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Quantum gravity workshop at perimeter institute

  1. Nov 6, 2004 #1

    john baez

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    This quantum gravity workshop at the Perimeter Institute was
    really cool:

    http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/

    It was their first conference in the new building. I'm writing an
    issue of This Week's Finds about some things I learned that made me
    much more optimistic about quantum gravity! But for now you can
    look at this talk I gave, which reflects the more pessimistic mood I
    had going into this conference - it focused on problems rather than solutions:

    The problem of dynamics in quantum gravity
    http://math.ucr.edu/home/baez/dynamics/
     
  2. jcsd
  3. Nov 6, 2004 #2

    marcus

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    It is really great that TWF #208 is gonna be about some things at
    the October "Quantum Gravity in the Americas" conference.

    In case someone reading has not seen the line-up of talks
    http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/participants.php

    the thing that struck me immediately is all the young people.

    the original two organizers were two postdocs Florian Girelli and Etera Livine, who have probably never worn ties in their life. The third organizer, T.T., was in Germany at the time and joined in after the two postdocs got it rolling, or so it seemed to me.

    and a large percentage of the talks were by postdocs and the like, rather than the seasoned veterans (as one may call them)

    cool is a well-chosen word for this conference (accuracy the first principle of good writing---Baez always exemplary)

    I notice these in the line-up:

    Freidel
    Particles in 3d quantum gravity

    Oriti
    Feynman propagator in spin foam QG: causality without time

    Pullin
    Semi-discrete solution to the dynamics of LQG

    Sahlmann
    String Theory with LQG methods

    Hanno Sahlmann's paper, if and when published, will hopefully have the beneficial side-effect of outraging Jacques Distler and prompting torrents of invective from Lubos Motl, who (with any luck) will busy themselves finding reasons to discredit it---as happened with T.T.'s Loop-String paper. Building bridges is threatening, it turns out.

    Pullin's talk is probably not too different from what GP just posted on arxiv.
    Gambini Pullin: Consistent discretization and loop quantum geometry
    http://arxiv.org/gr-qc/0409057

    Oriti's talk has the same title as what he just posted.
    Daniele Oriti: The Feynman propagator for spin foam quantum gravity
    http://arxiv.org/abs/gr-qc/0410134

    I wonder if Freidel's talk has something to do with the foam-perturbation that JB was talking about. Looked for recent Freidel papers but could not find any obvious tie-in with what JB mentioned.
     
    Last edited: Nov 6, 2004
  4. Nov 6, 2004 #3

    marcus

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    Getting in the mood for JB words on the "Quantum Gravity in the Americas" conference, I was thinking about his report (TWF #206) from the May 2004 Marseille Loop-Foam conference that Rovelli and friends organized.

    Too often there is part of the atmosphere that one doesnt get because one simply does not know the people and cant visualize.

    So I want to post these snapshots gleaned from Perimeter website, of what it looked like at the May 2004 conference. This is of people talking, mostly at the commons room, mostly just informal. But there is a shot of
    Julian Barbour going for a hike with Renate Loll and Don Marolf---things like that. So these are what some Quantum Gravity folk look like, and many of the same ones were at the October QG in A conference:

    at the May 2004 Marseille conference

    Carlo Rovelli
    http://perimeterinstitute.ca/images/marseille/marseille008.JPG
    Rodolfo Gambini
    http://perimeterinstitute.ca/images/marseille/marseille010.JPG
    Renate Loll ( leafy head-dress)
    http://perimeterinstitute.ca/images/marseille/marseille011.JPG
    Etera Livine (white gym-shirt)
    http://perimeterinstitute.ca/images/marseille/marseille015.JPG
    Martin Bojowald and Alejandro Perez
    http://perimeterinstitute.ca/images/marseille/marseille017.JPG
    Phillipe Roche
    http://perimeterinstitute.ca/images/marseille/marseille031.JPG
    Luisa Doplicher
    http://perimeterinstitute.ca/images/marseille/marseille038.JPG
    Ted Jacobson and Lee Smolin
    http://perimeterinstitute.ca/images/marseille/marseille041.JPG
    Thomas Thiemann
    http://perimeterinstitute.ca/images/marseille/marseille044.JPG
    Dans Christensen and Oriti
    http://perimeterinstitute.ca/images/marseille/marseille046.JPG
    (Oriti's Cambridge homepage: http://www.damtp.cam.ac.uk/user/do001/)
    Jerzy Kowalski-Glikman and Fotini Markopoulou at the seacoast near Marseille
    http://perimeterinstitute.ca/images/marseille/marseille059.JPG
    Jerzy Lewandowski
    http://perimeterinstitute.ca/images/marseille/marseille075.JPG
    Julian Barbour, Renate Loll, and Don Marolf
    http://perimeterinstitute.ca/images/marseille/marseille103.JPG
    Hanno Sahlmann
    http://perimeterinstitute.ca/images/marseille/marseille115.JPG
    Abhay Ashtekar
    http://perimeterinstitute.ca/images/marseille/marseille121.JPG
    John Baez
    http://perimeterinstitute.ca/images/marseille/marseille123.JPG
    Jerzy Kowalski-Glikman and Joao Magueijo
    http://perimeterinstitute.ca/images/marseille/marseille130.JPG
    Viqar Husain
    http://perimeterinstitute.ca/images/marseille/marseille146.JPG

