Why I am REALLY disappointed about string theory

[tom.stoer]

I know that these issues have already been identified and discussed since years. Nevertheless it was intersting to re-derive them in our discussion and to agree on them.

You can hear (quite frequently) statements like "string theory is the only theory of quantum gravity we currently have" and "string theory is fully background independent". So there was some value in our detour.

That brings me back to my question regarding the intrinsic obstabcles (I think I would add "mathematical complexity") and to atyy's question what the most promising research programs addressing these issues are.

 

[marcus]

Nevertheless it was intersting to re-derive them in our discussion and to agree on them.

Definitely!

 

[tom.stoer]

So before leaving I'll try to summarize the string theory internal obstacles

• background independence
• off-shell formalism
• mathematical complexity

 

[atyy]

http://arxiv.org/abs/1002.1120
String Theory and Water Waves
Ramakrishnan Iyer, Clifford V. Johnson, Jeffrey S. Pennington
"While string theory has had remarkable successes over the last several years, accelerated by the revolutions in understanding its non-perturbative properties, it is still very much the case that we do not yet know what the theory is. ... For the problems outlined above, it would be rather excellent to have the simplest possible string theories that still contain some of the marvellous non-perturbative physics we know and love, and be able to follow them as they connect to each other in ways that are entirely invisible in perturbation theory. Further icing on the cake would be to have the physics all captured in terms of relatively familiar structures for which there is an existing technology for its study. This is the subject of this paper (and a follow-up to appear later[4]), at least in part."

 

[Fra]

Tom, thanks for starting and interesting thread with many good posts. I've been in Rome for a week and just got back.

I'll just add a comment on one of the later disussed topics, from my own perspective (which aims for an intrinsic inference model; and physical interactions ~ inferences between observers; and that no interactions without context(observer) is possible).

I share a lot of what several have said but I just want to add a nuance in the discussion of B/I. A point where I disagree with some typical critics against ST background dependence. My point is to try to understand this in terms of measurements, and that what we are talking about here is not just mathematics, it's the quest for observer independence. And the real question is to what extent the belief in observer independence is rational and scientifically justified, and what it even MEANS? I don't think this is just a philosophical point.

If you start with only one background and if you only nw this background, you have no chance to explore the whole space of solutions.

That's what happened in string theory. There is no global picture that allows you to look at the whole theory. You can only look at individual pieces and hope to be able to constuct dualities or something like that.

I was thinking that string field theory would provide something like this global picture.

If we associate background ~ the context of an observer, the choice of background or vacuum etc, is physiclly the problem of specifying the observer.

(Anyone object to this association?)

I sense that you ask for a global observer independent picture? But does that make sense and resonate with the scientific idea of a measurement theory that information should be infered?

If inferrable/abducable "theories" require an inference context ~ an observer ~ background (in some general sense) then the observer invariant "supertheory" just wouldn't be inferrable, computable or representable? If this is the case, isn't the quest for observer invariant gods view, just a remnant desired from structural realism? is it scientifically justified?

I'm just raising the question, of what scientific status - in terms of measurement, computation and representation - such a supertheory or space of all theories would have?

My opinon is that the landscape in ST is a problem, but that I think the solution is not to seek some universal static observer independent view. Another solution may be to instead consider the physical world as interacting evolving observers with incomplete views, WITHOUT background (meaning also NO inferrable transformations that transforms deterministically between the set of observers).

This doesn't support string theory, I just want to add a different version of the critique against the lack of BI suggesting that BI in the sense of strucutral realist observer independence is hard to justify scientifically, since there is no way for any single observer to infer, compute, decided this. And that it may suggest a different way of looking at the "BI problem"?

/Fredrik

 

[atyy]

And the real question is to what extent the belief in observer independence is rational and scientifically justified, and what it even MEANS? I don't think this is just a philosophical point.

Smolin, http://arxiv.org/abs/gr-qc/9508064
Thus, our goal is not to eliminate the observer, it is instead, to relativize him. We would like a formalism that allows us to divide the universe arbitrarily into two parts, and call one part of it the observer and the other the system. We would like there to be something like a gauge symmetry, that expresses the arbitrariness of the split. And, most importantly, to satisfy the principle, we must do this in such a way that it is impossible to construct a single state space that would allow us the possibility of speaking in terms of a description of the whole system by an external observer. ... Thus, our slogan is “Not one state space and many worlds, but one world, described consistently by many state spaces.”

Van Raamsdonk, http://arxiv.org/abs/0907.2939
"we will argue that the “glue” connecting various parts of spacetime together is quantum entanglement between the corresponding degrees of freedom in the non-perturbative description. ... The mathematical structure that we observe in section 2 shares some features with an approach to quantum gravity called “relational quantum cosmology” [11], which also involves associating quantum states in a number of different systems with a single quantum spacetime. The association of specific Hilbert spaces to particular causal patches is also implicit in Bousso’s discussion of holography in general spacetimes [17, 18], and it is central to the holographic space-time proposal of Banks and Fischler [8]. "

 

[atyy]

It was me, see: https://www.physicsforums.com/showpost.php?p=2386391&postcount=9
This expresses my personal view, and the view of other colleagues but certainly not of all of them. And I am very glad that at least one can remember a statement over threads.

What did you have in mind here? I know you were being very speculative, but a few pointers to directions in the research literature for lay outsiders would be much appreciated.

 

[Fra]

Thanks Atyy, those views highlights one part of the problem - they reject the objective quantum state of the entires universe, in favour of sets of relative/subjective states.

So far so good and I agree.

But the next, more tricky question which was my main point is to know the structure of this "set of sets", and what transformations or evolution relations that exists within this set, and what inference status we have on this.

As far as I know, there aren't much published at all, in the more radical direction I have in mind, but the one coming closests is probably Smolin, in particular leaning towards the Unger angle, although they didn't publish anything beyond philosophical talks.

Some comments

And the real question is to what extent the belief in observer independence is rational and scientifically justified, and what it even MEANS? I don't think this is just a philosophical point

Smolin, http://arxiv.org/abs/gr-qc/9508064
Thus, our goal is not to eliminate the observer, it is instead, to relativize him. We would like a formalism that allows us to divide the universe arbitrarily into two parts, and call one part of it the observer and the other the system. We would like there to be something like a gauge symmetry, that expresses the arbitrariness of the split. And, most importantly, to satisfy the principle, we must do this in such a way that it is impossible to construct a single state space that would allow us the possibility of speaking in terms of a description of the whole system by an external observer. ... Thus, our slogan is “Not one state space and many worlds, but one world, described consistently by many state spaces.”

With one reservation I agree with their main message here, and it's in line with what I think. The problem is here

We would like there to be something like a gauge symmetry, that expresses the arbitrariness of the split. And, most importantly, to satisfy the principle, we must do this in such a way that it is impossible to construct a single state space that would allow us the possibility of speaking in terms of a description of the whole system by an external observer. ... Thus, our slogan is “Not one state space and many worlds, but one world, described consistently by many state spaces.”

It's clear what they mean here, and the first step is to my liking, but, the problem is that the "symmetry" that provides/defines consistency is not an inferrable/observable structure.

Edit: A clarification what I mean. The correct statement should be that the symmetry is inferrable as in inducable (ie it remains uncertain), but it's not deducable in the logical mathematical (non-uncertain) sense. For most practical purposes there is no difference, but it is a big different to the way you view this, and what implications it may have on the framework. So the emergence of the symmetry should be more like a statistical process, except there is no global objective probability space.

