Contemplating black holes

[dgarts]

I have a few questions about the nature of a black hole.

1. If its gravity and mass are constantly increasing, at what point should it become impossible for radiation or energy to be spit out of it? Again, if "even light" can't escape its gravitational force (rendering it "black") how does any other energy get spewed out in a jet?

2. With its gravity grabbing objects and pulling them into itself, shouldn't a black hole be easy to spot, as it'd be the place with all the stuff flowing into it? Even if we can't see the hole itself, shouldn't we be able to see a trail of objects all moving towards some unseen centre? (I picture "Wile E. Coyote's" electro-magnet dragging all manner of objects into his cave before the entire place explodes, for dramatic effect, but you get the idea). Apparently we're able to detect whether an object is moving towards us or away from us, based on line shift. Should we not be able to detect a large number of objects all moving to a single point?

3. I've never understood the concept of an "accretion disk". I could understand a disk if a black hole's gravity effected only its equator, but an object that becomes a black hole is originally some spherical body. As such, the "edge of accretion" - that "point of no return" periphery, is actually a bigger sphere that surrounds the entire black hole object. Thus, the proper term should be accretion sphere, shouldn't it? (perhaps we are limited because artists have failed to create an image of what this could possibly look like, but ought we not to drive conceptions further than what we have now since single plane accretion disks are misleading?).

4. Why don't objects orbit a black hole? We suggest that our moon is right at that point where earth's gravity is strong enough to keep it from spiralling off through centrifugal force, but not strong enough to pull it in and crash it into earth. No matter how strong a black hole's gravity is, there is a point away from it where the gravitational force is just like any other object - strong enough to hold something from leaving, but not quite strong enough to pull it right in?

5. What is the structure of a black hole? Should it not be an ever-increasing lump of material of immeasurable density? In which case, would new objects being sucked into a black hole simply adhere to its ever-expanding surface like a big magnet drawing metallic objects to its surface? Even if a black hole's gravity compresses material, the compressed material becomes much much denser until it...what? disappears? (I remember a cartoon where a vaccuum cleaner sucked itself out of existence! Is that the idea of what black hole gravity does to an object?)

Thanks for your thoughts. Perhaps a moderator would prefer these questions be broken up into their own individual posts. Fine. But I do think that all these questions provide an overall context for each response, and ought to be grouped to maintain that context.

[austintorn@aol.com]

This is a photograph that I took while vacationing in space last week:

[neutralino]

I have a few questions about the nature of a black hole.

1. If its gravity and mass are constantly increasing, at what point should it become impossible for radiation or energy to be spit out of it? Again, if "even light" can't escape its gravitational force (rendering it "black") how does any other energy get spewed out in a jet?

I'm not totally sure what you are thinking of here. Once anything (particle or light) has passed beyond the event horizon of a black hole there is no way for it to escape; that is there do not exist any timelike, or null, curves that start inside the event horizon of a black hole and end up outside it. What you are thinking of with the jets is matter or light from the accretion disk that is sent off into space in the form of jets.

2. With its gravity grabbing objects and pulling them into itself, shouldn't a black hole be easy to spot, as it'd be the place with all the stuff flowing into it? Even if we can't see the hole itself, shouldn't we be able to see a trail of objects all moving towards some unseen centre? (I picture "Wile E. Coyote's" electro-magnet dragging all manner of objects into his cave before the entire place explodes, for dramatic effect, but you get the idea). Apparently we're able to detect whether an object is moving towards us or away from us, based on line shift. Should we not be able to detect a large number of objects all moving to a single point?

If you look at an observer falling into a black hole, you will never see it cross the event horizon: that is, you will never see it disappear from view. This is due to the effects of time dilation.

3. I've never understood the concept of an "accretion disk". I could understand a disk if a black hole's gravity effected only its equator, but an object that becomes a black hole is originally some spherical body. As such, the "edge of accretion" - that "point of no return" periphery, is actually a bigger sphere that surrounds the entire black hole object. Thus, the proper term should be accretion sphere, shouldn't it? (perhaps we are limited because artists have failed to create an image of what this could possibly look like, but ought we not to drive conceptions further than what we have now since single plane accretion disks are misleading?).

Only a black hole with angular momentum (i.e. that is spinning) can accrete. I would imagine that any matter would be drawn to the equatorial plane of the spinning black hole, and thus form a disk.

4. Why don't objects orbit a black hole? We suggest that our moon is right at that point where earth's gravity is strong enough to keep it from spiralling off through centrifugal force, but not strong enough to pull it in and crash it into earth. No matter how strong a black hole's gravity is, there is a point away from it where the gravitational force is just like any other object - strong enough to hold something from leaving, but not quite strong enough to pull it right in?

I don't think there is anything to stop a body orbiting a black hole: however the question is will the orbit be stable? Black holes are strange objects whose mass is large enough to make the general relativistic effects of frame dragging very large indeed. I imagine that these will cause the object's orbit to become unstable after a few orbits. Then there's the fact that, since black holes have a very large mass, the acceleration required to keep an object in orbit would be immense.

5. What is the structure of a black hole? Should it not be an ever-increasing lump of material of immeasurable density? In which case, would new objects being sucked into a black hole simply adhere to its ever-expanding surface like a big magnet drawing metallic objects to its surface? Even if a black hole's gravity compresses material, the compressed material becomes much much denser until it...what? disappears? (I remember a cartoon where a vaccuum cleaner sucked itself out of existence! Is that the idea of what black hole gravity does to an object?)

No-one knows what a black hole is like behind its event horizon, since we can never see in there to take a look. So, your guess is as good as mine!

Thanks for your thoughts. Perhaps a moderator would prefer these questions be broken up into their own individual posts. Fine. But I do think that all these questions provide an overall context for each response, and ought to be grouped to maintain that context.

I think the thread is fine as it is. However, I've only given brief answers, since there are a lot of questions!

Good thread starter, though. Oh, and welcome to ToeQuest!

[mkirkpatrick]

Hello and welcome to the forum dgarts,I have been contemplating black holes for years now,helps you sleep at night,imagine you are being pulled in and wake up in another dimension!


warmest regards michael.