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What is singularity? [Black Hole ]

  1. Aug 27, 2008 #1
    What is singularity????? [Black Hole ]

    Hi,

    Can any one please tell me about Singularity [ Black Hole ].....
     
  2. jcsd
  3. Aug 27, 2008 #2

    tiny-tim

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    Welcome to PF!

    Hi jasmine! Welcome to PF! :smile:

    hmm … that's a big question …

    the singularity is the point at the very centre of the black hole, where our present understanding of space and time breaks down :cry:

    not to be confused with the event horizon, which is only the "surface of no-return".

    For loads more detail, see http://en.wikipedia.org/wiki/Gravitational_singularity

    Was there anything particular that was bothering you about singularities? :smile:
     
  4. Sep 12, 2008 #3
    Re: What is singularity????? [Black Hole ]

    What is the difference between black hole singularity and big bang singularity?
     
  5. Sep 12, 2008 #4
    Re: Welcome to PF!

    can we make a singularity without gravity?
     
  6. Sep 12, 2008 #5

    marcus

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    That actually depends on whose understanding :biggrin:
    A singularity is a breakdown in theory, not in nature. So at the center of a black hole the classic 1915 version of General Relativity breaks down.
    But a quantized version that some people are using does NOT break down.

    Both the old unquantized and the new quantized versions predict the same stuff away from BH or BB singularities so it's hard to say which is better empirically.

    In any case one can say that there is no scientific evidence that time stops, or space stops, or that there is infinite density or any kind of infinities, either down a black hole or at the big bang. Some old theory says there is, and some new theory says their ain't---and so far we can't decide. So no reason to believe either way.

    Tiny Tim, you provided a helpful Wiki link!

    Here is a quote from that article:
    Wikipedia is not reliable---kind of a grab-bag. But at least it gets it partially right. A singularity is not something in nature. It is a sign that a theory is broken and needs to be replaced by better theory. This has happened before and it now appears to be happening with BH and BB singularities. A lot of recent papers on this, if you want links please ask.
     
    Last edited: Sep 12, 2008
  7. Sep 12, 2008 #6

    wolram

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    Re: What is singularity????? [Black Hole ]

    The singularity is an abomination, but people love the sci fi maelstrom picture, i prefer to wait for physical evidence that these things exist.
     
  8. Sep 12, 2008 #7

    marcus

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    Well, singularities are not real. they don't exist in Nature as far as we know.
    You "make them" by having them occur in theories. Singularities have occurred in several physics theories having nothing to do with gravity!
    So sure, the old theory of thermal radiation had a singularity. The old theory of the hydrogen atom had a breakdown.

    But since they aren't real physical things, as far as we know, we cant say we can make them in the usual way one thinks of making something physical---like, say, poached egg on toast.
     
  9. Sep 12, 2008 #8
    Re: Welcome to PF!

    As far as I know, and correct me if I am wrong, the classic 1915 version of GR is pretty much as it is now. It is true that the contemporary mathematical treatment is different from Einstein's days but as far as I know the theory itself has not changed one single bit. Also, as far as I know, and correct me if I am wrong, there is no such thing as a quantized general theory of relativity. Many people hope to present such a theory but so far no one has been able to present it.
     
  10. Sep 12, 2008 #9

    marcus

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    Re: What is singularity????? [Black Hole ]

    Well you know theory is a human creation and never static. It's always being revised.
    You are asking your question at a good time, because a revolution is in progress in the way spacetime geomety is modeled, including BHs.

    The BH and BB singularities were features of the classic 1915 GR theory, places where the geometry went bad.

    The revolution is you add a little quantum uncertainty and kind of blur the geometry and then it gets all right.

    The main area of research where this is being done is called quantum cosmology.

    So when you ask people to tell you about BH singularities one part of replying adequately is to tell you what singularities are replaced by in quantum cosmology.

    Typically what replaces the BB singularity is a process called a bounce, where something collapses and contracts down to very dense---a high (but not infinite) density---and then quantum effects (resisting being pinned down to one place) take over and invert the force of gravity and it becomes briefly expansive. So the contraction bounces and begins an expansion. At least this happens at the Big Bang moment, in numerous models and cases*. But it is less clear with Black Holes. Different models indicate different outcomes. They get rid of the singularity OK but then they disagree about what happens instead, and what takes the singularity's place. It's an active research problem. Research grants are available, young postdoctoral fellows are making it their specialty.

    So that's why I'd say it is a good time to ask just that question as you did ask it, with at least six question marks: What is a BH singularity?????? :wink:

    *so far not verified by observation.
     