This is the point where rovelli resorts to structural realism. The idea to "relativize the observer" is of course right, but the problem is how: they implicitly assume that there MUST EXIST a representable mathematical transformation or symmetry that defines this consistency. This expectation is not justified - this is my objection. Instead I think the existence of an objective consistency condition only makes sense if you consider equiblirium, where a local group of observers are reasonably equilibrated.

So what they say makes good sense to me in the equilibrium approximation. As long as we keep that in mind, it's a good start.

The non-equilibrium problem them becomes that of how to infer these consistency transformations from the inside. They say that no observer can hold a complete view, and this is true. But each observer can still hold a reasonably complete view of the symmetry that exists in it's closest environment (where there is causal contact), and that this should yield an evolving symmetry, where consistency is violated off equilibrium. I think this requires some new mathematical framwork though, and I'm not aware of anything cleanly published in this direction. Conceptually, I think Roberto Ungers "social law" analogy is good.

Van Raamsdonk, http://arxiv.org/abs/0907.2939
"we will argue that the “glue” connecting various parts of spacetime together is quantum entanglement between the corresponding degrees of freedom in the non-perturbative description. ... The mathematical structure that we observe in section 2 shares some features with an approach to quantum gravity called “relational quantum cosmology” [11], which also involves associating quantum states in a number of different systems with a single quantum spacetime. The association of specific Hilbert spaces to particular causal patches is also implicit in Bousso’s discussion of holography in general spacetimes [17, 18], and it is central to the holographic space-time proposal of Banks and Fischler [8]. "

As far as I see, this suffers from the same structural realism. They resolve part of the point, but the problem of "relativize" the observer in a physical way, and not just mathematical way is missing.

It seems they are trying more or less the same trick we konw from SR, GR and gauge theories. The problem is that they seem to put in manually the choice of symmetry transformation. I think that the "abducable" symmetry, has to be emergent by means of a physical process and it's this physical process we need to describe (in terms of probably a new mathematical framework). The relativity defined by means of fixed transformation groups doesn't seem to have the right traits?

Edit: When picking on the fixed transformations, I also expect a solution to unification of forces, this is where Ithink the evolving symmetries will be useful. The unification could be accomplished in principle by scaling the observer complexity to zero. During this scaling various "phase" transitions will occurse that merges interactions into indistinguishable ones.

/Fredrik

 

[tom.stoer]

I am sure that observer-dependence (or -independence) is an interesting problem, but I don't think that it's at the heart of the string theory issues. If you look at the string theory Lagrangian it's nothing else but a Langrangian (spiced with some supersymmetry etc.). The problem is that treating it mathematically always requires a) to break somehow it's invariances and b) to check that nevertheless the invariance survives somehow.

The problem with background invariance seems to be that by introducing a background means to break the theory in different sectors or even in different theories. To check background invariance is much harder than to prove e.g. "gauge symmetry after gauge fixing" (BRST or something like that) because you have still one theory.

To me background invariance is not necessarily an ontological issue; it could very well be an issue regarding mathematical complexity only. It seems to be likely that we still do not have (or understand) the mathematical tools.

 

[Fra]

I am sure that observer-dependence (or -independence) is an interesting problem, but I don't think that it's at the heart of the string theory issues. If you look at the string theory Lagrangian it's nothing else but a Langrangian (spiced with some supersymmetry etc.). The problem is that treating it mathematically always requires a) to break somehow it's invariances and b) to check that nevertheless the invariance survives somehow.

Yes, I like that way of putting it.

If I may transcode what you sa into how I'd like to put it:

by introducing a background (ie by enforcing that the THEORY itself is abducable or in a generalized sense "measureable - ie by "introducing an observer" that implements this inference), we genereally might break any objective theory.

But this is exactly why my conclusion is that the theory itself evolves. Ie. there IS no objective theory in the strict sense. All objective theories are merely effective.

The timeless eternal picture of a mathematical fixed theory of everything is not compliant with the inference model since no inference machinery can establish such a picture for several reasons. If the theory is to be the result of an inference process from inside observes, the entire notion of "observer invariance" really needs new understanding. There is no objective fixed symmetry or set of transformations that defines this (which is what you would need to "check that invariance survives"), the "observer invariance" is perhaps better replaced by "observer DEMOCRACY", where a quasi-objective consensus is emergent just like social laws are.

So in the evolving pictures, the invariance may in fact not survive, but then what happens is that the population defining the observer democracy is changed, so that a new invariance is establish as a new steady state.

I like the analogy here to Einsteins static universe. I think the that nature of relations here, means that static laws of the universe are no more sensible than is Einsteins original quest for a static universe.

To me background invariance is not necessarily an ontological issue

Maybe to me, it's a bit also of an epistemological issue: since I question the nature of the process wherby the background invariance or non-invariance is established - processes in nature are not deductive by nature; they are inductive as there are always uncertainties. This is not reflected in our mathematical models of today.

It seems to be likely that we still do not have (or understand) the mathematical tools.

Yes that's a possibility, I agree. I too think that we need a new mathematical framework, but something that replaces partly the deductive approach with an inductive more flexible approach. Maybe we even need to unify not only forces, but also mathematics (or the abstraction of all computational and representation processes). In this sense one would marry math and physics more, and not just study the mathematics of physical systems but also study the physics of the actual realisations of mathematical systems and computation devices. Only then does things like fitness of algortihms and datacompression enter the picture due to finite resources. Compression ratio is always competing with decoding speed etc.

/Fredrik

 

[tom.stoer]

Maybe you are right - but your post is not regarding string theory, it's regarding ALL physical theories we have constructed so far ...

 

[Fra]

Maybe you are right - but your post is not regarding string theory, it's regarding ALL physical theories we have constructed so far ...

That's true.

But to get back to your focus, I guess what I tried to say (a little bit in defense of string theory) in the first post of mine in this thread does relate all this to the

Landscape problem and the lack of B/I formulation of ST.

My opinon is that sometimes the critique against hte lack of B/I in ST, is a little bit simple minded in that it ignores some of the issues I tried to illustrate. Namely that the nature of that B/I means, and how it can or can not fit into an measurement/inference perspective is not trivial.

There is somewhat of a paradox there; to required that we talk only about measurable things, and to required that we are independent of the measurement machinery. The two traits don't add upp consistently - thereof the quest for new ideas.

So maybe the landscape is just the set of observers, defining the democracy? Then maybe an evolutionary picture might help ST there.

That's the only small point I wanted to add, that has to do with ST.

(Still I don't want to give the impression that I like ST; I don't)

/Fredrik

 

[marcus]

I don't know if TomS and others actively want to continue this thread. It has been a very interesting thread and the main body of it may have reached a natural conclusion. The last post by Surprised contained a frank exchange of views worth quoting.
Since our system does not handle two levels of quotes, if I quote Surprised in the conventional automatic way the questions to which he responded will drop out! To avoid that form of incoherence I will just do it manually, with indent:

==excerpt from Surprised post #133==
...
...
It is simply not so that one is able to compute anything, even for a completely well-defined theory (try to analytically compute the hadron spectrum from the QCD langrangian, eg. And anything having to do with gravity is going to be much more complicated). So that's why supersymmetric toy models are so useful - as many things can be computed, sometimes even exactly. This is a quite non-trivial feat and source of a lot of excitement, as well as of many conceptual insights. Whether one would ever be able to get beyond studying toy models.. I don't know, but I doubt it.

Originally Posted by tom.stoer
; but what I still do not understand in all details is how one can argue that string theory fully incorporates gravity as dynamical background independent geometry.​

I don't think that anyone claims this!

