Notices
Results 1 to 14 of 14
Like Tree1Likes
  • 1 Post By Markus Hanke

Thread: How many hairs on a Black Hole

  1. #1 How many hairs on a Black Hole 
    Senior Member Boing3000's Avatar
    Join Date
    Jan 2013
    Location
    Belgium
    Posts
    194
    I know the standard response to none, and the correct is a few: mass / Hawkins-temperature / ergosphere (does it imply an eventual spin (of the ergosphere axis ?))

    Is there any other properties a black hole can emit (without breaking GR), and how much of them have been actually observed ?

    As a side note does GR, and its strong validation by the existence of black hole, imply that the gravitational field cannot be quantized, because graviton would then be able to exit a BH (which is impossible) ?
    Reply With Quote  
     

  2. #2  
    Banned
    Join Date
    Jul 2014
    Posts
    341
    In reply to Boing3000, re: your #1 post.

    I think you should consult the "Hogwarts School of Magic" for the answers you seek...or possibly "Spock" from "Star Trek". They will know...after all, if "blackholes" are actual real entities, then

    they too are "real".


    You cannot have GR and actual, real "blackholes" inhabiting the same Universe...as I see it. One set of conditions is paradoxical to the other. "GR" and "blackhole theory" deny each other!

    (Thanks for reading!)
    Reply With Quote  
     

  3. #3  
    Senior Member Boing3000's Avatar
    Join Date
    Jan 2013
    Location
    Belgium
    Posts
    194
    Quote Originally Posted by Gerry
    "GR" and "blackhole theory" deny each other!

    There is no such thing has black hole theory. BH is just a one of GR predictions, it is a small subset of GR.
    Another fact is that evidences for BH begins to pile atop of one another, so the only paradoxical thing is why you have post this.

    I have forgotten charge, has one of the properties that BH do "export" from inside to the outside of the Event Horizon.

    Does any real physicist can explain in some layman-like way how an electron could be attracted/repulsed by a BH, especially that photon (which, if my memory serves me well, is the boson carrier for EM force/field) cannot escape BH (by definition, if some here still don't get it ;-)

    Maybe string theory, or one of its gillions instances, have a explanation for that ?
    Reply With Quote  
     

  4. #4  
    Banned
    Join Date
    Jul 2014
    Posts
    341
    In reply to Boing3000, re: your#3 post.

    The conditions described for the "event horizon" of a BH constitute a pro forma "theory", the actuality that there IS a BH also constitute "theory...the act of writing what "electrons may or

    may not do" is also "theory!" (you mention "string theory" a completely hypothetical condition...yes, the word "theory" also appears.

    ......

    Anything described or attributed to "blackholes" is ALL "theory". (I have many difficulties in life...the proper context of the word "theory" IS NOT one of them)

    (also, Belgian girls tend to be VERY "fetching" as I remember...do you understand the context of the word "fetching" and it's implications within the declarative sentence?)


    (Thanks for reading!)
    Reply With Quote  
     

  5. #5  
    Administrator Markus Hanke's Avatar
    Join Date
    Jan 2013
    Location
    Ireland
    Posts
    1,378
    Quote Originally Posted by Boing3000
    Is there any other properties a black hole can emit
    "Emit" is probably a problematic term here, but I understand what you mean. The answer is that a classical (!) black hole is completely described by just three degrees of freedom : mass, angular momentum, and electric charge. Every black hole has mass, and every physically real black hole would have angular momentum ( since it is a conserved quantity during gravitational collapse ), but we would not expect a "real" black hole to have any significant net charge.

    As a side note does GR, and its strong validation by the existence of black hole, imply that the gravitational field cannot be quantized
    GR is a purely classical theory from the outset, so it does not really allow us to draw conclusions as to the possibility of quantisation. On the other hand, quantum field theories do not tell us anything about the geometry of space-time - this mutual exclusiveness is one of the many problems encountered in quantum gravity.
    Gerry Nightingale likes this.
    Reply With Quote  
     

  6. #6  
    Administrator Markus Hanke's Avatar
    Join Date
    Jan 2013
    Location
    Ireland
    Posts
    1,378
    Quote Originally Posted by Boing3000
    Does any real physicist can explain in some layman-like way how an electron could be attracted/repulsed by a BH
    The electric charge of a black hole would manifest itself in the presence of an electric field that extends away from the event horizon; for a far-away observer the charge would sit on the "surface" of the horizon. A moving electron would hence experience a Lorentz force.
    Reply With Quote  
     

  7. #7  
    Senior Member Boing3000's Avatar
    Join Date
    Jan 2013
    Location
    Belgium
    Posts
    194
    Quote Originally Posted by Markus Hanke
    for a far-away observer the charge would sit on the "surface" of the horizon

    Thanks for the response. Maybe this thread should have been put in QM. I did know that both GR and QM somewhat do not concern with each others, but I am wondering if they actually kind of exclude/invalidate each others...