    BTW if anyone sees a mistaken identification please let me know, either
    by post or by private message (there is a PM thing to click at upper right of screen)
     
    Last edited: Nov 7, 2004
  5. Nov 7, 2004 #4

    marcus

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    TWF #208 is out!

    TWF 208 has a report from the QG conference 29-31 October at Perimeter

    http://math.ucr.edu/home/baez/week208.html

    ---sample quote---
    The basic idea is to treat 4d general relativity with positive cosmological constant as a perturbation of a topological quantum field theory. The topological theory has a single state, which corresponds to a quantum version of "deSitter space": an exponentially expanding universe similar to the one we see today, but with no matter. To calculate in full-fledged gravity, we then use perturbation theory, getting answers as power series in a coupling constant. But the cool part is that unlike ordinary perturbative quantum gravity this perturbation theory is manifestly diffeomorphism invariant term by term. And each term is a sum over spin foams!

    Even better, the coupling constant in this theory is the cosmological constant in Planck units! That's an incredibly small dimensionless number: about 10-123. Physicists like perturbation theory when the coupling constant is small, since then the first few terms tend to give reasonably accurate answers - even if the whole series diverges. For example, quantum electrodynamics gives high-precision answers because the fine structure constant is about 1/137, which is pretty small. But 10-123 is really small.

    I'd seen Starodubtsev talk about this in Marseille (see "week206") but now he and Freidel have done calculations recovering Newton's law of gravity in an appropriate approximation from this theory. That may not seem like a big deal, but it's actually very cool to see Newton's law reemerge from a manifestly diffeomorphism-invariant theory of quantum gravity: no model had ever managed this feat before...
    ---end quote---

    I bolded a few words for emphasis in the quoted passage
     
    Last edited: Nov 7, 2004
  6. Nov 8, 2004 #5

    marcus

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    In that list of snapshots, one of the identifications is wrong.
    That is not Hanno Sahlmann. Sorry Hanno and the other person, for any confusion. Can anyone else supply corrections, if any are needed.
     
  7. Nov 8, 2004 #6
    Hi,

    There are some mistakes in the people that is supposed to be in the photos.
    -The lady at the photo is not Luisa Doplicher, but K. Giesel.
    - Jacobson is not with Smolin, but with J. Barret.
    -That is not Thiemann, but Krasnov
    -Gikman is not with Magueijo (I don´t know the guy at the photo)
     
  8. Nov 8, 2004 #7

    marcus

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    nonunitary, thanks for the help! I will make the corrections you suggest.
    Here is the corrected list:


