Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Where you going?

  1. May 26, 2005 #1

    wolram

    User Avatar
    Gold Member

    Where is loop quantum gravity going? im sure you few have left most in the
    dark.
     
  2. jcsd
  3. May 26, 2005 #2

    wolram

    User Avatar
    Gold Member

    It seems to me that LQG has more flavours than a flea can taste in a life time,
    Almost as spegetified as string theory.
     
  4. May 26, 2005 #3

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    no it only looks that way

    LQG is spaghettified, admittedly, but string is WAY more spaghettified


    LQG has about 100 researchers and still fairly coherent,
    they are working on similar sorts of things or a few varieties of things and
    you can get them all together in the same place once a year and they can still understand each other pretty much

    the actually mathematical theory or theories are SUPPOSED to change and adapt, as they explore new possibilities and improve. so "LQG" has no fixed definition except what LQG people are doing at the moment and call their field of research
     
  5. May 26, 2005 #4
    Also, apart from LQG, I wonder what will happen to string theory after the recent paper by Kachru which showed that the landscape has an infinite number of vacuums. Are there any other directions string theory can actually take, or is this just the signpost that it is really not valid anymore?
     
  6. May 26, 2005 #5

    wolram

    User Avatar
    Gold Member

    By Marcus.
    LQG has about 100 researchers and still fairly coherent,
    they are working on similar sorts of things or a few varieties of things and
    you can get them all together in the same place once a year and they can still understand each other pretty much.
    As you know i am skeptical ,but open to new ideas, but having read
    as much as i can," understanding some", i am lost as to the direction
    LQG is taking.
     
  7. May 26, 2005 #6

    wolram

    User Avatar
    Gold Member

    By the by i would love an audio
     
  8. May 26, 2005 #7

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    it is always a mistake to try to predict research, but since you ask I will try

    right now it is extremely urgent for LQG to link up with
    Renate Loll CDT-----the "triangulations" approach to quantum gravity that uses assemblages of "simplex" building blocks to approximate spacetime

    it possibly a year of crisis, and change, in LQG
    it will be fascinating to see how things sort out at the October 2005 conference
    there are even some fights brewing, or serious rivalries

    it has come time now for the full LQG theory to be applied to cosmology.
    the LQC of Martin Bojowald was a simplified version of the full theory (assuming the universe is uniform and looks roughly the same in all directions) and LQG got several dramatic results in the period 2001 to 2004.
    now they have to drop the simplifying assumptions and apply the full apparatus to cosmology and see if they can duplicate or modify those results. there may be fights (involving Bojowald and Thiemann) about this. but I think that Bojowald is basically a gentle mild personality who will not want to be quarreling, so it may not come out in the open.

    I think we can ignore the fracas over cosmology. it will follow well-established lines and come eventually to some satisfactory resolution with more in the way of testable predictions.

    What is much more explosive and unpredictable is the collision or merger between LQG and Loll's "triangulations" approach. this is even slightly scary to me.

    I think Loop people should make every possible effort to learn and assimilate CDT which I think has some new mathematics contained in it.

    sometimes the new mathematics comes from the humble applied grass roots and not from the monumental mathematically topheavy abstract oak trees.
    CDT is basically 3 people who found out how to run simulations of the universe.

    Bianca Dittrich has been Thomas Thiemann's righthand assistant for 2 years or so and has rendered him very valuable assistance on his Core-LQG program to construct official LQG dynamics. she is very smart. it would be a great blow to Thiemann to lose Dittrich as his assistant. But there are not enough smart people to go around. Suppose Bianca were to go over to the "Triangulations" group at Utrecht? Then there would be 4 CDT people

    Wolram, people go back and forth between core-LQG and CDT. The theories involve similar kinds of thinking (but some different symbols or formalism). It might not be too hard for them to assimilate each other. But I absolutely cannot see how this would happen, at the mathematical level. Only at the human level
     
    Last edited: May 26, 2005
  9. May 26, 2005 #8

    wolram

    User Avatar
    Gold Member

    What is much more explosive and unpredictable is the collision or merger between LQG and Loll's "triangulations" approach. this is even slightly scary to me.

    i have tried to follow this, but having just grasped some inkling of what
    spinfoams are, the math seems to take a quantum leap in some other
    direction, and left me, and im sure others gasping for breath.
     
  10. May 26, 2005 #9

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    all that means is you are trying to assimilate too much in a hurry.

    you already have some grasp of ordinary LQG and (you say) spin foams.


