Black Holes

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Fallingstar1971

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Could a galaxy form from just a spinning mass of gas, or is a black hole required?

Have any galaxies been found with no central black hole?

Could it be faked by forming a galaxy from a spinning cloud with no black hole? Stars would still appear to orbit the center even though there would be no object to orbit. There motion would be due to the cloud spinning when the galaxy formed. Since black holes are not directly observable, Im thinking that it could be possible, all the "quiet" (non-active) black holes may not even exist. What other tests could be used to detect a non-active black hole?

Star
 
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neilsox

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We are quite sure some galaxies have a black hole near the center. Most are subject to reasonable doubt. Likely some smaller galaxies lack a supermassive black hole, but likely most galaxies have several to many black holes that are several solar mass. Neil
 
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MeteorWayne

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You know neil, I have no idea where you get your "facts". They are wrong or distorted so often it's disconcerting. I hate to keep jumping in and correcting you, but I really can't allow casual readers to think you are correct.

The evidence is pretty strong that most (not all) galaxies have a black hole of some large size at their center.

I've seen nothing in any real scientific literature to suggest that most galaxies have "several to many" black holes near the center.

The black holes at the center of large galaxies are not "several" solar mass but millions of solar masses or more.
 
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ramparts

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Neil is just getting several different facts confused, all of which are right but not together :) Almost every galaxy seems to have a central black hole, or end up with one eventually. These are, as MW said, extremely massive. Much light - stellar mass - black holes are very common (all very massive stars end up that way), but they're scattered throughout the galaxies. A galaxy with more than one central black hole wouldn't last very long, the gravitational tug of war would cause the two to merge :) In fact, after large galaxies (each with their own central black holes) collide, this is what's believed to happen: the two black holes migrate to the centers of their respective galaxies, and then soon merge.

As for fallingstar's original question, I'm not entirely sure. In fact, neither are the pros :) Black holes play a crucial role in galactic formation, but I'm not 100% sure what the prevailing theories on that are. As for some specific questions...

Fallingstar1971":29zpz1sq said:
Could a galaxy form from just a spinning mass of gas, or is a black hole required?
The first galaxies definitely form without a supermassive black hole - there's just no way to build black holes THAT big before galaxies form. Galaxies first form from collapsing gas; the black hole is not a required component. The black hole is, however, important in the subsequent growth of the galaxy, mostly as a way of providing energy, rather than, as you suggest, for the fact that it's really big. In fact, though these central black holes are huge, they're still about 1/1000-1/10000 the mass of the host galaxy, so they aren't hugely important in keeping the galaxy together - the self-gravitation of the galaxy is more than sufficient.

Have any galaxies been found with no central black hole?
Believe so, yes.

Could it be faked by forming a galaxy from a spinning cloud with no black hole? Stars would still appear to orbit the center even though there would be no object to orbit. There motion would be due to the cloud spinning when the galaxy formed. Since black holes are not directly observable, Im thinking that it could be possible, all the "quiet" (non-active) black holes may not even exist. What other tests could be used to detect a non-active black hole?
See the above - the black hole in the center isn't exactly what the stars orbit. Stars on the outside are pulled in by the interior stars, and so on. Think about it like the Sun, which is also held together gravitationally, even though the particles in the Sun don't orbit around some central object.
 
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MeteorWayne

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ramparts":g1q5moxq said:
Neil is just getting several different facts confused, all of which are right but not together :)
The common phenomena I was referring to in his posts.

As for fallingstar's original question, I'm not entirely sure. In fact, neither are the pros :) Black holes play a crucial role in galactic formation, but I'm not 100% sure what the prevailing theories on that are. As for some specific questions...
This is a really new area of research, It's only within the last 5 years or so that it has been found that there appears to be a relationship between the central black hole mass and that of the galaxy around it. At this point, it is unresolved as to which is the chicken, and which is the egg :)

Fallingstar1971":g1q5moxq said:
Could a galaxy form from just a spinning mass of gas, or is a black hole required?
The first galaxies definitely form without a supermassive black hole - there's just no way to build black holes THAT big before galaxies form. Galaxies first form from collapsing gas; the black hole is not a required component. The black hole is, however, important in the subsequent growth of the galaxy, mostly as a way of providing energy, rather than, as you suggest, for the fact that it's really big. In fact, though these central black holes are huge, they're still about 1/1000-1/10000 the mass of the host galaxy, so they aren't hugely important in keeping the galaxy together - the self-gravitation of the galaxy is more than sufficient.
Agree with that.

