Uncovering the Mysteries of Black Holes

[danihel]

Hi,
I have to say that I'm not sufficiently educated in physics but I'm very interested in it so please excuse my silly questions.
First I'd like to ask what determines the quantity of space that the black hole occupies. Is it just the amount of matter that imploded during the super nova explosion or does the black hole grow bigger with everything that gets in it? If it grows bigger all the time doesn't it mean the matter doesn't completely disappear in the BH, nor it does appear again in other places?

Another question is: when the BH spews out energy it's because the heat and density along with magnetic field somehow overwhelms the gravitational pull of the BH? Does it spew out of the BH itself? does it spew out matter that's within the event horizon?

 

[DaveC426913]

Hi,
I have to say that I'm not sufficiently educated in physics but I'm very interested in it so please excuse my silly questions.
First I'd like to ask what determines the quantity of space that the black hole occupies. Is it just the amount of matter that imploded during the super nova explosion or does the black hole grow bigger with everything that gets in it? If it grows bigger all the time doesn't it mean the matter doesn't completely disappear in the BH, nor it does appear again in other places?

The black hole itself is theoretically zero volume - all the mass has been compressed to a point. But a BH is normally defined by its event horizon. This is nothing more than a radius. Anything less than this radius, including matter or energy can never escape.

This radius is entirely determined by the amount of mass in the black hole.

Another question is: when the BH spews out energy it's because the heat and density along with magnetic field somehow overwhelms the gravitational pull of the BH? Does it spew out of the BH itself? does it spew out matter that's within the event horizon?

A BH does not spew out energy. What happens is that infalling matter forms an accretion disc outside the EH. This disc radiates energy as it compresses.

 

[danihel]

Thank you very much for explanation, so if i understood well- every BH has this one infinitely small point of singularity but differs in the extent of gravitational deformation of space around it.
Does the event horizon grow with stuff falling into the hole?

 

[DaveC426913]

Thank you very much for explanation, so if i understood well- every BH has this one infinitely small point of singularity but differs in the extent of gravitational deformation of space around it.

Yes.

Does the event horizon grow with stuff falling into the hole?

Yes.

 

[Matterwave]

Black holes also radiate (theoretically) Hawking radiation. The simplest model for which is the creation of particle-anti-particle pairs near the Event horizon (allowed because of QM). One of the particles drop into the BH, the other particle escapes. This process steals energy from the black hole until the black hole completely evaporates. For a massive black hole, this would take a really really long time (something like a googol years for a supermassive black hole), but for theoretical micro-black holes, this process may evaporate the black hole within a matter of seconds.

 

[Prologue]

The black hole itself is theoretically zero volume - all the mass has been compressed to a point. ...

Why does the BH have to have zero volume? Why can't it just be a lump that is contained within its own event horizon? Is it because the highest counter-compression (that I know of) is neutron degeneracy pressure, and in order to be dense enough to be within it's own event horizon the pressure would be greater than this?

 

[Matterwave]

There are no known forces of nature which would prevent further gravitational collapse once the matter is squished within the Event horizon. We don't know if the matter really is concentrated at a point, we just don't know of any pressure or force that could "win" against gravity inside the EH.

 

[qraal]

Why does the BH have to have zero volume? Why can't it just be a lump that is contained within its own event horizon? Is it because the highest counter-compression (that I know of) is neutron degeneracy pressure, and in order to be dense enough to be within it's own event horizon the pressure would be greater than this?

Pressure is a measure of a system's internal energy - thus it adds to the gravity and so causes the necessary counter-pressure to increase. Confinement pressures from Pauli Exclusion and the like, mean the pressure-gravity feedback becomes positive and so runs away into a singularity. Thus the singularity is really where quantum and GR conspire to create the incomprehensible. A strong motivation to develop 'quantum gravity', but so far the proposed QGR theories have no clear front-runners we can confidently say describe/eliminate the singularity.

 

[qraal]

There are no known forces of nature which would prevent further gravitational collapse once the matter is squished within the Event horizon. We don't know if the matter really is concentrated at a point, we just don't know of any pressure or force that could "win" against gravity inside the EH.

