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Black hole ponder

  1. Jul 21, 2004 #1
    I was noticing the sink draining this morning and was wondering...If something gets caught up in the gimme-gimme attitude of a BH, does it HAVE to enter by getting "on plane" with the rest of the matter or can it enter from more of an angle? What if something were to approach from beneath the plane? Would it also have to get to the plane and then cross the event horizon OR could it enter further down the shaft? If something must cross the EH to enter, would that make the BH hour-glass in shape but with a real flat middle?
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  3. Jul 21, 2004 #2


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    A non-rotating black hole has a perfectly spherically symmetric event horizon. An object can cross the event horizon from any angle.

    Accretion discs only form when the cloud of matter collapsing toward the black hole already has some angular momentum. The contraction causes the cloud to rotate faster due to the conservation of angular momentum and become a disc.

    Not all black holes have discs, and there is no reason why an object must enter the disc before entering the black hole.

    - Warren
  4. Jul 21, 2004 #3
    Thanks. I didn't realize there were non-spinning black holes. I assumed they all had a rotating disc and a swirly-drain looking thing.

    For a non-rotating black hole, if I understand what you are saying, it sounds like material just moves toward a central sphere and goes away, correct? There's not an "edge" of any sort like is portrayed in so many sci-fi movies? And if it's a sphere, as it accrues matter does it grow in size or just density or both.

    I've read here and elsewhere that a black hole emits radiation and in graphic descriptions it's usually portrayed as some sort of jet emitting from the center of the black hole away from the event horizon. If this is true for spinning black holes, how would a non-spinning black hole do the same, if in fact it does? Would it be just a giant spheroid of radiation co-located with the central sphere?

    On a spinning black hole, after the event horizon is there just nothingness or is there any sense of depth to it?

    Thank you for your patience.
  5. Jul 21, 2004 #4
    How could you ever tell if a black hole were spinning or not, it's black? What, maybe the fact that there is a magnetic field mean that there are currents circulating inside? No, for no photons can escape so no magnetic field and develop. What then?
  6. Jul 21, 2004 #5


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    A spinning black hole has different geometry near it than a static one does (Kerr-Newman metric versus Schwartzschild). Presumably a sufficiently senstive gravity detector moved around (say in orbit around the BH) could tell the difference.
  7. Jul 21, 2004 #6


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    This would actually be a fairly rare event. tuff falling into the BH in a perfectly strait line toward the center would just cross the EH and be lost. Anything other than a perfect "bullseye", end the infalling matter swings past, probably in a very eliptical orbit, swings wide out and then in close again (closer than last time), and eventually crosses the EH.

    If there is other stuff doing the same thing at the same time, there's a good chance things will start to bump into each other, and as the collisions continue the whole big mess starts to rotate in one direction (like the draining sink you mentioned).

    So things can fall in on the first pass, but most things probably won't.
  8. Jul 21, 2004 #7


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    Also, regarding this;
    The radiation emitted by a BH, and the twin jets of radiation to which you refer are two seperate phenomina. Black Hole Radiation is an effect of QM and has to do with the generation of "virtual particals", and their seperation into a "real" partical/antipartcal pair. The jets coming from the poles of the BH originate from just outside the EH, not inside, and are a result of far more "ordinary" physics; a bunch of things going real, real fast crash into each other and there's a big release of energy (an explosion). They are not necessarily at the poles of rotation of the BH itself, but those of the accretion disk.
    Last edited: Jul 21, 2004
  9. Jul 22, 2004 #8


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    The kerr solution is the only practical solution to black holes. All mass possessing bodies in this universe must be rotating. This is the only sensible conclusion you can achieve if you accept the premise of a background independent universe. You may assume you are not rotating in relation to the universe, but, the universe will disagree.
  10. Jul 22, 2004 #9


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    Some popular depictions of BHs can be misleading - Echo 6 Sierra is not alone in having picked up the idea that the jets are from the BH!

    To add a few words about what's observed so far (and what's not): for BH at the centre of (some) galaxies, accretion disk phenomena and jets are what we see; analyses of these points to supermassive BH. But why conclude BHs? why not something else entirely different? Because no one has been able to come up with a model that matches the observations as well as ones with SMBH! The alternatives - e.g. completely new physics, or a titanic war of super-intelligent ETs - don't really survive Occam's razor. The motions of stars near the centre of the Milky Way are consistent with an SMBH there, tho' no accretion disk or jets are to be seen - the hole is on a diet.

    In the case of BH with masses of a few sol, the observations are the periods of binary objects, esp X-ray binaries, as well as accretion disk phenomena. Analyses of the periods gives a minimum value of the mass(es); as there are no models for compact objects with masses >~4 sol, other than BH, we conclude there's a BH.

    Interestingly, the gravitational redshift of matter near the BH usually requires careful observations to be detected; icing on the 'case for BH' cake so to speak.
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