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Emergent Gravity IV

  1. Aug 18, 2009 #1


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    http://www.emergentgravity.org/index.php?main=main_EGIV_about.php&banner=banner_default.php&sooter=footer_default.php&test=end%22 [Broken]
    I will be attending with the intent of improving my learning curve.

    Do you have any interesting questions that I could ask on your behalf?
    Last edited by a moderator: May 4, 2017
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  3. Aug 19, 2009 #2


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    Jal, you are lucky to be able to attend EG IV. Instead of giving you some questions I would like to be able to ask (if I could be there) I will, first of all anyway, list some talks that I would be especially interested in attending. I'll quote the abstract summary of each talk, to help explain why I'm interested in it. Other people may want to mention others of the talks. In this list I am omitting some talks by people whose work I know and whom I respect a lot---simply because I have read their papers already and I am not so interested in the topic at present. I have put a few folks on the list that I don't know, like the analog models people, whose laboratory work sounds interesting and I'm curious about it.

    Bilson--Thompson What a tangled web weaves us
    I shall discuss the prospect that randomly generated networks, such as those which occur in certain theories of quantum gravity, automatically lead to the emergence of matter and 3+1 dimensional spacetime.

    Bojowald Effective dynamics of loop quantum gravity
    Loop quantum gravity provides a discrete formulation of space-time in terms of elementary building blocks. This talk will discuss means to approach the semiclassical limit and the role of discreteness in corrections to the classical behavior.

    Conrady Simplicial complexes from condensation
    I will talk about work in progress on a statistical model of graphs. The configurations are arbitrary graphs with a fixed number of links. The Hamiltonian is a simple function of the graph that favors the formation of 2d simplicial complexes. Monte Carlo simulations show that at low temperatures the system is in an ordered phase, where the links condense to 2d simplicial complexes. At high temperatures one has a disordered phase and the links form graphs of high connectivity.

    Jain Numerical experiments in emergent spacetimes
    Emergent spacetimes in fluid-type systems present one possibility for observing and testing the predictions of curved spacetime quantum field theory. We present the results of numerical simulations for expanding spacetime models in Bose--Einstein condensates - ultracold quantum gases with a low energy fluid description. We further discuss the evidence of quasiparticle production in such systems as an analogue of cosmological particle production, referring to both past and current work.

    Kempf Spacetime could be simultaneously discrete and continuous, the same way that information can
    At the heart of information theory is the discipline called Shannon sampling theory, which establishes the equivalence of continuous and discrete representations of information. Sampling theory has recently been generalized and applied to physical fields on curved spacetimes. Here, we show how sampling theory can be generalized and applied to spacetime itself, yielding the equivalence of continuous and lattice representations of spacetimes that possess a suitable natural ultraviolet cutoff. The Einstein action is induced. The resulting methods could be useful in approaches to quantum gravity.

    Loll Self-Organized Quantum Spacetime and the Role of Time
    A novel approach to the quantization of gravity, using so-called 'Causal Dynamical Triangulations (CDT)', throws new light on the physical origins of spacetime. It describes the seemingly structureless, empty spacetime we see around us and its cosmological 'shape' - that of an exponentially expanding universe - as resulting from the complex, dynamical interplay of an enormous number of microscopic quantum fluctuations. The underlying mechanisms of 'emergence' and 'self-organization' are familiar from the theory of complex systems, but had so far not found a concrete application in high-energy physics. I will summarize the basic ingredients and construction method underlying this approach, and some of its achievements and outstanding challenges. Specifically, I will comment on what it teaches us about the role of time, its non-emergence, and the relation of the proper time present in CDT quantum gravity with stochastic time recently uncovered in two spacetime dimensions.

    Markopoulou Network-based toy models for gravity
    A number of different approaches to quantum gravity recently proposed are based on the idea that spacetime geometry and gravity are derivative concepts and only apply at an approximate level. Two fundamental challenges to any such approach are, at the conceptual level, the role of time in the emergent context and, technically, the fact that the lack of a spacetime makes difficult the straightforward application of well-known methods of statistical physics and quantum field theory to the problem. We study these using microscopic models for emergent space and gravity that are based on network evolution in which no a priori geometric notions are present. In this talk we present two models. In the first model, we are able to use methods from quantum information theory to derive features such as a variable speed of light from a non-geometric quantum system. The second model is a toy system for an interesting non-linear matter-geometry interaction in which geometry is defined by the behavior of matter.

