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Self-organizing quantum universe explained in July SciAm feature

  1. Jun 18, 2008 #1

    marcus

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    This is a good accessible introduction to one of the foremost quantum gravity approaches.
    http://www.scribd.com/doc/3366486/SelfOrganizing-Quantum-Universe-SCIAM-June-08

    It is a feature article in the July 2008 print issue of SciAm----pages 42-49.
    But SciAm put it out online already in June, so in this version it is dated June.

    This "scribd" version is set up so you cannot print it. You can only read it on the screen.
    But they make it easy. Click on "full screen" button, and on the enlarger "+" button to make it easy to read. The graphics help. The writing is for general audience and communicates effectively.

    This is the most efficient brief explanation of Renate Loll's causal dynamical triangulations approach that I have seen so far.
     
  2. jcsd
  3. Jun 18, 2008 #2

    marcus

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    the article mentions the physics of flocking
    in what I think is an enlightening analogy
    some readers may remember the cover of this October 2007 issue of Physics Today
     

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    Last edited: Jun 18, 2008
  4. Jun 18, 2008 #3

    marcus

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  5. Jun 18, 2008 #4

    MTd2

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    Marcus, use your imagination, please.

    Using Loll's formalism, do you think it is possible to define a general even horizon? For example, since the dimensions are free to very, an external observer sees the horizon as a 2-sphere surface made of triangles, right?
     
  6. Jun 18, 2008 #5

    marcus

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    How so? What size would the triangles be? In the Loll picture, the triangles do not exist in nature and they have no minimum size. They explain this fairly well in the SciAm article, for general audience. I thought you had read several of their papers.
     
  7. Jun 18, 2008 #6

    MTd2

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    I was not talking about the sizes of any triangles or anything. I was thinking about the spectral dimention, on page 49. And outside observer sees the horizon of a black hole as a 2-sphere. Would that make the spectral dimention fall to 2 exactly on the horizon?

    I am spaculating on new ideas.
     
  8. Jun 18, 2008 #7

    MTd2

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    Oh, I reread my 1st post... I didn't mean to be agressive, I just wanted to make an invitation for reflection... I'm sorry.
     
  9. Jun 19, 2008 #8

    Fra

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    My understanding of this is, that if the overall idea is to try to construct the optimum measure of expectations of possibilities based on given information. Choosing the right or the wrong construction is of utility or loss for the observer.

    So we need

    1) a general rating scheme (a logic of howto construct a predictive measure on possible new observations)
    2) we need a way to consruct the set of possibilities in the first place
    3) a way to assign some kind of weight to each possibility in relation to (1)

    The feynmann superposition path integral supposedly is a possible idea on (1) + (3). Wether it's the fundamentally correct one, is still open as I see it. But this is not questioned in the CDT approach as I see it.

    They do attacK 2 though, and they suggest that, unlike the euclidian style possibilities, only the set of possibilities that follow their construction of limit of the sets of - as per their reasoning - "causally glued triangulations". This consructions of theirs, generates in their argumentation the correct set of possibilities to apply the feynmann sum and EH action logic to.

    While I think it's interesting, I think all three points above need fundamental addressing. The logic implicit in the feynmann path integrals can't possible be unquestionable.

    Maybe in a spirit of their attempt to construct the set of valid possibilities (integration space), one could generalize this logic, and also find a way to construct from first principles a natural selection scheme and weighting to answer to (1) and (3)?

    Similarly to the idea that there are constraints on the generation of validly possible geometries, there may be constraints of the generation of validly possible actions and weighting rules?

    /Fredrik
     
  10. Jun 19, 2008 #9

    marcus

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    No problem! I appreciate your help. constructive comment, including critical, is essential. We need more dialog, not less!
     
  11. Jun 19, 2008 #10

    marcus

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    I know only one CDT black hole paper (Loll and Dittrich) and it does not get very far. Black holes is an area where the CDT progress is slow, compared to some others.

    You are raising interesting questions.

    I think there is a possible research paper to write here. The CDT team has gotten deSitter spacetime to emerge as an average out of quantum confusion. Why should they not also be able to get Schwarzschild spacetime to emerge, by changing some conditions?----by putting central matter into the picture for example.

    So far they are doing their computer simulations of universes with only dark energy but no matter. They need to start doing computer runs with matter included in the picture. (Or maybe they have already started doing this but have not yet results to report.)