Originally Posted by tom.stoer
Looking at the string theory action it uses a fixed metric in target space; there is no way how a propagating string can affect this geometry. Of course string theory contains all fixed geometries somehow, but it does not allow one to change from one to the other and to describe this via dynamical evolution. By that I mean that I cannot see how to formulate the collapse of a black hole in string theory; I cannot start with some geometry and then looks what will happen later. As far as I can see this is not due to technical problems, but due to conceptual one; I simply cannot formulate this question in the context of strings.
​

This is very true; at least for the on-shell formulation of string that we know. There is simply no known formulation which would allow to "compare" different backgrounds, describe tunnelings, etc, as all this would require an off-shell formulation that we don't have. Some limited toy models exist here and there, eg some insights can be gained by considering tachyon condensation, which is a model for relaxing to a ground state. Some other toy models for going off-shell are topological strings where one can identify on-shell vacua as critical points of off-shell superpotentials. AdS/CFT provides a background-independent setup in a certain sense, for a specific situation, but this also doesn't allow to address questions of vacuum selection or Calabi-Yau's, etc.

Obviously one of the major missing points in string theory is the lack of an off-shell, perhaps background independent formulation; I guess no one would contest this statement… it's hardly a point of disagreement for string physicists!

Originally Posted by tom.stoer
And if this is true gravitons ceased to exist since we a) do no longer study gravity in AdS with the help of "perturbative gravitons" but we b) we translated it to CFT where there are simply no gravitons :-)​

I would say if gravitons turn out not to exist, string theory is dead (in the sense of unification with gravity); it still would be relevant for gauge theories, and describe QCD strings etc.
==endquote post==

Here is the link to post #133

...
...
It is simply not so that one is able to compute anything, ...

Perhaps i could add my personal view that although I might find string publicity and behavior of individual theorists at times disappointing, I consider string to be a splendid extension of the great edifice of mathematics called differential geometry (the mathematics of smooth manifolds) and, as such, a valuable investigation in its own right even absent any definite expectation of relevance to physics. Since I look on stringy mathematics without any physics expectations, I do not find it disappointing. Many people do, but I do not, so my own thoughts don't fit in exactly with the stated topic of the thread.

 

[marcus]

In the light of what I just quoted, and the comment at the end, it is not actually disappointing to observe that stringy math depends to a large extent on an intensive use of manifolds---smooth continua---many different dimensions.
Manifolds come in any dimension, there are of course one dimensional manifolds. "Strings" (which come in several different dimensions) and their worldsheets are manifolds. The "branes" to which some strings must be attached are also manifolds. The whole works lives, in turn, in some larger "target" manifold of still higher dimension. Such a target manifold normally has a fixed metric geometry unable to respond to what is going on inside!
As retired mathematician, instead of being disappointed by such signs of the remarkable fecundity of differential geometry, I'm inclined to find them mildly satisfying. It's nice to see string mathematics give diff-geom and its manifolds such an extensive elaborate workout! (Not that I expect it to have anything special to do with the fundamental physical nature of space and matter. The signs are that manifoldless approaches are likely to take the lead there.)

 

[Fra]

I'll try keep the focus a little better. But to expand a little on the same point I already tried to make, relating to what surprised said that Marcus put forward in the light again.

I still have no ready solution, but I have some conceptual points that may guide the thinking.

This is very true; at least for the on-shell formulation of string that we know. There is simply no known formulation which would allow to "compare" different backgrounds, describe tunnelings, etc, as all this would require an off-shell formulation that we don't have.

They vision I have, does have a similar "problem", and by projecting ST to that, I would say that the missing "comparasion between different backgrounds" is the missing interaction or communication with different observers. This means that, given my personal association here that the "background" is part of specifying the observer (if I forget for a second about my objections to the continuum etc), what ST does is to describe the expected evolution of the environment with respect to an ny given observer (background), but that this background is fixed.

The sort of "B/I formulation" would correspond to how, TWO such observer interact with EACH OTHER. The exact same problem I see with what I think of as observer complexes. (The difference is that my picture of observer complexes are not "strings" but there are other similarities)

My thinking of this has come to the standpoint that, as rovelli puts it, the only way for two observer to LEVEL anything, is by a real physical interaction/communication. The problem is of course, how do you describe communication without a communication CHANNEL?

From the point of view of the two involved observers, there exists no description, and they have to resort to true evolution (darwin style). This means that part of the evolutio nis simply unpredictable, but it also means that the observers backgrounds are changed.

What can be, is that a third observer can give a partial prediction of how two observer complexes does interact, and how their actions CHANGE, and how this (but the connection between background~prior and their entropic actions) also how their backgrounds change.

This is in fact, exactly what we do already when one observer, observes how two particles interact in a lab. IT's just that we need to see, that the situation is the same, and that if we only could sort this out, also THOSE already known standard actions, should also follow from the same construction.

But I'm not suggesting some anthropic lame argument, I'm suggesting that maybe this could be made precise, and that the physical landscape, is MUCH smaller than the mathematical landscape AND that although there is no deductive scheme to navigate inthe landscape, there MAY be a inductive scheme.

The main problem I see is, that with the proposed conceptual model here, the continuum baggage makes it harder. But that leads to my other objections I already made here and hte other thread.

/Fredrik

 

[tom.stoer]

Marcus, all!

I think it's not up to us to close this thread as others may want to continue. But I agree with you that we have reached a "natural conclusion".

As a final comment from my side I would like to come back to my post #1: It was about disappointment and promises ...

One central statement was that the greatest achievement of string theory is that string theory turns most (all?) possible theories including gravity from theories into solutions derived from a (unique?) theory. Another central statement was that string theory comes with an enormous mathematical and physical apparatus, w/o being able to give us a hint why we should believe in this apparatus (10/11 dim., SUSY, CY, ...).

We identified central obstacles and problems which I would like to list again: lack of (full) background independence and off-shell formalism; mathematical complexity. No find is really new, so the discussion was more interesting than the final result :-)
Nevertheless we identified some good reasons why to believe in the theory even so it has this enormous complexity.

Regarding my personal impression: I think not so much has changed with my disappointment with the theory - but we (you!) established a much better understanding of the true nature of the achievements, problems and obstacles. So besides the problems I see (new) options for research programs in order to overcome these difficulties.

A last remark: Even so I am still (a little) disappointed with the theory I am not at all disappointed with this forum and this discussion! Thanks to all for their contribution and their patience!

Regards & Thanks
Tom

 

[suprised]

In the light of what I just quoted, and the comment at the end, it is not actually disappointing to observe that stringy math depends to a large extent on an intensive use of manifolds---smooth continua---many different dimensions.
Manifolds come in any dimension, there are of course one dimensional manifolds. "Strings" (which come in several different dimensions) and their worldsheets are manifolds. The "branes" to which some strings must be attached are also manifolds.

Well, no. Unfortunately this is confused by many string physicists as well. Whenever people talk about manifolds and strings, that applies only at a particualar region of parameter space (large radii, weak coupling). In other words, that amounts to the supergravity limit, where usual geometrical notions apply. But this is just a very small piece of the full parameter space, perhaps a subset of measure zero.

But away from this limit, these notions break down, and some kind of stringy quantum geometry emerges. For example, the notion of a D-brane wrapped around a p-dimensional cycle becomes ill-defined. What replaces this notion, is an object in some appropriate (derived, Fukaya etc) category. There is no other good way to describe a D-brane than in these abstract mathematical terms, generically (away from the geometrical regime).

