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Thread: Why do we need Gravitons?

  1. #1 Why do we need Gravitons? 
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    If gravity is the curvature of space-time then how can it ever be described by a quantum theory. If it is a graviton field then it is not the curvature of space-time but the interaction of the graviton force particles. Maybe i need to reread Three roads to quantum gravity. I don't understand it. They say it's one thing and then it's something else.

    Wouldn't it be better to describe a quantum theory of gravity as some kind of space-time atoms that interact on the quantum scale? If there is a quantum foam of space at quantum scales a better description for me would be a scaffolding of space-time geometries that are built up to create classical space-time.

    Maybe the density of the quantum foam is not high enough to constitute a classical non-quantum space-time at quantum distances and only on larger scales do the effects of those space-time interactions build up to construct the world we live in. I've never read anything like this. If someone could help me understand or point me in the right direction it would be greatly appreciated.

    Im just a lay person. I can't understand math so please no equations. And if it's something that can only be descibed mathematicaly ok.


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  3. #2  
    Moderator Moderator Markus Hanke's Avatar
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    If gravity is the curvature of space-time then how can it ever be described by a quantum theory. If it is a graviton field then it is not the curvature of space-time but the interaction of the graviton force particles. Maybe i need to reread Three roads to quantum gravity. I don't understand it. They say it's one thing and then it's something else.
    You pretty much pinpointed the crux of the problem - this is why it has proven so hard to formulate a theory of quantum gravity. The standard model of particle physics describes particles and their interactions, whereas General Relativity describes gravitation via geometrical properties of space-time. The two aren't immediately compatible.
    There are some good approaches however :

    1. String Theory suggests that particles are actually excited states of vibrating strings; these vibrations can only be consistently described in a curved space-time background. Thus both GR and SM emerge naturally, in principle anyway. There are still a lot of problems with this theory, mainly on the mathematical side of things.
    2. Loop quantum gravity attempts ( simply stated ) to quantize space & time itself on small scales. Again, GR can emerge in principle from this, but the mathematics haven't yet been fully worked out.

    And there are other approaches as well, but these two are probably the most prominent ones at present.


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    Actually there are plenty theories which don't quantize gravity in terms of gravitons. For instance, certain theories suggest that gravity is a psuedo-force a bit like the Coriolis force. It does not require a physical mediator.
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    Quote Originally Posted by Markus Hanke View Post
    If gravity is the curvature of space-time then how can it ever be described by a quantum theory. If it is a graviton field then it is not the curvature of space-time but the interaction of the graviton force particles. Maybe i need to reread Three roads to quantum gravity. I don't understand it. They say it's one thing and then it's something else.
    You pretty much pinpointed the crux of the problem - this is why it has proven so hard to formulate a theory of quantum gravity. The standard model of particle physics describes particles and their interactions, whereas General Relativity describes gravitation via geometrical properties of space-time. The two aren't immediately compatible.
    There are some good approaches however :

    1. String Theory suggests that particles are actually excited states of vibrating strings; these vibrations can only be consistently described in a curved space-time background. Thus both GR and SM emerge naturally, in principle anyway. There are still a lot of problems with this theory, mainly on the mathematical side of things.
    2. Loop quantum gravity attempts ( simply stated ) to quantize space & time itself on small scales. Again, GR can emerge in principle from this, but the mathematics haven't yet been fully worked out.

    And there are other approaches as well, but these two are probably the most prominent ones at present.
    I personally think LQG is the closest path to understanding everything. What are your opinions?
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    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by Geometrogenesis View Post
    I personally think LQG is the closest path to understanding everything. What are your opinions?
    I think it is too early to tell, since no complete mathematical descriptions exist for either one of these theories yet...
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    Brassica oleracea Strange's Avatar
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    Have we made no progress since I read Three Roads to Quantum Gravity? (about 10 years ago)
    ei incumbit probatio qui dicit, non qui negat
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    Quote Originally Posted by Strange View Post
    Have we made no progress since I read Three Roads to Quantum Gravity? (about 10 years ago)
    Well, to say we hadn't would be worrying. I'd say each day brings it's own understanding.


    We could cover loads of problems that even the three roads of quantum gravity presume, but the irony is that one of these approaches admits LQG which has been applied recently with some important work in the field of Geometrogenesis. This is my name, if you hadn't realized :P

    This ''Geometrogenesis'' theory explains the origin of the curvature and geometry of space as the emergent configuration phase space of matter. A mouthfull much?

    Interestingly however, the implications of unifying geometrogenesis with LQG leads to Fotini Markoupoulou's (PhD) Quantum Graphity model. In short, matter is a low energy phenomenon. It concerned with set theory relating to the facts that geometry and curvature and matter all appeared late in the universes history. However, the high energy theory is concerned with no matter or geometry, so it seems that gravity cannot be quantized.

    In fact, Fotini can prove that gravity or space is not fundmantal. Her approach in Quantum Graphity is using a model which eradicates geometry and these so-called ''spaceless'' models have been around for a while. Her model is the best approach in my eyes.
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    There's a reason why a particle is going back in time,and Why in Big3 times,each time returning particle event Duplicate itself into three-dimensional space,and the power that makes him break up and replicate back in time depends on the inverse squared R
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  10. #9  
    Comet Dust Collector Moderator
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    And what particles go backwards in time? This post belongs in another forum...
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  11. #10  
    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by Strange View Post
    Have we made no progress since I read Three Roads to Quantum Gravity? (about 10 years ago)
    Yes, progress has been made, but not enough to finalize any of the various QG models. This whole area is still open to further debate, even a decade on.
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  12. #11  
    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by Geometrogenesis View Post
    Well, to say we hadn't would be worrying. I'd say each day brings it's own understanding.


    We could cover loads of problems that even the three roads of quantum gravity presume, but the irony is that one of these approaches admits LQG which has been applied recently with some important work in the field of Geometrogenesis. This is my name, if you hadn't realized :P

    This ''Geometrogenesis'' theory explains the origin of the curvature and geometry of space as the emergent configuration phase space of matter. A mouthfull much?

    Interestingly however, the implications of unifying geometrogenesis with LQG leads to Fotini Markoupoulou's (PhD) Quantum Graphity model. In short, matter is a low energy phenomenon. It concerned with set theory relating to the facts that geometry and curvature and matter all appeared late in the universes history. However, the high energy theory is concerned with no matter or geometry, so it seems that gravity cannot be quantized.

    In fact, Fotini can prove that gravity or space is not fundmantal. Her approach in Quantum Graphity is using a model which eradicates geometry and these so-called ''spaceless'' models have been around for a while. Her model is the best approach in my eyes.
    This is an approach somewhat similar to Causal Dynamical Triangulations. I agree it is a very interesting model, and definitely worth further study.
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    It uses the Inequality in its phase space.
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