    You asked about the event horizon. I do not remember if the Loll/Dittrich paper had a CDT representation of the event horizon. I think it may have. But the paper was several years back and I don't remember clearly.
     
  12. Jun 19, 2008 #11

    MTd2

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    I may have a proposal of line of research to tackle this problem ... But I am afraid of developing this because I was called crackpot somewhere else... I promise that I am not trying to indulge in prepostorous ideas... I merely want to check the premises... :(

    So, instead of showing the idea, I must ask something before:

    Do you people, Marcus, et al., know a rigorous way to solve the Frozen Star paradox?

    http://www.mathpages.com/rr/s7-02/7-02.htm

    There is a thread here on this forum, https://www.physicsforums.com/showthread.php?t=132207&page=2 , but I am not convinced by anything posted there.

    EDIT.: An other thread here.
     
    Last edited: Jun 19, 2008
  13. Jun 20, 2008 #12

    Fra

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    How about if you start from by formulating a question where most of us can find a common reference. Then argue what your view of the question is and your scientific strategy of howto solve it?

    As I understand the rules here, there is no rules against discussing open questions if the reasoning is scientifically sound. What's banned as I understand, is publising or elaborating full blown solutions to problems (regardless of right or wrong) if those aren't already published where professional researchers usually publish.

    So if you keep your full blown theories to yourself (or publish it elsewhere) and just discuss parts of your reflections that does connect to commonly acknowledged issues I don't see how that can not be allowed as it's part of the scientific, creative and educational process.

    I didn't understand the premises and your question, are reasoning withing classical GR or what is your starting point, and how can you formulate the question relative to your starting point?

    I think your probe for the event horizon is interesting too, and I have some personal reflections on this, similarly immature, but I am not sure if it's related to what your thinking about. My starting points is a relative information concept that's always centered around an observer that is responding and action to survive.

    As I see it, I associate to the general question of accumulation and formations of mass. What is the logic behind an observer increasing it's mass? how is mass formed? I choose to ask, how is confidence formed? How can certainty spontaneously form, out of uncertainty? How does money grow on the bank? :) Somehow a black hole is an intuitive association here to an observer who doesn't need to compromise. It doesn't need to throw something out, to consume new info. It just eats it all and grows. The logic behind that is interesting but I think it's difficuly to find a consistent concencus.

    Do you see a connection between this and CDT?

    /Fredrik
     
  14. Jun 23, 2008 #13

    marcus

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    MTd2, you were asking about the CDT picture of a black hole and I want to get back to that. There is a paper by Loll and Dittrich:
    http://arxiv.org/abs/gr-qc/0506035
    Counting a black hole in Lorentzian product triangulations
    B. Dittrich (AEI, Golm), R. Loll (U. Utrecht)
    42 pages, 11 figures
    (Submitted on 6 Jun 2005)

    "We take a step toward a nonperturbative gravitational path integral for black-hole geometries by deriving an expression for the expansion rate of null geodesic congruences in the approach of causal dynamical triangulations. We propose to use the integrated expansion rate in building a quantum horizon finder in the sum over spacetime geometries. It takes the form of a counting formula for various types of discrete building blocks which differ in how they focus and defocus light rays. In the course of the derivation, we introduce the concept of a Lorentzian dynamical triangulation of product type, whose applicability goes beyond that of describing black-hole configurations."

    CDT is still a fairly new approach---the first CDT paper was in 1998. They have just scratched the surface in a lot of areas. Only the first steps have been taken in studying black holes, as far as I can see. Regretfully, there is not much I can say in response to your question.
    =========================

    Something that MAY be of interest is their beginning to include matter in CDT models. A paper was just posted today on arxiv about that.
    http://arxiv.org/abs/0806.3506
    Shaken, but not stirred – Potts model coupled to quantum gravity

    "We investigate the critical behaviour of both matter and geometry of the threestate
    Potts model coupled to two-dimensional Lorentzian quantum gravity in
    the framework of causal dynamical triangulations. Contrary to what general
    arguments on the effects of disorder suggest, we find strong numerical evidence
    that the critical exponents of the matter are not changed under the influence of
    quantum fluctuations in the geometry
    , compared to their values on fixed, regular
    lattices. This lends further support to previous findings that quantum gravity
    models based on causal dynamical triangulations are in many ways better behaved
    than their Euclidean counterparts."
    ====================

    I think the point here is that the paper serves to gauge progress in the CDT approach. The big recent news was December 2007 when they got deSitter universe to emerge at large scale out of a microscopic chaos. No geometry (no smooth metric manifold) is put in at the beginning, just a swarm of microscopic components each interacting locally with its neighbors. And an overall smooth classic spacetime emerges as a quantum average. This is one of the goals of any background independent approach to QG. And its achievement in the CDT context was something of a first. That's covered in the SciAm article we have a link for.