I think that the thinking in terms of manifolds has a lot of merits, mostly technical, but has also done a lot of conceptual damage to the understanding of strings, eg in terms of "compactifications" of some higher dimensional theories. As I keep repeating, this picture is not generic and applies only to a very small corner of all possible string "backgrounds".

 

[MTd2]

I must agree with suprised. If one looks at the action of superstring sigma model, there is nothing there that hints the SUSY fields, which yields the dimensions, will organize with any specific pattern except for general restrictions (branes) on the degrees of freedom of the worldsheet.

I wonder if there are any good arguments, besides string gas cosmology ( review here http://arxiv.org/abs/hep-th/0510022 ), to the emergence of any geometrical pattern on strings.

 

[suprised]

Perhaps I should add: I wrote this to emphasize that there is no simple notion to characterize string theory in one sentence, like the geometry of manifolds. Because it is so rich - depending on in which corner one looks, one finds interesting physical or mathematical structures, but none of them capture the whole thing in any seizable way.

Yes, one may say that geometrical string compactitications have shed a lot of light onto the algebraic geometry of Calabi-Yau's, for example, but as I said, this pertains only to some corner of the space of theories. Yes, one may say that string theory is background independent, in the sense of AdS/CFT; but that's again only a corner. Or that strings give insights to gauge theory; black holes and heavy ion physics. Or that it offers a playground for conformal field theory, and mathematical applications of it, like the theory of modular forms and variations on the monstrous moonshine, division algebras, loop groups, categories, K-theory, and <whatever>.

So there is no simple way to say that string theory is just "this" or "that", "purely mathematical" , or whatever. It is a very complex web of aspects and relationships that confuses many people, including experts. I see this here with amusement, where so many people try to guess in simple terms what it is. Well, there is no simple answer!

 

[Finbar]

String theory is more of a fame work than a theory. In fact its probably best to think of all of modern particle physics as a collection of models which use different frame works e.g. the standard model is a model in the frame work of QFT etc.

Maybe a theory should then be something like quantum theory which is based on a set of principles or maybe special relativity. These have a more universal meaning in that all models should be approximated by them in some limit.

QFT in flat spacetime then consists of a framework in which all models will naturally obey the principles of quantum theory and special relativity. However String theory also seems to be theory that its relativistic and quantum mechanical so it also is a frame work in which models maybe be built.

 

[tom.stoer]

So there is no simple way to say that string theory is just "this" or "that", ... It is a very complex web of aspects and relationships that confuses many people, ... I see this here with amusement, where so many people try to guess in simple terms what it is. Well, there is no simple answer!

This is similar to the situation in the early days of quantum mechanics = before Heisenberg and Schrödinger. There were magic numbers and magic formulas; there were hints regarding spectra, angular momentum and selection rules, half integer spin, Compton scattering, quanta of fields like photons etc. But there was no simple statement what quantum mechanics "is".

Nevertheless some clever people were able to work this out in detail, so today there is a complex web of aspects but with an underlying clear concept what qm "is". What we are discussing is what qm "means", how it can by applied to gravity, to the universe, ... But the basic rules are clear!

My hope is that this can be achieved within string theory as well. That does not mean that problems like the landscape will go away, that one can calculate (uniquely) the spectrum of elementary particles, ... But it means that there is a mathematical framework with a small set of formulas, guiding principles or even axioms which sets the rules how to apply the theory. Within such a framework certain dualities or the "web of axpects" should become as clear as the relation between the Heisenberg and the Schrödinger picture.

 

[Fra]

This is similar to the situation in the early days of quantum mechanics = before Heisenberg and Schrödinger. There were magic numbers and magic formulas; there were hints regarding spectra, angular momentum and selection rules, half integer spin, Compton scattering, quanta of fields like photons etc. But there was no simple statement what quantum mechanics "is".

Interesting analogy but maybe a key difference I see is that the birth of QM was driven by real physics (ie. unexpected experimental results that was unexplained). So the patchwork of magic relatins was backed up by real data, and the "logic" and "reasoning" essentially ending up with the idea of a "measurement theory" exemplified by Bohrs mantra etc came afterwards, as a result of processing and adapting to real data.

String theory seems to be different. The logic and reasoning came first, some some ideas to replace points with strings etc. And then the web of relations and dualites in ST certainly doesn't have the same epistemological status as did the web of magic relations that drove the development of quantum theory.

This is why I think all one can judge, is not a concrete result, but the plausability of the choice of reasoning, set of premises and abstractions that guiding string theory research.

But same same is true for LQG. This is why I try to abstract the basic constructing principles and methodology of theory building, specific to LQG vs ST, and try to in some intellectual spirit assess their soundness, beacuse that's all there is to judge or discuss until explicit connections to experiment is made. And who knows if that takes another 20 years.

/Fredrik

 

[suprised]

This is why I think all one can judge, is not a concrete result, but the plausability of the choice of reasoning, set of premises and abstractions that guiding string theory research.

Well there is more than armchair philosophy - there are plenty of very non-trivial computational results; hard facts, so to say, which _do_ mean something!

For example, counting states in black holes. What does it tell? It tells string theory provides just the right number of degrees of freedom that makes this work - unlike an ordinary QFT.

So string theory makes a lot of sense, and just this by itself is extremely non-trivial! This sets it apart from zillions of other possible, random ideas, which on an armchair philosophical level would seem plausible too.

 

[Fra]

Well there is more than armchair philosophy - there are plenty of very non-trivial computational results; hard facts, so to say, which _do_ mean something!

For example, counting states in black holes. What does it tell? It tells string theory provides just the right number of degrees of freedom that makes this work - unlike an ordinary QFT.

So string theory makes a lot of sense, and just this by itself is extremely non-trivial! This sets it apart from zillions of other possible, random ideas, which on an armchair philosophical level would seem plausible too.

Yes, I know there are a lot of hard results withing ST; various theorems etc. But that is something that is as I see it mainly physically significant withing the string framework, or if we just discuss the mathematics of string theory - to the mathematics. Nothing wrong with that of course, if you study mathematics itself. Mathematicians do real hard work all the time, but it's not physics.

I just meant that deducting a theorem, is indeed a hard result, but it's not a physical result ie. is not quite comparable to experimental data or the result of a physical process. The theorem is always and forever true in it's axiom system, but the question is the physical relevance of the choice of axiom system within the theorem lives.

So we still end up with a judgement, wether the "choice of axiomsystem" implicit in the string framework is the right one or not. Where right meaning, something that is "fit" as way of building models in nature.

About counting black hole states, I still consider that to be somewhat semi-classical and speculative as it's "results" arrived at my extrapolating things from different domains to some QG domain. ST has made some success there, to connect to semiclassical approximation results but I don't know how the entire notion and view of entropy, and states will be once we have a proper theory. Most treatments of that, make extrapolations of things into doubtful context - where we in fact lack experimental confirmation of methods.

/Fredrik

 

[Fra]

I just realized that maybe I made myself unclear:

Well there is more than armchair philosophy - there are plenty of very non-trivial computational results; hard facts, so to say, which _do_ mean something!

With "choice of reasoning, set of premises and abstractions that guiding string theory research" that you called armchar philosophy, I rather didn't mean that all ST do is sit an ponder! :) I know they don't.

On the contrary, with this I meant the choice of framework, axioms, and mathematical abstractions that characterize string research. Sure, once that choice is made, you do real work. You try to investigate connections between different results, prove theorems etc.

But the "signifiance" from the point of physics (not just mathematical truths) is still conditional upon wether the choice of framework is correct.