    Carlo Rovelli
    http://perimeterinstitute.ca/images/marseille/marseille008.JPG
    Rodolfo Gambini
    http://perimeterinstitute.ca/images/marseille/marseille010.JPG
    Renate Loll ( leafy head-dress)
    http://perimeterinstitute.ca/images/marseille/marseille011.JPG
    Etera Livine (white gym-shirt)
    http://perimeterinstitute.ca/images/marseille/marseille015.JPG
    Martin Bojowald and Alejandro Perez
    http://perimeterinstitute.ca/images/marseille/marseille017.JPG
    Phillipe Roche
    http://perimeterinstitute.ca/images/marseille/marseille031.JPG
    Kristina Giesel
    http://perimeterinstitute.ca/images/marseille/marseille038.JPG
    Ted Jacobson and John Barrett
    http://perimeterinstitute.ca/images/marseille/marseille041.JPG
    Kirill Krasnov
    http://perimeterinstitute.ca/images/marseille/marseille044.JPG
    Dan Christensen and Daniel Oriti
    http://perimeterinstitute.ca/images/marseille/marseille046.JPG
    (Oriti's Cambridge homepage: http://www.damtp.cam.ac.uk/user/do001/)
    Jerzy Kowalski-Glikman and Fotini Markopoulou at the seacoast near Marseille
    http://perimeterinstitute.ca/images/marseille/marseille059.JPG
    Jerzy Lewandowski
    http://perimeterinstitute.ca/images/marseille/marseille075.JPG
    Julian Barbour, Renate Loll, and Don Marolf
    http://perimeterinstitute.ca/images/marseille/marseille115.JPG
    Abhay Ashtekar
    http://perimeterinstitute.ca/images/marseille/marseille121.JPG
    John Baez
    http://perimeterinstitute.ca/images/marseille/marseille123.JPG
    Jerzy Kowalski-Glikman
    http://perimeterinstitute.ca/images/marseille/marseille130.JPG
    Viqar Husain
    http://perimeterinstitute.ca/images/marseille/marseille146.JPG

    BTW still if anyone else sees other mistaken identifications please let me know, either by post or by private message.
     
    Last edited: Nov 8, 2004
  9. Nov 9, 2004 #8

    john baez

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    Thanks! As you noticed, it's out now; one can read it here:

    http://math.ucr.edu/home/baez/week208.html

    Heh. I wore one once, when I went to a wedding. I bought it in the airport.

    I've never seen him in a tie either. Ashtekar is the only really dignified guy in this crowd: he always dresses impeccably, in a suit and tie. Someone has to get the NSF to fund this stuff, after all. But don't get me wrong: he's cool too. :cool: Heck, he got the whole game started!

    Freidel is working with a smart bushy-haired young guy named Artem Starodubtsev on this stuff, but they haven't come out with any joint papers yet. Don't underestimate Starodubtsev's role in this: he has been thinking about spin foam perturbation theory for some time now. You can find links to his two most relevant papers here:

    http://math.ucr.edu/home/baez/dynamics.html

    Most relevant of is the one he wrote with Smolin - that's where they came up with a Lagrangian of GR that's a perturbation of a Lagrangian for a topological theory whose only state is a quantum version of deSitter spacetime.

    (DeSitter spacetime describes a universe that's exponentially expanding thanks to a positive cosmological constant. That's like our unvierse. The string theorists have been spending most of their time thinking about anti-deSitter spacetime, which has a negative cosmological constant. Whoops.)

    I'll try to answer some more questions about this stuff when I get time... gotta run!
     
  10. Nov 9, 2004 #9

    marcus

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    I will fetch links to the two Artem papers which John Baez mentioned and try to say something about them, in case we could bring them into the discussion here or at least know of them


    Topological excitations around the vacuum of quantum gravity I: The symmetries of the vacuum
    Artem Starodubtsev
    17 pages
    http://arxiv.org/hep-th/0306135


    "This is the first paper in a series in which an attempt is made to formulate a perturbation theory around the the Chern-Simons state of quantum gravity discovered by Kodama. It is based on an extension of the theory of 't Hooft Deser and Jackew describing point particles in 3D gravity to four spacetime dimensions. General covariance now requires the basic excitations to be extended in one spatial dimension rather than pointlike. As a consequence the symmetry of the Kodama state, which is the (anti)deSitter symmetry, is realized 'holographically' on a timelike boundary, the generators of this symmetry being related to quasilocal energy-momenta. As GR induces a Chern-Simons theory on the boundary, the deSitter symmetry of the vacuum must be q-deformed with the deformation parameter related to the cosmological constant. It is proposed to introduce excitations around this vacuum by putting punctures on the boundary to each of which is associate a vector in some representation of deSitter group projected to the boundary. By equations of motion those punctures must be connected by continuous lines of fluxes of an SO(3,1) gauge field. It is also argued that quasilocal masses and spins of these excitations must satisfy a relation of Regge type, which may point on a possible relation between non-perturbative quantum gravity and string theory."