    Oriti On the emergence of spacetime and matter in the Group Field Theory approach to Quantum Gravity
    This talk is divided in two parts. In the first part, we introduce the Group Field Theory formalism for Quantum Gravity, and we present the arguments for considering it a promising candidate description of the microstructure of space. We also recapitulate the general strategy for investigating the emergence of a continuum spacetime from the GFT microstructures. In the second part we present the results of some work in progress showing how an effective field theory of matter on a non-commutative flat spacetime (of the 'Deformed Special relativity' type) emerges in fact from a simple GFT, with a procedure similar to that leading to effective field theories for quasi-particles on an emergent geometry in condensed matter systems.

    Penrice Black hole experiments laboratory tour
    A short 15-20 minute tour of our experimental facility. The tour will compose of a short description of the experimental setup and will then be followed by a demonstration. The demonstration will include the creation of an effective black hole and white hole horizon. Waves will then be sent into the white hole horizon where we believe that they are getting up converted and then dragged out of the system by the flow rate. This will be followed by the presentation of some preliminary results.

    Philbin Experimental demonstrations of horizons in fluid mechanics and optics
    The lecture discusses the lessons learned from some of our demonstrations of analogues of the event horizon. In one experiment we studied water waves in counter-flows and in another light propagation in photonic-crystal fibres influenced by ultrashort light pulses. In both experiments, we created white-hole horizons. As white holes are time-reversed black holes, their laboratory analogues exhibit essentially the same physics. In the water-wave experiment, we observed negative-frequency waves in a regime where a horizon for surface waves is not established. In the optical experiment, we measured the frequency shifting of light at the group-velocity horizon. We have not observed Hawking radiation yet, but we are working on a modification of the experiment that will allow us to do so. In both the water-wave and the optical experiment, negative-frequency waves occur in strongly-dispersive and non-stationary regimes, cases that have been largely neglected by theory until recently.

    Rideout deSitter Space from Causal Set Dynamics
    We present evidence that the early universe of sequential growth dynamics for causal sets begins with a phase of super-exponential expansion, followed by a deSitter-like period of expansion.

    Schuetzhold Analogue gravity
    This talk reviews some qualitative and quantitative analogies between gravity and condensed matter including analogues for black/white holes as well as the expanding/contracting cosmos. By studying these analogies, we might gain a deeper understanding of the effects of (quantum) gravity such as Hawking radiation.

    Uhlmann On the simulation of expanding spacetimes in trapped Bose-Einstein condensates
    The prospect of simulating cosmic quantum effects in laboratory systems has gained much attention in recent years. Low-energy phase fluctuations (phonons) in expanding Bose-Einstein condensates for instance obey the same evolution equations as a massless scalar field in a flat Friedman-Robertson-Walker-Lemaitre spacetime, which is believed to provide an adequate description of the universe during cosmic inflation. Hence, different features of these expanding spacetimes can be identified such as the formation of an effective sonic horizon as well as freezing and amplification of phonon fluctuations. In this talk, I will address the impact of a (harmonic) trap potential on their evolution and discuss the implications for an actual experiment.

    Unruh Dumb holes and water slides
    This talk will review some of the features of dumb holes, with particular emphasis on the behaviour of waves in a flume tank (the experimental situation being carried out here). In particular the alteration of the dispersion relation due to varying depth and velocity of the fluid will be examined, together with the various horizon phenomena expected.

    Visser Who's afraid of Lorentz symmetry breaking?
    Is Lorentz symmetry truly fundamental? Or is it just an "accidental" low-momentum emergent symmetry? Opinions on this issue have undergone a radical mutation over the last few years. Historically, Lorentz symmetry was considered absolutely fundamental --- not to be trifled with --- but for a number of independent reasons the modern viewpoint is more nuanced. What are the benefits of Lorentz symmetry breaking? What can we do with it? Why should we care?
    Last edited: Aug 19, 2009
  4. Aug 19, 2009 #3


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    I will be attending and will be taking the opportunity to improve my learning curve. (Listening and maybe get to ask some questions)
    I’m sure that everyone will want to talk, over a beer is nice, and explain their approaches.
    Hopefully, I will be able to get some insight on how they see the possibilities of using “an underlying condensed matter like theory.” that could explain the initial conditions prior expansion and prior inflation.

