spiral and the more turns from infinite space to the BH. But a radial line, however tightly curled, is still connected to its infinite outer end, with no breaks.
A picture would show what I mean, but I can't posts pics here, so I hope the words show what I 'see'.

John

astromark
2009-May-13, 07:41 AM
I noticed a tendency to talk of the event horizon of a fast rotating super massive black hole as if it were real. Its not there. The fact that at a given distance from a BH light can not escape. That's all there is to it. If a Black Hole has rotational velocity then it is spinning. That what might be at the very centre of the BH must be rotating very rapidly... As we have zero evadance of a non rotating galactic core,. We can make the informed opinion that all BH are spinning.

cbacba
2009-May-14, 03:36 PM
I noticed a tendency to talk of the event horizon of a fast rotating super massive black hole as if it were real. Its not there. The fact that at a given distance from a BH light can not escape. That's all there is to it. If a Black Hole has rotational velocity then it is spinning. That what might be at the very centre of the BH must be rotating very rapidly... As we have zero evadance of a non rotating galactic core,. We can make the informed opinion that all BH are spinning.

The classical dust collapse nonrotating model has been around a lot longer and is simpler to get a grasp on - despite the unreality of it. Were it to exist, one would be faced with the problem that everything falling in is still falling towards the event horizon and hasn't reached it yet (according to the distant observer or according to our universe). That leads to potentially one heck of a high density at the eh discontinuity.

I'm not into GR enough to have a good feel for whether the Kerr and the other metric (charge + rotation) get around this problem or not. Certainly, one has problems with the original as it will not be dust collapsing and would likely never form due to eddington limits on radiation supporting the collapse were it not noninteracting dust particles. I think the term is eternally collapsing objects ECOs.

However, if the kerr still has a true event horizon discontinuity as in the schwartzchild radius, then matter would not cross it in a finite time. However, I thought the wiki article indicated it did not have a true discontinuity problem at either the internal or external one.

grant hutchison
2009-May-14, 04:05 PM
However, if the kerr still has a true event horizon discontinuity as in the schwartzchild radius, then matter would not cross it in a finite time. However, I thought the wiki article indicated it did not have a true discontinuity problem at either the internal or external one.Can you give us the relevant quote? I'm not seeing that anywhere.

Grant Hutchison

SparkyVA
2009-May-14, 09:34 PM
The size of a black hole is usually measured at the event horizon. Peering into a black hole is a fairly difficult process. But consider the ice skater who spins, then draws her arms in to spin even faster. The pre-black hole, if it is spinning will spin even faster as it's actual diameter shrinks and its mass is concentrated as it evolves into a black hole. The wheelchair guy predicted radiation from the poles of a rotating black hole. Is there anything in that radiation that can give us an idea of the actual diameter of the concentrated mass? I.e. if you know the mass of an object and it's rotational speed, can't you then estimate the diameter/momentum even if it is hidden behind an event horizon? I am sure there are better brains than mine working on that problem. While many assume a black hole is a singularity, the rotational speed of the mass would go to infinity as it's actual diameter decreased to zero. Since that sounds rather impossible, there must then be an actual diameter. If there is an actual diameter, then there must be a density. If there is a density, then maybe we are looking at a quark star. - Enjoy

grant hutchison
2009-May-14, 09:39 PM
While many assume a black hole is a singularity, the rotational speed of the mass would go to infinity as it's actual diameter decreased to zero. Since that sounds rather impossible, there must then be an actual diameter. If there is an actual diameter, then there must be a density. If there is a density, then maybe we are looking at a quark star. - EnjoyWell, as pointed out above, the diameter of the singularity doesn't go to zero in a rotating black hole: it forms a ring instead. The ring singularity is of zero thickness, however, so we still have that vexed problem of infinite density and tidal gravity under GR.

Grant Hutchison

publius
2009-May-15, 03:03 AM
The classical dust collapse nonrotating model has been around a lot longer and is simpler to get a grasp on - despite the unreality of it. Were it to exist, one would be faced with the problem that everything falling in is still falling towards the event horizon and hasn't reached it yet (according to the distant observer or according to our universe). That leads to potentially one heck of a high density at the eh discontinuity.



It doesn't work quite that way. It's true that in coordinates of any external observer (one whose own world line does not cross the horizon), that the collapsing mass never crosses the horizon. That is best seen as a "defect" of those coordinates. In more appropriate coordinates, such as a co-moving observer riding the collapse down, the singular state is reached in finite time, and nothing strange happens at the horizon.

