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What is a singularity ?

  1. Aug 6, 2007 #1
    What is a "singularity"?

    I'm not sure I understand this concept? Isn't it just a "mathematical" concept, like "infinity" or "zero"? Couldn't it be equated with a similarly non-sensical concept as "God"? I know Hawking tries to eliminate the concept of a "singularity". But I'm not sure of what the alternative theory is? How, as atheistic materialistic hard/cold/logical scientific beasts, do we explain the big bang & black holes etc?

    I don't like the concept of the "singularity" - ie an unseeable "zero"-dimensional "point".

    To me, it reeks of superstition.

    So, what is the alternative?
     
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  3. Aug 6, 2007 #2

    marcus

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    singularities are where man-made theories fail.
    the answer to a singularity is to fix the theory (replace it by an improved version that doesn't break down in that particular situation)

    other theories (of electromagnetism, of thermal radiation,...) have had singularities---where they failed to apply, predicted nonsense, infinities, etc.
    And they have been fixed. People are in the process now of fixing gravity---replacing Gen Rel by an improved (quantum) version that will duplicate the successes of the classic theory but will not break down in the big bang and black hole contexts.

    One footnote to this: according to the tradition of empirical science, as soon as a research team comes up with a candidate theory that does what I said, they are supposed to derive predictions from it concerning FUTURE observation that could be made and which would provide a check. A candidate to replace Gen Rel, including quantum effects, must be made falsifiable---this involves deriving predictions about observations that could go against the theory and disprove it. People have candidates now, and they are beginning to study their observable consequences in detail so as to be able to make checkable predictions.

    If you want links to followup, just say.
     
    Last edited: Aug 6, 2007
  4. Aug 6, 2007 #3
    I love the sentiment, really I do, but can we have a few more sentences for us deeply disturbed laypeople? lol!

    Do you mean your Quantum Loop Theory? I really don't know where to start with this. Is it so new that nobody's done a "Paul Davies" with it yet?

    I need to picture something in my mortal brain. MY BRAIN IS VERY FLEXIBLE! TRY ME!

    To me, if it ain't a singularity (which is absurdist) then it is an infinity (which is absurdist). Is there a third road?
     
  5. Aug 6, 2007 #4

    marcus

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    Yes. Perhaps the trouble is that right at the moment there are several "third roads"

    You mentioned Loop Quantum Gravity. That is certainly one. A substantial part of the Loop community is working on the application to cosmology---Loop Quantum Cosmology (LQC)

    I don't want to seem to over-emphasize LQC because there are several different approaches being worked on. LQC is just a prominent example. It has 10 - 20 people working on it. Some concentrating pretty much all their research effort, others helping out now and then. Maybe more, that's just a guess.

    I wouldn't worry my head too much about singularities or infinities. LQC (the approach to quantum cosmology I am most familiar with) gets rid of them.
    Every time you take a classical (pre-quantum) theory and quantize it you get CORRECTION TERMS, little modifications of the equation that typically take effect in extreme circumstances outside the range of applicability of the classical theory.
    In the LQC case it just happens that due to the quantum correction terms gravity becomes repellent at very high density. It therefore leads to a BOUNCE. This was first noticed in 2001 by a young fellow named Martin Bojowald---some 15 years after Loop gravity was originally developed.

    The general point to make is THIS IS WHAT PEOPLE DO IN THE NORMAL COURSE OF THINGS: they remove singularities in their theories.

    If LQC doesnt turn out to be the right way to do it, some other extension of the classical theory will turn out to.

    Here is something you could do, Mattex, to get a feel for this. Go to GOOGLE and type in "before the big bang" and see what kind of stuff you get.

    or type in "what happened before the big bang"

    ==============
     
    Last edited: Aug 6, 2007
  6. Aug 6, 2007 #5

    Wallace

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    As marcus has alluded to, a singularity is not a physical 'thing' that you point to or touch. A singularity is a point or region where your theory throws up its hands and surrenders, being unable to describe physical reality at that point. This is the case with GR for black holes and the very very early universe which is why work on theories that do not break down for these cases but which essentially reduce to GR at other times is underway, as marcus has pointed out.
     
  7. Aug 10, 2007 #6
    I would think that if our universe were a finite part of an infinite universe then our theories ending as singularities would be a more accurate description of how our physical reality fits into the universe not the other way around.
     
  8. Aug 10, 2007 #7

    marcus

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    What makes you think that? Ordinary mainstream cosmologists don't assume that. they use a model called LCDM that comes in two versions finite and infinite.

    the universe is either finite, and that is all there is (it is not a "finite part of an infinite universe)

    or it is infinite, and that is all there is (again it is not a "finite part of an infinite universe)

    I am curious where you got that idea, Petm. Could you have read it somewhere---or perhaps misunderstood something you read?


    ======================

    I don't think you can mean the OBSERVED universe. the part of the universe from which we have gotten light so far (because it has had time to reach us) is constantly increasing across a broad front as we see more and more galaxies.

    the part that is observable, because its light has already reached us, is certainly a proper part of the whole, but it is certainly not joined to the whole by a singularity!

    it is separated from the yet-unseen part by an horizon that continually receeds.

    singularities are just bad descriptions of nature, which science gradually gets rid of by correcting its theories.
     
  9. Aug 11, 2007 #8
    Sorry Marcus that is my own thought, I don't remember reading about it anywhere. I have read about LCDM and the thinking that the universe is either finite or infinite, it just seems to me that being either, or, may not be the way it is, hence the use of the word "if" in my post.
     
  10. Aug 11, 2007 #9

    marcus

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    Well you aren't alone in thinking that. I don't think the multiverse picture is predominant in the mainstream, but there are plenty who espouse it or discuss it.

    The essential trouble with multiverse thinking is that the pros don't really NEED it in order to make progress understanding the universe.
    for the most part it is just a fascinating but irrelevant speculation.
    so since you don't need it, Occam Razor says to chuck it.

    Maybe someone else will see this thread and bring up some way in which the multiverse picture is needed (to explain something that has no alternative single-verse explanation).

    The only case of that which I know is Smolin's evolutionary reproductive cosmology hypothesis which gradually EVOLVES the fundamental numbers of physics so as to make universes more reproductively FERTILE at making baby universes.
    This hypothesis could be used to explain why the basic numbers of our universe are remarkably WELL-ADAPTED to the formation of stars of which many subsequently collapse to form black holes. It also allows for an empirical test: one can check to see if, in fact, the numbers really are as well-adapted as they look---are they really at a local optimum for making black holes abundant? If they are not, that would tend to refute the hypothesis. So it has the important virtue of being possible to falsify empirically--it actually predicts something measurable that prior theory doesn't.

    Other than that, I don't know of any multiverse scenario that fills any logical gap---that some single-verse explanation couldn't fill just as well. maybe you can suggest one. in any case thanks for putting out an interesting idea for discussion!

    As for replacing singularities with something closer to what actually happens in nature, it could turn out to be helpful in this connection as well as with other things---singularities are just signs that the classical theory is flawed, so they are definitely not something to keep around if you can help it! :smile:
     
    Last edited: Aug 11, 2007
  11. Aug 16, 2007 #10

    pervect

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    In standard LCDM cosmology, the observable universe is a finite subset of an infinite universe.

    Note that at the big bang the observable universe was a point (it takes inflation to explain this), but the universe itself was still infinite.
     
  12. Aug 16, 2007 #11

    marcus

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    the standard factor by which inflation is assumed to expand things is e60, right?

    as long as the "point" you have in mind is not a mathematical point, but has some (albeit small) spatial extent, what you say sound right to me.
     
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