    I’m eager to meet with Renate Loll.

    Their web page is giving me trouble. It is still in the setup phase.
    I have asked if they will be making the presentations on line and if there will be blogging with the participation of the public. (The fans!)
    No, to both questions.
    http://www.emergentgravity.org/index.php?main=main_EGIV_programme.php [Broken]

    I will try to give reports, and respect any confidentiality that are requested.
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  5. Aug 19, 2009 #4


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    Ask Viser what does he think of that last Fermi result that gives Mqg/Mplank>1? Ask him if that wouldn't rule any way for ruling out lorentz invariance.
  6. Aug 19, 2009 #5


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    Hi MTd2!
    I'll try to get a quote or get him to contribute an answer in this thread.
  7. Aug 19, 2009 #6


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    Really??? Please! If I could, I'd choose to work with Matt Visser. I guess he is the one closest to the truth. He is sensible enough to believe black holes do not exist, just its mimickers, and actualy research that.
  8. Aug 19, 2009 #7


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    Question for Markopoulou: she related quantum graphity to Levin and Wen's string net condensation which produced photons. Gu and Wen now have a model that maybe has gravitons - is quantum graphity related to the later Wen model also?
  9. Aug 23, 2009 #8


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    Jal, the conference starts tomorrow (24 August). What luck you can attend!
    Please let us know a few of your impressions (don't have to be complete there will be too much to give an exhaustive account.)

    Be sure you get a good look at Bill Unruh---who achieved physics immortality by recognizing the existence of the Unruh radiation (analog of a black hole's Hawking radiation) seen by an accelerating observer.

    He must be near retirement now, one of the Grand Old Men that you want to remember having met or seen in action.

    I just learned that Unruh will be chairing one of the parallel sessions at the Mexico City conference in 2010:

    D1 Loop Quantum Gravity and Spin Foams
    Chair: Alejandro Corichi

    D2 Strings, branes and M-theory
    Chair: (to be announced)

    D3 Causal sets, Causal dynamical triangulations, Non-commutative geometry, and other approaches to quantum gravity
    Chair: Fay Dowker

    D4 Quantum fields in curved space-time, semiclassical gravity, quantum gravity phenomenology, and analog models
    Chair: Bill Unruh
    He is currently interested in analog models, like hydraulic lab models of a black hole where the speed of light is replaced by the speed of sound in a moving medium.
    As you can see Emergent Gravity IV has a lot of talks about analog models. I think Unruh has helped to build that line of research up at Vancouver.
    It could overall be the most interesting single aspect of the EG4.
  10. Aug 23, 2009 #9


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    I'm checked into the resident accomodations.
    I ran into R. L. as she was getting out of the taxi. It was not a time to talk. Just a smile.
    I think that for the none science comments I will start a blog entry for people to read.
    I'm reviewing the written papers of the presenters so that I can have a better idea of what they will be discussing.
    Until tomorrow.
  11. Aug 23, 2009 #10


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    R.L. That's like royalty. Elizabeth the second smiled at you?
  12. Aug 23, 2009 #11


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    If anybody else wants to follow the adventures of Jal in this thread here is the main EG4 url:
    http://www.emergentgravity.org/index.php [Broken]
    and then if you want to see what talks are tomorrow you go to Programme in the menu on the right and get:
    http://www.emergentgravity.org/index.php [Broken]

    And it says that the Monday afternoon session is chaired by Fotini Markopoulou and the lineup is starting at 2 PM is Loll (who gets an hour)
    Florian Conrady (who gets half an hour)
    then coffee and cake
    then at 4 PM Bilson-Thompson (half hour)
    Rideout (half hour)
    followed by a discussion chaired by Fotini.

