This week there’s a large conference in Potsdam on non-perturbative/background independent quantum gravity called Loops ’05. The programme is on-line, and there is live-blogging from Robert Helling.
Update: String theorist Robert Helling has more coverage of the conference. This includes the hilarious criticism (quoted approvingly by Jacques Distler) that too many of the talks were “so vague and speculative that they are not even wrong.” Helling does notice that the string theorist speaker (Stefan Thiessen) delegated to talk about what is going on in string theory had nothing to say, and just repeated the failed dogma from more than 20 years ago. Maybe at Strings 2006 they’ll even let someone from the LQG camp speak, or at least there will be live-blogging from an LQGer.
Note: This is a reconstruction of my original posting. Unfortunately I accidentally hit the wrong button when trying to edit a typo in a comment, deleting the posting and all comments. Thanks to Steve and Aaron for helping me retrieve the content of the posting, but unfortunately most of the comments were lost. If you have copies of them, please send them to me or resubmit them yourself.
Update: For something truly bizarre, see Lubos Motl’s comments on Loops ’05, where he attacks the talks there as “not even wrong”, while in the same posting respectfully reporting on a talk by Cumrun Vafa at Radcliffe on the Swampland, a talk at which several people evidently expressed the opinion that it could never lead to an explanation of anything about physics.
Update: There’s a bit more about Loops ’05 in the latest issue of John Baez’s “This Week’s Finds”.
Update: The talks from the conference are now on-line.
great of Robert.
meets the hunger for news from the conference and
also the need always for a critical outside perspective.
real gift. thanks Robert! Looking forward to more
(e.g. Laurent Freidel on Tuesday, Loll and Reuter on Wednesday, and to Roberts choices of what talks to hear and his commentary on them)
Peter (in case you didn’t know) next week at Stony Brook there is an interesting symposium on geometry and universe which predominantly consists of relativists.
http://insti.physics.sunysb.edu/itp/conf/GR2005/ were you planning to go for this?
Maybe you can tell the readers about this conference and also let us know what the traditional GR community thinks of string theory (which will definitely be discussed at this conference).
Shantanu
“traditional GR community thinks of string theory�
Shantanu, what does the traditional GR community think of string theory?
I cannot reconstruct all I commented earlier. My main point was that Theissen gave a standard “strings for non-experts” talk rather than addressing the loop people and their prejudices directly. This way, they had seen it all before and he gave away the possibility of telling them something that is new for them and maybe even thought provoking.
Hi, if I can put a comment here I didnt figure out how to put on Robert’s blog:
Thanks for the comments. If I’d known you were here I could have answered some of these questions in person.
Let me emphasize first that the point of my talk was to emphasize important open issues. We are not afraid of emphasizing open problems but we do hope that people notice when they are solved. Hence, I would have hoped you noticed and reported that major progress was described concerning the problem of showing that classical spacetime honestly emerges from background independent theories. Rovelli derived the graviton propagator and hence Newton’s law. Freidel and Livine showed in detail that 2+1 quantum gravity with matter has a low energy limit which is an effective QFT on a non-commutative geometry with deformed Poincare invariance. Both results were derived from spin foam models.
Hence, one can no longer say that there is no understanding of how classical spacetime and low energy qft emerges from these theories.
There was still more. Perez showed how regularization ambiguities in the Hamiltonian constraint may be resolved. Markopoulou discussed a new approach to the low energy liimit based on her new paper with Kribs. Loll announced major results showing that 3+1 spacetime emerges from causal dynamical triangulations and that at short distances the theory scales as a 1+1 dimensional theory. Livine and Terno showed how to derive the log(area) corrections to black hole entropy and estimate the rate of Hawking radiation. Starodubtsev reported on work with Freidel in which quantum gravity is defined by a perturbation expansion around a topological quantum field theory where the expansion parameter is G Lambda. This is a very promising direction as there are indications (no proof yet) that this new pert. theory is renormalizable and the low energy limit reproduces QFT in DeSitter spcetime……
These, and other results were based on detailed calculations and are solid results. I would have hoped that your report would have focused on the presentation of these major result.