Good point. We don't know, but what theory tells us is that the mass is infinitely compressed because of the pressure-gravity runaway feedback... which makes more than a few people wonder if the theory is wrong.

There's two ways of handling that. One involves quantum gravity effects which prevent the singularity and smear it out or cause a daughter Universe to explode as a new space-time. The other option is to propose that a new kind of space-time is either just above the event horizon or just within it, and some kind of repulsive gravity is at work producing a 'gravastar' or similar non-singular space-time. The event horizon becomes a real physical boundary between two very different space-time states in the second option. The race amongst theorists working with either option is to produce real observational tests to distinguish between the options and/or classical GR black-holes.

 

[Chronos]

I would quibble over the density of a black hole. It is theoretically a point in space, but Heisenberg suggests it must have a finite, albeit miniscule, volume.

 

[danihel]

Is the fact that everything that falls into the black hole adds to its gravitational pull consistent with the notion that it appears in some other universe or bigbang or some other place in the universe?

 

[DaveC426913]

Is the fact that everything that falls into the black hole adds to its gravitational pull consistent with the notion that it appears in some other universe or bigbang or some other place in the universe?

In fact, no it seems pretty inconsistent. Whatever it has undergone, as far as we can tell, the mass it still there, at the centre of the BH.

 

[danihel]

thanks a lot again for such a quick answer
I was just wondering because in some documentaries I heard various ideas about black holes maybe being worm holes to other universes and stuff like that

 

[Chronos]

A wormhole to another universe would drain energy from this universe. No observations to date suggest loss of energy from this universe. Were this so, we would observe black holes eating matter without increasing in mass. Black holes would be all appear to be about the same mass if this was the case.

 

[DaveC426913]

I was just wondering because in some documentaries I heard various ideas about black holes maybe being worm holes to other universes and stuff like that

TV is a notoriously bad place to pick up knowledge. You just can't fit all the necessary foundational material in a half-hour or one-hour show. And almost all science shows go for gimmicky or flashy speculative stuff rather than good science.


Go with books.

 

[qraal]

A wormhole to another universe would drain energy from this universe. No observations to date suggest loss of energy from this universe. Were this so, we would observe black holes eating matter without increasing in mass. Black holes would be all appear to be about the same mass if this was the case.

Oddly enough we wouldn't see a mass-loss. The external field is "frozen" at the value set by infalling matter. In fact wormholes would exhibit a similar mass change with respect to inflow and outflow through them. A net negative flow would cause a wormhole to become repulsive.

But I wasn't referring to regular wormholes. Some theories - Lee Smolin's for example - have black holes producing "baby universes" from their mass.

 

[wofsy]

couple of questions:

- Why can't a BH be something other than a point?

- Someone once said to me that as a BH forms an outside observer will see the clocks in the forming hole slow down continually towards zero. So it would thus take an infinite amount of time on the outside observer's clock for the hole to form. If this is true how can a BH ever be observed? Put another way, how can a BH exist unless it already existed at the creation of the Universe?

 

[Frame Dragger]

I would quibble over the density of a black hole. It is theoretically a point in space, but Heisenberg suggests it must have a finite, albeit miniscule, volume.

A good point which is supported by theoretical existence of ring singularities.

@wofsy: I think as long as people are talking about Black Holes and the possible sigularity at its center interchangeably, finer points should be abandonded for the basics.

A black hole is never a point, it's an entity including an event horizon past which GR says there will be a point of zero volume. A singularity doesn't have to be a point... it can be a point, or a string, or a loop... maybe. If they exist. You have to understand, while there is observation of BHs, nothing has ruled out the possibility of something just at or below the radius of a BH existing with or without an event horizon. The margin of error in astronomical observations leave room for argument, and no one has detected the emission of phonons or the like from lab-bound analogues. The question is as up in the air as DaveC426913 pointed out.