    But notice that the work reported December 2007 and described in the SciAm has no matter in it. It is pure geometry, pure gravity. The deSitter universe is an ideal empty universe with nothing but "dark energy" in it, or in other words a positive cosmological constant. So the natural question was how are they going to follow it up by including matter?

    Indications are that there are several more papers in preparation. This one is a clue to how things are going.
    =====================
    Before 1998, one of the prominent approaches to QG was something promoted by Stephen Hawking, among other people, called Euclidean QG. And this was linked to work with non-causal dynamical triangulations. The CDT researchers see what they are doing as Lorentzian QG-----the earlier Euclidean approach but with a Lorentzian causal structure.
    The SciAm article goes into the history some, and explains this.

    It can be confusing that Lorentzian QG has the same initials as Loop QG. Have to watch out for that. At one point in this paper they abbreviate Lorentzian QG as LQG.

    Anyway the CDT group traces their history back to the approach used by Hawking and others in the 1980s and 1990s. They just found a way to make the earlier approach work better, in a sense.

    So part of this paper is making that point. Matter in a CDT context behaves right, more like in a regular lattice (even though the geometry can be highly irregular, and better than it behaves in the earlier non-causal Euclidean dynamical triagulations attempts.
     
  15. Jun 23, 2008 #14

    MTd2

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    She talks about 2d topology, but I think that is just a toy model to show the stability of the proposed space time, even without considering fixing a lattice. Above, I just meant that Hausdorff dimension would give you 2 dimensions, a 2 sphere, at the horizon, in a full 4d theory.
     
  16. Jun 24, 2008 #15

    xristy

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    I have been following the CDT work and have some questions:

    1) It seems that they use on the order of 100 - 300K elements (simplexes) in their simulations and that were the scale of the simplexes very short then they don't see a much volume - maybe not as much as a Planck volume - and may not see emergent effects; and on the other hand if the scale is larger then it may be too coarse (too few elements over a larger volume) to see interesting emergent effects.

    2) It doesn't seem like there is a thermodynamic element to the simulations that would represent the presumed cooling in the very earliest times of the universe that could exhibit any phase transitions. How would you associate a rate of interaction in an intrinsic manner with the CDTs?

    3) The CDT papers are essentially computational experiments. Is there enough information in the published papers to allow one to replicate the experiments?

    4) It has been mentioned on occasion that there is no matter in the CDT picture yet. This is somewhat puzzling. Isn't the idea that a picture of an emergent background of necessity includes the emergence of matter and the known forces and so on? In other words that the background and "stuff in the background" are just different facets of a single emerging phenomenon.

    Hopefully these are not totally idiotic questions. I was intrigued by the previous claims of emergence of large-scale 4d structure and have been trying to understand what the claims amount to.

    X
     
  17. Jun 24, 2008 #16

    nrqed

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    In the SciAm article they mention that they need to include a cosmological constant in thier simulation. Does anyone have a simple way to explain how a cosmological constant is implemented in CDT?
     
  18. Jun 24, 2008 #17

    marcus

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    Far from it, these are good questions!
    In nonstring QG the term "background" often just refers to a geometric background consisting of a smooth manifold with a metric (a distance function). Some approaches are perturbative in the sense that one puts in a standard geometric background at the start (like flat spacetime) and studies small variations of geometry----slight ripples on that background.
    So the answer to #4 is that in this, and most of the QG research I follow, "background" just refers to the geometric setup, it doesn't include specifying particles.

    #3 is about replication. I don't see why Loll's group wouldn't be willing to share their computer code.

    Loll's collaborators are spread out geographically----Athens, Tokyo, Reykjavik (Iceland), Crakow (Poland), Copenhagen,...
    Several of the people who are credited with computer work---doing the Monte Carlo runs---are at other universities besides Utrecht. So my guess would be that it would be natural for the CDT codes to be running at a number of different places. Not just at Utrecht.