    I have no problem with the notion of field, and did think that QM have replace this notion by boson/particle. Now I have a hard time imagining how a small charge (indeed near neutral) could be uniformly spread on the whole EH (except by field). Nor how, in the instant the Star create the horizon, somewhat all those random hairs suddenly (probably instantly (what does that mean?)) merge in one perfect and homogeneous value.

    I am going to think now the the EH is more like a point than a surface. Like if I am on one side of a BH, I throw a bunch of electron in it, and instantly on the other side the charge increase. The "other side" is some (many) miles away outside the EH. Have I send a message faster then light to my friend on the other side ? or is my friend actually zero miles away ;-)


    And you guess probably that by "emit" I meant "kind of causally related"
    Reply With Quote  
     

  8. #8  
    Administrator Markus Hanke's Avatar
    Join Date
    Jan 2013
    Location
    Ireland
    Posts
    1,378
    I would like to add to this a fine but important point - the free parameters appearing in the metrics that describe these black hole solutions ( i.e. mass, angular momentum, charge ) do not just measure properties of the black hole as an object, but they actually refer to space-time in its entirety. For example, the "M" that appears in the Schwarzschild metric is the mass content of the entire space-time, not just the black hole. Likewise, Q would be the charge present in the entire space-time, and so on. The distinction has few practical consequences for Kerr-Newman black holes ( since all of these solutions deal with isolated black holes in otherwise empty space ), but it does become important when one considers black hole solutions where the "hairs" are functions of time, for example Vaidya-Bonnet space-times, or Kinnersley black holes.
    Reply With Quote  
     

  9. #9  
    Senior Member Boing3000's Avatar
    Join Date
    Jan 2013
    Location
    Belgium
    Posts
    194
    Quote Originally Posted by Markus Hanke
    do not just measure properties of the black hole as an object, but they actually refer to space-time in its entirety
    Very interesting. The more I look at black hole (well not really possible ;-) the more I find they behave like particle. I do understand that a charge shot from an outside observer toward the black hole would seam to never reach it.
    But BH exist, and they DO "eat" things. So would the charge of a black hole change ? would that been instantaneous (no hair) ? The only way I can reconcile that instantaneous thing with GR causality is that somewhat consider the black hole like a "real" hole in space, of some size, but point like. It is a paradox I don't like...
    Reply With Quote  
     

  10. #10  
    Administrator Markus Hanke's Avatar
    Join Date
    Jan 2013
    Location
    Ireland
    Posts
    1,378
    I do understand that a charge shot from an outside observer toward the black hole would seam to never reach it.
    This is correct for Schwarzschild black holes; however, a charge falling towards a black hole is a scenario that cannot be modelled with Schwarzschild geometry ( being a vacuum solution ) - you would need other solutions for this, such as Vaidya geometry for example.

    So would the charge of a black hole change ?
    Just as with mass, the parameter Q in Reissner-Nordstroem or Kerr-Newman metrics signifies the total charge of the entire space-time, including the in-falling charge, so Q never changes. It is a constant. But again, neither of these metrics would be suitable to analyse this scenario, except perhaps as an approximation.

    It is a paradox I don't like...
    There is no paradox, because everything in GR is local. What an external stationary observer sees and calculates is true only for his local far-away frame; in the free-fall frame of the in-falling object itself, the event horizon is reached and crossed in finite time. This is not a contradiction, because notions of space and time are purely local.
    Reply With Quote  
     

  11. #11  
    Senior Member Boing3000's Avatar
    Join Date
    Jan 2013
    Location
    Belgium
    Posts
    194
    Quote Originally Posted by Markus
    you would need other solutions for this, such as Vaidya geometry for example.
    I am not going to be able to solves those solutions in this lifetime. Thus, I count on you.
    Am I right to say BH do change their property/hair, when absorbing (matter,charge, momemtum, whatever), for an EXTERNAL observer (the only ones that matter for me) ?

