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

Loop versus closed string?

  1. Jan 27, 2007 #1
    How is a loop in Loop Quantum Gravity physically differentiable from a closed string in String Theory?

    Aside from political, funding, and personality differences, what are the basic differences between LQG and String Theory?


  2. jcsd
  3. Jan 27, 2007 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    at the end of the SINGULARITIES WORKSHOP which you attended, the kitp guy in charge of the workshop, Gary Horowitz had a concluding discussion for an hour or so

    in which he wrote on one blackboard lists of strengths/weaknesses on the string side

    and then on the other blackboard the same lists of strengths/weaknesses on the LQG side


    it might be worthwhile trying to copy and transcribe these lists here for us, from the video or from your notes, including whatever they said but didnt write down that you think was important.

    the two approaches are extremely different mathematically, but this does not matter so much (I think) as the fact that they have extremely different practical results in terms of what they can and cannot do so far.
    For me, the most revealing thing that Horowitz said at this final discussion was that the AntiSitter framework is "not very interesting physically".

    This is a framework string-folk love to talk about because, as they see it, within that restricted framework their theories work out comparatively well. The downside is that nature is not AntiSitter. So it is a non-physical "sector" of the theory which is, however, intriguing and gratifying to investigate.

    At one point one of the people in the kitp final discussion said something like
    Let's suppose we lived in a universe where physics really was AdS/CFT, then wouldn't we be done already? I thought this sounded touchingly wistful---a kind of stringtheoretical pathos. He was saying imagine we lived in a universe where AdS/CFT applied, then wouldn't we have already solved these singularity problems and be all finished with our work? Then they all talked about that and speculated for a while.

    Anyway, probably a good way to answer your question is just to review that video----with concrete lists of how the two approaches have very different strengths and weaknesses (according to Gary Horowitz, a kitp string theorist).

    A non-string Quantum Gravitist would give a different discussion of the differences----like have a look at Lee Smolin's book
    "The Trouble...and What Comes Next"

    and by the time Horowitz was doing his wrap-up comparison, it looked like almost all the non-string QG people had left to catch their planes! IIRC only Martin Bojowald was still there and he seemed to be resting quietly and letting the string theorists react and summarize.

    So you would get a different perspective on the differences from a QG group, but I think Horowitz kitp discussion is a real good place to start.

    an interesting sidelight on this kitp workshop,
    it was advertised as a three-week event although more like two-and-a-fraction, but that is still a substantial block of time,
    so I was wondering about earlier kitp events where you get some loop and some string folks together----what are precedents for comparison?
    well there was this one-week thing in 1999

    and then there were a bunch of talks by various people scattered over a six month period leading up to that but it did not appear that it gathered everybody into the same room at the same time
    That was more like several talks each month, spread out over 6 months, so you didn't necessarily have lots people from different lines of research talking to each other and trying to explain their research to each other.

    Some of that must have happened in the earlier meetings, but I think this concentrated 2+ week thing is a big improvement.
    Last edited: Jan 27, 2007
  4. Jan 27, 2007 #3
    Hi Marcus

    I was traveling much of the time before I got to KITP to audit the last two days of the mini-program, and have only managed to watch a few of the videos. I hope you will understand that I was only there as a tourist, and only got to be present at the last four seminars. However I do hope to relate my personal adventures in some detail here and elsewhere.

    Of course I did try to take notes. Every one talks very fast. I wish I had you at my elbow to tell me who was who. I typed out my notes and will have to listen to the video carefully to make sure I got it right.

    I will gladly copy and transcribe these differences and similarities. That may take a couple hours.

    I have the sense that the differences are mathematical, and think I get it that the maths are formulated on different kinds of space. I am working to understand the different maths, but still recognize them about as easily as I did the participants of the discussion.

    Ted Jacobson was still there, and Martin Bojowald, and Abhay Ashtekar was sitting quietly in the back row.