/Fredrik

 

[suprised]

About counting black hole states, I still consider that to be somewhat semi-classical and speculative as it's "results" arrived at my extrapolating things from different domains to some QG domain. ST has made some success there, to connect to semiclassical approximation results but I don't know how the entire notion and view of entropy, and states will be once we have a proper theory. Most treatments of that, make extrapolations of things into doubtful context - where we in fact lack experimental confirmation of methods.

The whole point of the extrapolations done in these computations is that they are exact in the full quantum theory. This goes far beyond semiclassical analysis!

One thing is clear that string theory works extremely well and makes sense as a _physical model_. It is based on physical principles and produces, for example, scattering amplitudes that can be measured in principle, so it is _not_ just a mathematical game. Whether it actually describes nature is a different question.

I would view it analogous to "gauge theory". The detailed study of the various incarnations of gauge theories (susy eg) is certainly a subject in both physics and mathematics, and has provided many important insights into the real world gauge theories like QCD.
"String theory" is simply the natural generalization of gauge theory when including gravity; and as said before, it is intimately tied to it due to dualities. One should consider both, gauge and string theory, as part of one package.

 

[Fra]

In despite of my critique, I want to note that I appreciate the discussion and this thread. We all want the same thing in the end.

The whole point of the extrapolations done in these computations is that they are exact in the full quantum theory. This goes far beyond semiclassical analysis!

Just to make sure I understand - What do you mean by "full quantum theory"?

Correct me if I am wrong, but part of ST reasoning or premises, contains the premise that the mathematical formalism of QM as the framework for a measurement theory is correct, and need no revision. Ie. unitarity, timless hilbert spaces, linear operators etc as the correct abstraction for measurements. Right?

This is part of the "extrapolation" I refer to, and to which I object. But this isn't unique to string theory, it's indeed inherited from normal QFT. But then again, we don't know to what extent QFT makes sense beyond the experimentally verified domains. Ie. where the extrapolation is valid.

When it comes to things like very massive or extended systems (cosmological scale), we do not have a clean exprimental verification of wether the structure QM holds. So extrapolating things we know are right at laboratory scale, to QG scale is I think at least speculative. I personally am of the opinion that some of the rigid framework of QM can't make sense in the "full theory". Instead current formalism may be a special case where the observer is non-dynamical, and it studies a small subsystem (sufficiently small so that it does not deform the overall environment). This is why I think that extrapolating this to the general case is to formulate myself diplomatic at least not obvious! There is no experimental confirmation of the QM structure in this domain.

One thing is clear that string theory works extremely well and makes sense as a _physical model_. It is based on physical principles and produces, for example, scattering amplitudes that can be measured in principle, so it is _not_ just a mathematical game. Whether it actually describes nature is a different question.

Don't you rather have a whole set of possible predictons? ie. that for each (consistent) choice of background (ie observer in my assoication) you get different predictions?

Here I like to add one thing: I previously "defended" this possibility and I would like to have a balanced discussion. So from my perspective I don't object to the fact that there is no manifest B/I (I see reasons for why this is not possible), I object to the fact that you've constructed a set of possibilities without navigation scheme.

/Fredrik

 

[suprised]

Just to make sure I understand - What do you mean by "full quantum theory"?

The extact quantum theory including all perturbative and non-perturbative corrections to the classical one.

Correct me if I am wrong, but part of ST reasoning or premises, contains the premise that the mathematical formalism of QM as the framework for a measurement theory is correct, and need no revision. Ie. unitarity, timless hilbert spaces, linear operators etc as the correct abstraction for measurements. Right?

Of course. If you like to challenge the basic axioms of quantum theory.. uh oh...good luck. Well unless there is a good reason for doing this, we rather prefer to go on with something constructive, and get somewhere with that.

There is no experimental confirmation of the QM structure in this domain.

Indeed very much so. And what shell we conclude from this - stop research?

Don't you rather have a whole set of possible predictons? ie. that for each (consistent) choice of background (ie observer in my assoication) you get different predictions?

I don't see what you mean. It is rather the other way around - "different" backgrounds can give rise to the same predictions. That is what underlies duality.

, I object to the fact that you've constructed a set of possibilities without navigation scheme.

I am not sure to understand what you mean - you object to, or agree with, the statement that string theory constructs a set of possibilities without navigation scheme?

I myself do agree with it.

 

[Fra]

If you like to challenge the basic axioms of quantum theory.. uh oh...good luck. Well unless there is a good reason for doing this, we rather prefer to go on with something constructive, and get somewhere with that.

Thanks, I'll probably need it. Of course I do see good reasons for it, otherwise I wouldn't suggest it. The details are separate discussion.

Indeed very much so. And what shell we conclude from this - stop research?

No, of course not. My response to this will get me into WHY I think QM structure needs to change. It may be an interesting discussion, where I've argued in some other threads already, so I won't comment further in here.

I don't see what you mean. It is rather the other way around - "different" backgrounds can give rise to the same predictions. That is what underlies duality.

Again please correct me if I'm wrong (I'm certainly no string expert) but there are dualities that connects SOME choices of background. This is of course a good thing, I agree!

But the full claim that there is some kind of picture, where EVERY background is equivalent (predictionwise) to EVERY othre background is I think not true because then there would be not landscape problem? That full equivalence is probably more like the string theorist dream or "vision"? But we aren't yet there, right?

I am not sure to understand what you mean - you object to, or agree with, the statement that string theory constructs a set of possibilities without navigation scheme?

Both. I agree that is the way You do it. But I object to that it makes sense.

Here I think my objection differs to the usual critique against the lack of manifest background independence. I hold the opinon, that any measurement, and any predction needs a context to be formulated. A physical prediction that lacks any reference, is to me not possible.

So I do not per see object to the fact that you need a background, to reference a prediction. It's not more strange han the fact that you need an observer/measurement device (a real observer) to make a (real) observation or measurement.

What I find strange, and which I think is a symptom of the way ST is constructed, is that fact that you have "deducted" a gigantic set of possibilities, to the point where the progress stalls because there seems to be no competitive way to sort or scan this set in a rational way. I think this should not happen.

I guess from your response above that your idea is that the landscape is really somehow a gauge, and that you will eventually see that the big set of options is rather just the one and the same, so that you do get unique predictions?

I personally find that idea doubtful, but I understand the idea.

Another idea is that the landscape is "somehow" real, BUT that it's not really that large, AND that maybe there is a way to navigate in it. I just find it strange that if the construction of the landscape was "physical" then there should have been some kind of probability measure on it, where a given observer would then drift in the landscape like an evolution. But I'm not aware of anything in this direction from ST. There are these people who refer to anthropic principles, but that is more like an excuse, it does not solve the real problem of the making a choice, or explain the paradox of how it can be rational to generate such a landscap without navigation.

/Fredrik

 

[suprised]

But the full claim that there is some kind of picture, where EVERY background is equivalent (predictionwise) to EVERY othre background is I think not true because then there would be not landscape problem? That full equivalence is probably more like the string theorist dream or "vision"? But we aren't yet there, right?

Not at all and no one wants to be there! It would imply to saying eg that all the solutions of the Maxwell equations and GR would be equivalent and give the same physical answer!

Here I think my objection differs to the usual critique against the lack of manifest background independence. I hold the opinon, that any measurement, and any predction needs a context to be formulated. A physical prediction that lacks any reference, is to me not possible.

So I do not per see object to the fact that you need a background, to reference a prediction. It's not more strange han the fact that you need an observer/measurement device (a real observer) to make a (real) observation or measurement.

True… we do live (approx) in Minkowski space and that’s a flat backround of GR
and certainly in order to make measurements in our world we’d like to have a space-time around...