In the face of this kinds of impressive progress, I was not embaressed to emphasize open issues or speculate a bit about future directions. So my talk was certainly not representative. By the way, I would have thought that as a particle physicist you would have recognized that what I presented was just a preon model. It was translated into the language of LQG with the help of recent work by Bilson-Thompson. This is new stuff and much remains to be done. But you can also no longer say that there is no proposal for unification of matter and geometry in LQG.
Still to come are new results on rigorous formulations of the theory and at least one striking new results on quantum cosmology, of relevence for upcoming observations.
a friend of ours was in Berlin and happened to attend Lee Smolin’s public lecture. Here is Ratzinger’s report:
—quote—
I’m in Berlin right now and found out that Lee Smolin is giving a public lecture, named “The unfinished revolution: finishing what Einstein started”. So how could I resist?
He started with the question “what is at stake?”. Answer: all the big questions (what is time, space, physical law? why is the universe hospitable to life?). After going through the three revolutions (Aristotle, Newton, Einstein) and their notions of space, time, etc., he stressed the importance of relationality in present-day world view, both in qm and gr.
(He said Leibniz was right about relationality, but he had no workable physical theory, so scientist followed Newton for 200 years. Mach and then Einstein rediscovered relational thinking.)
So what are the approaches to attack quantum gravity? There are two, according to Smolin.
1. Einstein’s way (rethinking the concepts of space and time, and especially reworking qm)
2. everybody else’s way (string theory, loops, etc.)
He admitted that he researches everybody else’s way, but pointed out the need for radical and rebellious thinking the Einstein way. He also praised Penrose in that context.
He ended with saying that physicists are standing in the lights and shadows of Einstein. Lights: Einstein’s theories, that they work with. Shadows: ignoring what is at stake and not sharing Einstein vision of a complete understanding of the universe by rejecting qm.
Edit: 1. I think Renata Loll was also sitting in the audience.
2. Smolin was very excited by the coming representation of Winkler and another guy on Friday.
—endquote—
http://www.physicsforums.com/showthread.php?p=785298#post785298
[my comment: the other guy is probably Abhay Ashtekar, whose talk on Friday will discuss what replaces the hole and bang singularities when the classical singularities are removed by quantum gravity methods. This would probably interest Lee Smolin since he has conjectured that the two regimes could be related—black hole pit expanding to form another spacetime region.]
or the other guy, whose name Ratzinger didnt remember, could be Viqar Husain—-Husain is also giving a talk Friday and he and Winkler have a new QG way to remove the BH singularity.
a: ‘…what does the traditional GR community think of string theory? ‘
I earlier posted that Sir Roger Penrose who is the only person that actually understands GR physically – or at least the only person who lets on that he does – thinks Witten is talking out of his hat where Witten says in a leading journal in April 1996 that ST: ‘has the wonderful property of predicting gravity’ (not an exact quote, but close enough).
Penrose has this issue that ST is at best just a perturbative calculating procedure which cannot predict – or lead to – the prediction of the strength of gravity.
Witten’s praise of his own work on M-theory by calling it a wonderful prediction of gravity reminds me of the controversy when Edward Teller around 1983 hyped the ‘wonderful’ space-based nuclear explosion pumped x-ray laser as a thing the size of a suitcase which can shoot down the entire Soviet missile force, ‘if in the field of view’. (After checking the equations, it turned out that the x-ray laser would need to be about 1000 metres long, and that as such it would only be able to shoot one thing at a time, and would be hard to send into space anyway.)
Sorry if I’ve gone off topic a bit 😉 On my weblog, I’ve got all kids of rubbish I can’t delete because you have to keep pressing ‘confirm to proceed to deletion’ and ‘are you really sure you want to delete this post, it looks so brilliant? please click here again before it can be deleted!’ So I just give up trying.
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Lee,
please reread what I wrote. I said I liked Rovelli’s talk an recommended reading his paper. I gave some extended coverage of what you were saying but you have to admit your remarks on ‘quarks’, your remarks on CMB and the Poineer anomaly were quite bold to say the least. I expect to read some popular texts on LQG where these topics will be mentioned as ‘understood/solved by the canonical approach’.
What Markopoulou said sounded to me (but that might be my fault) as an introductory lecture in block spin transformations and RG group in the 1D Ising model. The remarks on how evolution is like a channel in quantum information theory again were not very concrete.