    David Rideout has mostly done causal sets but also some (super?)computer simulations involving other kind(s) of QG.
    Sundance Bilson-Thompson is the young Aussie who brought up the idea of matter particles being composed of braids in a network. That started a group at Perimeter working on how matter could be implemented in the spin networks that describe the quantum state of geometry in Loop---by knots or braids in the geometry. I don't know how that initiative is going. Maybe he will give a status report.
    Florian Conrady has co-authored with Laurent Freidel on several major recent spinfoam papers, where they seem to have gotten a grip on the semiclassical limit. That could be wrapped up by now or it could still have loose ends. He could have decided to talk about that but he seems instead to have chosen to present some of his own solo work on a completely different topic. To me as a reader of his research papers, he is impressive. I don't know what he is like as a communicator, in front of an audience.
    Loll is Loll. I watched her video at the Planck Scale conference in Poland, she gave a strong talk.

    I just looked at the abstracts again. Loll and Bilson-T are not surprises. They are presenting work they are known for, where there may also be new progress to report.

    But Conrady and Rideout are both unexpectedly talking about (so-far unreported) QG work relevant to the early universe.

    Florian Conrady Simplicial complexes from condensation
    I will talk about work in progress on a statistical model of graphs. The configurations are arbitrary graphs with a fixed number of links. The Hamiltonian is a simple function of the graph that favors the formation of 2d simplicial complexes. Monte Carlo simulations show that at low temperatures the system is in an ordered phase, where the links condense to 2d simplicial complexes. At high temperatures one has a disordered phase and the links form graphs of high connectivity.

    David Rideout deSitter Space from Causal Set Dynamics
    We present evidence that the early universe of sequential growth dynamics for causal sets begins with a phase of super-exponential expansion, followed by a deSitter-like period of expansion.

    My guess about Rideout is that he is running a Causal Set simulation of the early universe on a computer (he did computer work that was reported a year or two ago, involving socalled Cactus network or cluster or framework for massive computation. I have no further information about this.) And the "present evidence" means evidence of how the model behaves that is gathered by running computer simulations. That is just my guess, it could easily be something different.
    Here is a picture of Rideout with a list of his recent publications:
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  13. Aug 24, 2009 #12


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    One of the things I've been wondering about, it's not really emergent, but since you may be able to speak to Loll and Conrady is: CDT and spinfoams are quite different formalisms, but both are derived from actions that classically give rise to the Einstein field equations. In CDT, there is no proper classical limit unless the sum is restricted in Lorentzian space before rotating to Euclidean space. Conrady and Freidel have recently shown that a Riemannian (non-Lorentzian) spinfoam amplitude has the proper semiclassical limit, but left open whether the sum of the amplitudes also behaves nicely. Given that in DT, there is no nice semiclassical limit without using some Lorentzian properties, what is different about spinfoams that might give hope for the Riemannian case?

  14. Aug 25, 2009 #13


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    Wheee! What an exhauting long day.
    Marcus got everything just about right. I can only add a few comments.

    I did mention that this thread is existing and I am encouraging them to come and read ... maybe give some personnel comments.

    I'm glad that I took the time to review the papers that are availabe at http://arxiv.org/
    As a result, It improved my understanding of their presentations.
    I can say that I did not hear of anything new in their representations.

    A few quick comments

    A new paper has been presented on arxiv.org.

    His presentation, very well done by the way, (I'm brown nosing in case he is reading this), centered on try to find a way of analysing spectral observables in a way that could show us the shape of the manifold, (spacetime).


    Tensor network + Global symmetry = Spin network
    There are plans to have his presentation recorded by perimeter. If I recollect, this fall?

    Unfortunately, his model way beyond my ability to understand and make comments.


    Presented a Qbits model that he thought might have the potential to get spacetime without putting it in. The approach/model/concept is in the development stage.

    R. Loll

    Did mention including "noise" in her model which did not make any demonstrable changes. I don't recall if she had written about it.
    (The picture on her web page makes her look way too harsh.)
    She is a very competent presenter.


    Presented a toy model from graphity using tetras with the final intent to have spacetime and the Langerian arizing.

    S. Bilson-Thompson

    Again, another model in progress.
    I finally got an explanation on the nodes (located at the center of the figure) are connected at right angle to the surfaces of the figures (tetras).
    He has a picture in his paper at arxiv.org.