    Last edited: Sep 12, 2008
  11. Sep 12, 2008 #10

    marcus

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    Much has been presented, none has been adequately tested by astronomical observation (though the GLAST satellite may enable some tests).
    The current leading theory as regards BB and BH is Loop Quantum Cosmology LQC.
    Hundreds of papers, thousands of citations, completely solvable analytic models. Computer models being run. Very active. Here is a keyword search for the published literature since 2005---keywords "quantum cosmology". Ranked by how often the papers have been cited in other research.
    http://www.slac.stanford.edu/spires...+DATE+>+2005&FORMAT=www&SEQUENCE=citecount(d)

    It just happens that the type of quantum gravity research that deals with the BB and BH singularities is quantum cosmology, and its development has recently advanced more rapidly than quantum gravity in full generality.
     
    Last edited: Sep 12, 2008
  12. Sep 12, 2008 #11
    Re: Welcome to PF!

    Yes, but I would not wager that any leading LQC theoretician is going to make a claim soon he or she developed a quantized general theory of relativity.
     
  13. Sep 15, 2008 #12
  14. Sep 15, 2008 #13

    marcus

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    Re: What is singularity????? [Black Hole ]

    Did you check out the numbers:
    http://www.slac.stanford.edu/spires...+DATE+>+2005&FORMAT=www&SEQUENCE=citecount(d)

    I wouldn't say leading if the citation numbers didn't corroborate that. I didn't intend to give my own subjective judgment. I don't know what else to say when the top 20 recent papers are almost all Loop---I guess 18 out of 20 last time I looked.

    Thing about Kerr's analysis is it is classical. The singularity exists in classical General Relativity---I absolutely agree. And it is very neat that it is a ring (when the hole rotates).
    But there is no indication as far as I know that a singularity exists in nature.

    Anyway, to be fair why don't you look at the spires link and see for yourself what the proportion is of the highly cited papers (say since 2005 to make it recent)?

    If you look in other timeframes like the late 1990s you will find other quantum cosmology approaches are leading or dominant citation-wise.
     
  15. Sep 16, 2008 #14
    Re: What is singularity????? [Black Hole ]

    Thanks, I'll give them a look. I no longer have the link, but about four years ago NASA imaged an accretion disk for a suspected black hole and determined that if there was a singularity involved, then it must be rotating. I remember, because I used the info in my observation that a Kerr ring singularity has a remarkable similarity to a one dimensional stack of closed strings.
    https://www.physicsforums.com/showthread.php?t=247439
     
  16. Feb 25, 2009 #15
    Re: What is singularity????? [Black Hole ]

    hi jasmine and welcome to PF

    the singularity of a black hole is a theoretical place where all matter comes to an end, where it is all crushed. the singularity has infinite density and infinite gravity, supposedly. so when an object is sucked into the black hole, it is first stretched into sphagetti because of the very strong pull and then it goes to the singularity. in a singularity, the fabric of matter is broken down and space and time ceaes to exist.

    this is just a basic definition of a singularity. if you need more info visit,
    http://en.wikipedia.org/wiki/Gravitational_singularity
    http://archive.ncsa.uiuc.edu/Cyberia/NumRel/BlackHoleAnat.html

    hope you find it useful
     
  17. Feb 25, 2009 #16

    marcus

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    Re: What is singularity????? [Black Hole ]

    I should post a warning about the Wikipedia article. It is a mixture of good and bad, like many Wikipedia articles, a mixture of stuff contributed by a variety of authors.

    The above post by Val is misleading---talks about singularities as if they part of nature.
    This excerpt from the Wikipedia article is more reliable:
    ==quote==
    Interpretation

    Many theories in physics have mathematical singularities of one kind or another. Equations for these physical theories predict that the rate of change of some quantity becomes infinite or increases without limit. This is generally a sign for a missing piece in the theory, as in the Ultraviolet Catastrophe and in renormalization.
    ==endquote==

    I think an even better, professional and up-to-date discussion of spacetime singularity in GR is at the Einstein-Online website, which has a page that straightens out some popular confusions and misconceptions about the cosmological Big Bang singularity. The page is called A Tale of Two Big Bangs.
    http://www.einstein-online.info/en/spotlights/big_bangs/index.html

    The point is that a singularity is a failure of some man-made mathematical model, which defines the boundary of applicability of that model.
    If you want to explore beyond the limits of applicability of a certain model, you have to replace or improve the model so it does not break down.
    And that improvement must also be empirically tested to make sure it fits the available data better.

    From the standpoint of GR cosmology, all one can say is that GR fails (stops computing meaningful numbers) as one approaches a certain "t = 0" or whatever you want to call it. So GR does not actually describe the initial conditions of expansion. Any initial state is outside the domain of applicability of that particular (vintage 1915) theory.
    Likewise with the GR black hole models.

    There are newer mathematical models of black hole and of conditions around the start of expansion---and these models, unlike GR, do not break down. Part of the job of researchers is to find ways to test the new models. There are a lot of professional journal articles about this, but not much accurate popularization.

    In the popular literature there are examples of poetically worded language about conditions at the start of expansion that are not linked to any particular mathematical model---basically it is philosophy/mythology, something to stimulate people's imaginations without giving them testable information about nature.
     
    Last edited: Feb 25, 2009
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