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Back ground independant

  1. May 15, 2006 #1

    wolram

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    Occams razor and logic implies that there has never been nothing, so why back ground independance ?
     
  2. jcsd
  3. May 15, 2006 #2
    i don't know what's the strict definition of the term background independence, but from what it entails, i think that it suggests quite otherwise, that space is independent of the matter which it contains in it.

    but i may be wrong.
     
  4. May 15, 2006 #3
    Of course there's NEVER been nothing. For time itself is something. If there is time then there is something.

    But you haven't presented the logic which would deny a beginning. Logic permits a valid conclusion from a false premise. If anything that exists (or even existence itself) is taken as a premise that is true, then a false premise would be non-existence. Logic allows a true premise (existence) from a false premise (non-existence). Logic does not allow a false premise from a true one. Thus we conclude that reality will never disappear.
     
  5. May 15, 2006 #4
    when you consider any kind of interaction (so everything you can see in our world) you do a calculation in a specific kind of (mathematically talking) space, with its signature, metrics and so on..
    but in a background indipendent theory, you have not to choose a particular space, the interaction is the space...
    Philosofically it would be a great saving of thought ;-)
    well, this is what I imagine...
    please someone correct me if I'm in wrong!

     
  6. May 15, 2006 #5

    wolram

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    I am sure you guys are double glazing salesmen :smile: only they could convince some one that a back ground is independant of some thing that has all way existed.
     
  7. May 15, 2006 #6
    what really exist would be relationship, not what we see as space...
    we can also talk about the no-existence of time ;-)
    the problem isn't that we see space, this is a fact, but the way we realize it in physics! I can't ensure that this would be important for all mankind, but this is so for high energy physics!
     
    Last edited: May 16, 2006
  8. May 15, 2006 #7

    marcus

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    I am glad you mentioned that wolram, it just happens that we have a special price on storm windows this month. We can do your whole house and the cost will be recovered in the savings on your first heating bill!*

    *if you live in Antarctica.
     
  9. May 15, 2006 #8

    wolram

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    No i am not going to give you a laughthy, the question is serious :smile:
     
  10. May 15, 2006 #9

    marcus

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    wolram, I think you have attatched some meaning which I don't understand to the words "background independence"


    a "background METRIC" is a technical device which is employed in some theories and not others.

    the ordinary meaning of "background independence" is as a shortened way of saying "background metric independence"

    meaning that the theory is one of those that is formulated without using a background metric.

    It does NOT mean that in such a theory the UNIVERSE ARISES OUT OF NOTHING:smile: That is way off.

    A background metric is a technical specialized thing that you really can do without. People shouldn't make such a big deal.

    A theory can be constructed without ever using a background metric and it can STILL have plenty of stuff assumed at the outset for the universe to arise out of.

    So you don't have to imagine that because a theory is independent of a background metric it is having the universe arise out of nothing----there will still be varying amounts of gear in the initial assumptions.

    I guess you can GENERALIZE the idea of background independence to where you throw out more and more of the initial assumptions, so that it becomes a relative concept------one theory can be MORE B.I. than another (in the generalized sense) if it uses less initial assumptions. But even in that generalized sense you would never get to some ABSOLUTE absence of initial conditions or absolute independence of assumptions.

    Every theory has stuff that it assumes as its materials to work with.
    ========================

    In the most common meaning, where you mean independence of a background metric----the reason quantum gravitists tend to insist on B.I. is basically because 1915 GENERAL RELATIVITY was a B.I. theory. It does not use a background metric.

    this means that spacetime in Gen Rel is more dynamic and more free to bend than if a background metric was assumed. With a background metric you can bend SOME but you are more constrained and more pre-determined. You are limited to some (normally small) "perturbations" around the basic shape dictated by that background metric.
    So the quantum gravitists have the opinion that a theory that needs a background metric in its formulation is NOT TRUE TO GEN REL

    like a starched collar and necktie does not allow as much freedom to your neck. the collar has some preconception of how you are going to be holding your head (which you are only allowed to disagree with slightly, by small perturbations)

    like, if your theory is built using a preconceived background metric then you are not really studying NATURE, you are studying your own preconceptions

    there is more to the discussion, but that's a starter.

    anyway it is not about the universe arising out of nothing
     
    Last edited: May 15, 2006
  11. May 15, 2006 #10

    wolram

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    If i accept that a back ground can be independant of an eternal thing, then i must execept that any thing that arises from that thing is a subset, and so not independat.
     
  12. May 15, 2006 #11

    marcus

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    I am not sure I understand you.
    But I think the main thing to realize is that most of the time when you hear people discussing the issue of QG theories being "background independent" what they are talking about is whether or not the theory is METRIC-independent

    (able to be formulated without specifying a fixed geometry)

    a metric is merely a particular type of distance-function that can be set up on a certain kind of artificial picture of space(time) called a manifold.
    There are other things you can use to depict space(time) besides a manifold. If you don't use a manifold then you can't define a metric, in the technical sense. And even if you do use a manifold you don't HAVE to start by fixing on some particular metric (called the "background" metric)

    If, for one reason or another, your theory doesnt depend on a fixed metric then it should really be called "metric-independent" but instead by some verbal accident they call it "background-independent".

    ================
    Wolram please don't confuse the limited technical notion of "background metric" with something big and important, like Nature.
    that is a whole other issue.
     
    Last edited: May 15, 2006
  13. May 15, 2006 #12

    hellfire

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