    David Rideout

    Yes, marcus, that is the model that he present.
    He did his presentation on crutches. Broke his ankle at a camping trip. That his story and we accept it. There might be a funny story behind it but it would not be proper to laugh at such a misfortune.

    Different small groups went out to different places for supper.
    The sushi was raw and the conversation entertaining.

    I'm on an old slow laptop and have to try to review tomorrow's papers.
  15. Aug 26, 2009 #14


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    Day # 2
    The conference is composed of experts and students from many approaches. Therefore, you can imagine that some of the presentations are not of interest or understandable enough to generate a flurry of questions or discussion.

    A comment given to me was that the student fear asking questions that would appear to demonstrated their lack of understanding. This is especially true when the conference is being recorded.

    This, therefore, increases the value of physics forums in distributing information.

    The organizers offered all of the presenters to submit their presentation and they would put them online. Let’s watch for the development.

    A few comments on the presentations


    Bosonization … successes and limitations
    He was a speed talker and I could not take notes fast enough and pay attention at the same time.
    In condense matter Lorentz invariance is often broken ( eg. 3d crystals)

    Example: Fail Green function

    Depending on the shape of the cos curvature from the Fermi surface to the next energy level.

    Those who didn’t need to look up what I said might be able to furnish more detailed info.

    Van Raamsdonk

    Here is his paper
    Comments on quantum gravity and entanglement
    (Submitted on 17 Jul 2009)


    His talk was based upon
    AdS/CFT v.s. String Loops
    Authors: Gianluca Grignani, Marta Orselli, Bojan Ramadanovic, Gordon W. Semenoff, Donovan Young
    (Submitted on 8 May 2006)

    As you can see … a mixture of experts trying to communicate to experts in other fields.
    Now we come to the anticipated discussions of Horava's recent article
    Quantum Gravity at a Lifgarbagez Point
    Authors: Petr Horava
    (Submitted on 26 Jan 2009 (v1), last revised 2 Mar 2009 (this version, v2))

    Matt Visser was the first to of 4 to speak. (very eloquent and passionate )
    Lorentz symmetry breaking as a quantum field theory regulator
    Authors: Matt Visser (Victoria University of Wellington)
    (Submitted on 3 Feb 2009 (v1), last revised 17 Jul 2009 (this version, v3))

    Short notes:
    Planck scale and Lorentz symmetry scale breaking will be different.
    There has been a proliferation of papers. ( cited 120 times) This subject is hot, hot!
    Horava introduced “a preferred foliation”. There are questions of “Projectability’, “detail balance”
    Matt Visser will hold judgment for another 6 month of analysis.

    Robert Brandenberger
    He has two personalities. He becomes a completely different person when presenting his paper to an audience. Wow! He comes alive!

    Here is his paper.
    Matter Bounce in Horava-Lifgarbagez Cosmology
    Authors: Robert Brandenberger (McGill University and CERN)
    (Submitted on 18 Apr 2009 (v1), last revised 22 Apr 2009 (this version, v2))
    Abstract: Horava-Lifgarbagez gravity, a recent proposal for a UV-complete renormalizable gravity theory, may lead to a bouncing cosmology. In this note we argue that Horava-Lifgarbagez cosmology may yield a concrete realization of the matter bounce scenario, and thus give rise to an alternative to inflation for producing a scale-invariant spectrum of cosmological perturbations. In this scenario, quantum vacuum fluctuations exit the Hubble radius in the pre-bounce phase and the spectrum is transformed into a scale-invariant one on super-Hubble scales before the bounce because the long wavelength modes undergo squeezing of their wave-functions for a longer period of time than shorter wavelength modes. The scale-invariance of the spectrum of curvature fluctuations is preserved during and after the bounce. A distinctive prediction of this scenario is the amplitude and shape of the bispectrum.

    If you have not figured it out The Horava Lipggarbagez gravity is an alternative “driver” for inflation.