As for the red shift you're talking about, you're ignoring relativity. To an observer it may appear that infalling matter fades to unobservability at the event horizon (after a long redshift) assuming you could see through the radiation from the accretion disk (and maybe Hawking Radiation or other thermal-like process). To the subject B falling into the black hole, they can pass the event horizon without noticing anything until tidal forces, radiation, etc, tear them to bits. This requires a VERY large black hole, and a hardy observer, but you get the idea. The whole problem with black holes and the event horizon is that it marks a boundary condition on reality if the theory is formalist in GR.

This is why some people believe that a black hole is a series of events which eventually comes to a complete halt (not as in frozen, as in oblitered), and not an "object". The Singularity by definition never interacts with the universe, only the event horizon, and surrounding accretion disc. Gravitic effects are the result of within the radius of the event horizon. If GR is correct, then black holes represent a boundary condition on anything like Classical and Relative space-time. Whatever describes an object with a radius of or less than the event horizon requires physics other than GR or SQM in their current state, assuming they are open to description or observation... which is an iffy assumption. Some String Theorists and many other have ideas, but they are completely untested and theoretically also untestable. Fantastic and amazing ideas are born around the topic of black holes, but in terms of hard science they are still pretty damned enigmatic.

 

[Frame Dragger]

Oddly enough we wouldn't see a mass-loss. The external field is "frozen" at the value set by infalling matter. In fact wormholes would exhibit a similar mass change with respect to inflow and outflow through them. A net negative flow would cause a wormhole to become repulsive.

But I wasn't referring to regular wormholes. Some theories - Lee Smolin's for example - have black holes producing "baby universes" from their mass.

You're right and wrong. A TRAVERSABLE wormhole to another part of the same universe might not show mass loss, but one to another universe surely would. I'm not sure exactly what kind of wormhole you're talking about... and ERB? Something far more stable presumably... As for the idea that black holes spawn baby universes, I think that is just people enjoying the apparent similarity to the initial state of the universe and the mathematics of a gravitational singulariy. Most resptable "multiverse" theories involve a primordial vacuum, brane cosmology, etc... not our universe blowing inverted bubbles out of its own existence.

 

[Chronos]

The external observer will see the event horizon form, but, never the singularity.

 

[wofsy]

The external observer will see the event horizon form, but, never the singularity.

so how can a BH exist unless it existed at the creation?

 

[sylas]

so how can a BH exist unless it existed at the creation?

Just put enough mass together in one place; the collapse of a big star should do it. Not seeing past the horizon has nothing to do with whether it exists or not. Horizons are limits of visibility or influence; not limits of existence.

Cheers -- sylas

 

[wofsy]

Just put enough mass together in one place; the collapse of a big star should do it. Not seeing past the horizon has nothing to do with whether it exists or not. Horizons are limits of visibility or influence; not limits of existence.

Cheers -- sylas

but sylas - the outside observer will never see the clocks go to zero time - it will take infinitely long - so how can the singularity form?

 

[Dmitry67]

it will take infinitely long for the outside observer only
For freely falling observer it takes a short time

 

[Frame Dragger]

Just put enough mass together in one place; the collapse of a big star should do it. Not seeing past the horizon has nothing to do with whether it exists or not. Horizons are limits of visibility or influence; not limits of existence.

Cheers -- sylas

Remember wofsy, what the central tenants of GR mean. Along with Mass-Energy equivalence, the concept that there is no universally valid "clock" is central to this issue. Imagine GIANT body of gas collapsing to form a black hole, such as the ones that exist in Active Galaxies. You could be there for the formation of such a hole, pass the "point of no return" that is the Event Horizon and you would NOT NOTICE A THING.

To the person watching you from a distance, it would be a VERY different story. You would appear to become dimmer and dimmer... redder and redder... the closer you came to the event horizon. You wouldn't experience that however as the person falling inward until you began to experience tidal forces that turned you into subatomic spahgetti.

This is what "Relativity" is. Each entity, person, particle, space-like slice... etc... has its own unique "clock" that tells local time. No two clocks will agree. The event horizon of a black hole represents an end to the ability of GR to predict any further behaviour beyond speculation, or the concept of a literal singularity that truly exists. An event horizon is the boundary at which a photon's possible degrees of freedom ALL take it within the BH, and make escape impossible. As nothing is believed to be FTL, presumably all "information" is lost to observers (unless there is a Holographic Principle that works, some kind of Beckenstein-busting remnant, etc... who knows), but that means NOTHING to the person passing that horizon except that they are inevitably going to encounter the singularity. And be torn apart. You get the idea.