    General relativity with a topological phase: an action principle
    Lee Smolin, Artem Starodubtsev
    8 pages
    http://arxiv.org/hep-th/0311163

    "An action principle is described which unifies general relativity and topological field theory. An additional degree of freedom is introduced and depending on the value it takes the theory has solutions that reduce it to
    1) general relativity in Palatini form,
    2) general relativity in the Ashtekar form,
    3) [tex]\inline{F\wedge F}[/tex] theory for SO(5) and
    4) BF theory for SO(5).
    This theory then makes it possible to describe explicitly the dynamics of phase transition between a topological phase and a gravitational phase where the theory has local degrees of freedom. We also find that a boundary between a dynamical and topological phase resembles an horizon."
     
    Last edited: Nov 10, 2004
  11. Nov 9, 2004 #10

    marcus

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    Well, this is not about Artem's papers----but about some other fall-out from the conference.
    One of the poster sessions, given by Kevin Vandersloot, was about some work done with Karim Noui and Alejandro Perez on the Hilbert space of Loop Quantum Cosmology, and their paper just came out today

    The poster session was titled: A model of loop quantum cosmology and its physical Hilbert space

    The preprint on arxiv that appeared today is:
    On the Physical Hilbert Space of Loop Quantum Cosmology
    http://arxiv.org/gr-qc/0411039

    "In this paper we present a model of Riemannian loop quantum cosmology with a self-adjoint quantum scalar constraint. The physical Hilbert space is constructed using refined algebraic quantization. When matter is included in the form of a cosmological constant, the model is exactly solvable and we show explicitly that the physical Hilbert space is separable consisting of a single physical state. We extend the model to the Lorentzian sector and discuss important implications for standard loop quantum cosmology."


    ------
     
    Last edited: Nov 9, 2004
  12. Nov 11, 2004 #11

    john baez

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    Thanks for pointing out these pictures, Marcus! I hadn't seen these shots of my pals.

    There are just a few errors, I think:

    These are actually Daniel Oriti and Alejandro Perez, from left to right.
    Here's a picture of Dan Christensen:

    http://perimeterinstitute.ca/images/marseille/marseille117.JPG

    This is actually a shot of some grad student whose name I forget right
    now. I don't see a shot of Barbour, Loll and Marolf.

    Also, Renate Loll is not really wearing a leafy headdress in this shot...

    http://perimeterinstitute.ca/images/marseille/marseille011.JPG

    That's not her style. It's just a plant on the window sill behind her head. (Maybe you knew that, in which case I apologize for my missing sense of humor.)

    There are a few more pictures from Marseille on this webpage:

    http://math.ucr.edu/home/baez/calanque/

    but I knew Lee Smolin had taken a whole bunch more, and I guess you've found them!
     
  13. Nov 11, 2004 #12

    marcus

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    thanks for the help!
    Here is the corrected list:


    Carlo Rovelli
    http://perimeterinstitute.ca/images/marseille/marseille008.JPG
    Rodolfo Gambini
    http://perimeterinstitute.ca/images/marseille/marseille010.JPG
    Renate Loll (leafy head-dress, just kidding :))
    http://perimeterinstitute.ca/images/marseille/marseille011.JPG
    Etera Livine (white gym-shirt)
    http://perimeterinstitute.ca/images/marseille/marseille015.JPG
    Martin Bojowald and Alejandro Perez
    http://perimeterinstitute.ca/images/marseille/marseille017.JPG
    Thomas Thiemann talking with Renate Loll
    http://perimeterinstitute.ca/images/marseille/marseille028.JPG
    Phillipe Roche
    http://perimeterinstitute.ca/images/marseille/marseille031.JPG
    Kristina Giesel
    http://perimeterinstitute.ca/images/marseille/marseille038.JPG
    Ted Jacobson and John Barrett
    http://perimeterinstitute.ca/images/marseille/marseille041.JPG
    Kirill Krasnov
    http://perimeterinstitute.ca/images/marseille/marseille044.JPG
    Daniel Oriti and Alejandro Perez
    http://perimeterinstitute.ca/images/marseille/marseille046.JPG
    (Oriti's Cambridge homepage: http://www.damtp.cam.ac.uk/user/do001/)
    Jerzy Kowalski-Glikman and Fotini Markopoulou at the seacoast near Marseille
    http://perimeterinstitute.ca/images/marseille/marseille059.JPG
    Jerzy Lewandowski
    http://perimeterinstitute.ca/images/marseille/marseille075.JPG
    Julian Barbour, Renate Loll, and Don Marolf
    http://perimeterinstitute.ca/images/marseille/marseille103.JPG
    Dan Christensen
    http://perimeterinstitute.ca/images/marseille/marseille117.JPG
    Abhay Ashtekar
    http://perimeterinstitute.ca/images/marseille/marseille121.JPG
    John Baez
    http://perimeterinstitute.ca/images/marseille/marseille123.JPG
    Jerzy Kowalski-Glikman
    http://perimeterinstitute.ca/images/marseille/marseille130.JPG
    Viqar Husain
    http://perimeterinstitute.ca/images/marseille/marseille146.JPG