  5. Jan 27, 2007 #4


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Oh no! I would be embarrassed to have caused such an expenditure of time!
    I can read the lists of "strengths and weaknesses" of loop and string quite easily for myself. Perhaps I should transcribe for PF, since I brought it up.
    Or we can just forget about it.

    Anyway don't take on any unwanted burdens. You have already given a nice firsthand impression.

    I didnt realize that Abhay was still around Friday afternoon. He is normally voluble and this time, as you say, he was being quiet.

  6. Jan 27, 2007 #5
    No, I am glad to work through this. I want to get something out of the experience aside from memories of my little social blunders. I may have recollections of something or other that might be useful to you. For example, I kind of remember what was being said by audience members, and that doesn't come through well, if at all, on the soundtrack. Maybe I can identify some of who-said-what.

    Along that line, there is a gentleman seated at the left corner of the camera angle who had authoritative things to say toward the end of the concluding session. Who is he? It is a shame that I didn't get to meet people, but I am sure you understand they were busy working and I didn't want to disturb anyone for my picayune requirements. If I had been there the whole three weeks I might have figured out who they all were, but it just wasn't possible in the few hours I had.

    More later....

  7. Jan 27, 2007 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I mostly just go by the pictures
    http://www.kitp.ucsb.edu/directory/photos/?activity=singular_m07&all [Broken]

    this has a picture of Kristina Giesel that you almost spilled your coffee but luckily didn't

    I can't be of much help identifying these people by the back of their heads and baldspots
    Last edited by a moderator: May 2, 2017
  8. Jan 27, 2007 #7
    Yes well. I try. I think it may have been David, the director. THere was another grayback also in the front row, and I can't swear which was which.
  9. Jan 27, 2007 #8
    String Theory strengths:

    1.Beautiful and complete AdS/CFT correspondence, a non-perturbative quantum description of quantum gravity in terms of an ordinary field theory, providing some tools for probing the question of what happens to black holes and cosmological singularities in AdS/CFT, albeit not in the most interesting sector of the theory.

    2.Perturbative string theory can shed light on some singularities, as in Eva Silverstein’s talk on Tachyon condensation.

    String Theory weaknesses:

    1.AdS/CFT correspondence is very indirect, involving answers to Gravity questions coded in a complicated way in ordinary gauge theory variables. The dictionary is very complicated, and we only understand a small part of it.

    2. String theory is lacking, even in principle, a non-perturbative way of addressing cosmological singularities in a closed universe, or anything else that can not be pushed into an asymptotic AdS/CFT framework. String theory does not currently offer any non-perturbative framework for describing closed cosmologies. Even perturbative string theory is not well understood in time dependent backgrounds.
  10. Jan 27, 2007 #9
    Behind the cush raised auditorium seats there is an aisle with folding chairs for overflow seating. Joe Polchinski was hiding on one of those seats with his face in his hands during the listing and discussion of string theory weaknesses. When Tom Jacobson started asking questions about transforming calculations on the Riemannian worldsheet to Lorentzian space-time, Gary Horowitz seemed to have trouble answering. Gary then called for help from any string experts in the room. Joe seemed reluctant at first, but perked up rather quickly when prodded by Tom Jacobson to come to the defence of the stringy cause, and he left his redoubt behind the seats to join the main group on the cushions.

    Some of my notes have sequences which make me wonder if they have been edited out of the video, but perhaps it was just that I could hear what was going on in the seating and it does not come through on the vid. For example, at about this point, Joe began by wondering out loud what would happen if space-time is time dependent, and not a natural analytic continuation. Hui then started talking about how it is not clear what are the right observables in a time-dependent background.
    Last edited: Jan 27, 2007
  11. Jan 27, 2007 #10
    Loop Quantum Gravity strengths:
    1. Clear model of universe bounce in isotropic gravity coupled to free scalar field.