What I find strange, and which I think is a symptom of the way ST is constructed, is that fact that you have "deducted" a gigantic set of possibilities, to the point where the progress stalls because there seems to be no competitive way to sort or scan this set in a rational way. I think this should not happen.

How would you like to sort eg, the solutions of the Einstein equations of GR?
That’d be an “easy” sub-part of the problem.

I guess from your response above that your idea is that the landscape is really somehow a gauge, and that you will eventually see that the big set of options is rather just the one and the same, so that you do get unique predictions?

No no.. unique predictions are not at all wanted.

I sense some confusion about landscape and background independence. I don’t have the time right now, but may like expand on this a bit later. In the mean time, I recommend: http://arxiv.org/pdf/0807.3249v3 for some food for thoughts.

 

[Fra]

Not at all and no one wants to be there! It would imply to saying eg that all the solutions of the Maxwell equations and GR would be equivalent and give the same physical answer!

Mmm, that wasn't what I meant. I guess by equivalence I don't mean that all backgrounds give the same "numbers", I mean that they are different but "consistent", just like different SR observer get different numbers, but they are related in a "consistent way" - but the symmetry transformations.

Anyway, this was just ONE possibility I saw (I wouldn't share it). But I'll get to it more later... I'll try to skim the paper you suggested and see if it answers any questions about the String logic. Thanks for the reference!

How would you like to sort eg, the solutions of the Einstein equations of GR? That’d be an “easy” sub-part of the problem.

It's not that easy as Einsteins equation and GR as it stands (ie classical GR) is simply not cast in an appropriate form. They way I envision it, the corresponding information contained in "Einsteins equation" should be coded in an observer view. Anyway, all possibili solutions - in a given perspective - should come with a "probability" and be countable. I would expect gravity and Einsteins equation to be an emergent entropic phenomomen in this reconsturction. GR is not a starting point for me. But this this is as much a "vision" as string theory. I certainly have no solution on the table. But each will have to judge the plausability of the approach.

/Fredrik

 

[tom.stoer]

I understand what you are saying and I am with you in the sense that string theory has a lot of physical guiding principles - except for missing hard experimental facts. The latter one is due to the fact that string theory tries to reproduce the SM at reachable low energies and also tries to produce new results at unreachable higher energies. But somehow this seems to be natural for any approach towards a completion of SM + gravity and does apply in some sense it to LQG, NCG, ... as well.

Unfortunately the picture is a little bit more complex than that: there are two different experimental facts:
a) know facts string theory is not able to post-dict
b) new (and possibly experimentally inaccessable) facts string theory is not able to predict (again this applies to LQG, NCG, ... as well).

So there is a paradigm shift we see in modern theoretical physics: stronger focus on mathematics, less stress on phenomenology. This is a rather strange situation, but I see no way out.

Anyway - what I am expecting from string theory (seen as a framework) is more than just something like the framework of "gauge theory + SUSY". There should be some kind of uniqueness, some hint like "the swampland", "solutions / vacua of the landscape instead of theories", something like a definition of string theory.

Having a non-unique framework w/o any predictability in practice (not in principle) does not lead anywhere. So string theory either must be able make physical predictions which are unique to string theory (= which are not just something that could be derived from xMSSM w/o strings) and which can be tested in practice - or string theory must be able to go one step further than just providing a collection of losely coupled frameworks which contain both gauge symmetry and gravity.

As I do not expect a unique new low-energy prediction (post-diction would already be nice) from string theory, I guess it's more the conceptual side that should be stressed.

Coming back to my example regarding the early days of quantum mechanics. I still think that the situations are comparable, but on a different level. String theory is a broader framework addressing different issues, but we already know a lot of magic numbers and structures to be addressed within this framework: U(1)*SU(2)*SU(3), 6 flavors / 3 generations, Higgs particle (?) Weinberg angle, Yukawa couplings, 4 dimensions, ...

It would be phantastic if string theory could produce a results like
- a list or category of allowed low-energy theories (*)
- one theory very closed to the SM
- a list or category of forbidden low-energy theories (~ selection rules)
- the statement that the theories (*) are not just frameworks or "theories" but "solutions"

We have quite a lot of these results in our hands - just like Einstein, Planck, Bohr etc. had the photoelectric effect, black body radiation, the hydrogen spectrum, selection rules, half-integer spin (has been introduced by Pauli on purely phenomenological reasons and has been critizised by Sommerfeld :-)

Therefore the next logical steps seem to be
- understand more details (e.g. come even closer to the SM, proof of perturbative finiteness, ...)
- make predictions which can be addressed experimentally in principle or even in practice
- provide a unique definition of string theory = a guiding principle, set of axioms or rules

In QM we had exactly that:
- spectra, fine structure, Zeeman and Stark effect, ...
- formulations developed by Heisenberg, Schrödinger, Dirac and last but not least Feynman

In string theory there is a lot of progress regarding model building: D-branes, F-theory, xMSSM-like theories. There is also some progress regarding new predictions (unfortunately not testable in practice afaik). And there are even different concepts which one could compare with the Schrödinger and the Heisenberg picture, Feynman's formulation etc. - of course on a different level. What is missing is a guiding principle, a sound proof of dualities, an axiomatic derivation of the different concepts, formulations, sectors, ... This is exactly what string theorists promised to be provided by M-theory!

So I end up with the same conclusions as David Gross:

"What is the fundamental formulation of string theory? or: What is string theory?
This is a strange question since we clearly know what string theory is to the extent that we can construct the theory and calculate some of its properties. However our construction of the theory has proceeded in an ad hoc fashion, often producing, for apparently mysterious reasons, structures that appear miraculous. It is evident that we are far from fully understanding the deep symmetries and physical principles that must underlie these theories."

 

[Haelfix]

Tom, everything you say is more or less correct. But I mean at one point you do have to throw your hands up in the air.

Quantum gravity is something like 15 orders of magnitude away from anything we might be able to test. Making some sort of testable and practical prediction, is a running issue the entire field faces, and everyone is accutely aware of it. Even if you could figure out some observable that was possible to measure, you would likely end up facing a maxwell demon type situation. Eg the scale difference is so huge, that invariably any practical approximation you might make at the Planck scale, gets amplified and distorted many times over.

So absent a theoretical breakthrough (eg a vacuum selection method), it seems like the only hope is to get lucky with astrophysics and cosmology. For instance the observation of a cosmic string (in the stringy sense) would be a relatively clear and smoking gun (although of course it would take years for astronomers to be sure).

So at this time, it seems like only mathematical consistency has any hope of guiding us to a theory of qg, and I insist that st is the best candidate currently out there under those guidelines. It is really the only candidate that takes the minimal amount of physical inputs and outputs something that smells roughly correct. There is no alteration of the Rules of Quantum Mechanics or special/general relativity, and no steps are avoided, there is only a simple statement (strings are extended objects) and many of the miracles of modern physics seem to fall out of that.

I think perhaps its just one of those things you actually have to see being done to appreciate, as it sounds like snake oil when its being explained.

 

[ensabah6]

So at this time, it seems like only mathematical consistency has any hope of guiding us to a theory of qg, and I insist that st is the best candidate currently out there under those guidelines. .

More so than LQG?

 

[Fra]

(The paper surprise referred to is 87 pages, I'll try to get time to skim some of it during the next days and get back on that.)

Tom, I guess I don't disagree with anything particular your wrote.
I just have some minor comments.

So there is a paradigm shift we see in modern theoretical physics: stronger focus on mathematics, less stress on phenomenology. This is a rather strange situation, but I see no way out.

