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Black holes again

  1. Jan 12, 2010 #1
    Hi,
    I have to say that I'm not sufficiently educated in physics but I'm very interested in it so please excuse my silly questions.
    First I'd like to ask what determines the quantity of space that the black hole occupies. Is it just the amount of matter that imploded during the super nova explosion or does the black hole grow bigger with everything that gets in it? If it grows bigger all the time doesn't it mean the matter doesn't completely disappear in the BH, nor it does appear again in other places?

    Another question is: when the BH spews out energy it's because the heat and density along with magnetic field somehow overwhelms the gravitational pull of the BH? Does it spew out of the BH itself? does it spew out matter that's within the event horizon?
     
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  3. Jan 12, 2010 #2

    DaveC426913

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    The black hole itself is theoretically zero volume - all the mass has been compressed to a point. But a BH is normally defined by its event horizon. This is nothing more than a radius. Anything less than this radius, including matter or energy can never escape.

    This radius is entirely determined by the amount of mass in the black hole.

    A BH does not spew out energy. What happens is that infalling matter forms an accretion disc outside the EH. This disc radiates energy as it compresses.
     
  4. Jan 12, 2010 #3
    Thank you very much for explanation, so if i understood well- every BH has this one infinitely small point of singularity but differs in the extent of gravitational deformation of space around it.
    Does the event horizon grow with stuff falling into the hole?
     
  5. Jan 12, 2010 #4

    DaveC426913

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    Yes.
    Yes.
     
  6. Jan 12, 2010 #5

    Matterwave

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    Black holes also radiate (theoretically) Hawking radiation. The simplest model for which is the creation of particle-anti-particle pairs near the Event horizon (allowed because of QM). One of the particles drop into the BH, the other particle escapes. This process steals energy from the black hole until the black hole completely evaporates. For a massive black hole, this would take a really really long time (something like a googol years for a supermassive black hole), but for theoretical micro-black holes, this process may evaporate the black hole within a matter of seconds.
     
  7. Jan 13, 2010 #6
    Why does the BH have to have zero volume? Why can't it just be a lump that is contained within its own event horizon? Is it because the highest counter-compression (that I know of) is neutron degeneracy pressure, and in order to be dense enough to be within it's own event horizon the pressure would be greater than this?
     
  8. Jan 13, 2010 #7

    Matterwave

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    There are no known forces of nature which would prevent further gravitational collapse once the matter is squished within the Event horizon. We don't know if the matter really is concentrated at a point, we just don't know of any pressure or force that could "win" against gravity inside the EH.
     
  9. Jan 13, 2010 #8
    Pressure is a measure of a system's internal energy - thus it adds to the gravity and so causes the necessary counter-pressure to increase. Confinement pressures from Pauli Exclusion and the like, mean the pressure-gravity feedback becomes positive and so runs away into a singularity. Thus the singularity is really where quantum and GR conspire to create the incomprehensible. A strong motivation to develop 'quantum gravity', but so far the proposed QGR theories have no clear front-runners we can confidently say describe/eliminate the singularity.
     
  10. Jan 13, 2010 #9
    Good point. We don't know, but what theory tells us is that the mass is infinitely compressed because of the pressure-gravity runaway feedback... which makes more than a few people wonder if the theory is wrong.

    There's two ways of handling that. One involves quantum gravity effects which prevent the singularity and smear it out or cause a daughter Universe to explode as a new space-time. The other option is to propose that a new kind of space-time is either just above the event horizon or just within it, and some kind of repulsive gravity is at work producing a 'gravastar' or similar non-singular space-time. The event horizon becomes a real physical boundary between two very different space-time states in the second option. The race amongst theorists working with either option is to produce real observational tests to distinguish between the options and/or classical GR black-holes.
     
  11. Jan 14, 2010 #10

    Chronos

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    I would quibble over the density of a black hole. It is theoretically a point in space, but heisenberg suggests it must have a finite, albeit miniscule, volume.
     
  12. Jan 14, 2010 #11
    Is the fact that everything that falls into the black hole adds to its gravitational pull consistent with the notion that it appears in some other universe or bigbang or some other place in the universe?
     
  13. Jan 14, 2010 #12

    DaveC426913

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    In fact, no it seems pretty inconsistent. Whatever it has undergone, as far as we can tell, the mass it still there, at the centre of the BH.
     
  14. Jan 14, 2010 #13
    thanks a lot again for such a quick answer
    I was just wondering because in some documentaries I heard various ideas about black holes maybe being worm holes to other universes and stuff like that
     
  15. Jan 14, 2010 #14

    Chronos

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    A wormhole to another universe would drain energy from this universe. No observations to date suggest loss of energy from this universe. Were this so, we would observe black holes eating matter without increasing in mass. Black holes would be all appear to be about the same mass if this was the case.
     
  16. Jan 14, 2010 #15

    DaveC426913

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    TV is a notoriously bad place to pick up knowledge. You just can't fit all the necessary foundational material in a half-hour or one-hour show. And almost all science shows go for gimmicky or flashy speculative stuff rather than good science.


    Go with books.
     
  17. Jan 15, 2010 #16
    Oddly enough we wouldn't see a mass-loss. The external field is "frozen" at the value set by infalling matter. In fact wormholes would exhibit a similar mass change with respect to inflow and outflow through them. A net negative flow would cause a wormhole to become repulsive.

    But I wasn't referring to regular wormholes. Some theories - Lee Smolin's for example - have black holes producing "baby universes" from their mass.
     
  18. Jan 15, 2010 #17
    couple of questions:

    - Why can't a BH be something other than a point?

    - Someone once said to me that as a BH forms an outside observer will see the clocks in the forming hole slow down continually towards zero. So it would thus take an infinite amount of time on the outside observer's clock for the hole to form. If this is true how can a BH ever be observed? Put another way, how can a BH exist unless it already existed at the creation of the Universe?
     
    Last edited: Jan 15, 2010
  19. Jan 15, 2010 #18
    A good point which is supported by theoretical existence of ring singularities.

    @wofsy: I think as long as people are talking about Black Holes and the possible sigularity at its center interchangeably, finer points should be abandonded for the basics.

    A black hole is never a point, it's an entity including an event horizon past which GR says there will be a point of zero volume. A singularity doesn't have to be a point... it can be a point, or a string, or a loop... maybe. If they exist. You have to understand, while there is observation of BHs, nothing has ruled out the possibility of something just at or below the radius of a BH existing with or without an event horizon. The margin of error in astronomical observations leave room for argument, and no one has detected the emission of phonons or the like from lab-bound analogues. The question is as up in the air as DaveC426913 pointed out.

    As for the red shift you're talking about, you're ignoring relativity. To an observer it may appear that infalling matter fades to unobservability at the event horizon (after a long redshift) assuming you could see through the radiation from the accretion disk (and maybe Hawking Radiation or other thermal-like process). To the subject B falling into the black hole, they can pass the event horizon without noticing anything until tidal forces, radiation, etc, tear them to bits. This requires a VERY large black hole, and a hardy observer, but you get the idea. The whole problem with black holes and the event horizon is that it marks a boundary condition on reality if the theory is formalist in GR.

    This is why some people believe that a black hole is a series of events which eventually comes to a complete halt (not as in frozen, as in oblitered), and not an "object". The Singularity by definition never interacts with the universe, only the event horizon, and surrounding accretion disc. Gravitic effects are the result of within the radius of the event horizon. If GR is correct, then black holes represent a boundary condition on anything like Classical and Relative space-time. Whatever describes an object with a radius of or less than the event horizon requires physics other than GR or SQM in their current state, assuming they are open to description or observation... which is an iffy assumption. Some String Theorists and many other have ideas, but they are completely untested and theoretically also untestable. Fantastic and amazing ideas are born around the topic of black holes, but in terms of hard science they are still pretty damned enigmatic.
     
  20. Jan 15, 2010 #19
    You're right and wrong. A TRAVERSABLE wormhole to another part of the same universe might not show mass loss, but one to another universe surely would. I'm not sure exactly what kind of wormhole you're talking about... and ERB? Something far more stable presumably... As for the idea that black holes spawn baby universes, I think that is just people enjoying the apparant similarity to the initial state of the universe and the mathematics of a gravitational singulariy. Most resptable "multiverse" theories involve a primordial vacuum, brane cosmology, etc... not our universe blowing inverted bubbles out of its own existence.
     
  21. Jan 15, 2010 #20

    Chronos

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    The external observer will see the event horizon form, but, never the singularity.
     
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