Even in those external coordinates, the density at the horizon doesn't become infinite. You see, the notion of volume depends on the metric, which is getting compressed radially in Schwarzschild coordinates. So even though the mass looks like it is getting compressed into a smaller and smaller radial coordinate, the "proper distance" and the effect on volume keeps the density finite. And then there is the regular Lorentz contraction of a moving object, compressing free-falling objects, which are moving relative to the external observer.

The radial "length" element in Schwarzschild is dr/(1 - R/r), where R is the Schwarzschild radius, 2GM/c^2. As r --> R, a small increment of 'r' represents a large metric distance, and it blows up at R = r. A ruler of constant length "appears" to be getting shorter and shorter, approaching zero as it approaches the horizon and "freezes" there. But nothing strange happens from the perspective of the ruler itself.

-Richard

publius
2009-May-15, 03:21 AM
Now, rotating black holes. They are much more complex. :lol: The representive asymptotic state is the Kerr metric. A real rotating collapse (perfectly symmetric idealization, of course)is something that approaches that state, in the same fashion as the cold dust collapse model reaches Schwarzschild.

The Kerr metric is not static (but is stationary), meaning there is an invariant notion of frame dragging there that can't be transformed away via any coordinate transform. This means there are space-time cross terms in the metric, the calling card of frame dragging effects.

There are three boundaries of interest in that metric. The first, a characteristic of frame dragging space-times, is a "stationary limit", also called the ergosphere (but it isn't spherically shaped). Past this point nothing can remain stationary in the coordinates. It has to move.

If we think of frame dragging as "space moving", then something stationary would be "moving against space faster than light". The g_00 part of the metric goes to zero there. But, because of the space-time cross terms, (ie dr*dt, dphi*dt, etc), if something is moving, the clock rate can still be greater than zero because of the offsetting effect.

The next boundary is the first event horizon, and no matter how fast it's moving, there are no radially stationary observers possible there. That is distinguished by the g_11 component of the metric blowing up, just as it does in Schwarszchild. Radial space and time "flip" there, then flip back again at a second horizon (a spot where g_11 again blows up).

Because of the frame dragging, "frozen matter" asymptotically approaching the horizon appears to be moving, rotating around with space itself. So, in external coordinates, the "freezing" still happens, with clock rates appearing to stop, but that freezing occurs as a rotating shell. :)

-Richard

publius
2009-May-15, 03:37 AM
A word about the stress-energy tensor. It is a tensor, that is an invariant object. The components, which give you things that the energy density, energy current density, and the momentum density and its current, do change with the coordinates.

There can be problems with the coordinates that make terms blow up or act funny, but the invariant question is if there any problem with the tensor in the invariant sense.

The stress-energy tensor in a cold dust collapse model only becomes singular at r = 0. In Kerr, it's only singular at a ring locus. In static Schwarzschild, the tensor is zero everywhere save for r = 0, where it blows up. In Kerr, it's zero everywhere save at the ring, where it is singular.

In the collapse models, you have a dynamic process approaching those states. The stress energy tensor never becomes singular in external coordinates.

-Richard

AndrewJ
2009-May-15, 03:59 AM
Everything in space rotates unless it's stuck to or frictionally locked to something bigger. Things don't have to rotate, they just get angular momentum from a knock and there might not ever be anything to resist it. An object that receives no accelerations in its creation seems improbable.

But black holes are queer things...

Jens
2009-May-15, 04:07 AM
A slowly rotating black hole interior may be possible, but at some very slow rotational rate, we have to ask: Rotating with respect to what, as all things are relative?

Movement is relative, but isn't rotational movement absolute?

Jens
2009-May-15, 04:08 AM
An object that receives no accelerations in its creation seems improbable.


It wouldn't require that. It would be enough if all the accelerations it has received balance out to zero. But of course your point still hold: that would be improbable.

nauthiz
2009-May-15, 04:14 AM
Movement is relative, but isn't rotational movement absolute?

Pretty much. Rotational movement is just a way of saying that different parts of the object are moving relative to each other. Or that they're all circling the object's axis of rotation.


But of course your point still hold: that would be improbable.
Infinitely improbable, even. :D

cbacba
2009-May-15, 12:37 PM
Can you give us the relevant quote? I'm not seeing that anywhere.