    I don't know the answer for sure, though. Maybe someone knows and will tell us.

    #2 is about cooling. This is hard to answer because the published work does not yet include matter. Or does so only in a preliminary fashion in lower dimension models.
    The way the CDT researchers have proceeded in the past is to try every new result first in 2D and then work up in dimension, from 2D to 3D, from 3D to 4D.

    I assume that the inclusion of matter will follow the same pattern. They will study it in 2D for a couple of years and then extend to higher dimensions. (The first CDT papers, in 1998, dealt with the simplest 2D case----3D came in 2001 if I remember correctly.)

    Perhaps I'm wrong but meaningful results about cooling would seem to require the inclusion of matter.
    (A 2D+matter paper came out yesterday, by the way. Anagnostopolous et al. He's the CDT guy in Athens.)

    #1 is about scale. You can read about that yourself in Planckian Birth
    http://arxiv.org/abs/0712.2485
    They say the volume of their largest spacetimes is up to 173,000 planck volumes.

    The linear size would be up to about 28 planck lengths.

    Interestingly, they find evidence of semiclassical behavior already at linear scales of a few planck lengths. If their models are right, then space continues to act in a somewhat conventional way---as we expect it to act----at least in the quantum average----even down to scales of a few (on the order of ten) planck lengths!

    I have to go and do some other things, so can't respond completely to this. But this scale issue is really interesting. The problem seems to be how to to push the simulation down to sub-planck scales where highly unclassical stuff might be revealed. Some discussion of this in their recent papers.
     
  19. Jun 24, 2008 #18

    xristy

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    I'm somewhat puzzled here. It seems there is more at stake than just the geometric setup without particles. Per Smolin (hep-th/0507235) dependent is
    He goes on to say that in the independent view:
    In short, I thought that the over arching hypothesis is that there is effectively no distinction between the background and the particles emerging in the background. That is, particles and such are local configurations of the background. This is how I took the comment by Smolin and Wan (0710.1548):

    And in Ashtekar & Lewandowski (gr-qc/0404018):

    So this is the source of my wondering about the "absence" of matter in the CDT approach. As I understand the background independent quantum gravity program there should be an expectation that the CDT approach generates matter along with the spacetime itself. It seems that in the LQG approach and off-spring involving braids, ribbons and such there's some a priori content that isn't as minimal as CDT, but I fail to see how the CDT will give rise to anything more than a non-physical background unless the simplexes are endowed with a bit more content than just the causal primitive.
     
  20. Jun 24, 2008 #19

    Chronos

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    Two very important accomplishments pointed out in the sciam article is they derive

    - 4.02 spacetime dimensions under the
    . Hausdorff–Besicovitch [fractal] definition

    - a stable DeSitter space by inserting causality [arrow of time]
    . and a cosmological constant

    These results are not only astounding, but, robust. No 'background' is required [it is nonperturbative], few assumptions are made, and the assumption are consistent with observational evidence.
     
    Last edited: Jun 24, 2008
  21. Jun 25, 2008 #20

    Fra

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    I personally associate the issues of matter with dynamical actions. If the action is put in as a "background action" then it seems hard to see where relational actions will come from. If characterize particles in the way they interact, a particle might be seen as a kind of partly localized quasi stable action formation that responds as per a particular logic encoded in the action. These gives it's properties.

    Perhaps the CDT people can find a way to not only doing random walks as per a given selection rule, but a random walk where the section rules are also emergent in the same spirit by random walks in the space of selection rules. until the point there further choices can't be distinguished.

    Ie. they construct a sample space, and they take the path integral with the EH action, and they find emergent spacetimes that make sense.

    Wouldn't the same logic be applied to the path integral and the EH action itself? Could the EH-action itself be found by a similar principle?

    If we ask, how is spacetime constructed?
    Why not also ask, how are the "construction rules" constructed?

    Perhaps some construction rules, will imply emergent spacetimes, by the same logic some construction rules will also imply formation of particles? And if the construction of the construction rules find a common logic, so should spacetime and matter. And perhaps we could derived GR from even deeper first principles rather than put in manually the EH action.

    Perhaps such a construction will generate natural corrections or complementes to classical EH action, and this corrections will be found to be idenfitied with the expected particle phenomenology? Something lhow I would expect the extension of their program to incorporate matter. IF that can be done I think it will be really beautiful.

    /Fredrik
     
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