Have any galaxies been found with no central black hole?
Believe so, yes.
Definately yes... so far :)

Could it be faked by forming a galaxy from a spinning cloud with no black hole? Stars would still appear to orbit the center even though there would be no object to orbit. There motion would be due to the cloud spinning when the galaxy formed. Since black holes are not directly observable, Im thinking that it could be possible, all the "quiet" (non-active) black holes may not even exist. What other tests could be used to detect a non-active black hole?
See the above - the black hole in the center isn't exactly what the stars orbit. Stars on the outside are pulled in by the interior stars, and so on. Think about it like the Sun, which is also held together gravitationally, even though the particles in the Sun don't orbit around some central object.
No, spin has nothing to do with it. It's all mass. Black holes are shown to exist by the orbits of stars around them...there's too much mass in too small a space for any other explanation.

In fact, as I've repeated so many times, even the black holes themselves orbit around the center of mass of the whole galaxy, which may be well removed from the black hole itself.
 
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ramparts

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MW, do you know how close the black hole in Sgr A* is to the galactic center?

I have a bit of experience in the field of how galaxies form, but we didn't focus so much on the role black holes play, so my knowledge has some gaps there ;)
 
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MeteorWayne

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No, and I'm not sure we could determine that with any certainty. How could we really measure it's motion?

The point I was making is just that in any system, all objects orbit the center of mass of the whole system, not the most massive object.
 
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ramparts

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Depends - if we could measure background stars, and if the center of mass were sufficiently close to the black hole that it moved as quickly as those stars in the region do, then we'd be able to see motion. This is something about which I know little, though :)
 
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5hot6un

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In the media, us laymen hear pretty much the same story about black holes. They are weird.

Is there anything at all familiar going on inside? Is there still fusion? Are there protons and neutrons? Is the whole thing one lump of the heaviest element imaginable?
 
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ramparts

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None of the above, in all likelihood. There's certainly no fusion or normal particles. What we do know about black holes comes from the description from Einstein's theory of general relativity (GR): according to GR, black holes are infinitely dense. It wouldn't make sense to ask what particles exist there, because everything is squeezed into an inconceivably (hell, infinitely) small volume.

But then again, GR isn't the right setting to ask those questions in the first place, because after the black hole gets small/dense enough, the effects of quantum mechanics - another theory, which describes very small things, on the scale of particles - takes over. Unfortunately, GR and quantum mechanics don't mesh so well, so we have no clue what happens in the center of black holes, where both theories are necessary to describe what's happening. The result just from GR, that it is infinitely dense, is completely untrustworthy since quantum mechanics effects aren't taken into account.
 
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5hot6un

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ramparts":30veebni said:
None of the above, in all likelihood. There's certainly no fusion or normal particles. What we do know about black holes comes from the description from Einstein's theory of general relativity (GR): according to GR, black holes are infinitely dense. It wouldn't make sense to ask what particles exist there, because everything is squeezed into an inconceivably (hell, infinitely) small volume.

But then again, GR isn't the right setting to ask those questions in the first place, because after the black hole gets small/dense enough, the effects of quantum mechanics - another theory, which describes very small things, on the scale of particles - takes over. Unfortunately, GR and quantum mechanics don't mesh so well, so we have no clue what happens in the center of black holes, where both theories are necessary to describe what's happening. The result just from GR, that it is infinitely dense, is completely untrustworthy since quantum mechanics effects aren't taken into account.
I see. So is it fair to say that black holes are the "center" of our need to merge GR and quantum mechanics?

I saw in another post a question that asked if a black hole has 3 dimensions. You answer here implies it does not.

It just seems so illogical to me. Maybe that 4th grade science lesson about two objects occupying the same space at the same time prejudiced me.

No wonder they dumb black holes down to "not even light can escape" on discovery channel. :D

So Newton description of gravity fit what astronomers observed. (Among many other things)
Einstein rewrote "our" Newtonian understanding of gravity and in the process predicted black holes. (Among many other things)

Now we are stuck trying to explain how the very big becomes infinitely small? Is that right? Is it possible that, like Newton, Einstein's work just falls short?

And now the big question. If humankind does find the answers that merge GR with QM, what will be some of the more profound implications?
 
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origin

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So Newton description of gravity fit what astronomers observed. (Among many other things)
Einstein rewrote "our" Newtonian understanding of gravity and in the process predicted black holes. (Among many other things)

Now we are stuck trying to explain how the very big becomes infinitely small? Is that right? Is it possible that, like Newton, Einstein's work just falls short?
I would not say his work falls short, it is more like:

Newton and Einstein were two of the many rungs on the ladder of discovery and knowledge.

