Over the last couple years there has been a large amount of hype about cosmic strings, including press releases from Santa Barbara, and a story in New Scientist about The First Evidence for String Theory?.
The Santa Barbara press release from June 2004 concerned a paper by Polchinski and others about potentially observable fundamental strings of cosmic size. It stated that “LIGO… could provide support for string theory within two years.” There are five months left for this prediction to work out.
The New Scientist story was about an astronomical object optimistically given the name “CSL-1” (for Capodimonte-Sternberg-Lens candidate), that supposedly might be a galaxy lensed by a cosmic string, causing it to appear doubled. Of course the much less exotic and much more likely possibility was always that it was just two similar looking neighboring galaxies. Recently the Hubble space telescope was used to take a closer look at CSL-1, and as reported here and here the Hubble image clearly shows that it’s not a cosmic string, just two nearby galaxies.
Peter, can’t you stop reporting string theory hoaxes?
O.K. Pauli made a point of exposing the empty claims of Heisenberg’s grand unified theory and many others, but we all know string theory doesn’t hold water now!
Just because the string theory band is still playing loudly, doesn’t mean anything. (The band on the Titanic continued playing until the end.)
It’s interesting that the initial hype was over the image being evidence for either cosmic strings OR string theory. Given the Borg-like ability of string reasearch to lay claim to virtually any and all phenomena hinting at new high-energy physics (as low-energy manifestations of some-or-other version of string theory, I guess), I’m surprised CSL-1 wasn’t touted as potential evidence of both at once. Then again, maybe it was…
Dumb Biologist,
The hype was for both. It will be interesting to see how much this effect this negative result will have on the hype going on in this particular area.
Peter:
Predictably this negative result will have virtually zero effect on the enthusiasm of cosmic string theorists. The hype will continue on, so will the search.
One has got to understand the universe is pretty big and there are more number of galaxies than the number of sand grains on a beach. You can find anything you are looking for out of this huge collection of samples. Look a little bit harder and I am sure you will find many more of such similar galaxies in close proximity so it’s predictable there will be many more CSL-x type of cosmic strings “discovered” and be hyped by string theorists.
This time in this particular case HST is JUST slightly sensitive enough to be able to make a small distinction between the two close by galaxies. Interesting question to ask is what if it doesn’t, or what if the CSL-2 or CSL-3 is slightly beyond HST’s the ability to resolve. How long will the hype continue on. Or are they going to claim “String Theory Confirmed by HST”?
Quantoken
anon said ”Peter, can’t you stop reporting string theory hoaxes?”
I don’t think there is any danger of Peter acceeding to this request, but I’d like to vote for him continuing as he is. It is very useful to have an honest broker in a field where there is so much…well, I guess we’ve all agreed to call it ‘hype’.
What’s the difference between a “cosmic” string and a “regular” string?
Matt,
Cosmic strings are strings of macroscopic size, perhaps even big enough that you can see them or their effects in a telescope. It turns out that in some kinds of gauge theories with Higgs fields, there are topologically stable string-like configurations of the classical fields (basically solitons). You don’t get these in the standard model, but you do get them in some GUT theories. If they were produced in the big bang, they might still be around since they can’t decay for topological reasons. But most kinds have been ruled out since if they existed they would have had effects on cosmology that disagree with observations.
The standard fundamental strings are quite different. They are fundamental dynamical variables, not classical configurations of a Higgs field, and their size is generally assume to be the Planck scale (or, if you believe in some extra dimensional models, they could be as big as the TeV scale, but this is still extremely small). In recent years, Polchinski and others have found variants of string theory where the fundamental strings can get to macroscopic sizes, and there has been a lot of hype promoting these models as ones where strings would be experimentally visible. There’s no experimental evidence whatsoever that such models have anything to do with the real world.
Dear Peter,
half of what you write is a complete nonsense.
The fact that strings – if they carry enough energy – can be macroscopic has been appreciated for several decades, certainly since the mid 1980s and probably since the early string theory in the late 1960s. It’s just you and your naive readers who has infinite problems to realize that the shape and size of a string is a dynamical observable.