Grant Hutchison
"
The Kerr metric is often used to describe rotating black holes, which exhibit even more exotic phenomena. Such black holes have two surfaces where the metric appears to have a singularity; the size and shape of these surfaces depends on the black hole's mass and angular momentum. The outer surface encloses the ergosphere and has a shape similar to a flattened sphere. The inner surface is spherical and marks the "radius of no return" also called the "event horizon"; objects passing through this radius can never again communicate with the world outside that radius. However, neither surface is a true singularity, since their apparent singularity can be eliminated in a different coordinate system. Objects between these two horizons must co-rotate with the rotating body, as noted above; this feature can be used to extract energy from a rotating black hole, up to its invariant mass energy, Mc2.
"

from http://en.wikipedia.org/wiki/Kerr_metric

grant hutchison
2009-May-15, 01:00 PM
However, neither surface is a true singularity, since their apparent singularity can be eliminated in a different coordinate system. Ah, OK.
That quote applies to the ergosphere and the outer event horizon, not to the two event horizons of the Kerr metric. But the text immediately preceeding my quote above also makes it clear that the outer horizon is a true event horizon, which prevents events inside the horizon communicating with the rest of spacetime.
The matter of the "apparent singularity" at the horizon, which goes away with a more appropriate choice of coordinates, applies equally to the Schwarzschild metric of a non-rotating black hole. It doesn't affect the surface's status as an event horizon.

Grant Hutchison

cbacba
2009-May-15, 01:19 PM
I read the quote above the bold highlight as indicating the inner surface as being the event horizon or point of no return.

However, at this level, I don't trust the wikipedia to always be correct and I haven't studied anything in this realm for over 30 years so I don't have a text to refer to.

grant hutchison
2009-May-15, 03:36 PM
I read the quote above the bold highlight as indicating the inner surface as being the event horizon or point of no return.Yes, the inner surface mentioned in your Wikipedia quote is in fact the outer event horizon of the Kerr black hole. Below that surface, the radial direction becomes timelike, just as in the non-rotating Schwarzschild black hole. So we have a point-of-no-return in both types of black hole. In the Kerr black hole there's another event horizon, farther in, beneath which the radial direction becomes spacelike again. Travellers have the theoretical option of flying around in there indefinitely, if they can survive the journey, but they can't get back out again through the timelike region between the two event horizons.

Grant Hutchison

tommac
2009-May-17, 07:07 PM
Actually if a Black Hole warps space-time so much that nothing can escape how can a CHANGE in topology escape?

I understand that the topology itself is there because the BH is there. I realize that there is no need for the gravitational waves to propogate from inside a BH outward ... however I dont get how what is going on inside of BH can propogate outwards IF gravity itself can only propogate at the speed of light AND that space-time is so warped that even light cant escape.

raptorthang
2009-May-17, 07:23 PM
Curious: I assume that smaller black holes orbit larger lack holes. Can the rotation of smaller black hole be 'locked' in place like the Moon to the Earth? If so, does this impact the 'stuff' that is happening around the event horizon of the smaller black hole?

grant hutchison
2009-May-17, 08:18 PM
I understand that the topology itself is there because the BH is there. I realize that there is no need for the gravitational waves to propogate from inside a BH outward ... however I dont get how what is going on inside of BH can propogate outwards IF gravity itself can only propogate at the speed of light AND that space-time is so warped that even light cant escape.I think you mean "curvature", not "topology". :)
All the stuff that happens to black holes comes from outside the event horizon: the addition of mass, charge, angular momentum; the emission of Hawking radiation and gravitational waves. The inside of the black hole doesn't need to (and can't) send signals outwards.
What do you think is "going on" inside the black hole from the point of view of external observers?

Grant Hutchison

tommac
2009-May-17, 08:41 PM
I think you mean "curvature", not "topology". :)
All the stuff that happens to black holes comes from outside the event horizon: the addition of mass, charge, angular momentum; the emission of Hawking radiation and gravitational waves. The inside of the black hole doesn't need to (and can't) send signals outwards.
What do you think is "going on" inside the black hole from the point of view of external observers?

Grant Hutchison


I think nothing is going on inside the EH. Or at least nothing that can effect us.

But what I dont get about is the momentum. The momentum of what?

grant hutchison
2009-May-17, 08:49 PM
But what I dont get about is the momentum. The momentum of what?Are you asking about the angular momentum I mentioned? The angular momentum of the black hole: its rotation, which we can modify by chucking stuff in from the outside.

Grant Hutchison

tommac
2009-May-17, 08:50 PM
Are you asking about the angular momentum I mentioned? The angular momentum of the black hole: its rotation, which we can modify by chucking stuff in from the outside.

Grant Hutchison

So the effect is only from the stuff that is hanging out around it.

robross
2009-May-17, 10:27 PM
Are you asking about the angular momentum I mentioned? The angular momentum of the black hole: its rotation, which we can modify by chucking stuff in from the outside.

Grant Hutchison

I can understand how charge and mass attributes of the black hole are manifest outside the event horizon, and how we can measure these, and what their effects "look like" to us observers of the black hole. On the issue of angular momentum, the only external manifestation of this attribute that has been mentioned is "frame dragging." Is this the only way to observe the effect of a black hole's angular momentum? Or are there other external effects that we can see and say " this happens because the black hole is rotating", just like we can say "this mass is attracted to the black hole because of its gravity", or "this electron is affected by the black hole because of its charge"?

Rob

grant hutchison
2009-May-17, 10:37 PM
I can understand how charge and mass attributes of the black hole are manifest outside the event horizon, and how we can measure these, and what their effects "look like" to us observers of the black hole. On the issue of angular momentum, the only external manifestation of this attribute that has been mentioned is "frame dragging." Is this the only way to observe the effect of a black hole's angular momentum?Well, other effects mainly derive from frame dragging. Frame dragging gives us the ergosphere region above the event horizon of the black hole, which lets us harvest energy from the black hole's rotation. Frame dragging separates out photon orbits, so that the photon sphere occupies a distinct volume around the black hole, threaded by complicated looping orbits. Frame dragging distorts the orbits of objects moving around the black hole. Frame dragging alters the way the gravitational lens around the black hole works, changing our view of distant objects.
The size and shape of the event horizon of a black hole is also altered by its rotation: that's deeply linked to the frame dragging, but I don't know enough about GR to say whether or not it's caused by frame dragging.

So there are lots of observables. Does that answer your question?

Grant Hutchison

tommac
2009-May-17, 10:51 PM
I can understand how charge and mass attributes of the black hole are manifest outside the event horizon, and how we can measure these, and what their effects "look like" to us observers of the black hole. On the issue of angular momentum, the only external manifestation of this attribute that has been mentioned is "frame dragging." Is this the only way to observe the effect of a black hole's angular momentum? Or are there other external effects that we can see and say " this happens because the black hole is rotating", just like we can say "this mass is attracted to the black hole because of its gravity", or "this electron is affected by the black hole because of its charge"?

Rob

Even frame dragging .... it could only be dragged from "stuff" outside of the EH. Anything inside the EH could not frame drag anything.

grant hutchison
2009-May-17, 11:49 PM
Even frame dragging .... it could only be dragged from "stuff" outside of the EH. Anything inside the EH could not frame drag anything.Well, the metric works just fine with nothing outside the event horizon. The Kerr black hole leaves its rotating imprint on the curvature of spacetime in just the same way the Schwarzschild black hole leaves its stationary imprint.
The distant observer's view, in which nothing is ever seen to cross the event horizon, is just one possible way of parsing things.

Grant Hutchison

tommac
2009-May-18, 12:56 AM
Well, the metric works just fine with nothing outside the event horizon. The Kerr black hole leaves its rotating imprint on the curvature of spacetime in just the same way the Schwarzschild black hole leaves its stationary imprint.
The distant observer's view, in which nothing is ever seen to cross the event horizon, is just one possible way of parsing things.

Grant Hutchison

Sorry if I am hijacking the thread but how does can gravity propogate to outside of the EH if it only travels at C? I think I understand why a black hole has gravity ( even though a gravitational wave couldnt escape the EH ) but I dont understand how frame dragging could happen without some sort of wave coming from within the EH.

Do you understand my confusion?

grant hutchison
2009-May-18, 01:06 AM
Sorry if I am hijacking the thread but how does can gravity propogate to outside of the EH if it only travels at C? I think I understand why a black hole has gravity ( even though a gravitational wave couldnt escape the EH ) but I dont understand how frame dragging could happen without some sort of wave coming from within the EH.

Do you understand my confusion?Not really. :)
Spacetime is curved in the vicinity of a mass. That alters the free-fall geodesics so that stuff moves towards the mass. For a rotating mass, the geodesic for an infaller is also skewed off the radial direction: you fall towards and around the mass. That effect becomes very noticeable in the close vicinity of a Kerr black hole. That's frame dragging.

Grant Hutchison

tommac
2009-May-18, 01:37 AM
Not really. :)
Spacetime is curved in the vicinity of a mass. That alters the free-fall geodesics so that stuff moves towards the mass. For a rotating mass, the geodesic for an infaller is also skewed off the radial direction: you fall towards and around the mass. That effect becomes very noticeable in the close vicinity of a Kerr black hole. That's frame dragging.

Grant Hutchison

I sort of get it. But again how could something that is outside of our universe spin and have an effect inside of our universe? I understand how frame dragging would work if all of the BHs mass was just outside of the EH. But that is not what you are saying right?

grant hutchison
2009-May-18, 08:41 AM
I sort of get it. But again how could something that is outside of our universe spin and have an effect inside of our universe?The phrase "outside our Universe" doesn't really do the job, here.
But the Kerr black hole does its frame dragging in exactly the way a Schwarzschild black hole does its radial gravity: they both leave behind a self-sustaining imprint on the curvature of spacetime.

Grant Hutchison