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All things aside, how big could a black hole actually get?

  1. May 6, 2012 #1
    Say if one were to meet a nice little black hole and just feed it, everything one could throw at it....

    Galaxies, other black holes, just everything..... how BIG could it actually get, before things started to not be so "black holeish" - like the gravity started to weaken or light started to escape etc., or would it just grow and grow more intense, only with a bigger size.
     
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  3. May 6, 2012 #2

    mfb

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    It would just grow and increase its mass and size. While some effects like spaghettification get smaller, it stays a black hole.
     
  4. May 6, 2012 #3
    50 billion suns could be lurking out there - but that's the limit.
    http://www.newscientist.com/article/dn14653-how-big-can-a-black-hole-grow.html

    Two astronmers reached that conclusion. But the reasons provided have nothing to do with any internal inability to become bigger. Instead, they are based on the limiting factors that the black hole creates in its own vicinity via blasting away available additional material until it sits starved of new matter. Barring that there seems to be no reason why there should be a limit. In fact, The Big Crunch Theory of how the universe could end had all matter in the unviverse collapsing into a singularity or black hole.
     
  5. May 6, 2012 #4

    mfb

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    I think this limit could be exceeded via feeding with controlled angular momentum. In that case, the matter would just fall into the black hole, without forming an accreation disk.
     
  6. May 6, 2012 #5
    As pointed out, the limit is based on the observation of how a black hole clears out the immediate vicinity until there isn't any more material available to consume. Also, control takes the situation out of the natural phenomenon category into the speculative or what-if of sci fi. To bring in a hypothetical situation where the natural process isn’t present because someone is control-feeding the black hole might be useful for a Sci Fi story. But on a forum that limits itself to reality, it is really out of place.
     
  7. May 6, 2012 #6
    Your statement seems to suggest that black holes work like vacuum cleaners.

    That is not the case, it is actually hard to fall into a black hole, it is far more likely one zooms past it or orbits one (or more) times and travels away.

    From the perspective of the dynamics of objects there are no differences between a black hole and any other mass.
     
  8. May 6, 2012 #7
    I think a more interesting question is, what is the maximum size the universe could be, at its observed density, without becoming a black hole?
     
  9. May 6, 2012 #8
    That would require the universe to have a center of gravity (think balloon and dots, no single dot is in the center).
     
  10. May 6, 2012 #9
    The balloon and dots represents a closed universe. Even a closed universe without a center of gravity and without dark energy should contract under its own gravity.

    I think our best measurements indicate the universe is flat or open. Could an infinite universe at the same density we observe exist?
     
  11. May 7, 2012 #10
    The likelihood of falling into a black hole has nothing to do with the question that was asked. Neither does the differences between a black hole and any other mass.

    The question is limited to how large a black hole is able to become and I limited my response to that question.

    Your comments might be relevant in response to the issues you have added but are off topic to this one in my opinion.

    In any case, let's momentarily consider your claims.

    First, the analogy between a black hole and a vacuum cleaner is inappropriate because the attempted comparison would break down in too many places due to the obvious dissimilarity of the twain. Clearly a vacuum cleaner isn't interacting with objects in motion that might resist being pulled in or go into orbit around it. A vacuum cleaner doesn't attract from all directions as a black hole does. Things that are drawn in don't undergo the same transformations. It doesn't spaghettify things, for example. The vacuum cleaner itself isn't in either orbital motion or rapidly spinning on its axis. So even if we imagine it as a planet sized vacuum cleaner the forces involved and the dynamics just do not mesh.


    Second, your statement seems to imply that it is always difficult to fall into a black hole. That seems to contradict your other statement that in terms of mass and gravity black holes do not differ from any other massive objects. Whether or not it is hard to fall into a black hole, of course, would depend on trajectory and velocity and mass as it would with any other celestial body of significant mass.

    Your statement that black holes are essentially no different from other massive bodies is also misleading since most mass isn't as densely compressed as it is in a black hole and neither are singularities found within non-black-hole masses. In fact, that's what makes study of black holes so interesting, their difference from regular celestial objects, a difference which you seem to claim to be virtually almost non-existent.

    Third, at present our knowledge of what happens to matter once it reaches that singularity is completely absent because the laws of physics break down at that point.


    So no, I am not comparing a black hole to a vacuum cleaner. The comparison simply breaks down in too many places to function as an effective analogy.


    BTW

    I think you misunderstod the expression "clearing out" as a vacuming action. The clearing out also involves the blasting away of surrounding material caused by the black hole's activitiies. It is briefely mentioned in the article provided.
     
    Last edited: May 7, 2012
  12. May 7, 2012 #11

    Chronos

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    The case remains that a black hole is no different than any other gravitating mass. Unless you are near the event horizon it is indistinguishable from any other equally massive body.
     
  13. May 7, 2012 #12
    What about a diffuse, yet very, very very large gas cloud that exists within its self Schwarzschild radius? As I have read before, those were the precursors of supermassive black holes in the early universe.

    What limited their size to a few million solar masses? Why not an arbitrarily large diffuse gas cloud? In fact, just intuitively speaking it should be even easier to have billion, or trillion solar mass clouds, as Schwarzschild radius increases to proportionally to the mass but the actual radius only increases proportional to the cube root of mass.
     
  14. May 7, 2012 #13
    Please show me where I said it was gravitationally different in my original response to this question.
     
    Last edited: May 7, 2012
  15. May 7, 2012 #14
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    Last edited: May 7, 2012
  16. May 7, 2012 #15


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  17. May 7, 2012 #16

    mfb

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    Not dense enough without some massive attraction in the center, e.g. the remnants of early stars, which fused to a black hole of significant size.


    Talking about a fed black hole is like talking about relativistic trains: It is a method to explore science, without performing actual experiments (because we don't have access to relativistic trains or black holes to perform experiments with).
     
  18. May 7, 2012 #17
    Very true. My apologies. Hypotheticals are definitely useful. But the fact remains that there isn't anything that indicates that such a fed black hole has a limit to what it can devour. However, the Big Crunch Theory seems to make room for it by postulating a cyclical universe. But that rebound is purely conjecture and not based on the laws of physics as we know them.

    Big Crunch
    http://science.howstuffworks.com/dictionary/astronomy-terms/big-crunch.htm

    Note please that in this scenario the universe becomes a singularity. Which seems to support the idea that at a certain point in feeding a black hole it will rebound with a big bang.
     
    Last edited: May 7, 2012
  19. May 7, 2012 #18

    Nabeshin

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    I do not understand this logic at all. If some quantum gravitational correction produces some bounce near the singularity of a big crunch, the mechanism should work equally well for any mass. That is, it would resolve the singularity of any size black hole into a non-singular object. Even so, it seems very difficult to escape that there still must exist an event horizon, and there is no reason to suppose that this disappears at any particular size.
     
  20. May 7, 2012 #19
    Please show me where I said that the event horizon disappears.
     
  21. May 7, 2012 #20

    Nabeshin

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    Well you did not, you said that there would be a 'big bang'. I simply assumed this meant the structure of the hole, i.e. the horizon, would be eliminated by this. What did you mean, then?
     
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