    2. Even with anisotropy, still allows evolution past classical singularity.

    3. Expectation value of curvature stayed finite in some kinematical states approaching singularity

    Loop Quantum Gravity Weakness: The whole theory still murky or ill-defined.
    1. How unique is this quantization approach?

    2. There are issues in local Lorentz invariance. Is it preserved in evolution?

    3. There are worries about fundamental discreteness in loop approach. How can LQG recover Lorentz invariance if it is built on a lattice-like structure?
    Last edited: Jan 27, 2007
  12. Jan 27, 2007 #11


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    this is very helpful what you are doing, Richard, thanks!
  13. Jan 27, 2007 #12
    From Thursday:

    "Afterwards there was coffee and cookies in the lounge. Dr. Einhorn was very courteous and practically took me by the arm to make sure I got some coffee and a cookie. The cookies were grocery store variety and not the top shelf either. The coffee was good and strong, as I like it. But when I was making my way out of the crowd by the table, Dr. Hui bumped my elbow and I nearly got coffee all over Dr. Giesel, which would have embarrassed me no end, as she is easily the prettiest doctor in the place."

    Social blunders. I certainly should have tried to blend in, but you know I live as a hermit in the north woods and my social blending skills are rusty, not having ever been very sharp.

    I don't want to offend anyone of course. I am no Lubos Motl. I did offer to send any reports for review to Dr. Einhorn before going public, but he did not respond to my offer so I assume I am free to write any nonsense as I see fit.

    I haven’t told you about my one sally into the fray. Friday morning before things got going I was in the lounge all by myself. There are blackboards all over KITP, just in case anyone needs one for purposes of illustrated discussion. Well, no one was talking to me, so I did feel a little like a graffiti artist as I made a sketch of an idea I had while listening to one of the talks.

    Imagine a light cone, future and past, and at the vertex a black hole. I was thinking of one of the very small variety that might be expected at CERN in the next year or so. It occurred to me that if the duration of the hole was very short, and an energy impulse was very strong and of a sufficiently long wavelength, some of it might actually be transmitted through the hole.

    Then I drew a graph of what I thought the result might be.

    When I came back later, after the close of the program and my hour or two in the library, someone had picked up the chalk! I was thrilled. They erased my graph, and drew a different one, along with a bunch of formula bits, but they left my original drawing untouched. There were two new drawings and a curve showing how the energy might be able to pass through. I would be so happy if this little thought were to show up in a paper somewhere.

    By the way, Dr. Veronica Hubeny was also there, and she was equally worth trying to observe without being caught staring. I doubt if she will find out but I don't want anyone to think I slighted her.

    I can't recall that I heard Dr. Giesel say anything at all, but Dr. Hubeny was a participant of several conversations which I was interested in. Well, it was a more or less public forum. I don't want to be an eavesdropper but they all have private offices to go to if they don't want to be overheard. Anyway I heard Dr. Hubeny say that she didn't think there was correspondence between black holes and the big bang. The Black holes are local. I would have like to have had the opportunity to ask her if she thought the locality question would be reversed for an observer behind the horizon. But I am shy around women in general, especially when they are geniuses.

    I have reviewed about half the final discussion, and will carry on tomorrow.


  14. Jan 27, 2007 #13


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    this is a good kind of reporting that lets an ordinary shy nonspecialist guy feel how it was at the conference. I wish selfAdjoint were checking in these days. he would enjoy reading this.
  15. Jan 28, 2007 #14
    Thank you Marcus. I have noticed that sA has not been on the boards since early in December. But who knows? Perhaps he is curled up some where watching all this and enjoying a good laugh about it. Certainly he deserves a good vacation, and you know how he loves to travel.

    I have had some congestion since I got here to California and did not sleep well last night, as it sounded like an alien harmonica band partying in my chest. I hope you will understand that I may be even slower than usual today.