And, as you will find stated in my second post, I had to leave Golm during Friedel’s lecture to return to teaching duties so I could not report on what happened after Tuesday 10:40.
As far as the announced results on quantum cosmology go, I have to admit, I am sceptical. Loop quantum cosmology had one big result in the past, the fact, that the initial singularity is not there and the inverse area operator was bounded (i.e. the universe cannot get arbitrarily small). This result was celebrated as a huge step forward and all the publicity was collected in favour of LQG.
In the meantime however, Thomas Thiemann has shown, that this result was an artefact of the minisuperspace like approximation and that it the inverse area is unbounded in the full theory. As always everybody is encouraged to draw their own conclusions.
Off-topic: there’s a short article on string theory featuring Gabriele Veneziano (now chair at Collège de France) in today’s edition of Le Monde at http://www.lemonde.fr/web/article/0,1-0@2-3244,36-698934@51-699012,0.html
Another one features Brian greene and Stephen Hawking and their popular science books.
This pops up because this week is the nationwide Sciencefest in France, and there’s “a large public eagerly interested in theoretical physics” who read (part of) those books.
Of course this is just a tiny aspect of the Sciencefest, most other things are on less controversial topics, all the activities are listed at http://www.fetedelascience.education.gouv.fr/
If one goes to amazon.com and looks up David Lindley’s book on Boltzmann ( “Boltzmann’s Atom: The Great Debate That Launched A Revolution In Physics”), then, at the end of the editorial reviews is an excerpt from the book. We learn of James Waterston who discovered some of the ideas of the kinetic theory of gases, but was ignored.
And the following contains probably useful advice for the 21st century as well:
Great advice Arun. Surely it would have been much better for everybody involved if young Albert Einstein had concentrated on establishing a solid track record in molecular physics till he was 40 or so, to then tackle the big vision task of unifying mechanics and electromagnetism.
BWAHAHAHAHAHA!
Unfortunately Arun’s advice is all too sound: if you concentrate your mind on how not to be a threat to the has-beens or never-weres who run the physics establishment, then they might, after forcing you to jump through a lot of hoops, offer you a permanent job. If your brain has not completely atrophied by then, then who knows, you might even have some infinitesimal contribution to make to the subject.
Relativity and Quantum Mechanics only happened because there were enough bright 23-25 year olds around in the early part of the last century brave enough to think for themselves. Although the research establishment then was just as much a gerontocracy as it is now, it was one of those situations where the old timers simply could not afford to ignore the youngsters.
Arun’s advice is sound only if the goal is to get a job in academia. Why anyone would be willing to sacrifice his or her intellect and creativity for such a lowly reward is beyond me. There are far more rewarding occupations which will let you exercise both to the fullest *and* which offer the very real possibility of reaching financial independence well before 40 (especially if you don’t mind living on a theoretical physicist’s budget). And then it’s off to the races. 🙂
Actually, I did want a job in theoretical physics. I was prepared to put up with the low salary (less than a schoolteacher, by the way) in exchange for doing what I love doing. I will never have moral qualms about going in search of £££ in the City of London as leaving academia was a question of being pushed rather than jumping.
“[..] Of course, Peter Woit followed up on this line, because it is his argument that superstring theory is not even wrong. [..]”
Dissident,
Albert Einstein did concentrate on molecular physics – that was his Brownian motion paper. Anyway, considering that Boltzmann committed suicide in 1906, in part we think because of opposition by Mach and Ostwald to atomic theory in general and Boltzmann’s work in particular, molecular physics was not the place to be, either.
Anyway, Einstein’s Nobel citation was “for services to Theoretical Physics, and especially of the law of the photoelectric effect.” Perhaps another sign of the conservative nature of the establishment.
-Arun
That’s why I mentioned molecular physics, Arun (if memory serves, it was his thesis subject). But for how long did he “concentrate” on it?
And yes, that silly Swedish academy used the photoelectric effect to motivate his Nobel prize, so as to avoid openly endorsing relativity. It wasn’t really controversial in the world at large by then, but apparently there were some local “heavyweights” in Uppsala who just didn’t get it.
