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Black holes and their guts

  1. Nov 20, 2003 #1
    I've been doing some writing in my physics journal about black holes and baby universes. I'm stuck with near nothing because of a lack of ideas. I have quite a few questions, but would like some other opinions on the matter.
    What happens to the matter in a black hole after it has evaporated? I was thinking something like it all going to another universe like our own. (Even though I don't belong in the same universe as most people here. )
    So, post away!
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  3. Nov 20, 2003 #2


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    Are you talking about this Hawking radiation?

    I don't know very much about it, but I've gotten the impression that it is indicatrive of one of those situations where QM wins against GR. That is, the gravity won't even allow light to escape, classically, but the uncertainty allows for a finite probability of a trapped photon being found outside the black hole (a very "red" photon).

    Another thing that I'm not sure about but have understood to be the case is that, when stuff (i.e. matter) gets trapped by the black hole, it no longer exists in our universe as matter, but it still contributes to the curvature of space-time in our univserse. So, I conclude from this (in a very simple-minded way) that the matter becomes energy, and that all the "stuff" in a black hole looks like energy to our universe. It is this energy (I conjecture) that fuels the Hawking radiation.
  4. Nov 20, 2003 #3
    Assuming the black hole has matter in it, and that the black hole evaporates, then we might suppose that the matter in the black hole gets converted to Hawking radiation in the process of evaporation. More likely, the matter will hit the singularity before the hole evaporates, and we don't know at all what happens to it then.

    Some people have proposed that the matter that reaches the singularity goes into another universe, but nobody has any idea.
  5. Nov 20, 2003 #4
    The matter or radiation ("energy") that falls into a black hole doesn't have to continue to exist in for the black hole itself to exist. So we don't know whether anything continues to exist in any form once it hits the singularity.

    Hawking radiation isn't produced by anything that fell into a black hole; it's a vacuum effect.
  6. Nov 20, 2003 #5
    I'd always just assumed that any matter entering a black hole is spat into another time and or universe depending on it characteristics.
    I'm leaning towards a definite relationship with Kaluza-Klein Theory here and his mention of a fifth dimension. It is in this dimension that the sieve/filter type processing of matter in the black hole determines how and where matter/energy is transferred to. Perhaps the Hawkins radiation is simply matter of the observable universe being ejected back into that same universe, i.e. not processed or transported within the black hole and which might include matter transported by the black holes dimensional twin.

    I haven't fully got my head around how a black hole might treat matter at the subatomic level and whether atoms are pulled apart or if they remain stream like i.e. retain their topographical shape. I'm inclined to think at the very minimum that the topographical signature of any matter entering the black hole is not compromised. My reasoning behind this is that otherwise we would have far to many anomalies (a bit like the Fly film) and physics would not be able to maintain it's consistency. Just as a side note I suspect that a black hole can only be closed once it's corresponding piece of space-time (it's lid) enters into the black hole. This lid I suspect is unique in its topographical, dimensional nature and is the lid that allows our universe to shape shift and cross over into other dimensions when needed or appropriate. The big bang would be a good example of this phenomenon at work.

    Last edited: Nov 20, 2003
  7. Nov 20, 2003 #6

    jimmy p

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    Well i would also assume that the mass would be converted to energy and emitted... but then that leads to the problem that if light cant escape a black hole, how does any emissions? i know that black holes emit x-rays so that how we know where they are, but them thar waves must be very (as in EXTREMELY) energised. im prob wrong then.
  8. Nov 20, 2003 #7


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    Light can't escape from WITHIN the black hole's event horizon, but it can escape when it's very close (but outside) the event horizon.

    The general description of Hawking radiation is that a virtual pair of particles is spontaneously created very close to the event horizon. One falls in, the other flies escapes. It is as if the black hole created the escaping particle, and thus evaporated.