Geez, ain't that poetic?
 
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ramparts

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5hot6un":3obtu3uh said:
I see. So is it fair to say that black holes are the "center" of our need to merge GR and quantum mechanics?
Exactly right. We know of two situations where the conflict between GR and quantum mechanics becomes manifest because both are important: the centers of black holes, and the big bang.

I saw in another post a question that asked if a black hole has 3 dimensions. You answer here implies it does not.
Does it? A black hole certainly has a position in space and time which can be labeled using four dimensions. According to general relativity, it's a point. In fact, particle physics says that elementary particles are also points. Both are likely wrong. We don't know yet ;)

No wonder they dumb black holes down to "not even light can escape" on discovery channel. :D
But that's also true! :)

So Newton description of gravity fit what astronomers observed. (Among many other things)
Einstein rewrote "our" Newtonian understanding of gravity and in the process predicted black holes. (Among many other things)

Now we are stuck trying to explain how the very big becomes infinitely small? Is that right? Is it possible that, like Newton, Einstein's work just falls short?
It certainly does fall short, as we know by the fact that it's currently incompatible with quantum theory.

And now the big question. If humankind does find the answers that merge GR with QM, what will be some of the more profound implications?
I think that would be rather profound in itself, don't you?
 
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5hot6un

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ramparts":29mqljt2 said:
Does it? A black hole certainly has a position in space and time which can be labeled using four dimensions. According to general relativity, it's a point. In fact, particle physics says that elementary particles are also points. Both are likely wrong. We don't know yet ;)
I don't know why, but I spend way too much time pondering this stuff. I know full well that I am ill equipped for I lack the math skills. But the fascination is there.

So... While the singularity is infinitely small, the even horizon is not. It is a 3 dimensional sphere. A gradient where matter as we know it transitions to something we can not yet fully describe. Is that right?

I think that would be rather profound in itself, don't you?
Indeed! The mechanism of creation understood by a product of that creation.
 
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csmyth3025

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On the subject of the black hole singularity, I know that it's described as an infinitesimal point of infinite density. Would it be more correct to say that once a given mass is compressed to its Schwarzschild radius we know of no process that would prevent continued collapse?

I ask this question because the concept of "infinite" density is hard to grasp and hard to believe. I wonder if the mass inside the event horizon becomes energetic massless particles by a process we don't yet understand. Such an equivalent concentration of energy could still produce an equivalent gravitational field, I believe. Correct me if I'm wrong on this.

I wonder if the conditions inside the event horizon are similar to those proposed for the first moments after the Big Bang.

Are there reasons to believe that mass cannot be converted back into energetic photons and, perhaps, "free" quarks?

Chris
 
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ramparts

Guest
It's definitely fair to say that once all of an object's mass is within its Schwarzschild radius, nothing stops its collapse. It's not just that we don't know of anything that could; it's that, by the laws of physics as we know them, it's physically impossible, the reason being that the escape velocity within the Schwarzschild radius is by definition greater than any possible travel speed, meaning no force can possibly overcome the gravity within there.

The one possible exception has to do with quantum effects; unfortunately, we have no clue how gravity acts on quantum scales, so it's hard to say :)
 
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Gravity_Ray

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Is the event horizon the actual edge of the black hole? Because if the black hole is only a single point in space seems to me that the event horizon seems to drag whole chucks of matter around it which appears to be much larger than a single point in space. If so, then what is between the event horizon and the single point that makes up the black hole? Strechee strings of matter?

Is there any place on the web that one can find information about 2 black holes circling each other? Actual not theoretical.
 
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ramparts

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Well, between the black hole and the event horizon is space. The event horizon is just the radius at which the escape speed is equal to the speed of light. There's nothing physically significant there; if you were to fall into a black hole, you'd feel nothing different at the event horizon.
 
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bdewoody

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What I'm curious about is whether it's possible that there are enough black holes spread across the universe to make up most of the so called missing mass. Since they can only be detected by the activity of visible matter in their vicinity or the distortion of light from objects behind them it seems at least possible that they are responsible for some of that missing matter.
 
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MeteorWayne

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As I understand it, that possibility has been considered and rejected as not supported by the evidence.
 