I’m disappointed that the coverage of Loops ’05 has stopped after the first couple of days of talks. I would have liked to know what happened next. Can one find that anywhere?
Dear Robert,
Thanks, perhaps I misread you. I was perhaps over-reacting to your opening: “I sneaked into the Loops 05 conference…” I hope this was not how you really felt. Certainly, all were welcome, no one was checking badges.
But I also don’t understand why, after doing a good job of just reporting, you feel the need to add comments like, “I expect to read some popular texts on LQG where these topics will be mentioned as ‘understood/solved by the canonical approach’?” Perhaps you are used to an atmosphere in which people are not careful to distinguish conjecture, evidence and proof, but this is not us.
As to the other points, why not read the papers? Kribs and Markopoulou is gr-qc/0510052. Regarding singularity avoidance in the full QFT, the papers are by Johannes Brunnemann and Thomas Thiemann, gr-qc/0505032 and 033. Not surprisingly, the situation is quite a bit more complicated in the full theory than it is in the models, but their conclusion is not pessimistic. They do find states on which the inverse volume is not bounded. But they show it is bounded on a large class of coherent states. The last line of their abstract states, “After outlining what would be required, we present the results of a calculation for LQG which could be a first indication that our criteria at least for curvature singularity avoidance are satisfied in LQG.”
If I may add, the atmosphere in the quantum gravity community is pretty open. Most of us know and easily acknowledge that what we are doing is high risk. Most of us are very self-critical and any honest critic will be made to feel very welcome because you cannot be more critical to us than we are to ourselves.
Looking forward to seeing you next time,
Lee
John Baez TWF #222 is out.
Scroll 3/4 of the way down the page for his report on a couple of things of interest from Loops ’05.
http://math.ucr.edu/home/baez/week222.html
Unofficial word about when the VIDEO from Loops ’05 will be made available at the website:
My understanding, based only on indirect report, is that they plan to have the videos of the talks up by beginning of next week.
Worth keeping an eye out for it. Thanks to John Baez for originally mentioning something about this.
will there be a loop ’06? incidentally, shouldn’t all LQG researchers work on the semiclassical limit problem, for if it doesn;t reduce to GR, then it is not a viable theory of QG?
will there be a loop ‘06? incidentally, shouldn’t all LQG researchers work on the semiclassical limit problem,…?
Normally it might be better to wait until John Baez responds—since your question is one he can answer best. But he might not still be reading this thread, now that it is overlayed by other discussion. So I will take a shot at the question.
I havent heard anything official about a conference next year. It seems to me that enough is going on in the various allied research lines to make people want to have one next year. Last year it was at Marseille so it probably would not be there again so soon. It could be at Penn State (Ashtekar’s institute) or a Perimeter in Waterloo, or it could be at several places in the UK, or possibly at Utrecht.
My private guess is that there will be Loops ’06 and it will be at Utrecht. (Ashtekar seems to be taking his sabbatical there for at least part of 2006—the place has become a QG center.)
about the semiclassical limit problem, dan—-it does seem to me that there is a lot of focus on that now in several of the main QG research lines. I think your guess that they should is approximatley right and that they ARE focusing effort on that.
To get an overview, go to Loops ’05 programme and take a quick look at several leading people’s ABSTRACTS. Look at the abstracts of Rovelli, and Baez and Freidel and Loll.
All these people are focusing on this and related issues like including matter. Rovelli is explicit about this, also Baez, as you can see from the abstracts. Freidel has just finished showing that the 3D version of his spinfoam model DOES include matter and have the right largescale behavior. His program is to achieve success in 3D and then extend the results to 4D. He talked about the prospects for 4D at the conference. Still other nonperturbative QG approaches are those of Loll (CDT) and of Reuter (QEG)—where considerable effort is focused on these same issues.
So on the whole you are right, but there are exceptions. LQC (loop quantum cosmology) is almost a separate field. It may be possible to test it separately. There the semiclassical limit has already been shown. So the emphasis has shifted towards phenomenology—what might be some observable effects of LQC? Cosmology involves symmetry-reduced degrees of freedom (assuming isotropy and homogeneity simplifies the picture). One thing that can be explored is the relation to the full QG theory—-how similar will the predictions of full LQG (say in Thiemann’s Master Constraint version) turn out to be to the “toy model” symmetry-reduced LQC?
