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GFT as the microscopic description of the quantum spacetime fluid - Dan Oriti

  1. Sep 29, 2008 #1

    MTd2

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    I just heard about Oriti when 3 days ago. But it seems he has interesting ideas. I will try to understand what Oriti is trying to accomplish, helpe please. His last article http://arxiv.org/abs/0710.3276 talks about the realization, or better, the emergence of gravity from fundamental scales.

    The novelty here it is that Oriti doesn't merely take the low limit of a parameter, instead, he looks for an phenomenological analogue in the real world. Then, he sugests that the continuum classical limit appears as a Bose Eisntein condensate from Planck scale, in the sameway that we see the lattice of atoms emerging as a a continum condesate in millionth of kelvin.

    A similar this point of view is present in a superstring theory, where one can view a brane as a low energy condensate of their energy spectrum, whose geometry is determined by the degrees of freedom of allowed colective motion of the open strings. Brane here, is the classical stuff, where GR can be found somewhere. Note that gravitons can't condensate, just the strings, since there isn't a multi graviton state, at least as they are described by string theory, according to a theorem of Weinberg-Witten., I think

    The crucial difference it is that in String theory, you can see explicitly particle that will be part of the gravity in the condensate, the graviton, in the perturbative expansion of the string, whereas in GFT, the graviton is nowhere to be found as a particle, because it will by itself be a fundamental piece of colective motion, that is a multiparticle state. So, in GFT, gravitons may condensate, but I am not sure...
     
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  3. Sep 30, 2008 #2

    marcus

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    I'd like to get down some context and detail about this paper. It is one that Oriti presented at the June 2007 Trieste conference (Francesca was there BTW) on quantum to emergent gravity---and then he presented it again at the August 2008 emergent gravity conference at MIT---Bee Hossenfelder blogged about that MIT conference (which brought together quantum gravity and condensed matter theorists.)

    Group field theory as the microscopic description of the quantum spacetime fluid: a new perspective on the continuum in quantum gravity
    38 pages, 6 figures; contribution to the proceedings of the conference 'From quantum to emergent gravity: theory and phenomenology', SISSA, Trieste, Italy, June 11-15 (2007)
    Daniele Oriti
    (Submitted on 17 Oct 2007)

    "We introduce the group field theory (GFT) formalism for non-perturbative quantum gravity, and present it as a potential unifying framework for several other quantum gravity approaches, i.e. loop quantum gravity and simplicial quantum gravity ones. We then argue in favor of and present in detail what we believe is a new GFT perspective on the emergence of continuum spacetime from discrete quantum structures, based on the idea of quantum space as a condensed matter system. We put forward a more specific, albeit still very much tentative, proposal for the relevant phase of the GFT corresponding to the continuum: a Bose-Einstein condensate of GFT quanta. Finally, we sketch how the proposal may be realised and its effective dynamics could be extracted in the GFT setting and compared with continuum gravity theories."

    Bee Hossenfelder's blog entry about the August 2008 Emergent Gravity conference gives some context.
    http://backreaction.blogspot.com/2008/08/emergent-gravity.html
     
  4. Sep 30, 2008 #3

    marcus

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    MTd2, let's bring several different resources to bear on this.
    The essential message of Wilczek's new book The Lightness of Being is to look for empty space to be a condensate.

    He gives several examples of a condensate because, while he does not have the answer, he thinks he knows what it will look like and he wants to prepare our minds to recognize the answer.

    So the heart of the book is around pages 91-96 where he says what is a chiral condensate.
    He first talks about the QQ (quark antiquark) concentrate---using colorful almost violent language. And then he tells how he imagines the HIGGS concentrate. And then he talks about the metric of General Relativity, that describes geometry, and I don't think this is as clear perhaps he doesnt see what the next layer of concentrate should look like. But his message is to tell us what to be looking out for.

    So we should perhaps keep several things in mind at once. Oriti GFT, Yidun Wan Braid matter, Loll and Freidel's path integrals (which Oriti thinks can be fused in GFT formalism)...

    If we are trying to watch a convergence of these several research lines then we should get some basic WikiP definitions like for chiral condensate (short expression for chiral symmetry breaking condensate). fermionic condensate.
    http://en.wikipedia.org/wiki/Fermionic_condensate
    http://en.wikipedia.org/wiki/Chiral_symmetry

    My feeling is that Wilczek has a passionate sense of conviction, almost a quiet rage, about how quantum empty space will find a formal human expression, but he does not have a mathematical formalism yet.

    The formalism that we see taking shape around Loll-Freidel-Oriti, or in the shadows adjacent to them, may possibly not be the correct one! But it is one possible formalism and some people have bet their careers on it and some have bet some money on it, and so on. So we can watch and see how it goes.

    And there was just that September conference at MIT, which I didn't think on the whole was so exciting but which might have had some good parts. Let's see what Bee Hossenfelder said about it.
    some context.
    http://backreaction.blogspot.com/2008/08/emergent-gravity.html
     
    Last edited: Sep 30, 2008
  5. Sep 30, 2008 #4

    marcus

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    Here is part of what Bee said in her 29 August blog:
    ==quote Bee==
    I am on my way back from the Emergent Gravity conference at MIT, contemplating what I've heard and learned. The aim of this meeting was to bring together condensed matter physicists with those tireless seekers looking for a fundamental theory unifying classical gravity with quantum field theory. Should such a theory exist then the features we observe might only be collective variables, emerging from a more basic underlying structure. Much like the properties of liquids are eventually a consequence of the dynamics of its molecules, the spacetime we live in might only arise in a macroscopic limit from a more fundamental theory. One would expect then both areas, condensed matter and quantum gravity, to share common approaches when going from a microscopic to a macroscopic description, and there to possibly be similar features like phase transitions or modifications of symmetries in the small distance limit.

    This past week we have heard about emergence of gravitons on a quantum bosonic model (Zheng-Cheng Gu, arXiv:gr-qc/0606100v1), the emergence of diffeomorphism (Jorge Pullin, arXiv:gr-qc/0606121v1) and the emergence of spacetime and matter in Group Field Theory (Daniele Oriti, arXiv:0710.3276v1), to only mention a few...
    ==endquote==

    My personal opinion is that Wilczek has a clear mental image of what he wants the (layered) condensate that is empty space to look like---and (IMHO) the reason he does not yet have a formalism is because the correct formalism should be background independent---free of an assumed background geometry.

    the geometry should emerge from the squirming mess, as it does with Loll, and not be put down at the beginning.
    so he knows what the fields are doing but he does not know what mathematical scaffold they are defined on.

    so this gives an opportunity for some people in the QG community to arrive at what is basically a background independent way of describing a stew of gluons, a background independent picture of QCD, or at least of some part of QCD, a chiral condensate perhaps.

    you know with Yidun Wan's stuff, it is all chiral. This turned out to be inevitable, it is just how braids are. At the end of his paper, Yidun was talking about a meet-up with GFT. I wonder how that would work, and if it has a chance.

    I hope these reactions to your post are at least in some part helpful, and not too scattered.
     
  6. Sep 30, 2008 #5

    MTd2

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    That's why I proposed Tsallis entropy. I just know it from the wikipedia entry and vague comments from old university professors. I guess I should just start studying this.

    And I also found the analogy of Planck -> low energy with Bose Einstein really amazing. I wonder why no one, as far as I know, thought of that analogy in a so straightforwardrd way. I guess, it boils down to look how the transition from discrete to continuum happens when you make things really cold.

    I will think more about this, mainly about the Fermionic condensate.

    BTW, I still don't have Franck's book...
     
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