    As for ordinary generalist guys, you may be amused by my one attempt to get around the caution I was given not to bother the participants. It was just before the closing discussion and Ted Jacobson and some others were sitting in the courtyard enjoying their lunch. It was time to gather in the auditorium and Ted was left behind a moment, as he had not finished eating and was having some difficulty juggling utensils, cup, and two of those clear plastic salad-to-go boxes.

    Aha, I thought, seizing the moment. Well, I was specifically told not to bother the participants in their offices, and he wasn't, so I thought I might get away with stretching my rights a little. After all, Dr. Einhorn had given me a cookie.

    So I came up behind him and said something to the effect of "Excuse me, you are Dr. Jacobson, aren't you?" Now there is a snappy conversation starter.

    Dr. Jacobson, who seems to have a somewhat habitually apprehensive expression to his face, turned around and nodded quickly, holding food in both hands and a fork in his mouth.

    "I was going to ask you if LQG had held its own in the conference," I said, paralleling his flight toward the door. "But I don't think you can talk with that thing in your mouth." He smiled at me as well as he could under the circumstances, shook his head, and I had the honor of opening the door for him.

    I suppose one could say this shows that Dr. Jacobson is not one to speak with a forked tongue.

    Dr. Jacobson sat in the front on the left side of the auditorium, and I had the audacity to sit on one of the cushioned seats on the other side of the room. I didn't want him to think I was following him.

    After the session there were some moments when an enterprising reporter might have pushed upon this brief and clumsy introduction to approach him again, but I am by nature more of a passive observer. He did look in my direction a couple times with that slight smile and nod people use with chance acquaintences who have not totally displeased them, but there was a conversation group between us and I did not have the courage to drag myself and my baggage through the middle of it, so I missed the gold.

    Oh well. Perhaps there will be another time.

  16. Jan 29, 2007 #15
    In my view, the singularities program at KITP USCB mostly had one area of consensus.....namely that a new view of space-time is probably needed. Dr. Hiu got rather excited in the final discussions when it was mentioned that time-like singularities need to be explored. He said that we don't even know what the observables would be.

    Backing up a little, most approaches to singularity resolution up to now have been to space-like situations. That is, the study has been directed at what happens when space is scrunched down to the Planck limit. Evidently it is thought to be much less clear what happens as time collapses near the singularity.

    So we have to think about this.

    My first thought is that space and time is one thing, perhaps in some way analogous to electricity and magnetism. In the Lorentzian metric, the quantities of interest are not the space-like arms or the time-like arms, but the displacement along the resulting geodesic. I am not sure why the space-like results are not directly translatable to the time-like situation.

    It is an old story to everyone here by now, but I will say again that it seems to me we have to accept that there are as many dimensions of time as there are of space. If I have understood correctly, the objection to this is mainly that we have no way of observing conditions along any of the time dimensions that we are not on. That is fair enough. If a time dimension is at right angles to the one we are on, the intersection would be nothing more than a single instant, and we would certainly have no way even in principle to detect such a thing.

    It isn't very satisfactory to suggest that we have no reason to believe that physics is any different along the orthogonal. So what? Even if they are there, they would have to be of negligible import. We would gain nothing by adding in a bunch of extra time elements. The one or two we have are all we need for anything we can observe.

    What could we hope to observe about a time-like singularity? It has no duration. Thinking in this fashion clearly leads to a dead end.

    Let’s go back to what we do know. Our best approximations (from string anyway) are in the uninteresting realm of deSitter space, perfectly flat and undisturbed by clumpy masses. We can calculate, model, and watch what happens as such a space collapses to a single point. This is our space-like singularity. But in fact, space-like or not, this model does have a dimension of time. Wheeler deWitt equation allows us to calculate how long in time a black hole will last. That is certainly a time dimension. Perhaps it would be useful to think about time-like singularities which preserve one, two, or three space-like dimensions. What would they look like? A time-like singularity in one dimension of space would be an object with extension along a line, but would still have no duration….not observable.