I still think it's important to distinguish between not only physics and mathematics, but also between mathematical modelling and mathematics itself.

You're right about the paradigm shift, and it's in particular why I chose to focus stronly on the guiding principles, logic of reasoning and methodology, as this is the only thing you can assess in different programs. The "mathematics itself" is not really something to comment on, beyond consistency issues and level of stringency. To discuss CHOICES of axioms is not a mathematical problem. To discuss "unification" of mathematical branches is also indeed interesting, which some mathematicians do. I find that interesting, but it's somehow not what theoretical physics should be about IMO.

So My focus is really on inference models! In a sense this is reall not physics! It's actually about learning models. This is how I secretly think of many things, and it certainly have applications outside of theoretical physics. This is best IMO seen as ET Jaynes puts it - an extension to logic. The normal language for this is probability theory, usually in some bayesian form or so. There are also other inference rules, such as entropic reasoning.

If you listened to Ariel Caticha's talk on perimeter not too long ago about nature of laws, he points out during the first explanation that "he is not doing physics, he is doing inference".

This is quite different from mathematics. Inference models can be tested without physics. It can be tested in different ways.

One of my conjectures which I share with Ariel and those that work in this direction, is that the laws of physics are really just "rules of inference". And that physical processes, and observations, backreactions from environment etc, can all be described abstractly in terms of inference processing, where each subsystems constantly tries to infer and learn and take control of it's environment.

It's in this light, I critique some of the current researhc programs. From this point of view, I simply find their methodology to not be quite rational. (ie. from the point of view of inference! not phenomenology).

After all, physicists do make inference on nature, this is what we do when we model and construct experiments. I put this on par with physical processes, where one atom makes inference about a neighbouting atom for example.

All this is, in line with the paradigm shift you mention. However it's not just "mathematics" - it's inference, or mathematical models for inference or learning. An inference model is judged not by truthness of theorems or consistency, it's judged on it's fitness. A good inference models, simply makes efficient and good inferences.

Coming back to my example regarding the early days of quantum mechanics. I still think that the situations are comparable, but on a different level. String theory is a broader framework addressing different issues, but we already know a lot of magic numbers and structures to be addressed within this framework: U(1)*SU(2)*SU(3), 6 flavors / 3 generations, Higgs particle (?) Weinberg angle, Yukawa couplings, 4 dimensions, ...

I agree that there are these "magic things" but you refer more or less to the structure of the forces, particle families etc. I too think this calls for some unification, but just because string theory is one of the few "mainstream-candidates" doesn't mean I think it's even close to the only option.

/Fredrik

 

[Fra]

So at this time, it seems like only mathematical consistency has any hope of guiding us to a theory of qg, and I insist that st is the best candidate currently out there under those guidelines.

I know you're very knowledgeable as you typically come with great posts, but I still think a good alternativ to "mathematical consistency" as a guide, is to instead too at our methodology, and in particular formalise it and study inference models. This is also I think a very good guide, as it focuses on "rational behaviour". That can in fact be seen as "consistent reasoning", which is not the same as "consistent results", because it may in fact be that inconsistent states DRIVES development.

I think it IS rational to FIRST try minimal extensions to what we know (ie keep QM intact etc) but OTOH long time has passed with many unsolved problems, and there are also good reasons to doubt the validity of extrapolation of current frameworks. So it is rational to try this first. But at some point, I think it's irrational to not start to question our first conjectures and consider alternatives.

/Fredrik

 

[suprised]

More so than LQG?

Can't you realize that ST plays in a different league?

Otherwise I mostly agree to what has been said esp. by tom, Haelfix, finbar.

 

[Fra]

I realize that I just don't have time to comment on this paper in one session the nearest time as it's too long, so I'll try to skim it in parts and comment.

I recommend: http://arxiv.org/pdf/0807.3249v3 for some food for thoughts.

A first comment (without having started reading) on the abstract on what seems to be the main point

We are in the middle of a remarkable paradigm shift in particle physics, a shift of opinion
that occurred so slowly that some even try to deny that they changed their minds at all.
It concerns a very basic question: can we expect to derive the laws of particle physics
from a fundamental theory? The Standard Model of particle physics as well as the 1984
string theory revolution provided ample food for thought about this. The reason this was
ignored for so long can be traced back to an old fallacy: a misguided idea about our own
importance.

IMO, taking it litteraly, I do not think it's possible to _derive_ (ie. deduce) the laws of particle physics from a static fundamental theory.

But I do think it's possible to makes a very could guess and infer the best expecations of the laws of physics as per some inference scheme, and that this working may in fact reflect not only how science works, but how nature itself works. Ie, the uncertainty in the inference is not an "approximation" it is in fact corresponding to real physics uncertaintes. And we know these things exists in general (just think QM).

For me, from the inference perspective, it's important to distinguish between deduction and induction or abduction. All of them quality as inferences, but deduction is a certain, fault free idealized form of it that doesn't correspond to most real life situations, which are almost without except inference under conditions of uncertainty, where moreover there is no objective independent way of MEASURE the uncertainty.

(The remaning part of the paper, I will see if they expand on htis, but I see conceptual associations to what I suggest here and the meaning of the landscape, and observers. It's just my prior experience that string theorists really does not (genereally at least) see it like this - this is why I think this could be interesting to discuss)

/Fredrik

 

[tom.stoer]

I agree that there are these "magic things" but you refer more or less to the structure of the forces, particle families etc. I too think this calls for some unification, but just because string theory is one of the few "mainstream-candidates" doesn't mean I think it's even close to the only option. /Fredrik

I only wanted to stress that both today and in the early days of qm there are / were this magic things, so the situations are comparable.

 

[tom.stoer]

More so than LQG?

Of course!

LQG is not aiming to unify gravity with anything else. LQG just says that the very nature of gravity is different both mathematically and physically (just read Rovelli's book or Smolin's and Ashtekar's summaries) that one must first understand how to quantize gravity in order to be prepared to couple it to other forces. LQG does not deny the need for unification, but it postpones it (discoveries like algebraic structures similar to particles emerging from quantum-deformed / framed loops are not mainstream).

 

[tom.stoer]

Tom, everything you say is more or less correct.

Thanks for the agreement.

Quantum gravity is something like 15 orders of magnitude away from anything we might be able to test. ... it seems like the only hope is to get lucky with astrophysics and cosmology.

yes

So at this time, it seems like only mathematical consistency has any hope of guiding us to a theory of qg, ...

yes

... and I insist that st is the best candidate currently out there under those guidelines.

yes and no;
it's not the best candidate w.r.t. these guidelines, it's the only candidate which truly aims for unification ...

It is really the only candidate that takes the minimal amount of physical inputs and outputs something that smells roughly correct.

... and/or the only candidate that mainstream physicists are able to understand; I still try to understand what NCG means and I always come to the conclusion that it is some magic trick; Connes is too clever for me.

Regarding qg, there are a couple of viable approaches (LQG, asymptotic safety, ...), regarding unification I only know ST and NCG.

 

[Haelfix]

"More so than LQG?"

My opinion of LQG is not even wrong. I am not a gravitational theorist, and I have zero experience with it.

I looked at it briefly several years ago in my spare time when I was a graduate student, but it ceased to interest me when it became clear that it involved a lot more than standard Dirac quantization and that the models seemed to change every six months.