    "Triangulations" (Loll style) is a bit like spin foams but here is a difference.

    spin foams are mapped or projected or imbedded into some surrounding 4D continuum (technically a differentiable manifold, damn George Riemann for making up such a clumsy name for it)

    spinfoams are made of pieces (triangles and stuff) that "live" in some surrounding t,x,y,z space

    In Loll-style, the blocks dont live in a surrounding manifold. They ARE it. You use a whole lot of identical building blocks (actually two kinds, slightly different, think of them as male and females, but otherwise identical) and the assemblage of all these block IS the spacetime.

    technically there is a topological space R x S3 which the union of this assemblage of half a million blocks is supposed to equal, but the topological space has no differential structure, no calculus to it, just a formality.

    morally and intuitively the assemblage of glued together blocks is the space itself.

    and then two things happen
    1. you consider all the other ways the blocks could be glued together and you get this fantastic blur, this swarm of possible geometries. (they invented a "shuffling" process in the computer that imitates this blur)

    2. you imagine reducing the size of each block and increasing the number of blocks, and you make this quantum swarm of geometries, getting finer and finer, APPROXIMATE the real spacetime you want to know about.


    that is it,


    so the upshot is that WHEN IT COMES TIME TO CALCULATE you can accept a finite degree of approximation and use enough blocks of sufficiently small size and simply dont worry about going to the limit. You just calculate with some finite degree of precsion. And then, all you need to consider is this assemblage of blocks!
    And it turns out that Loll and the others figured out how to calculate with that assemblage like a sona*****, they can calculate stuff to beat the Dutch, oh my mistake, they ARE the Dutch.

    for this reason it is cannot be permitted for core-LQG not to make contact with this little project they have at Utrecht.
     
    Last edited: May 26, 2005
  11. May 26, 2005 #10

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    I have been gradually been working on understanding CDT since April 2004.
    You will just give yourself fits if you try to understand it too much in a hurry.

    Relax, it is fairly basic. It will soak in to the wolram brain, in time.

    I remember April last year when "Emergence of a 4D world from Causal quantum gravity" appeared
    http://arxiv.org/hep-th/0404156
    that was my first exposure to it.
    at that time John Baez had not even started calling attention to it (that was later, in May)

    It has been a year and I am still soaking it in. Not good to be impatient.
     
    Last edited: May 26, 2005
  12. May 26, 2005 #11

    wolram

    User Avatar
    Gold Member

    technically there is a topological space R x S3 which the union of this assemblage of half a million blocks is supposed to equal, but the topological space has no differential structure, no calculus to it, just a formality.

    A swarm going to infinity?
     
  13. May 26, 2005 #12

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    when mathematicians talk about the limit as something goes to infinity or goes to zero, what they PRACTICALLY mean is you take it large enough or small enough to get close to a realistic answer, and decide to be satisfied


    dont worry about mathematical formalities

    you take enough blocks, (right now they are up to a third of a million)
    or. what for a given volume amounts to essentially the same thing, you take small enough blocks
    and you decide to calculate with that and be satisfied

    the business of imagining being able to put an infinite number of block in the computer is a formality or a fantasy.
    you want to have ENOUGH so your results can be realistic.

    right now Ambjorn and Loll are using a WORK STATION and one of their runs will often take several weeks.
    It is infuriating that some of the major research establishments, with supercomputers, have not realized how interesting this work is and offered them time on a big fast parallel process computer.

    this kind of work is MADE for a fast parallel process computer. like they simulate airplane wings and nuclear explosions etc.

    it is crazy that the CDT authors have to do this work like on a desktop workstation.

    i would like to see the results of modeling spacetime with a couple million blocks, not just 1/3 million. and then put matter into the blocks (which would make the computing heavier)
     
  14. May 26, 2005 #13

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    once you decide on how many blocks, then you load that many blocks into the computer and you make the "swarm" or "blur" of geometries by
    SHUFFLING
    you cause the blocks to put themselves together differently again and again so that the range of possible geometries is explored
     
  15. May 26, 2005 #14

    wolram

    User Avatar
    Gold Member

    A desk top ? kick the weather forecasters out and use theirs, im sure we
    would not mind not knowing if it is going to rain or shine for a week or two
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Where you going?
  1. Where is the string? (Replies: 6)

  2. Where is the energy (Replies: 5)

  3. No-go theorems (Replies: 3)

Loading...