Again... anything COULD be crushed into a singularity if you compress it past its Schwarzschild Radius. In nature this happens to massive bodies (probably) undergoing collapse or accretion. A neutron star accretes matter and then collapses further, or a star or other gas cloud collapses under its own gravity beyond the force that even degeneracy pressures can overcome.

Remember, a white dwarf = Atoms stripped of their electrons in a soup of degenerate matter. A Neutron Star is one in which those electrons and protons can no longer overcome gravity and are forced to fuse. They become neutrons, so a neutron star is probably varying configurations and densities of degenerate neutron matter. In THEORY, you could have a quark star, or a gravstar and they might, or might not have an event horizon. In theory, it may be event horizons do not exist, and neither do singularities. A BH is a BH, not so much because of the singularity at its heart... which is still open to TONS of theoretical speculation... but the event horizon that separates anything within its radius from the universe outside.

 

[wofsy]

it will take infinitely long for the outside observer only
For freely falling observer it takes a short time

Ok but I am an outside observer say the center of of a distant galaxy. Why then do astronomy web sites say that there are Bh's in their center. that seems impossible.

 

[Frame Dragger]

Ok but I am an outside observer say the center of of a distant galaxy. Why then do astronomy web sites say that there are Bh's in their center. that seems impossible.

I don't mean this as an insult, but if you haven't understood the principle of what relativity means, you need to study this subject more. You're not at this point yet. People are trying to tell you as simply as possible: THE EVENT HORIZON IS LIKE ANY OTHER HORIZON. You can't see beyond the horizon on the ocean... right? Does that mean the ocean beyond the horizon doesn't exist? This is a different configuration and type of horizon, but the meaning is VERY SIMILAR.

 

[wofsy]

I don't mean this as an insult, but if you haven't understood the principle of what relativity means, you need to study this subject more. You're not at this point yet. People are trying to tell you as simply as possible: THE EVENT HORIZON IS LIKE ANY OTHER HORIZON. You can't see beyond the horizon on the ocean... right? Does that mean the ocean beyond the horizon doesn't exist? This is a different configuration and type of horizon, but the meaning is VERY SIMILAR.

i understand more than you think. think about my question.
the presence of an event horizon does not mean that there is a black hole.

 

[Frame Dragger]

i understand more than you think. think about my question.

*facepalm*. Look man... Relitivity = event horizon forms and everything beyond that is unknowable. Add mass to a black hole, and its radius increases. Allow it to radiate, and the radius shrinks. What you're describing is only paradoxical from the point of view of a particular observer. It's understanding that both are valid viewpoints forming only PART of a whole that is at the core of Relativity.

Your experience as the observer is precisely why a black hole would be BLACK. The event horizon would be a true black-body in theory, but detection would fail before that point. Send in 10,000 observers at intervals, and you'd either get their mass added to the black hole, or in another theory their information is sort of... smeared... across the Event Horizon. Either way, in principle the varying accounts of each observer would differ from an external observer, and each other.

GR says that space and time are an inseprable fabric, and that everyone at any distance will agree on the outcome of an event. They will percieve it slightly (or in extreme cases such as a black hole VERY) differently, but the event itself must be universally THE SAME... until the Event Horizon, which is the point past which all of those theories cease to make meaningful predictions!

EDIT: you added another statement... An event horizon doesn't mean there is a black hole, as I and others have said previously many times on this thread. However, anything beyond the event horizon doens't interact with the universe outside, so in essence a Black Hole as studied and understood is a gravtiational field bound by the region of the horizon beyond which there could be, "green slime and socks." :)

 

[DaveC426913]

Ok but I am an outside observer say the center of of a distant galaxy. Why then do astronomy web sites say that there are Bh's in their center. that seems impossible.

You're imagining this happening a bit too idealistically. It's not not like have a perfect view into the BH.