It is in that regard that I claim that it is highly nonminimal. You have to learn a new type of quantum mechanics in order to begin understanding it, and its totally unclear if it makes contact with general or special relativity (this is the issue of the semiclassical limit and lorentz invariance respectively). Heck the new version of LQG is not even about the Einstein Hilbert action, so again I have no idea what they're really trying to do.

And much more importantly, it makes absolutely no contact with anything that I work on in my own research (unlike string theory, which often provides a toolset that is valuable well outside the field of quantum gravity).

 

[suprised]

Well I guess that LQG is represented here at this forum very well, IMHO much more than it deserves for its merits and prospects, so perhaps we can try to stick to the title of the thread at least here.

 

[tom.stoer]

As I said: LQG in its current fashion is of no interest for unification. NCG may be interesting, but not under the topic "string theory" (which is rather nice because it may be the first theory which cannot be called string theory :-)

=> I agree with suprised.

 

[ensabah6]

Can't you realize that ST plays in a different league?

Otherwise I mostly agree to what has been said esp. by tom, Haelfix, finbar.

Of course!

LQG is not aiming to unify gravity with anything else. LQG just says that the very nature of gravity is different both mathematically and physically (just read Rovelli's book or Smolin's and Ashtekar's summaries) that one must first understand how to quantize gravity in order to be prepared to couple it to other forces. LQG does not deny the need for unification, but it postpones it (discoveries like algebraic structures similar to particles emerging from quantum-deformed / framed loops are not mainstream).

"More so than LQG?"

My opinion of LQG is not even wrong. I am not a gravitational theorist, and I have zero experience with it.

I looked at it briefly several years ago in my spare time when I was a graduate student, but it ceased to interest me when it became clear that it involved a lot more than standard Dirac quantization and that the models seemed to change every six months.

It is in that regard that I claim that it is highly nonminimal. You have to learn a new type of quantum mechanics in order to begin understanding it, and its totally unclear if it makes contact with general or special relativity (this is the issue of the semiclassical limit and lorentz invariance respectively). Heck the new version of LQG is not even about the Einstein Hilbert action, so again I have no idea what they're really trying to do.

And much more importantly, it makes absolutely no contact with anything that I work on in my own research (unlike string theory, which often provides a toolset that is valuable well outside the field of quantum gravity).

Suppose that future evidence coming from the LHC, SuperCDMS, Fermi Gamma-ray Space Telescope, Super-Kamiokande etc., is empirically consistent only with the theory that nature is only 4D and SUSY is unrealized in nature, and GUT such as SU(5) and SO(10) are falsified. Do you think string theory is, under the additional, experimentally observed guidelines of no-SUSY, 4D, no SO(10) unification, still the "st is the best candidate currently out there under those guidelines"

 

[ensabah6]

As I said: LQG in its current fashion is of no interest for unification. NCG may be interesting, but not under the topic "string theory" (which is rather nice because it may be the first theory which cannot be called string theory :-)

=> I agree with suprised.

Is ST a more promising QG than LQG ?

 

[suprised]

Suppose that future evidence coming from the LHC, SuperCDMS, Fermi Gamma-ray Space Telescope, etc., is empirically consistent only with the theory that nature is only 4D and SUSY is unrealized in nature. Do you think string theory is, in this scenario, still the "st is the best candidate currently out there under those guidelines"

Try to read what was written above, about the meaning of higher dimensions and the role of SUSY in string theory. I had commented on this in the past several times as well. It makes no sense to repeat it all the time, again and again, esp. if poeple don't want to read and/or understand.

The answer to your question is yes, as I don't see a good reason, as of today, to think otherwise.

Is ST a more promising QG than LQG ?

LQG hasn't even convincingly demonstrated that it describes gravity in 4d in the first place; nor that it's quantization makes any sense; nor that it is a single theory with a clear definition (rather it is a collection of attempts whose starting points change all the time and, after decades of research, didn't get anywhere near to what has been promised).

So again, please let's keep at least this thread clean from this fog, ok?

We could open other thread about what is wrong with LQG, why so few people believe in its merits, why there is so much hype involved promoting it, and so on. Though I suspect you wouldn't like to read it.

 

[tom.stoer]

I do not agree with suprised's statements regarding LQG, but I will not respond here in this thread.

The situation with strings and the LHC is the following:

A) even if the LHC does not produce a single mini black hole in the TeV range, even if there are no large extradimensions, and even if the LHC does not find a single SUSY particle, nearly nothing will happen to the basic principles of string theory. String theory can still be right or wrong. Our interpretation of the situation will be different; the position of string theory regarding fashion, acceptance, support, money, ... will be affected, but not its fundamental principles.

B) if the LHC finds SUSY particles fitting perfectly to some (x)MSSM the same applies; of course this will support string theory in some sense, but it does not affect its basic principles. Verifying SUSY does not help in identifying new underlying principles or axioms of string theory, it does not help in developing an off-shell formalism, pushing string field theory forward, establishing a background independent framework, identifying a vacuum selection principle, ...

Regardless what the LHC finds, the situation regarding basic principles will stay the same.

 

[suprised]

I can't answer all what I wanted but a few comments on those:

(LQG just says) that the very nature of gravity is different both mathematically and physically (just read Rovelli's book or Smolin's and Ashtekar's summaries) that one must first understand how to quantize gravity in order to be prepared to couple it to other forces. LQG does not deny the need for unification, but it postpones it (discoveries like algebraic structures similar to particles emerging from quantum-deformed / framed loops are not mainstream).

Let's see whether they will ever succeed. If string theory is right, then this program should fail, because there matter IS needed for consistency. This is a nice dividing line between those approaches. If matter is not needed, even worse for them, because how could they ever hope to get constraints on particle physics then?



Fully agreed on:

Regardless what the LHC finds, the situation regarding basic principles will stay the same.

But not quite on:

A..NCG may be interesting, but not under the topic "string theory" (which is rather nice because it may be the first theory which cannot be called string theory :-)

Why cannot? They present a cute way to parametrize (part of) the standard model. You want to imply that the standard model is not contained in string theory?

More specifically: as written before, the dualities discovered during the last 15 years have shown that there are in general many different ways to geometrically represent a given theory; there is no absolute meaning of a given background geometry, be it continuous or discrete. Those authors have discovered still another way, by using a non-commutative structure. There is no a priori reason why that would not be contained in string theory.

 

[tom.stoer]

It's rather simple: I am referring to a statement "whatever ... it may be ... it may look like, we will still call it string theory" I know from Smolin's book ... I am not sure so I have to check ...

What I am sying is this: NCG is not related mathematically to string theory.

If you have the QCD beta function which contains 11/3 and if I give you an 11-faced geometrical object with 3 yellow faces, that does not automatically mean that the QCD beta function is somehow related to this 11-faced object; it does not mean that this object appears somewhere in QCD; nor does it mean that probability theory is dual to QCD.

So even if both ST (for which it is not proven) and NCG converge in some appropriate limit to the SM that does not automatically mean that NCG is related to ST mathematically. What you are saying here is rather dangerous - and I have the feeling that this applies to the web of dualities as well: the fact that certain theories or formulations match in some appropriate limit does not automatically mean that they really match. It is dangerous to think about theories like coordinate patches for which a small smooth overlapping region is sufficient. Low-energy effective theories (chiral perturbation theory, nonrelativistic quark model) are in some appropriate limit related to each other. But they are not identical; they can only be identified via QCD; w/o QCD the essential unifying structure is missing.

For ST this means that the web of approximate dualities is perhaps too weak to be called a theory. It is perhaps not sufficient to stay with this web of approximate dualities w/o being able to identify the underlying unifying structure. The initial idea and program of M-theory is still waiting for completion!