    thanks to J.B. and nonunitary for correcting my mistakes, if anyone else sees other mistaken identifications please let me know.
     
    Last edited: Nov 11, 2004
  14. Nov 11, 2004 #13

    marcus

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    AFAIK we havent had any comment on Hanno Sahlmann's paper. And I havent seen anything in arxiv that I could recognize as foreshadowing it.
    I will get a picture-link for Hanno.

    Here is Hanno Sahlmann's homepage at AEI-Gölm
    http://www.aei.mpg.de/cgi-bin/interface/people.cgi?key=sahlmann

    While we are at it, here is Thomas Thiemann's AEI-Gölm homepage
    http://www.aei.mpg.de/cgi-bin/interface/people.cgi?key=thiemann

    both of their AEI pages have snapshots
    both Sahlmann and Thiemann have researched the possibility of applying LQG methods to string----one way of getting explicit background independence into string theory, something that has been discussed a lot (by string people from Witten on down) and which would have major repercussions on the theory, I would suppose.
     
    Last edited: Nov 11, 2004
  15. Nov 11, 2004 #14

    marcus

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    Oriti's causal spin foams

    Back in post #2 of this thread I mentioned Oriti paper about causal spinfoam models.

    this could be one of the most exciting developments showing up at this conference

    It will take some work for me to understand---and only then will I be able to form my own opinion about if it is important.

    Oriti has defined a new class of spinfoam models. It involves putting some extra labels on the edges and vertices. this is a very short paper---only 5 pages. how hard can it be? but it is new, so it will take some work.

    I am working on the copy I printed out, with pencil and highlighter felt-tip.

    Here is a picture of Daniele Oriti, John Barrett, and Julian Barbour.
    http://perimeterinstitute.ca/images/marseille/marseille019.JPG


    Maybe causality is more basic than time and
    time should only emerge at macroscopic scale in a model which has causality built in at the ground level.
     
    Last edited: Nov 12, 2004
  16. Nov 14, 2004 #15

    marcus

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    There is a tension between two allied approaches:

    On the one hand, Loop Gravity and Loop Cosmology (loop cosmology is a specialized application of the full LQG----more calculation has been possible and possibly more definite results, than in the full theory)

    On the other hand, Spin Foam models.

    ===========
    Back some 5 years ago, Loop was encountering some difficulty with describing its dynamics, by means of a hamiltonian constraint.
    this led people to shift over to the then new Foam approach. Baez was
    important in initiating this line of research.

    The foam approach has been having a real struggle getting to the point
    of calculating stuff. The objects are purely combinatorial and even with supercomputers you can get bogged.

    So now it looks as if the pendulum has swung back and the original Loop approach is making headway----not just in the specialized application to cosmology, which has been doing great all along, but also in the full theory.

    One sign of this is recent Gambini, Pullin, Porto papers about
    "consistent discretization"

    Another sign is recent work by Bianca Dittrich and Thomas Thiemann.


    Eventually it is probably win-win. But for now I feel a tension.
     
  17. Nov 15, 2004 #16
    Did not J Barbour 'admit' the concept of 'Time' had defeated all of his motions of understanding it?..he produced a number of papers for the "Removal" of Time from all of Physic's..or maybe it was just Cosmological Models?

    Maybe it was J Barbour who was at fault?..not a criticism of one that I do admire..just a thought?

    Time waits for no Man?
     