There’s a lot of experimental evidence that these models have something to do with the real world because there is a lot of evidence for the Standard Model and gravity which is implied by these models.
But your comments are addressed to likes of Quantoken – who obviously thinks that all string theorists are really “cosmic string theorists” 🙂 – so the fact that the quality of your comments is 3 orders of magnitude beneath the quality that would be acceptable in science does not really matter.
Best
Luboš
Lubos,
What I wrote is obviously not an exhaustive treatment of the subject, but it is perfectly correct. Your delusion that string theory implies the Standard Model is just wishful thinking.
In the years before 1984, there was quite a lot of discussion of string-like solutions in gauge theories and whether they might be produced in the early universe, discussions which I was well aware of since I was working on topological aspects of gauge theories. No one was talking about fundamental strings in this context, actually virtually no one except Green and Schwarz was talking about fundamental strings at all.
In 1985, Witten wrote a paper about the possibility of fundamental strings produced in the early universe reaching cosmic sizes as the universe expands, but he identified three problems with the idea. I’m no expert on this, but my understanding is that it is those three problems that Polchinski et. al. claim to be able to overcome.
Again, there’s not a shred of evidence for the particular scenario Polchinski et. al. work with, as far as I can tell it was developed purely in order to overcome the problems Witten identified and allow cosmic size strings to be produced and survive.
Oh well. Though I can’t quite get my head around the idea of a fundamental constituent of matter blown up to cosmic scales, I have absorbed as much as my pea brain can from sundry popularizations about cosmic strings, and found the idea of some hyper-dense filament of primordial matter/energy floating around out there to be pretty wild and fun. Too bad this ain’t it.
There are some statements which are hard to prove, but easy to disprove. Others are hard to disprove but easy to prove.
A statement of the form
“For all x in S we have A(X)”.
Is usually harder to prove than to disprove. Proving it requires knowledge about all x of S. Disproving it requires knowledge only about one x.
A statement of the form
“For at least one x in S we have A(X)”.
Is usually harder to disprove than to prove. Disproving it requires knowledge over all x of S. Proving it requires knowledge only about one x.
I gather this is well known to philosophers of science.
The situation with string theory is currently that it makes statements of the second kind.
Up to some well-known caveats, the claim of perturbative string theory is something like
“There exists at least one x in the set of real numbers such that at an energy scale of x eV we see stringy effects.”
As Peter is fond of pointing out, this is hard to disprove. You can check for higher and higher x (and we cannot check really high). But finding nothing at these x does not disprove this statement. Simply because it is a statement of the form “there is at least one x…”.
Proving the statement (if true) would in a sense be much easier. Observe at least one stringy effect and you are done.
Driven by the desire to make contact with experiment, people like Polchinski began scanning the theory for possibities of such stringy effects that might be observed.
They argue that one such possible effect could be cosmic fundamental strings.
It should be noted that nobody is claiming that string theory demands that we observe cosmic strings.
Just like GR does not demand that we observe a Goedel universe, or Einstein’s instable static solution.
But if you want to prove an existence statement of the form “there is at least one x in S” what you usually do is scan the space of all elements of S for those that might be candidates which make the statement true.
So people investigating stable cosmic fundamental strings could be said to be studying the claim:
“There is at least one x in the set of stringy effects such that x can be observed with contemporary technology.”
Disproving this is hard. Proving it (if true) would be easier. It suffices to find one example.
So people are checking plausible candidates for x. They argue one good candidate is “x = cosmic fundamental string”.
I guess one could have a scientific argument about if this is a “good” candidate or not. But the simple fact is that, indeed, if we did observe a cosmic string which could be identified as a “fundamental string” the above statement would be proven.
I don’t think this is likely. But I do think it is scientific.
Of course, one can state many existence theorems which are highly unlikely but nevertheless very hard to disprove.
“There exists at least one extraterrestial life form with a genome the same as terrestrial groundhogs”, I claim.
You don’t believe that? Prove me wrong!
So if you feel the same about the statement that there is at least one observable stringy effect, there is nothing much one can do about it except that you should feel free to study some other theory.
But whoever does see a chance that a certain existence statement is true should, in order to prove it, try to find an example that solves it. That’s what people interested in cosmic fundamental strings are doing.
You may feel that it is hopeless and that money should be spent elsewhere. Fine. But it is not non-scientific.
I haven’t said that thinking about cosmic strings is non-scientific (that applies to something else Polchinski promotes, the anthropic landscape). What I object to is the way this work has been over-hyped. Working on something extremely speculative, something for which you don’t have any evidence and that is very unlikely to really exist is fine, but I don’t think you should be issuing misleading press releases about it.
I don’t know why but this strikes me as being very funny! I couldn’t help but think of the Star Trek movie where Kirk dies saving the Universe from a superstring.
-drl
That’s sad. I always thought superstrings were at least nice fiction, but horrible murderers!
While it is hard to say what “anthropic landscape” really refers to, I could make a similar discussion as the above on cosmic string solutions but with respect to the space of all global solutions. It’s not a priori and by itself non-scientific to study the space of solutions of a theory.
But I agree that there tends to be too much hype about things too little understood.
cosmic string = superstring?
Sure? 🙂
—
Juan R.
Center for CANONICAL |SCIENCE)
DRL wrote: “I couldn’t help but think of the Star Trek movie where Kirk dies saving the Universe from a superstring.”
Oh my God! The Nexus ín ST:Generations (1994)!
http://www.startrek.com/startrek/view/series/MOV/007/synopsis/87.html
It wasn’t a superstring, it was quite clearly a brane intersection. So THAT is where Randall got the braneworld idea!
I hereby nominate writers Brannon Braga and Ronald D. Moore for the Nobel Prize in physics.
Urs, “Driven by the desire to make contact with experiment, people like Polchinski began scanning the theory for possibities of such stringy effects that might be observed.”
Wouldn’t one of the most impressive way for string theory to do this, would be as Lubos Motl says, to show you can derive most, if not all, of the particles of the standard model from string tension?
BTW: Lubos states it is possible to derive the standard model from first principles of string theory, Peter says it is not. Who is right on this specific issue?
It would certainly be impressive. It would also be impressive to have a theory which would allow to derive most, if not all, distances of the planets from our sun. We do, however, have no particular reason to assume that such theories exist.
We may hope such theories exist. Kepler hoped a theory exists which explains the distances from the planets to the sun as given by spheres inside perfect polyhedra.
Some people hope that the parameters of the standard model are unique solutions to a theory beyond the standard model. It might be so, or it might not be so. Who knows.
What I am already impressed, by, however, is that string theory is a theory which in principle allows to address this question. String theory may well be wrong. But it is the only theory we have ever known which has solutions that determine these parameters.
On this blog I shouldn’t say something like this without giving an example from outside of string theory which illustrates this by analogy:
Analogously, it is impressive how GR is a theory which, while it does not fix the curvature of spacetime, it has solutions which determine it. If we perfectly understand the space of all solutions of FRW-models in GR we can check if it contains any solution which matches the observed structure of the cosmos at large scales. The theory does not predict it, but it puts a structure on the space of all possibilities.
Hi Urs,
Does String theory predict the Standard Model from first principles?
The stringy attempts I have seen are adhoc!
An amateur mathematician
No.
That was the short answer. There is a longer one which mentions how there is an issue of perturbative versus nonperturbative, how things are too little understood to fully answer this question, and so on. But I have to run now.
The problem with this is that the evidence is that there are an infinite number of solutions, enough to give you almost any values of the standard model parameters. If this is really the case, the theory has zero predictive value, can’t be falsified, and continuing to pursue it doesn’t make any sense. Some string theorists believe the theory is still not well-enough understood and maybe there is some way around this to get a predictive theory. I think they’re engaged in wishful thinking, but at least they haven’t given up on science. What I really don’t understand are those who accept the idea that all these solutions are out there, can’t come up with any evidence that there is enough structure on the space of these solutions to allow predictions, and yet keep working on this.
That prejudice is called a hypothesis. The structure it implies is called a prediction, implied by the hypothesis.
There is every reason to be unsatisfied with the progress string theory has made on the phenomenological side and with some of its sociological aspects. What I find somewhat deplorable is, however, that on this blog the party line has established that “string theory is not science”. That’s unfruitful anti-hype (co-hype, for the mathematicians) and not suited to bring anyone closer to the truth than the equally unfruitful string hype is.
This is, by the way, the generic situation for any theory. The irony of history is that the “landscape affair” was ignited by an argument that there are only finitely many solutions to string theory (under some conditions).
This might be the expectation of some people, concerning an unknown space known as the “landscape”. But if you check with those people who actually construct solutions which come close to the standard model, the picture is rather the opposite.
Urs,
I don’t think there’s a party-line here, except that this blog does reflect my views. I’ve never said “string theory is not science”, but have time and time again made a quite precise point about the particular thing that I see some string theorists doing that is not science: See my review of Susskind’s book for the details of this.
By the way, I’m starting to get annoyed by what I have been hearing and seeing in how some quite prominent string theorists are dealing with the existence of this blog and the arguments I have been making. Instead of dealing with my arguments, people have been attributing to me stupid or mean blanket statements about string theory or string theorists that I have never made. I try to be quite clear and precise in what I write here, and I wish that anyone who wants to discuss any of this would make an attempt to deal with what I actually write, not something else.
Sure. I think I was engaged in discussion with other commenter’s here and certainly had them in mind.
I get the impression that your personal view, while often expressed in a provocative form (“string theory is not even wrong” is the intended provocative title of this blog, no?) is pretty much that people should stick to doing solid research. Nobody could disagree with that.
But it is also true that the comment sections here display a lot the point of view that “string theory is simply plain nonsense”, which I would like to see rectified a little.
And I do think it is good to criticize things which ought to be critized. It is, not the least, helpful for those being criticized.
Urs,
The problem isn’t that there are an infinite number of solutions, the problem is that there is no evidence that the structure of the set of solutions has any useful predictivity. If there were an infinite number of solutions, all with different CCs, but all with a specific neutrino mass matrix, you would have a highly predictive theory.
Sure, it is hard to construct particular examples that do exactly what you want. The main reason for this is that simple examples that you can analyze all don’t look like the standard model. To get something that looks more and more like the standard model, you have to make your construction more and more complicated. This is an obvious clue that you’re on the wrong track.
Correct me if I’m wrong, but of the 20 odd parameters of the SM, and its choice of gauge groups and fermion representations, I know of no plausible proposal of a particular piece of structure on the landscape that will allow you to predict a single one of them. The closest thing to an attempted prediction I have seen was the attempt a year or two ago by Douglas and others to predict whether the supersymmetry breaking scale was low or not, and that effort seems to have quickly collapsed in failure.
Urs,
There certainly is a sizable amount of nonsense in the comment section here, including uninformed blanket denunciations of string theory.
I already delete some of this sort of thing, in the future may start deleting more, to keep the noise level down to a more tolerable level.
urs said:
Yes, it would be impressive. Let me propose such a theory…F=ma… As input we need the position and momenta of the particles comprising the gas cloud surrounding our proto-sun.
-or-
In principal we could use the SM+GR, using the conditions shortly after the big bang, and derive the radii exactly.
The point, of course, being that the planetary radii are not anthropically-determined random parameters and are entirely derivable from other quantities.
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Urs said:
“But it is also true that the comment sections here display a lot the point of view that “string theory is simply plain nonsense”, which I would like to see rectified a little.”
As a noiseless reader of this blog, I’d say what Peter and others mostly do here is criticizing the hype and unjustified funding and support that string theory is receiving from the Establishment compared to its physical achievements. It’s this absurdly large imbalance that is often denounced as plain nonsense in this blog, not the whole string theory program.
String theory has always been a joke.
No, let’s be clear. String theory provides one possible frame for quantum gravity since it would allow a spin-2 gauge boson (graviton) if that theory could ever be found (like the empty frame which Heisenberg called his grand unified theory), and it provides a landscape of 10^500 vacua, one of which might be real.
Whatever you do, don’t laugh. It is not supposed to be a joke.
dan said,
It is easy. Cite a single paper, manual, talk, etc. where the Standard Model is derived from string theory.
Of course, Lubos Motl is not correct. The whole string M-theory framework is very outated and wrong in many technical details.
P.S: “Derived” here means that all mathematical and conceptual structure of the SM is a special case of the axiomatic and conceptual structure of ST. Of course derivation does not mean to know previously the answer and use it as input in the ‘derivation’, not obtain models simlar but not equal to SM, for example models with certain simetries resembling SM but based in massless states, etc.
—
Juan R.
Center for CANONICAL |SCIENCE)
Even if string theory worked as advertised, the resulting world-picture would be so repulsive that no one would get interested in science. That it fails so thoroughly is evidence that the world is not repulsive, and people will remain curious about it.
-drl
Lubos is on record stating ” claim such nonsenses such as that LQG only has the same problems like string theory”
His position as well as Witten Greene Weinberg, is that the alternatives such as LQG suffer from such serious problems that the only research program worth pursuing is string theory. the argument seems persuasive — given a speculative theory, if the alternatives suffer from even more serious problems, work on the one with the least (in this case string theory).
While i cannot answer Lubos’ criticisms of LQG, i cannot help but wonder, if the greatest physcists like Witten worked on LQG’s oustanding issues for the past 30 years, and abandoned string theory in the early 80’s, where would LQG be today?
Some kind of loop quantum gravity is going to be the right theory, since it is a spin foam vacuum. People at present are obsessed with the particles that string theory deals with, to the exclusion of the force mediating vacuum. Once prejudices are overcome, proper funding of LQG should produce results.
Dear Dan,
Actually LQG and related research programs (such as CDT or programs based on quantum information ideas) are progressing rapidly, see for example the talks at the recent Loops 05 conference: http://loops05.aei.mpg.de/
Lobus’s criticisms have been aswered in detail several places, you might look for example at the FAQ section of my review hep-th/0408048 (although this review needs updating in view of some developments since then.)
While we would of course be happy with anyone’s contribution to the subject, the fact is that there is a community consisting of about 150 mostly young physicists and mathematicians working in the general area of LQG and other background independent approaches and the results show that many of them are of equal quality scientifically to the best young people in other fields. The only concern I have for the future of quantum gravity is that these people be rewarded for their contributions with positions that will let them continue to contribute; for the present people in these fields have much harder careers than equally accomplished or talented string theorists. This does mean that those who have gone into the field have significantly more courage and intellectual independence, which I believe is one reason so much has been accomplished by relatively few people.
Thanks also to Nigel for those supporting comments. Of course more support will lead to more results, but I would stress that I don’t care nearly as much that LQG gets more support as that young people are rewarded for taking the risk to develop new ideas and proposals. To go from a situation where a young person’s career was tied to string theory to one in which it was tied to LQG would not be good enough. Instead, what is needed overall is that support for young scientists is not tied to their loyalty to particular research programs set out by we older people decades ago, but rather is on the basis only of the quality of their own ideas and work as well as their intellectual independence. If young people were in a situation where they knew they were to be supported based on their ability to invent and develop new ideas, and were discounted for working on older ideas, then they would themselves choose the most promising ideas and directions. I suspect that science has slowed down these last three decades partly as a result of a reduced level of intellectual and creative independence avaialble to young people.
Thanks,
Lee
Thanks for that description, WOIT
Hi Lee,
Thanks for the response. I do think your suggestion is admirable in principle, but i wonder to what extent it could be realized in practice. For example, Igor and Grichka Bogdanov would be an example of two intellectual creative young people who developed and invented new ideas, but whether they created some kind of Sokal hoax remains disputed. If a young researcher publishes string theory, the value of the article can be appreciated by other sting theorists. If a young researcher publishes an article suggesting a new program, it may be difficult to find qualified individuals to make a proper evaluation.
I am curious: how close is LQG on its semiclassical description, dynamics and contact with standard model particle physics?
Lee,
you say
I note that this research community does not have much visibility on the Web: for instance there is no QG blog and I don’t know of any central bulletin-board for QG news information and comment.
You mention the archived Loops ’05 talks as a good indication of current progress. That would be an excellent resource except the recorded talks are now off-line. They were up for a month or so around late-November-early-December but are now inaccessible. One can still download slides in some cases.
I would particularly like to be able to refer people to the video of the talk by Laurent Freidel, which I found quite impressive. But there are no slides and the recording (like the others from Loops ’05) is no longer available. I have my own copy, but nowhere to post it for others.
You mention the intellectual independence of the best young QG researchers. It is a strong point. None of them, that I can think of, are merely working on some senior person’s program. They tend each to have a definite individual research direction.
I don’t see why universities are not scrambling to hire these people in junior faculty positions. It looks like where the future action is, but apparently deans and department chairmen haven’t realized this.
It sounds good, in principle, to say that the academic rewards and support should go to those who make independent progress and not be attached to this or that program. However it would probably help to create prospects for the present crop of postdocs if non-string QG approaches were validated in visible ways.
QG could probably use a web-magazine of some kind. Not limited to scholarly papers but open to comment, news items, usual blog-stuff.
And I mean non-string QG.
There isn’t a good collective name for it—-simply saying LQG can lead to confusion (people cite Nicolai et al thinking that is a critique of current LQG work)—-and I see that you resort to circumlocutions such as LQG and related research programs (such as CDT or programs based on quantum information ideas)
or as you say elsewhere LQG and other background independent approaches
and when you even just say LQG you mean, I believe, to include spinfoams—which many people don’t seem to realize under that heading. The Nicolai et al paper purported to review LQG but did not discuss spinfoam—or any path integral approach—as i recall.
So there is a bothersome communication problem. To improve prospects for young researchers you need to publically validate the field and point out the rapid progress being made. but you do not even have a NAME for the field!
Much of the progress is in Path Integral approaches but if you say LQG then stringy-people will make a lot of noise about strict canonical LQG (as defined 5 or 10 years ago and reviewed by Nicolai et al) and this will tend to drown out whatever you say about LQG.
You need a name for this allied group of QG approaches which is short and immediately recognizable and which people cannot intentionally or unintentionally confuse with circa 1995 canonical LQG.
Something like QG*
where the asterisk indicates non-string, background independent, non-perturbative, and that spinfoam and other path-integral approaches are included
and you need a blog with a name like QG*, which reports all the progress being made and emphasizes all that is happening in the field, what different postdocs are doing and where they are going, what the most active centers are. (which turns out to be mostly outside the US, so less visible to people in US habituated to think of Harvard and Princeton etc. as where the important physics research happens)
and I think maybe you need to drum into people the message that
if a physics department is not strong in QG* then it is apt to lose standing the next time round.
But agreeing with your point that QG* is not one specific program but a hodge-podge of independent directions which various independentminded individuals have gone in, off the beaten path, just (this is a delicate point) the successful ones who have made a real contribution and real progress by doing that. Because your main point is that support should not be tied to some senior person’s research agenda.
No time to edit this and make organized and brief. Hope helpful anyway.
Lee Smolin said:
LQG and related research programs (such as CDT
What does causal dynamical triangulations have to do with loop quantum gravity? They seem completely disparate approaches to me.
For Marcus, what you don’t seem to realize is that many people have looked at these approaches and they don’t think they’re fruitful. Lee has his FAQ, but people disagree with him.