'The singularity forming taking infinite time' is really only in principle. What you really see is a fade-to-black, because, very rapidly, the number of photons reaching you drops to near zero. If, in principle, you could crank up the gain on those few photons, you'd see that each successive photon showed less and less change. Eventually, photons rising out of the BH would become few and far between - so few that you'd no longer have an image, you'd simply have discrete photons. Additionally, each photon would show very little change from the last. Ultimately, you'd get bored waiting for photons, especially since the information they brought with them would be horribly out-of-date.

Think about watching a video. At first, the video looks fine, with full pixel information and 30fps frame rate. But very rapidly the frame rate drops to 1fps and then 1 frame every ten seconds. Additionally, each frame begins rapidly dropping information, so that only a few pixels are being updated. You' quickly reach a point where there's only a frame change every minute, and that frame change only updates a couple of pixels. The movie you are trying to watch will take an infinite length of time to play out - not so much because it's slowing down, but because your view of it has virtually ground to a halt.

 

[wofsy]

*facepalm*. Look man... Relitivity = event horizon forms and everything beyond that is unknowable. Add mass to a black hole, and its radius increases. Allow it to radiate, and the radius shrinks. What you're describing is only paradoxical from the point of view of a particular observer. It's understanding that both are valid viewpoints forming only PART of a whole that is at the core of Relativity.

Your experience as the observer is precisely why a black hole would be BLACK. The event horizon would be a true black-body in theory, but detection would fail before that point. Send in 10,000 observers at intervals, and you'd either get their mass added to the black hole, or in another theory their information is sort of... smeared... across the Event Horizon. Either way, in principle the varying accounts of each observer would differ from an external observer, and each other.

GR says that space and time are an inseprable fabric, and that everyone at any distance will agree on the outcome of an event. They will percieve it slightly (or in extreme cases such as a black hole VERY) differently, but the event itself must be universally THE SAME... until the Event Horizon, which is the point past which all of those theories cease to make meaningful predictions!

EDIT: you added another statement... An event horizon doesn't mean there is a black hole, as I and others have said previously many times on this thread. However, anything beyond the event horizon doens't interact with the universe outside, so in essence a Black Hole as studied and understood is a gravtiational field bound by the region of the horizon beyond which there could be, "green slime and socks." :)

you are very poetical

 

[wofsy]

You're imagining this happening a bit too idealistically. It's not not like have a perfect view into the BH.

'The singularity forming taking infinite time' is really only in principle. What you really see is a fade-to-black, because, very rapidly, the number of photons reaching you drops to near zero. If, in principle, you could crank up the gain on those few photons, you'd see that each successive photon showed less and less change. Eventually, photons rising out of the BH would become few and far between - so few that you'd no longer have an image, you'd simply have discrete photons. Additionally, each photon would show very little change from the last. Ultimately, you'd get bored waiting for photons, especially since the information they brought with them would be horribly out-of-date.

In other words, there really aren't any new black holes that have formed in this incarnation of the Universe

 

[Frame Dragger]

you are very poetical

I'm a really *****y poet... I appreciate the irony/insult... and while I shouldn't laugh along with it, I am. Probably that isn't a good sign of my mental health. Ah well. :tongue2:

You are at the very least, extremely curious and persistant. Not bad qualtities at all.

 

[DaveC426913]

the presence of an event horizon does not mean that there is a black hole.

Yes it does, pretty much by definition.

You have a horizon out of which light cannot escape. You have mass inside. Regardless of what's actually happening inside, that's a black hole.

Now, the issue is whether you have a singularity. Well, you do. Same way.

When we project known physics (GR) into the space inside a BH, our understanding tells us there's no force that can withstand gravitational collapse. But: it doesn't matter what actually happens, what matters is that our current physics is unable to describe it. That is the definition of singularity.

 

[DaveC426913]

In other words, there really aren't any new black holes that have formed in this incarnation of the Universe

Why do you say this? This is nonsensical.

Are you trying to say that, because we don't observe it from our current coordinates, it does not exist?


By the same logic there are no stars outside our observable universe. If I flew in my spaceship 1 billion light years East (i.e. l change my coordinates), I would see an empty void at the end of the universe.