    - Warren
  9. Nov 20, 2003 #8


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    Doesn't there need to be stress-energy to curve space-time? Are you saying that, once space-time curves enough to make a black hole, that the curvature will support itself? That would be some new level of understanding for me. Also, I wasn't talking about the matter being at the singularity, just inside the event horizon. Would that make a difference?

    Does it happen here on earth? What are the requirements?
  10. Nov 20, 2003 #9
    If I remember correctly, the particle that falls into the hole will have negative mass/energy causing the black hole to shrink. Both particles then become real particles because they can no longer turn into energy.
  11. Nov 20, 2003 #10
    No; there are plenty of curved solutions to the vacuum Einstein equations; the Schwarzschild solution is one of them.

    Yes, exactly: general relativity is a nonlinear theory in which gravity gravitates.

    Any matter inside the horizon ends up at the singularity very quickly --- in most cases, and maybe always, before the hole can evaporate.

    Hawking radiation needs an event horizon to be produced. You could produce something analogous to it on Earth, namely Unruh radiation, merely by accelerating (which results in a Rindler horizon) --- but it is too weak to detect.
  12. Nov 20, 2003 #11
    What are the properties of a singlarity?
  13. Nov 20, 2003 #12
    A singularity doesn't really have meaningful physical properties. That's why people don't like them when they appear in their theories; the theory can't predict what happens after you hit one.
  14. Nov 20, 2003 #13


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    I read on John Baez' website that GR shows that the vacuum has very close to zero energy.

    I assumed that was because the vacuum is very close to being flat, and so there cannot be very much stress-energy in it, or else it would not be flat.

    Am I jumping to the wrong conclusion? I know this is the other side of the coin, but that new revelation made me second-guess my assumption.
  15. Nov 20, 2003 #14
    Well, as you will also see on Baez's website, there are many different ways of speaking of the energy of the vacuum in quantum field theory.

    It's not at all clear right now what relation the vacuum energy has to the curvature of spacetime: this is part of the famous cosmological constant problem.
  16. Nov 21, 2003 #15


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    Don't get deffensive, I'm just trying to resolve the conflict in my mind. I don't see why QFT should enter into the discussion.

    So does GR just say that the vacuum has almost zero energy as a postulate? That doesn't seem right. From what in GR is almost zero vacuum energy concluded?
  17. Nov 21, 2003 #16
    In classical physics (not just GR), the vacuum never has energy: energy arises from particles, fields, etc. Vacuum energy is a quantum phenomenon.
  18. Nov 21, 2003 #17


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    I guess I need to get straight on what the vacuum is. I have been thinking of it as empty (absent of mass) space-time. I thought that, for instance, E&M radiation propogated through the vacuum, but that it was still vacuum. What do you have to say about this?
  19. Nov 21, 2003 #18
    Classically, that's fine. (Quantum mechanically, it can be debated whether the vacuum is "empty", or seething with virtual particles and fluctuations of spacetime foam.)
  20. Nov 21, 2003 #19


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    OK, maybe you can straighten me out on this one. When I first say Einstein's equation (not too long ago, in case you haven't noticed), I thought that the nonlinearity was the diff. eq. being nonlinear in the sense that, if the metric is gμν, and Einstein's tensor is Gμν, then if the metric is Ag(1)μν + g(2)μν, then Einstein's tensor is not necessarily AG(1)μν + G(2)μν. Then, after some forum chat, I thought that I began to realize the nonlinearity was the fact that Rμν (or gμν, or something else related to the curvature) contributed to Tμν, in which case, Tμν would not be zero, even though there was no mass or energy from anything (other than the curvature), sort of like recursion. I am not at all clear why there is a contribution, though.
  21. Nov 21, 2003 #20
    Yes, that's what it means.

    I'm not sure in what sense the Riemann tensor can be said to contribute to the stress-energy tensor.

    The nonlinearity means that gravity can gravitate, but that doesn't mean that the stress-energy tensor is nonzero. You can have curved spacetimes that gravitate (such as the Schwarzschild solution), but the stress-energy tensor is everywhere zero.
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