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bdewoody

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MeteorWayne":1vkgaa8u said:
As I understand it, that possibility has been considered and rejected as not supported by the evidence.
How can it be rejected when they can hardly be detected? It was said Tuesday night on the science channel that there are possibly thousands of 10 solar mass black holes in our galaxy. And if all galaxies have super massive black holes at their cores it seems to me they should at least remain a candidate.
 
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SpaceTas

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bdewoody":cohprtco said:
What I'm curious about is whether it's possible that there are enough black holes spread across the universe to make up most of the so called missing mass. Since they can only be detected by the activity of visible matter in their vicinity or the distortion of light from objects behind them it seems at least possible that they are responsible for some of that missing matter.

The estimate of about 10,000 - 100,000 stellar mass black holes in the galaxy comes from the measured number of stars versus mass. Only a small fraction of the stars in the galaxy have enough mass to become black holes.

There was a detailed search for dark matter in the form of MAssive Compact Halo Objects MACHO's (black holes, white dwarfs, brown dwarfs, planets ...). The method used gravitational (micro)-lensing, where a foreground object passes very close to the direct line of sight to a background star. The gravity of the foreground star creates a magnified and distorted image of the background star. This image cannot be resolved, but because it is larger in angular size than the star, makes the combined star+image look brighter. So by monitoring the brightness of millions of stars in the Large and Small Magellanic clouds (next door galaxies) it was possible to put limits on the number of black holes etc in the outer regions of our galaxy. There were very few gravitational micro-lensing events observed. There is no where near enough massive compact objects to make up the mass of dark matter require to explain the rotation curve of our galaxy.

This is why WIMPS (Weakly Interacting Massive Particles == so far undiscovered sub-atomic particle) is the favored general explanation for dark matter.

So the MACHO's v WIMP's debate has been won by the WIMP's.
 
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SpaceTas

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As for the infinite density etc at the center of a black hole; this is an artifact of the very simple ideas and maths used to describe a black hole without any physics. The argument that leads to this is: there is no known force mechanism that can stop a the collapse of a over-massive neutron star; therefore it will continue to collapse to a point; ie no size, infinite density.

Beyond the event horizon there is no satisfactory theory for what happens. It requires some melding of general relativity and quantum mechanics, in regimes where both theories are very unknown and untested, For GR for very strong gravity, and on very small scales, and for QM for very large masses on extremely small scales of quantum foam. The string theory people claimed a major breakthrough when they were able to predict Hawking radiation from a black hole. Hawking radiation was postulated by Stephen Hawking based upon entropy arguments. So the string theory agree with a completely different theory on only one aspect of black holes. The string theory have a nest of resonant strings at the center of black holes ....

From an observers point of view we cannot hope to test anything inside the event horizon (a 3d dimensional ellipsoidal or even donut shaped region). There is a possibility of differentiating different forms of GR in strong gravity. This is done by observing quasi periodic brightness changes (in X-rays) of matter in the last stable orbit of a black hole. There are different relationships between the period of this orbit and the mass of the black hole depending on the theory.
 
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csmyth3025

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Leaving aside for the moment the question of what exactly happens to matter within a black hole, is there any consensus on what happens to electromagnetic radiation that crosses the event horizon?

As a layman who's only knowledge of what lies inside the event horizon is gleaned from non-technical descriptions, I can only guess that my understanding in this area is either entirely wrong or, at best, seriously flawed. Still, I'll lay out what I think is one of the accepted notions in the hope that the responses I receive will give me a better understanding.

First, I understand that within the event horizon, the vector that describes "future" time always points towards the singularity. Thus, anything at the event horizon must move towards the singularity regardless of any translational (rotational) motion it may possess when it crosses the event horizon.

I'm guessing that the singularity itself must be very hot and, thus radiating heat energy. The problem I have with this notion is that this radiated energy would have to move backward through time in order to radiate away from the singularity. Even if a photon of radiated energy was a very energetic gamma ray and it only traversed a nanometer before loosing all its energy - the remnants of the photon (before dissipating entirely) would exist before it was emitted.

The only way out of this situation that I can imagine is that the photon must be emitted forward in time, but not spatially
(radially) outward towards the event horizon. I'm at a total loss trying to figure out when (or if) such a photon would arrive at its future destination.

Chris
 
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Saiph

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From what I recall, estimations of BH numbers aren't enough to account for what we see. Further observations and models also don't match with the sort of distribution you'd expect. I.e. the way stars move and clump and the distortions dark matter create...don't fit the patterns expected from BH's strewn through the galaxy.
 
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