Another complication is that, as is often pointed out, the field of (nonperturbative, non-string) QG includes several approaches—it isnt just the canonical LQG (described, say, in Hermann Nicolai’s “outside view” paper). Offhand I can’t think of anyone at Loops ’05 whose abstract indicates they were talking about canonical LQG. The guy from Beijing Normal was discussing Thiemann’s Master Constraint which is really a new QG approach. If you look over the programme and happen to see someone who is actually doing LQG proper, please let me know!
The terminology is really frustrating. Can of worms! People should really not ever say LQG. They should say “nonperturbative QG” (meaning Rovelli’s spinfoam, Freidel’s spinfoam, Loll CDT, Reuter QEG, Gambini CD, Thiemann MC,…) and when all these people get together they call it “Loops”.
“So on the whole you are right, but there are exceptions. LQC (loop quantum cosmology) is almost a separate field. It may be possible to test it separately. There the semiclassical limit has already been shown.”
What is LQG and how is it there’s so little word on its semiclassical limit? So it reproduces GR with quantum corrections on the planck scale?
dan said:
will there be a Loops ’06?
Probably; the idea of calling it Loops ’05 was to make this an annual thing. However, we need someone to agree to run Loops ’06 – and I don’t think it’s gonna be me!
Some obvious possibilities include Penn State, the Perimeter Institute, and Marseille, but they’ve all run conferences like this quite recently. So, Mexico and Utrecht are being mentioned.
I’m not sure Renate Loll will want to run something called Loops ’05, since she considers her own approach – causal dynamical triangulations – quite distinct from loop quantum gravity, and more successful! Personally I think this year’s conference should have been called something like QG ’05, since there were talks on almost every approach to quantum gravity. Or maybe NOT VERY MUCH STRINGS ’05. 🙂
Anyway, we’ll see what happens.
dan said:
incidentally, shouldn’t all LQG researchers work on the semiclassical limit problem, for if it doesn’t reduce to GR, then it is not a viable theory of QG?
I think all loop quantum gravity researchers should work on this problem. That’s why I keep talking about it every time I get a chance! I spoke about it at Marseille, at the Perimeter Institute, and at Loops ’05. However, it’s hard to get people to work on a very hard problem, when there are easier problems out there.
Similarly, I think all string theorists should be working on a background-free approach to this theory, and on finding a way for it to make specific predictions about particle physics. But at any given moment there are lots of easier things to do.
I’m not sure Renate Loll will want to run something called Loops… since she considers her own approach… quite distinct from loop quantum gravity… Personally I think this year’s conference should have been called something like QG ‘05…
QG ’05 would have been a better choice (it seems now)
Names matter (you pointed this out somewhere not long ago) and a little thing like a name can be a bigger obstacle or source of trouble than one would expect—-I will spare everybody the Shakespeare quote.
For the first time I understand why they didnt simply announce at the end of Loops ’05 that Loops ’06 would be held at Utrecht Spinoza Institute. Damn. It is a shame. But I understand how Loll might object.
To me it looks different. If the Utrecht people would simply have Loops ’06 at Utrecht and call it Loops, this would prove to the world that CDT IS LOOPS. It would show everybody that there is a collective research drive towards the goal of non-perturbative quantum theory of gravity which is not confined or characterized by one specific approach like canonical loop quantum gravity, or any particular path integral tactic either. So I see it for Loll as a good opportunity to establish a broad inclusive definition of the term “Loops”.
hello john & “who”
re: “However, it’s hard to get people to work on a very hard problem, when there are easier problems out there.”
from the standpoint of physics, if LQG & friends do not reproduce GR as its semiclassical limit, then is there any reason to continue researching it? so i’m not sure if it is wortwhile to work on the easier problems if LQG does not reproduce GR. Perhaps no-go thereoms that show background-independent QG does not produce GR might be an easier problem everyone should work on! has lubos published to this effect?
“who” would you call SST/m-theory “qg”?
incidentally John, you once wrote about de-emphasizing QG in your research, has loops 05 changed that?