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?'s about Black Holes

  1. Aug 26, 2004 #1
    I am not a student of Physics although it is a very interesting subject to me. I watch alot of science shows about Physics subjects and have had some questions about Black Holes. I just came across this forum yesterday and thought I could get some answers here.
    When you see a graphic of a Black Hole it is usually a funnel shaped thing sucking everything around it in like a whirlpool, but I have thought wouldn't things circle into it more like the orbit of planets on a pretty much flat plain? Like if a star was on the same vertical plain as the Black Hole wouldn't the stuff from the star be spiralling around and into it like a giant disk instead of a funnel? Like the rings of Saturn but steadily being drawn into the Black Hole. Then if there were 2 stars 1 above it and 1 on the side wouldn't the spirals be perpindicular to each other an then the stuff from both stars would collide? I also read somewhere that Black Holes warp space and time and that is what creates the funnels, but in diagrams I have seen of this they show 2 funnels 1 above the Black Hole and 1 below. To me though this doesn't seem exactly right. Doesn't the gravity from the Black Hole pull equally from all directions? So if it is that Black Holes warp space in this manner and you imagine a giant cube in space with a thin membrain stretched around it, ( to represent the space that would be warped) and a Black Hole in the center, and the cube was the exact size so that the sides of the cube were in the the range of the Black Holes gravitational pull but the corners of the cube were out of range wouldn't this create 6 funnels? Then if that is true what if the membrain was stretched around a giant sphere? Wouldn't then the whole sphere just be sucked in and callapse? Anyway these are just a few questions that I have the time to ask right now. I am really glad I came across this forum cause I have alot more questions to ask and it looks like I have found the right people to mb get some answers. So I thank you in advance for you help with these questions that I have. One more thing like I said I am not a student of Physics so if possible pls don't use to many terms exclusive to Physics. I have read through some of these threads and there are alot of terms that I do not know right now.

    Thanks Ed,
  2. jcsd
  3. Aug 26, 2004 #2
    Just from my own reading I get the sense that most scientists view black holes as a very massive sphere surrounded by a disk shaped cloud of debris orbiting into it.

  4. Aug 26, 2004 #3
    I believe the cone/funnel shape more accurately depicts a black-hole having a singularity... therefore large at the top, tapering to a point (the singularity).

    If i'm not mistaken, Kerr holes - rotating black holes - are formed (in theory at least) by a rotating ring of neutrons with the centrifugal force preventing the formation of a singularity and so no funnel shape(?).
  5. Aug 26, 2004 #4


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    The funnel is a 3d representation of a 4d black hole. It represents the curvature of space-time, which we can't really see directly (though you can "see" it via gravitational lensing of light).

    Welcome to PF!
  6. Aug 26, 2004 #5


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    There are some cool animations available on the Internet. I suggest you check out these two:

    http://www.spaceref.com/tools/vi.html?id=139&cat=blackholes&imgs=movie [Broken]

    The gravitational pull from a black hole is the same on all sides, unless the black hole is rotating. If it's rotating, the axis of rotation defines a direction of special significance, so the gravitational field will not be spherically symmetric. It will however still be symmetric around the axis of rotation.

    All six sides of a cube would experience the same force, unless the black hole is rotating. If it's rotating, two sides would feel one force and four sides would feel another force (assuming that the first two are perpendicular to the axis of rotation).

    Yes, a sphere would collapse, unless there are outward forces that compensate for the gravitational pull.

    When you see a diagram that is supposed to show the curvature of space, you should remember that people who draw those diagrams are "cheating" a little. To make sense of the diagrams, you have to pretend that everything has one dimension less than it actually has. You have to imagine that space is 2-dimensional instead of 3-dimensional, and that the event horizon is a circle (1-dimensional) instead of a sphere (2-dimensional).

    Imagine that space is a (2-dimensional) sheet of some elastic material. The singularity is represented by a hole in this material, and the event horizon is represented by a circle around it. Now grab this sheet by the hole and pull it until the material has been stretched into a funnel shape. Now the shape of the material is a visual representation of the curvature of space.

    Of course, the space we live in is 3-dimensional, but we can still think of the funnel diagram as a visual representation of the curvature of two of those dimensions.
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  7. Aug 27, 2004 #6
    Thanks for the information all. Thanks Fredrik for the links. The visuals helped alot. I didnt know about spinning Black Holes. I have some more questions. If you dont mind. I want to know if I am on the right track to knowing how BH are thought to be. Is it figured that all BH are spinning? Or are there non spinning ones? I think I get the description of the sheet. Is the sheet supposed to represent 1 plane in space? Do only BH or does all matter warp space-time some? Is is correct to think of a BH as an infinitely small sponge that sucks up everything around it? Including space and time? Thanks in advance for helping menderstand this. I have some more question that, but I will ask them later. I don't want to make this feel like a test :)

    Thanks, Ed
  8. Aug 27, 2004 #7


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    All matter that has mass does indeed warp space-time. This warping of space-time is the phenomenon we are observing when we experience "gravity". The "sheet" used in illustrations of gravity does not so much represent a plane within space-time, but rather it represents space-time as if it were a plane. The surface of a plane as two dimensions, and so things can move side-to-side or front-to-back. Place a heavy object on that plane, and it will bend in a third direct and (up-and-down), which it did not pose us before.

    The idea is that space, which has three dimensions, is warped by the presence of any mass. This warping is in a fourth direction, but how the heck do you draw a picture of that?! So you draw a picture with two dimensions, and then add a third. This represents looking at three-dimensional space, and then adding a fourth dimension. On the plane, this warping in the third direction has the effect that objects trying to travel past the warp in either of the two previously existing directions (left-to-right or front-to-back) have their pass curved toward the object causing the warp. Adding one dimension, we see that they mass sitting in space warp's space in a fourth direction (which we are unable to see). This warping in a fourth direction causes objects trying to travel past the warp in any of the three more familiar directions (left-to-right, front-to-back or up-and-down), will have their pass curved toward the object causing the warpage. Clear as mud?

    Nonrotating black holes are not necessarily impossible, they're just highly improbable. I can't imagine any way for a star to form, go through its entire life cycle, then collapse into a black hole, all without ever picking up any kind of spin.
  9. Aug 28, 2004 #8
    Thanks for clearing that up for me. I had thought b4 that the graphic with the elastic sheet was supposed to represent 1 plane in space. It didn't seem to make since, now I know why. That wasn't what it was supposed to be. I also didnt think of BH as spining but yea it does seem that they would be spinning and very fast too. I seen another graphic of it now that shows a 3 diminsional grid like a cubes and how BH and other massive bodies warp these cubes and pull them toward the objects. This makes since to me. So the BH warps all these cubes in its range into a single point, and would the spinning of the BH also twist them to? That is why things would spin into the BH in the same direction that the BH is spinning?

  10. Aug 28, 2004 #9
    The orbiting debris form what are known as an accretion rings. Not all black holes have accretion rings. Those rings are formed when there is matter captured by the black hole. This does not mean that all black holes have them.

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