    I think we must find a way to look at space-time unified dimensions, so that we can regain something observable. Anything extended in space must then be extended in time, and likewise, anything extended in time must be extended in space. A black hole is not a singularity in this sense, because of the clear fact that it does have temporal extension. It also has a horizon, which gives it a kind of extension in space, although it is difficult to pin down just where this horizon actually is. We can see this problem best by trying to imagine what the internal radius of the black hole is. Yes, we can use triangulations from the outside, at a sufficient distance, to get a radial measure indirectly. But this is state dependent. It changes depending on where you are. If you are inside the horizon, all exterior coordinates collapse to a point, and your gauge, your ruler, your measuring stick gets smaller and smaller as you approach the center. Try counting how many of them are between the horizon and the center. In a perspective sense, the center of the black hole, from within the horizon is not a point, but a direction.

    This is why entropy of a black hole must be calculated as a feature corresponding to surface area. There is simply no way to calculate the volume of the black hole from the perspective of an observer inside the horizon.

    I am going for a walk. I hope to hear from you, Marcus, or any other readers, on what if any thoughts this may engender.


    Last edited: Jan 29, 2007
  17. Jan 29, 2007 #16


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Richard post #14 is a lovely vignette,
    indeed Dr. Jacobson is not one to speak with a forked tongue :smile:

    It is a bit as if by accident they had let in Carl Sandburg.
    Or Robert Frost, sometimes I am not sure which is which. And solemnly cautioned him not to disturb the guests.

    If I remember right, the original meaning of a vignette was a little curlicue of vine that they used to paint or sculpt. It gives serious furniture or mansion hallways a slight suggestion of Bacchus----if there are grapes on the vine----or at least an air of naturalness to relieve pure geometry.


    you mentioned the reedy sound of an alien harmonica band inside your lungs. I used to get a low-grade pneumonia or a bronchial infection nearly every year until I started taking the flu shot. What happens, at least with me, is that one gets the flu and some secondary infection takes advantage of the opportunity and gets firmly lodged. It wheezes at the bottom of a deep breath and can last over a month until good sunny weather comes.
    I started taking the flu shot and it hasnt happened for over 5 years now.
    Last edited: Jan 29, 2007
  18. Feb 22, 2007 #17

    Hans de Vries

    User Avatar
    Science Advisor
    Gold Member

    selfAdjoint's last visit here was Dec 3 2006, after posting 15 times on the
    1st and the 2nd of December.

    His age shows 73, I'm afraid we may have lost a good friend....

    Regards, Hans

  19. Feb 22, 2007 #18


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    I'm still hoping his absence is only temporary. Amazing guy:
    dignity, humor, sensitivity, gentleness, thorough grasp of large parts of physics and mathematics, a knowledge of the classics too.
    I never saw a limit to his curiosity.
  20. Feb 22, 2007 #19
    Marcus, just out of curiosity, what type of researcher are you? String, loop, or something else?
  21. Feb 22, 2007 #20

    Hans de Vries

    User Avatar
    Science Advisor
    Gold Member

    Let's hope so indeed. An individual so full of live on a never ending quest.

    From some of selfAdjoint's posts:

    He’s a widower, living in the town of Greenville (Fox cities) Wisconsin,
    probably the community of Menominee Falls, born in Maryland, August
    7, 1933. Followed grad school at the University of Wisconsin in Mathematics
    in the early 60's. He left grad school in 1966 and took a job as a programmer-
    analyst at a manufacturing company, He met his wife Mary Ann at the
    Christmas party of the department manager. Has a daughter named Pam born
    on Aug 9th who did Industrial engineering at Marquette. He has a married son
    with a 7 year old daughter Nikki born in Sep 1999 and 1 year old daughter
    Elizabeth born on Dec 21 2005. He has lived also in Southern California,
    Indiana and Illinois. He has been a Catholic for a major part of his live.

    Regards, Hans.
    Last edited: Feb 22, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook