|Nov10-07, 12:05 PM||#1|
Gang of four--Lisi, Loll, Reuter, Rovelli--make trouble for stringery
String thinking has been described as a framework for constructing theories---although one which has not as yet produced mature testable theory. As such there are some assumptions which serve as justification---assumptions about the NEED for and about the GOALS of the stringy way. As far as I can tell there are two main string-related big news stories going on at present
A. the string research effort in USA is dwindling due to internal reasons. Citations of recent work are down. Cutbacks are planned. The researchers themselves seem less motivated and productive than, say, 5-10 years ago.
B. the justifying assumptions are being challenged. This is causing occasional outbursts of reaction which are instructive to watch.
I think you can learn a lot from watching academic turmoil---key points that you might otherwise miss---what issues the hulabaloo and outrage seem to center around. Also it is just interesting.
So I've chosen four researchers whom I see CHALLENGING those justifying assumptions---whether about the need for stringery or its goals. In most cases the name I give is just one standing for many. There are substantial groups in these cases so that I should really put "et al." after the names. These are people that we should be alert to reaction against them, because their work creates some form of cognitive dissonance.
1. Loll makes spacetime emerge from a deeper description.
In recent years at conference talks, David Gross has been saying "We don't know what string theory is." And he usually continues by saying something like "We are missing some essential idea. Perhaps space and time are not fundamental realities, but appearances which emerge from some more fundamental [descriptive] degrees of freedom."
Loll et al have discovered fundamental dynamical degrees of freedom from which a spacetime emerges whose very dimensionality is a quantum observable and varies with the scale at which it is measured. At macroscopic scale the emergent spacetime has the expected 4D dimensionality.
A theory is a human artifact. If you don't know what it is, then it does not exist. Loll's theory is not the dream of a theory, it is a theory. And like any real scientific theory is testable (=falsifiable). It predicts that LHC will not see evidence of extra spatial dimensions. If such dimensions are demonstrated, Loll's theory is wrong.
2. Lisi proposes a Theory of Everything with 18 new particles and no free parameters
The importance of the Lisi E8 theory now taking shape is not merely that it challenges the "only game in town" assumption. The challenge to stringy mindset is only a side issue. But that side issue is what this thread is about! I want to point out that various developments ARE creating a stir by challenging assumptions basic to that mindset, so we can better understand the reaction.
E8 theory is predictive (that is to say falsifiable) because it has no free parameters to adjust. It will say what it will say---and if that is shown to be wrong, then the theory's wrong.
As development proceeds changes might be made to the action and to the way E8 symmetry is broken, but a good many features are already locked in as unalterable predictions. Like the 18 new particles---which might serve to resolve the astrophysical dark---or might serve to trip the theory up!
I don't see how E8 theory could accommodate extra spatial dimensions. So I believe it too would be falsified if such were discovered at LHC. In that sense also it is predictive. (When you make a prediction you have to specify the energy. The string framework does not predict that extra dimensions will be seen at LHC and it does not predict that they will NOT be seen---it is not at risk either way and is therefore not predictive regarding this.)
E8 theory predicts what reactions are allowed for both the new and the already observed standard particles. So even though it is just taking shape the theory is already offering the prospect of something experimentalists can look for. Traditionally this is what hep-th is supposed to do.
An unidentified moderator at ArXiv has removed Lisi's paper from the arxiv hep-th section.
3. Reuter presents evidence that quantum gravity is renormalizable after all
Reuter et al are accumulating an increasing weight of evidence that gravity has a renorm group fixed point at high energy and thus can be SAFELY TREATED BY ORDINARY QUANTUM FIELD THEORY. This contradicts a major justification which has been assumed by string thinkers and others.
It has been assumed that there is some NEED to take drastic steps and invent novel structures. The perceived need was based on the idea going back to at least 1970s that Einstein gravity was not renormalizable, could not be extended to high energies, was therefore not fundamental and would therefore be replaced by a more fundamental theory of gravity possibly arrived at by unification with other forces---hence the HOPE and faith placed in stringy approaches was based in large part on the imagined non-renormalizability.
So Reuter et al challenge a major premise.
Reuter can certainly turn out to be WRONG but so far the evidence continues to mount up. The approach is also predictive in the obvious sense that it would be falsified if extra spatial dimensions were exhibited. Interestingly, Reuter's approach converges with that of Loll in what it says about dimensionality at very small scale. Both theories predict scale-dependent dimensionality---4D at large scale declining to around 2D at small.
Reuter's approach is (metric) background independent. i.e. does not depend on prior choice of geometric background.
Reuter's approach has been subjected to scathing attack.
4. Rovelli et al progress on the dynamics and large scale limit front
As a challenge this is lower key and longer term than the others. Speakers for the "only game in town" point of view have typically dismissed possibility that LQG would achieve a successful dynamics with correct largescale (Newtonian or classical GR) limit. The point is Rovelli et al are making visible progress!
The essential in LQG has always been that it focuses on a manifold CONNECTION. In LQG proper, the geometry of an underlying manifold is always seen and described by a connection (rather than a metric or simplex triangulation, as in other approaches.)
The configuration space of the quantum theory, on which quantum states are defined, has always been a space of connections.
How you define the quantum states and their evolution (say by spinnetworks spinfoams or some other means) is secondary, as I see it, to the main goal of arriving at at background independent quantum dynamics of connections.
One thing that Lisi's work does is confirm what has always been suspected namely that this goal is worth pursuing. To unite matter with geometry, Lisi's program is to have a quantum "connectio-dynamics" where the connection has a larger gauge group than just for gravity. It should be an E8 connection instead of just a deSitter connection or whatever. Do LQG but with a much larger Lie-algebra.
Well Rovelli et al, which is a lot of people not necessarily all at Marseille or all collaborators, are doing is basically working out the details of "connectio-dynamics".
Correct me if you think I'm wrong, but that's what I think the main idea of their work is.
This year there have been a lot of papers about the "new LQG vertex" which is basically a way to do a PATH INTEGRAL QUANTIZATION of how the quantum state of the connection evolves dynamically.
Loll and Reuter each show a different face of gravity---Reuter uses the 1915 einstein metric to describe geometry and Loll uses a triangulation with identical blocks---these are both very interesting and show stuff about how spacetime could actually emerge, but they don't have the valences to bond with Lisi. He wants a connection to be in the picture, and that is what Rovelli et al have.
I should say what a connection is, as a technical idea---a mathematical construct in differential geometry. But this post is already long so I'll put it off till next.
|Nov10-07, 12:28 PM||#2|
I should give some links to papers representative of these people's lines of research.
For Rovelli here's something on the Marseille new vertex:
Here's Renate Loll's latest on the Utrecht Legoblock path integral:
Reuter's latest survey/introduction paper is http://arxiv.org/abs/0708.1317
Lisi's paper that is causing such a vigorous response is http://arxiv.org/abs/0711.0770
Some fascinating discussion has occurred here
|Nov10-07, 12:51 PM||#3|
|Nov10-07, 01:15 PM||#4|
Gang of four--Lisi, Loll, Reuter, Rovelli--make trouble for stringery
Hi Marcus, I agree with a lot of what you say, but need to suggest a few things.
First, please don't name it "Lisi theory." As flattering as this is, I didn't just pull it out of my, err, head -- it's come from bringing together the work of many others. Calling it "E8 Theory" is fine -- that's what Baez referred to it as, and I like that. I don't consider it "my" theory, and if it's right I suspect it will develop beyond where I could take it.
Second, I'm not committed to predicting 18 new particles yet. There are parts of the theory I don't understand perfectly, and until I do I don't think there's reason to predict new particles with any confidence. The LHC does set a nice soft deadline for this.
Third, I am excited about these ideas too, but there are many big "if"'s that need to work out right before they come together as a fully successful ToE. I appreciate and enjoy your enthusiasm, but try not to sensationalize this stuff too much -- we're scientists, not football teams.
And you've forgotten many other people working on these or similar ideas, but you mentioned that, and I won't try to make a list. I do agree and think it's great that the quantum gravity community is prospering, and may lead to theories (possibly this one) that succeed where strings have failed.
|Nov10-07, 01:52 PM||#5|
But i will try to cultivate a little restraint.
It would be strange if something came of it, but I'm hopeful we will hear something in the next couple of months.
|Nov10-07, 01:56 PM||#6|
OK, great. Carry on. :)
|Nov10-07, 03:39 PM||#7|
Blog Entries: 1
|Nov10-07, 05:35 PM||#8|
you refer to Weinberg's 1995 paper with the notion of "modern renormalizability"
That may not be what Reuter and Percacci are talking about. I don't recall them ever citing the 1995 paper, or mentioning "modern renormalizability".
Instead, they always cite a 1979 piece by Weinberg which defines "asymptotic safety". This seems to be different, and different from what you are describing.
Reuter often refers to gravity as perturbatively non-renormalizable but NONperturbatively renormalizable. I think it is a useful distinction.
Your post seems to be embedded in a perturbative context---you speak of terms in a perturbation series, and counterterms. Reuter considers gravity to be NON-renorm in that context. If you take the theory out of a perturbative context, then according to him it can be PREDICTIVE AND FUNDAMENTAL.
predictive once one experimentally determines the values of some finite number of parameters, like three say.
fundamental if the UV fixed point is in fact real----mounting evidence now suggests that it is.
Percacci, in his 2007 paper titled Asymptotic Safety discusses the issues of predictive and fundamental. You might be interested in reading it.
Asymptotic safety is a program that Weinberg initiated around 1976-1979 and made some progress on, but gave up on. Reuter picked up around 1996-1998 where Weinberg left off.
the doctrine had already been laid out by Weinberg. thing was, it depended on finding a nontrivial (UV) fixed point of the renormalization group flow,
sitting in a finite dimensional critical hypersurface (in which the fix point was an attractor),
and demonstrating that the fixed point is really there.
if you can do that, it's beautiful---but doing that can be a formidable job. Reuter Percacci and friends look as if they may have succeeded.
You mention not being expert---I should hasten to add that I'm not either, I just have read some of Reuter's papers and a couple of Percacci's. A good way to get clear about definitions would be to look at pages 5 and 7 of Percacci
page 7 gives the definition of A.S., critical surface, predictive
page 5 says what they mean by the theory being fundamental rather than merely effective (if indeed it is, this has not yet been determined)
Although not expert and not having a complete understanding, AFAICS there's nothing hard about the basic ideas, they are just unfamiliar because do not fit with ordinary perturbative context where you perturb around flat Minkowski space and develop a series.
Several people here at PF have taken a short course from Reuter earlier this year, at the Zakopane QG school, so they might help if you have further questions. f-h is one, francesca is another.
|Nov10-07, 06:54 PM||#9|
|Nov10-07, 07:21 PM||#10|
But this has been written about quite a lot. I particularly recall a paper by JoAnne Hewett of SLAC/Stanford. She was enthusiastically imagining finding signature of extra dimensions at LHC and presenting this as a possible test of stringy ideas. But AFAIK the string approach has not predicted finding indications of extra D at LHC energy, nor has it predicted NOT finding. So I found JoAnne's paper disappointing. It was intended as a serious paper but at best it offered a prospect of testing LQG and other 4D theories.
She is a prominent and influential experimentalist who serves on national committees etc, so I should probably suspend disbelief and take her word for it regarding prospects for seeing extra D.
|Nov10-07, 07:29 PM||#11|
we must NOT keep optimism. your most optimistic moment was when you posted the photo of Hilbert's grave
as you point out ironically it does NOT work to blur the distinction.
what I value is that Garrett is clearly a researcher and I am clearly a spectator, yet not only can I meet him online but we can even discuss some things.
there are rare individuals who are both---a kind of philosophical self-consciousness allows them to be scientists and also to introspect about what that means, and witness what they are doing. maybe you are both. but they are rare enough that we don't have to allow for them and can have a simple dichotomy.
|Nov10-07, 07:41 PM||#12|
But a point about, for instance, football spectators is that they acknowledge their lack of training to play the league, but they usually have a good knowledge of the internals of the game; even they play in minor leagues. Even, with some handicap, the amateurs can do a good play with/against the pro, for instance in non competitive matchs. I can not see we I can not expect the same of science "spectators". The problem, I think, is that the internet only provides information about the rules, but not incentive for the practice of the game.
Hmm, that picture... it is not only that I posted it; I traveled to Gottinga to take the picture in 1990. I think to remember I even left some scrap of paper in the grave, sort of Jerusalem Temple Wall.
|Nov10-07, 08:00 PM||#13|
I would have liked to read that scrap of paper.
it is right about the internet having no adequate minorleague peer-review playing fields for the spectator who wants to practice the game.
peer-review publication is the key binder of the science community
to some extent PF moderators do filtering here but it does not achieve the level of a bush-league science community. bush-league is an idiom for minor minor league---almost like kids in the neighborhood playing in a vacant field.
originality is the hang-up. I dont want to be original, I just want to see how to paraphrase work I respect.
you are different. you do original work (with Hans and others) and post it on arxiv. so you are already in minor league play===as well as being a thoughtful spectator.
well it is an interesting idea
We should try to get back on topic. How did you react to the reclassification of the ToE paper from hep-th to gen?
I was depressed by it. I want to respect arxiv and this was such a base action. I don't like to experience such contempt (for Distler probably).
UPDATE: I see that Fabrizio Nesti showed up at Bee blog and joined the discussion. I hear a friendly crunching noise.
Here is a link to Nesti's first post.
|Nov10-07, 11:39 PM||#14|
None of these ideas penetrate the 'darkness' of hep, IMO. Loll probes more boldly than most, but does not cross the 'line' of reason. Let the LHC chips fall where they may. I do not foresee any grandiose results, but, 'baby' steps are plausible.
|Nov11-07, 01:46 PM||#15|
One can hear, here and there, rumours about arguments originating the existence of hep-th in first place, and depending who tells the history it can happen that hep-th is seen, from hep-ph, as gen-ph is seen from hep-ph.
Were I to choose about organizing the arxiv, I would stress HEP against VHEP or UHEP. Any paper depending on the Planck mass is not HEP.
|Nov12-07, 09:30 AM||#16|
It's interesting to hear your ideas about electronic science media. You've had more experience and have thought about the issues more than most. Scientific media help define scientific communities----and essential media filtering is arrived at only thru some kind of formal or informal community consensus. so media and community interact and shape each other the way matter and geometry interact in Garrett Lisi's E8 connection picture.
I want this thread to contain INTUITIVE THUMBNAILS OF EACH OF THE FOUR approaches mentioned at the start. here is the best short intuitive description of the main thrust of E8 theory that I have seen:
==quote Woit blog comments==
Because all these fields are parts of E8, we can assemble an E8 principal bundle connection (technically a superconnection) which consists of 1-forms and Grassmann fields valued in this E8 Lie algebra, and use the curvature of this big connection to get the dynamics. The curvature, in the action, determines how E8 interacts with itself. And since everything is part of E8, this corresponds with how all the fields of the standard model and gravity interact with each other. This action is built by hand to match the standard model, which is an inadequacy of the theory, but it’s very concise. And I think it’s bloody amazing that this works at all.
“I’m meanwhile somewhat confused as to the physical significance we are meant to take from the various features of E_8’s structure.”
Physically, as this theory develops it should make definite predictions for the coupling constants, predict a handful of new particles and some non-standard interactions, and (if things go astoundingly well) have something to say about the particle masses. These predictions may end up being wrong, killing the theory, but so far things are looking good.
“I’m particularly baffled by what to make of the big root diagram”
The root diagrams correspond to the structure of the Lie algebra. If two E8 roots add to give a third (in eight dimensional Euclidean root space) then the Lie bracket of the corresponding two Lie algebra basis elements give the third. In the paper, I describe which roots correspond to which elementary particles. Also, since the projection used to plot the eight dimensional root system is linear, you can determine particle interactions by adding these roots together as two dimensional vectors, extending from the origin — it’s fun, try it. This is standard representation theory, and it’s very pretty. In my opinion, Peter doesn’t push his own subject hard enough — I wish I had learned about this stuff earlier than I did, it’s wonderful.
“are the rotations just showing the root system diagram in different ways to make the different groups the system breaks down into visually clear?”
“Hm, okay, so additions of E8 roots produce interactions between? What are those “interactions”? Or is this specified by the yang-mills action or something?”
Yes, the addition of the roots corresponds to the Lie brackets between fields, which is in the curvature, which is in the action, and this gives the interactions between particles, appearing as Feynman vertices in QFT calculations.
|Nov12-07, 10:35 PM||#17|
About the topic of the internet as a way of becoming a player my position is very clear. Yes, it allows it.
In fact with avaiability of arxiv, and other sources to have access to sophisticated science maeterial and blogs and phorums to discuss the only big difference of beeing in a university (or similar) is that you are payed for doing research.
There are still some dependence in the personality of people. I have friends who like to got to conferences and learn a lot in them. I go to somes ocassionaly, but mor my taste I prefer to search articles in the topic and read them.
Other advantage of not being in a university is that you have not the pressure to publish things that you actually know that are not of any mayor interest but that will fit in a review and will give you the points you could need to get a tenure (or a better position in the academic burocracy). Moreover, you can study whatever you want and not depende on the "environment".
To be honest, nowadays the only reason (I agree, a very important one) I find to worry about an academic carrer is the money.
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