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B Black hole question

  1. Dec 18, 2016 #1
    Is a black hole a hole? What I mean by that, the word hole implies a structure that is shall we say like a plug hole in a sink. But a B/H is the result of an imploding star so surely it would be spherical. So really we are talking about black spheres which make much more sense in my head, also it rules out them being a 'gateway' to somewhere else. As matter would be coming in from all direction not only on a plane that the word hole implies. But, that word again, it would imply that it can be filled up, could we assume that Hawkings 'evaporation' would take care of that?
    Now shoot me down in flames!!!!!
     
  2. jcsd
  3. Dec 18, 2016 #2

    phyzguy

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    The Cambridge English Dictionary defines a hole as "an empty space or opening in an object". I don't think there is any rule that a hole has to be two dimensional. The event horizon of a non-rotating black hole is in fact spherical, but I think "hole" is still a good description.
     
  4. Dec 18, 2016 #3

    phinds

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    One way in which it is very pragmatic to think of a black hole as a true hole is that while the event horizon is detectable in our universe, nothing inside it is in causal connection to the rest of the universe except via gravity and the only thing that tells us is how much stuff went in, not what is happening to it now. So it's a hole in our knowledge of the universe in a more direct way than just being something we haven't figured out yet.
     
  5. Dec 18, 2016 #4
    Can someone describe to me exactly how a black hole travels through the fabric of space?
     
  6. Dec 18, 2016 #5

    phinds

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    First of all, there IS no "fabric" of space. That's just pop-sci jargon. A black hole travels through space just like everything else travels through space.
     
  7. Dec 18, 2016 #6
    Someone on here has this picture as their avatar, which visually helped me escape the pop-sci notion of a space fabric.
     
  8. Dec 19, 2016 #7

    Drakkith

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    A more accurate term to use would be the metric of spacetime, not the fabric. A metric is a way of representing the curvature at any location, which is important because a black hole is predicted and described by General Relativity, which describes gravity as a curving of spacetime. The matter and mass that makes up a black hole travels through spacetime exactly like anything else. Unfortunately I'm not sure how to describe how this process works.
     
  9. Dec 19, 2016 #8

    Ssnow

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    I think the word hole is justified by the fact that if you pass on it you fall inside (and it is impossible to turn back) in analogy to a real hole ( where in the major part of the cases you can turn back :smile:). I prefer the word singularity for the black hole because from the geometrical point of view it is a special place where the continuity of the space-time is dropped, but it is a matter of taste :smile:
     
  10. Dec 19, 2016 #9
    Watch Interstellar, the principal is the same, but with black holes =)
     
  11. Dec 19, 2016 #10
    Reminds me of the question as to why we call or consider e.g. the Sun as "Black Body" ... [in black body radiation ...]
    {actually a spherical integral over 4π of indeed infinitesimal black bodies, small openings-holes ...}
     
  12. Dec 19, 2016 #11

    Labguy

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    The Event Horizon (EH) calculates to spherical, but all Black Holes (BH) rotate; none can be "non-rotating". That was shown long ago. Actually, Hawking Radiation (HR) "comes from" the edge of the EH which is the classical 2GM/c2. This EH is at the same radius for a rotating BH as it is in the classical (non-rotating) BH. But, all BH's rotate and that is where the Ergosphere comes in. Roy Kerr showed that a rotating BH also has a "second" EH, the Ergosphere, in the shape of an oblate spheroid with the Ergosphere and the EH meeting at the poles of the axis of rotation. Anywhere off the poles and the EH is "inside" the bulge of the Ergosphere, so you can visualize the BH as having two EH's. A particle, and photons, between the EH and the Ergosphere can escape the BH since the "inside" EH is actually where the radius = the escape velocity of c.

    Also, there is no "tub" to fill up so a BH has no upper mass limit. There is at least one BH known to have a mass of 18 billion suns. It is OJ287 and Craig Wheeler of the University of Texas in Austin, US, says "it depends only on how long a black hole has been around and how fast it has swallowed matter in order to grow. There is no theoretical upper limit.”

    Labguy
     
  13. Dec 19, 2016 #12
    A black hole sure is a hole in terms of spacetime curvature (that could suck things in it) ... like a gravity well, i.e. a hole. [one can draw the diagram]
    I think that's why it was named 'black hole', and 'black' because it also attracts light.
     
    Last edited: Dec 19, 2016
  14. Dec 19, 2016 #13

    Drakkith

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    That's not quite accurate. They are termed black holes because they have an event horizon which not even light can get out of. Black holes do not suck things in any more than the Sun or the Earth sucks things in. One can safely orbit a black hole provided one stays outside the event horizon.
     
  15. Dec 19, 2016 #14
    I completely agree (I skipped the full analysis). I just wanted to visualize and emphasize the space-time distortion due to extreme gravity, as it affects curvature, like you also have mentioned earlier:
     
  16. Dec 19, 2016 #15
    Strictly speaking, the metric is not the same with curvature; i.e. metric tensor gμν vs Riemann tensor R..... etc.
     
  17. Dec 20, 2016 #16
    well think swiss cheese everyone agrees that the cheese has holes in it but the holes are spherical in shape yet we all still think and call them holes in a block
     
  18. Dec 20, 2016 #17
    How small can a BH be?? Is there a theoretical lower limit to the size of a BH?
     
  19. Dec 20, 2016 #18
    But, hey, swiss cheese is white (or yellow?)!! [but then again so my sun is shiny and bright in my 'black body' analogy earlier above ...]

    But the holes in swiss cheese are holow empty spheres, i.e. holes. [While] Black holes are not empty and holow, are they?

    I think that you just have to admit that you like swiss cheese (as much as I do) !!!
    ["Projection" and "Reframing" ... (in psychology - e.g. when you don't have something that you so badly want) - I think you better get to the store! ... (I know I am after this ...)]
     
  20. Dec 20, 2016 #19
    i was just trying to make it easier to think of a hole in 3d and not 2d is all
     
  21. Dec 20, 2016 #20
    Point taken
     
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