    Paul Saffin
    Strong coupling in Horava gravity
    Authors: Christos Charmousis, Gustavo Niz, Antonio Padilla, Paul M. Saffin
    (Submitted on 15 May 2009 (v1), last revised 21 May 2009 (this version, v2))
    Abstract: By studying perturbations about the vacuum, we show that Horava gravity suffers from two different strong coupling problems, extending all the way into the deep infra-red. The first of these is associated with the principle of detailed balance and explains why solutions to General Relativity are typically not recovered in models that preserve this structure. The second of these occurs even without detailed balance and is associated with the breaking of diffeomorphism invariance, required for anisotropic scaling in the UV. Since there is a reduced symmetry group there are additional degrees of freedom, which need not decouple in the infra-red. Indeed, we use the Stuckelberg trick to show that one of these extra modes become strongly coupled as the parameters approach their desired infra-red fixed point. Whilst we can evade the first strong coupling problem by breaking detailed balance, we cannot avoid the second, whatever the form of the potential. Therefore the original Horava model, and its "phenomenologically viable" extensions do not have a perturbative General Relativity limit at any scale. Experiments which confirm the perturbative gravitational wave prediction of General Relativity, such as the cumulative shift of the periastron time of binary pulsars, will presumably rule out the theory.

    Jiawei Mei
    Solutions to Horava Gravity
    Authors: H. Lu, Jianwei Mei, C.N. Pope
    (Submitted on 10 Apr 2009 (v1), last revised 31 Jul 2009 (this version, v4))
    Abstract: Recently Horava proposed a non-relativistic renormalisable theory of gravitation, which reduces to Einstein's general relativity at large distances, and that may provide a candidate for a UV completion of Einstein's theory. In this paper, we derive the full set of equations of motion, and then we obtain spherically symmetric solutions and discuss their properties. We also obtain solutions for the Friedman-Lemaitre-Robertson-Walker cosmological metric.
    As you can see, my comments are brief. The importance of these papers overrule my comments.


    Ps. I was allowed in the group picture. :-)
  16. Aug 26, 2009 #15


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    This morning's session consisted of the presentation of 5 different models/approaches.

    Everyone has studied for years to master their particular approach. It took up a lot of energy.

    I was watching the crowd ... the reaction ... polite, quiet, but not attentive.

    humm...humm ... As if to say yah, yah, sorry, but unless you are announcing a major discovery, then i will not spend the energy to learn the details of your approach.

    It will be up to the "leaders" to come up with new papers on applying their model/approach to solve "How the universe is made" in such away to make everyone take notice.

    Being a good saleman is a definite asset.

    If you are a regular reader at this forum, then you are already aware of those approaches.
  17. Aug 26, 2009 #16


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    Please let us know where we can find digital feeds if any are available. I would like to see a good deal of this. Thanks, and I hope you're having a ball there.
  18. Aug 26, 2009 #17


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    The organizers have, once again asked that the speakers to hand in their presentations so that they can be put online. (see link to the web page)

    I found that most of what they talked about was already in arxiv.org
    I'm having a ball. The guys and gals are great.

    I'm the only, "non-affiliated" participant.

    Since, I assume, that this is not a usual occurrence for small conference, I do not want to "spoil the waters" for future conferences for other "interested amateurs".
  19. Aug 26, 2009 #18


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    jal, thanks for the insights. I hope the presenters and organizers will post some links where we can get the whole thing on-line,
  20. Aug 28, 2009 #19


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    Day #4

    Today was about “analogue”. To put it another way, what kind of experiment setup could be made that would help to demonstrate the mathematic and or even simulate the mathematic (eg. Black holes, white holes, Hawking radiation etc. )

    Ralf Schutzhold gave me a copy of his presentation which I wanted to present here but … I can’t make it happen. He has some good links that I wanted to share with you.

    We will just need to wait for the organizers to put them up on their web page.

    Xiao-Gang Wen presented his Qbit model. (http://arxiv.org/abs/0907.1203 ) Emergence of helicity +/- 2 modes (gravitons) from qbit models.

    It’s a way of representing space as a collection of qbits. If there is no qbits there is no space. Empty space (vacuum) are the ground state of the qbits.
    There was a wonderful dinner last night which was attended by all who did not have to leave due to other obligations.

    Due to previous commitment I will not be attending Friday, hum that’s today,

    I’ll post or someone will post when the papers have been put up on the web site.
  21. Aug 28, 2009 #20


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    Thanks Jal! It's been great to have reports from someone right there at the conference. I'm looking forward to seeing some of the talks when they are put on line. I forget, are they posting video? Or will we just see PDF of the slides?
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