  18. Nov 17, 2004 #17

    marcus

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    Oriti's causal spin foams again----possible connection to AJL

    A possible importance attached to Daniele Oriti's paper that he gave at the late October QG conference at Perimeter is that...according to his argument (which makes sense to me, at least)...the version of spinfoam that he has invented (maybe with Livine) is the only version that is causal----and therefore, I surmise, would connect with AJL causal dynamical triangulations.

    that they are doing Monte Carlo computer runs and getting statistics on the random evolution of the geometry of the universe.

    there are currently 4 or more closely allied approaches that are pursuing Quantum Gravity

    Canonical LQG (Ashtekar, Rovelli, Smolin, Thiemann, ...many others)

    Consistent discretizations + LQG (Gambini, Pullin, Porto,...others)

    Spin Foam (Oriti, Baez, Christensen, Livine,....many others)

    Causal Dynamical Triangulations (Jan Ambjorn, Renate Loll, et al)

    Hawking's approach (Hawking)

    This may be a very haphazard incomplete list. It is not clear to me that Hawking's approach should be included because I thought the people at Cambridge gave up on it some 10 years ago and it was not an active line of research.

    now it seems that not uncommonly things happen in real-world research that people were not anticipating
    and it now looks like the AJL approach Causal Dynamical Triangulations may have hit the jackpot

    And in the process it sort of dug up the buried Hawking approach, because AJL looked at their computer output and noticed that their random evolutions of cosmic geometry seem to be following the Lagrangian that Hawking was using back in early 1990s.

    Right! Now which of you blokes forgot the stake of mistletoe?

    And besides that, dynamical triangulations looks very much like spinfoam. Both are like Feynmannian path integrals except for geometry instead of particles.

    So since spinfoam is a close relative of dynamical triangulations (aka Simplicial Gravity because it uses little 4D analogs of tetrahedrons called four-symplexes)-----since spinfom and "dynamations" are alike---the big question is:

    What kind of spinfoam is like CAUSAL dynamations?

    Both Oriti in his latest, and AJL in their latest, make a big point of integrating forwards, and not backwards. they have some causal direction in their model.

    It seems like Oriti's new type of spin foam because it is causal---has some orientation or order or extra structure-----it might be more closely connected to Ambjorn Jurkiewicz Loll simplicial model, which ALSO has this element of extra order or structure.

    something to check out
     
    Last edited: Nov 17, 2004
  19. Nov 17, 2004 #18

    marcus

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    Whoever wants to check connections between the two approaches has, i guess, to look for analogy between these three papers:

    Daniele Oriti
    The Feynman propagator for spin foam quantum gravity
    http://arxiv.org/abs/gr-qc/0410134

    J. Ambjorn, J. Jurkiewicz, R. Loll
    Emergence of a 4D World from Causal Quantum Gravity
    http://arxiv.org/hep-th/0404156

    Semiclassical Universe from First Principles
    J. Ambjorn, J. Jurkiewicz, R. Loll
    15 pages, 4 figures
    http://arxiv.org/abs/hep-th/0411152

    Abstract:
    "Causal Dynamical Triangulations in four dimensions provide a background-independent definition of the sum over space-time geometries in nonperturbative quantum gravity. We show that the macroscopic four-dimensional world which emerges in the Euclidean sector of this theory is a bounce which satisfies a semiclassical equation. After integrating out all degrees of freedom except for a global scale factor, we obtain the ground state wave function of the universe as a function of this scale factor."

    =========
    It looks to me like AJL are stabilizing their nomenclature
    they are calling what they do "causal dynamical triangulations"

    over the years research in this direction has been called various things: simplicial gravity, simplicial quantum geometry, quantum gravity---sometimes Lorentzian instead of causal---as in
    "Lorentzian dynamical triangulations".

    =====
    the idea is simple

    they gamble with the geometry of the universe

    or more precisely with the whole history of the geometry of the universe
    =======

    imagine you have 200,000 small tetrahedra---little 3sided pyramids---jumbled into a box

    and there are coils so you can suddenly turn on a magnetic field in the box and make the tets want to stick together

    so you say "Tets, cohere!" and you throw the switch and they all jump up out of their heap and fit together (triangular side to triangular side, vertex touching vertex) and this gives you a random geometry

    and as an experiment, you do that over and over again and photograph the result each time-----so eventually you get an idea of the probabilities of various configurations and an idea of average properties etc.
    ======
    they have an idea of causal ordering, so they only allow a certain kind of assembly.
    so their game is not quite as free as what i described. but it is a game and falls in the category of "Monte Carlo methods"

    =======
    Notice that a 3sided pyramid has FOUR triangular faces because you have to count the bottom too. so it is called a Tet (to indicate the four-ness)

    A triangle is what you use to triangulate a 2D surface
    A tetrahedron is what you use to triangulate a 3D region
    What is it called that you use to triangulate a 4D spacetime

    what is the analog of a tetrahedron in 4D? It could be called a "pent" because it has
    FIVE faces (each of which is a tetrahedron) and pent indicates five-ness.
    Technically it is called a 4-simplex. and a 4-simplex has 5 faces, each of which is a 3D solid, namely a tetrahedron.