    Quote Originally Posted by Markus
    Just as with mass, the parameter Q in Reissner-Nordstroem or Kerr-Newman metrics signifies the total charge of the entire space-time, including the in-falling charge, so Q never changes. It is a constant.
    Indeed, and the more you dicribe that to me, the more BH look like particle to me. But...

    Quote Originally Posted by Markus
    There is no paradox, because everything in GR is local.
    I don't beleive in paradox, my brain have no taste for them
    I don't mean that pseudo paradox (different observer), but another.

    If BH have no hairs, and only have some UNIQUE property, they also take up space. Am I right to think that every (somewhat stationary) far away observer will agree on its SIZE. one KM, a few thouthands ?

    The pseudo paradox is to imagine that an in falling charge will be located at some point on one side of the horizon, and then in 'an instant' will be spread across the hole BH volume (changing the EM field uniformly everywhere at the horizon at once). The everywhere not being a point like particle, but a huge surface in space...I cannot reconcile that with my weak understanding of causality.

    I have seen simulation of black hole coliding. The merged BH, at first a least, display a huge amount of twist and curls. I would not consider that to be hairless. BH may be mostly bald, but not hairless
    Reply With Quote  
     

  12. #12  
    Administrator Markus Hanke's Avatar
    Join Date
    Jan 2013
    Location
    Ireland
    Posts
    1,378
    Am I right to say BH do change their property/hair, when absorbing (matter,charge, momemtum, whatever), for an EXTERNAL observer (the only ones that matter for me) ?
    Yes, because even though such an observer never "sees" anything reach the horizon, he can still go anywhere at the exterior region of the black hole and directly measure space-time curvature ( e.g. with a Pound-Rebka style experiment ) before and after the in-fall, and he will find that there is a change.

    Am I right to think that every (somewhat stationary) far away observer will agree on its SIZE.
    So long as size is defined as being the event horizon radius, then yes, all observers agree on this.

    The pseudo paradox is to imagine that an in falling charge will be located at some point on one side of the horizon, and then in 'an instant' will be spread across the hole BH volume (changing the EM field uniformly everywhere at the horizon at once).
    You cannot directly observe a property such as "charge" - all you can detect is the resulting electromagnetic field, as well as the geometry of space-time that goes with it. Both of these change smoothly without any instantaneous jumps; this is described by the so-called ingoing Vaidya-Bonnet metric. I have to reiterate that this is not a Schwarzschild black hole - a Vaidya-Bonnet black hole is far more complicated, and in addition to an event horizon contains other physically significant "surfaces" as well.

    I must admit that I have not studied this metric in any detail, so I don't have all the answers for you. All I'd like to say is don't make the mistake of thinking that every black hole scenario is automatically Schwarzschild - that is not the case.
    Reply With Quote  
     

  13. #13  
    Administrator Markus Hanke's Avatar
    Join Date
    Jan 2013
    Location
    Ireland
    Posts
    1,378
    I have seen simulation of black hole coliding. The merged BH, at first a least, display a huge amount of twist and curls. I would not consider that to be hairless. BH may be mostly bald, but not hairless
    The no-hair theorem in its traditional form applies only to one isolated black hole in otherwise empty space; add a second black hole, or indeed any other object, and the situation becomes much more complex.
    Reply With Quote  
     

  14. #14  
    Senior Member Boing3000's Avatar
    Join Date
    Jan 2013
    Location
    Belgium
    Posts
    194
    Quote Originally Posted by Markus
    I must admit that I have not studied this metric in any detail, so I don't have all the answers for you.
    I am afraid you are wrong. Beside I'll keep the nasty questions for myself
    Quote Originally Posted by Markus
    All I'd like to say is don't make the mistake of thinking that every black hole scenario is automatically Schwarzschild - that is not the case.
    I am not, everywhere I look Schwarzschild BH are said to be limit case of over implication. But a featureless hole in space literaly give me the creep, or more precisely some kind a cosmic vertigo.
    From now on, I'll imagine even horizon like a fractal structure of space-time, where a charge could vanish, and somehow seem to split and split until it has encompass the surface, observably at cosmic speed.
    That picture will help me sleep again, thank you
    Reply With Quote  
     

Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •