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What happens to matter inside a black hole?

  1. Aug 24, 2008 #1
    does it destroy it or does it get sent back in time like movies suggest?
     
  2. jcsd
  3. Aug 24, 2008 #2

    Fredrik

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    If we're talking about the simplest kind of black hole (non-rotating, non-charged), all matter that falls inside the event horizon will get crushed into the singularity within a finite time, which is typically very short (less than 0.001 seconds I think, but I'm not sure I remember that right).

    There are however solutions of Einsteins equation that describe rotating black holes with a geometry such that there are paths that "miss" the singularity and end up in a region of spacetime that can be described as a "white hole". A white hole is in many ways the opposite of a black hole, so a particle could escape from it but not fall into it, but no white holes have ever been observed.

    Perhaps someone who has studied this recently (or just knows this better than me) can remind me if that white hole stuff is a part of every solution that describes a rotating black hole, or just a part of some really weird ones.
     
  4. Aug 27, 2008 #3
    I've actually wondered about this - does the black hole consist of matter or is it purely localized energy? Are there still quarks/other particles inside the black hole or are they an entirely different animal from anything in the standard model?
     
  5. Aug 27, 2008 #4

    JesseM

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    According to GR, the collapsing star that formed the black hole will continue to collapse inside the event horizon, turning into a singularity of infinite density, everything between the singularity and the event horizon is just empty space (aside from any objects that fall through the horizon later). A theory of quantum gravity would probably get rid of the singularity, replacing it by who-knows-what, but it's expected that quantum gravity would probably only differ significantly from GR at the Planck scale (when the collapsing star has crunched down to such a small size that it reaches the Planck density, perhaps), so at macroscopic distances from the point where GR predicts a singularity I think that means you'd probably see about the same things as would be predicted by GR plus ordinary quantum field theory.
     
  6. Sep 3, 2008 #5
    Hi All,

    I presume conservation of momentum and angular momentum apply inside a black hole. If a black hole is to spin, it can't disappear to a singularity. A singularity has 0 radius (definition) so has 0 angular momentum (definition) therefore does not spin.

    If a black hole is to spin, it must have a radius at the "singularity" in order to exert angular momentum. Don't go telling me 0 times infinity is going to give the exact right angular momentum for each black hole by magic... it's not going to convince me :-)

    Have I missed something somewhere...? Most black holes will spin to some degree or another but maybe at slow rates - the stars that made them were all spinning after all. Is there any observational evidence of black hole candidates that exhibit spin activity?

    Regards, Wallis
     
  7. Sep 3, 2008 #6

    George Jones

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    The singularity inside a spinning black hole is a ring, not a point.

    There is some evidence for spinning black holes. For example, the extreme energy output of quasars had been modeled using spinning black holes.
     
  8. Sep 3, 2008 #7

    Fredrik

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    Quantum gravity may prevent the singularity from ever forming, but that really doesn't matter here. The spin of a black hole is a geometrical property of spacetime, so it doesn't matter if singularities exist or not. Black holes can still spin. One effect caused by the spin is that falling matter is dragged "sideways". The world line of a falling piece of matter is still "straight" (a geodesic) but a different set of lines are straight in this geometry.
     
  9. Sep 3, 2008 #8
    Thanks but that's not really my question. My question is, what is the "stuff" inside the black hole? Is it:
    a) An infinitely dense mesh of standard model particles (if so, why doens't the pauli exclusion principle apply?)
    b) Photons converted from mass
    c) Some other kind of energy
    d) Something else
    e) Unknown
     
  10. Sep 3, 2008 #9

    JesseM

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    But by "inside" do you mean everything inside the event horizon, or just the immediate neighborhood of the singularity? As I said, the region between the singularity is mostly empty space aside from anything that falls in after the black hole forms, and falling objects would continue to be made of the same materials behaving the same way, until they reached the singularity.
    GR is not a quantum theory, it treats matter in a classical way and doesn't address any quantum concerns. If you want to talk about what's going on quantum-mechanically at the center of the black hole, you'd need a theory of quantum gravity, and as I said no one really knows what such a theory would say about the center once you reach the Planck scale (but I think most physicists would bet that you'd no longer have a singularity of infinite density...it might not be meaningful to talk about densities higher than the Planck density in quantum gravity).
    Yes, that's the one. That's what I meant when I said "A theory of quantum gravity would probably get rid of the singularity, replacing it by who-knows-what".
     
  11. Sep 4, 2008 #10
    Gotcha. So in GR, we'd not worry about Pauli and we'd just say that the matter is packed infinitely tightly because the electrostatic and other forces are overtaken by gravity. But because of QM, we cannot reconcile that idea with the exclusion principle so we're not sure what the signularity is "made of."
     
  12. Sep 4, 2008 #11
    i think the real answer is e) unknown
     
  13. Sep 4, 2008 #12
    I would not say that at all.
     
  14. Sep 5, 2008 #13

    Borek

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    You can calculate Schwarzschild radius for any given density. That means that any volume of the space can be a black hole if it is contains enough matter. It may as well mean that the black hole is pretty huge (in terms of radius) but relatively light (in terms of mass). If the density inside is low anough matter inside should not differ much from what we observe around us. Could be we already are in the black hole :wink:

    Seriously, I don't have time now to play with numbers, but perhaps someone already tried that approach - assume some high but reasonable matter density with closely spaced stars, and check the radius? Is it possible to construct a stable volume (stable in terms of not collapsing, but just one stars orbiting other stars) that will have an event horizon?
     
  15. Sep 5, 2008 #14

    atyy

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    Oh no, Borek!:bugeye:

    http://people.maths.ox.ac.uk/~nwoodh/gr/index.html:
    "One might not think this matters: for a star of the mass of the sun, the Schwarzschild radius is 3 km, which would make the star an apparently impossibly dense object with a density 1016 times that of water. However, such stars exist not much more massive than the sun. If one increases the mass to that of a galaxy, then the critical density goes down to that of air. So the issue must be addressed as a potentially real physical problem, and not simply as an absurd extrapolation."

    After reading this, I have decided the LHC creating black holes is inconsequential.:devil:
     
  16. Sep 5, 2008 #15

    atyy

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    Champion, with respect to the time travel, look up wormhole.

    Kip Thorne has a nice book "Black Holes and Time Warps: Einstein's Outrageous Legacy".

    http://arxiv.org/abs/gr-qc/9803098
     
    Last edited: Sep 5, 2008
  17. Sep 5, 2008 #16
    Care to elaborate?
     
  18. Sep 5, 2008 #17
  19. Sep 5, 2008 #18
    You might want to explain to me what that has to do with the composition of the singularity.
     
  20. Sep 5, 2008 #19

    JesseM

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    Electron (and neutron) degeneracy pressure is a direct consequence of the Pauli exclusion principle, and the Chandrasekhar limit tells you that for sufficiently massive white dwarfs the electron degeneracy pressure is insufficient to overcome their gravity, so they collapse further into a neutron star. There is a similar limit to how large neutron stars can be before gravity overcomes neutron degeneracy pressure, beyond that point they'll collapse into black holes. Presumably as the star continues to collapse inside the event horizon, the degeneracy pressure continues to get higher and higher as the matter is packed more and more tightly, but the inward gravitational pull continues to become greater too. In the limit as you approach the singularity, I imagine the degeneracy pressure due to the exclusion principle would approach infinity, but the gravitational pull would approach infinity at an even greater rate so the matter would keep collapsing. Perhaps it would even be possible to deal with the exclusion principle by just treating it as a type of pressure which you could feed into the equations of GR--my understanding is that the stress-energy tensor does include a notion of pressure, although I'm not actually sure how free you are to let the pressure vary in any way you want (and I'm also not sure whether the degeneracy pressure would vary with compression in exactly the same way as more classical notions of pressure).
     
    Last edited: Sep 5, 2008
  21. Sep 5, 2008 #20
    Except quantum physicists are pretty adamant that the PEP is not a force, and therefore it should not increase in magnitude as densities increase. Thus there is a serious conflict here unelss I'm smoking something.
     
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