    So if you want to play the AJL game in 4D then you have to have a heap of "pents" in a box. you have to have a heap of 4-simplexes.

    A computer finds it very easy to imagine a 4-simplex. But I do not. therefore I imagine them as little tetrahedra----a bit like hard cream-colored plastic dice.

    AJL use large numbers of them: 100 thousand or 200 thousand, the most this time seems to have been 360 thousand

    When you say "Pents! Cohere!" they all jump together and form a random 4D shape.

    Then the computer can project that for you into lower dimension so you can have the experience of viewing it. Or it can calculate statistics of certain measures and features, and keep a record. then you do it over and over again and get an idea of the probabilities.
    =======

    and AJL have some extra rules in how they can fit together
    having to do with causality.

    =======
    So the question is HOW CLOSE IS Oriti spin foam model to this
    AJL Causal Dynamations "pent" random geometry game?
    Can you get similar results?

    AJL report a kind of Epiphany when a semiclassical Lagrangian for the scalefactor of the universe appeared to them out of the midst of Monte Carlo randomness. they didnt put it in.
    A familiar Lagrangian came to dinner uninvited. This is quite interesting.
    I didnt yet hear of anything like this happening with Spin Foam.
    So it seems like an urgent question to put to Daniele Oriti. Can he assimilate or link to anything in the AJL program?
     
    Last edited: Nov 17, 2004
  20. Nov 22, 2004 #19
    Here is a revised version of interest:http://uk.arxiv.org/abs/gr-qc/0406063
     
  21. Dec 8, 2004 #20

    marcus

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    the papers presented at that conference are gradually being posted on ArXiv.

    today there were two that appeared. by Daniele Oriti and by V.Husain and O.Winkler. In both cases the titles of the papers are virtually identical to those of the talks given at the Halloween conference at perimeter

    1.
    http://arxiv.org/gr-qc/0412035
    The Feynman propagator for quantum gravity: spin foams, proper time, orientation, causality and timeless-ordering
    Daniele Oriti
    8 pages; to appear in the Proceedings of the DICE 2004 Workshop "From Decoherence and Emergent Classicality to Emergent Quantum Mechanics"

    "We discuss the notion of causality in Quantum Gravity in the context of sum-over-histories approaches, in the absence therefore of any background time parameter. In the spin foam formulation of Quantum Gravity, we identify the appropriate causal structure in the orientation of the spin foam 2-complex and the data that characterize it; we construct a generalised version of spin foam models introducing an extra variable with the interpretation of proper time and show that different ranges of integration for this proper time give two separate classes of spin foam models: one corresponds to the spin foam models currently studied, that are independent of the underlying orientation/causal structure and are therefore interpreted as a-causal transition amplitudes; the second corresponds to a general definition of causal or orientation dependent spin foam models, interpreted as causal transition amplitudes or as the Quantum Gravity analogue of the Feynman propagator of field theory, implying a notion of ''timeless ordering''."
    *

    2.

    http://arxiv.org/gr-qc/0412039
    Quantum black holes
    Viqar Husain, Oliver Winkler
    4 pages

    "Using a recently developed quantization of spherically symmetric gravity coupled to a scalar field, we give a construction of horizon operators that allow a definition of general, fully dynamical quantum black holes. These operators capture the intuitive idea that classical black holes are defined by the presence of trapped surfaces, that is surfaces from which light cannot escape outward. They thus provide a mechanism for classifying quantum states of the system into those that describe quantum black holes and those that do not. We find that quantum horizons fluctuate, confirming long-held heuristic expectations. We also give explicit examples of quantum black hole states. The stage is thus set for addressing all the puzzles of black hole physics in a fully quantized dynamical setting."
    *
     
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