Hi Aaron,
**…seem to realize is that many people have looked at these approaches and they don’t think they’re fruitful. Lee has his FAQ, but people disagree with him…**
I can see for myself that certain of the QG* approaches are fruitful. I don’t need your authorities. Nor, actually, do I need Lee Smolin’s FAQ to tell me about recent results—several of those I have in mind came after his paper with the FAQ was written.
I’m not attempting to argue by authority. I’m trying to explain to you that the issue isn’t one of publicity or prejudice or whatever. It’s a disagreement on the merits.
Yes, but how are we going to get there? The relics who run the research establishment are not going to willingly give up power and young researchers are generally streetwise enough to know that however small their chance of getting tenure, it is going to be even less if they follow the Sinatra doctrine.
Dear Dan,
It is no problem to assess even very independent and creative young physicists, and distinguish those with promising ideas and results from those of non-professionals like the Bugdanovs. Their work is not in a vacuum, it is based on and uses established physics and mathematics and is aimed at solving long standing problems. The work can be judged both on the quality of the work and presentation and the promise of solving outstanding problems. A good test is surprise, is the idea new and potentially signficant? Is it an idea I wish I’d had, that I would like to work on? Has the person pulled off more than once an original idea that had an impact on the field? These are the kinds of tests we use when assessing the promise of young people in quantum gravity and foundations of quantum theory and we seem to have done well with them.
To Chris, There is no problem getting there. What I described is an ethic which is followed in some fields of physics and mathematics. It was the governing philosophy in what used to be called “the relativity community” which was why there were so many important results from relatively few people from the 1960s on. I would claim that those communities that don’t adhear to it are anomalous and suffer from slow progress because of much duplication of effort and “me too” science and not enough risk taken by people trying to solve the hard open issues.
To Who, Thanks for the suggestion of QG*. We generally speak of just quantum gravity. Of course LQG includes spin foam models. There was a LQG email news and debate distribution list run for a few years by Fotini Markopoulou, this could certainly be now web based. Thaniks for the suggestion, Ill pass it on.
To Aaron,
At the level of statements of results, the basic claims of LQG and CDT are generally accepted. I assume you agree with this as no critic has shown a technical error in an important result-such as those listed in my review. There is of course disagreement on their significance.
Let us put the question this way: it is a non-trivial question whether there could be any examples of background independent QFT’s. In fact there are some examples. There are of course the TQFT’s but there are others including 2+1 gravity with matter, which we now know a lot about and there are a number of rigorous results about 3+1 including the existence and uniqueness theorems, the finiteness results about spin foam models and the CDT results. So you may disagree that nature should be background independent. But you cannot disagree that there is good evidence that such theories exist.
As to what CDT and LQG have to do with each other, they are both background independent approaches. CDT can be seen as very simple spin foam models. A number of people have worked on both.
Thanks, Lee
Lee,
I believe the non-trivial question for LQG and similar approaches is whether they are indeed quantum theories of gravity, in other words do they contain semi-classical states and propagating gravitons in 4dim. I think lots of people will become interested in any quantum model that can be shown to contain gravity as we know it, whether it is BI or not. Since there is some progress on this recently, maybe one can be cautiously optimistic.
I’m entirely too tired to go through the usual rigmarole yet again, so I’ll just stand by my statement that people disagree on the merits.
I will suggest a citation as footnote to something in Lee’s post:
http://arxiv.org/abs/hep-th/0512113
Effective 3d Quantum Gravity and Non-Commutative Quantum Field Theory
Laurent Freidel, Etera R. Livine
9 pages, Proceeding of the conference “Quantum Theory and Symmetries 4” 2005 (Varna, Bulgaria)
“We show that the effective dynamics of matter fields coupled to 3d quantum gravity is described after integration over the gravitational degrees of freedom by a braided non-commutative quantum field theory symmetric under a kappa-deformation of the Poincaré group.”
This paper summarizes work described elsewhere in more detail. Here are some related papers including one that Freidel recently co-authored with Shahn Majid
http://arxiv.org/hep-th/0502016
http://arxiv.org/gr-qc/0506067
http://arxiv.org/hep-th/0601004