Why I am REALLY disappointed about string theory


by tom.stoer
Tags: disappointed, string, theory
suprised
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Mar29-11, 03:48 AM
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Fra, I really can't answer your questions, I barely understand them.

But I comment on this:

Quote Quote by Fra View Post
For example, has any string theorist ever tried to justfiy the basic string action, from a pure inferencial perspective?
There is no such thing like a basic string action. There are various actions, with different symmetries (like heterotic string world sheet, like type II string world sheet, like open type I world sheet...). They are all different, and each one refers to some particular perturbative approximation centered at a different regime. Moreover, for F-theory or M-theory such "world-sheet" actions are not known or may not exist; as we have discussed earlier, there are quantum theories which are strongly coupled and no lagrangian or action description of them exists.

So the string world-sheet perspective (Polyakov action and generalizations), while very useful in many situations (eg see the above discussion about CFT and internal degrees of freedom), is hardly fundamental. Trying to find a deeper meaning of it had been another of many blind ally's.

That's one of the most important conceptional riddles: does a "fundamental" action that would universally describe strings in every corner of the parameter space exist at all? I don't know but I doubt it.
Fra
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Mar29-11, 07:01 AM
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I know what I asked is fuzzy, but thanks for trying to answer.

Quote Quote by suprised View Post
There is no such thing like a basic string action. There are various actions, with different symmetries (like heterotic string world sheet, like type II string world sheet, like open type I world sheet...). They are all different, and each one refers to some particular perturbative approximation centered at a different regime. Moreover, for F-theory or M-theory such "world-sheet" actions are not known or may not exist; as we have discussed earlier, there are quantum theories which are strongly coupled and no lagrangian or action description of them exists.

So the string world-sheet perspective (Polyakov action and generalizations), while very useful in many situations (eg see the above discussion about CFT and internal degrees of freedom), is hardly fundamental. Trying to find a deeper meaning of it had been another of many blind ally's.
Yes there are different string actions dependong on what string theory you consider, but that doesn't avoid my question:

Since you might have figured from my strange comments that I'm slowly working on an inference perspective to physics, and in this context, one can talk about actions as a way to measure the information divergence of possible futures relative to present. The idea is to define expected change not as dynamics realtive to external time, but with respect to a observer dependent entropic flow. IE to understand the concepts w/o referencing mechanical or geometrical visualisations.

As far as I know (even though yes there are different string actions) the actions is understood at least originally simply from the CLASSICAL ACTION you would expect from a litterally oscillating string. Then this is put in a background and you quantize etc.

The reason what I keep asking this because I sincerly think that there IS a deeper way to understand strings (or a way to at least connect string theory to something else). But this would require a deeper understanding of string actions and background beyond the classical geometric "picture" it started out as.

Maybe this is included in the open issue you already defined, but the basic string itself and the string action is a good starting point.


Quote Quote by suprised View Post
That's one of the most important conceptional riddles: does a "fundamental" action that would universally describe strings in every corner of the parameter space exist at all? I don't know but I doubt it.
I don't think so either it wasn't what I meant.

I meant that you can only "measure" one theory with respect to another one; by including a renormalized version of the first in the second one in a holographic sense.

But maybe we can in this way understand how theories interact. If I understand you, you also seek a way to understand how say transitions between different theories work, right?

What I am suggesting, and that does connect to the question I asked about the meaning of string actions, is that instead of thinkg in terms of a gigantic state space where you have transitions between theories, maybe the better way is to think of the "transitions" in terms of INTERACTING theories, that are negotiating.

Ie. the transitions are then simply internal revision in the light of new information. There is a good change to connect then the understanding of a string (seen as a simple measure on it's environment) to the foundations of measurement theory.

This means that the "background of the string" is defined by the interaction context (ie. neighbouring strings). But the difference is that, this "background space" only exists from the point of view of the string itself.

Ie if we thinkg of a string as an observer! then the string can "as far as it cna infer" conlude that it lives in this background space, and thus the rational action of the string (defined in the way I SEEK in the original question) is then merely doing a random walk in this effective background.

Transitions from different string theories would then (maybe?) correspong to the string observer remapping it's internal structure, so that giving instnatly "consistent" expectations, it becomes more stable.

What comes to my mind first is to tro "reproduce" or connect the ordinary string actions to some probabilistic measure based on permutations of string configurations - assuming ou can count it, maybe starting with discrete strings?

If such a deeper understanding of the string, and the string action as observers resp rational actions, I think it would be a major boost and it would help solve many questions. It would also force a new way of thinking about this.

Totally relased from the simple "geometrical pictures" you also mention you want to loose.

So the question is, what do we replace that with? I propose the inferentical perspective, but the connection to string seems to be in sight, but yet I'm not sure of anyone works in this direction.

Edit: Thinking in the direction is this http://math.ucr.edu/home/baez/nth_quantization.html. This is related to probabilities of probabilities which in turn related to renormalization of theories.

Could be generate string from something else, that does not come with the ad hoc or classical pictures to it? Something purely inferential?

/Fredrik
marcus
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Mar30-11, 12:02 PM
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FOR CONTINUITY since we're on a new page, it may help to carry over some essential posts. This of Suprised was seminal:
Quote Quote by suprised View Post
I guess there were many potentially wrong turns - at least in the sense of bias towards certain ways of thinking about string theory. Here a partial list of traditional ideas/beliefs/claims that have their merits but that potentially did great damage by providing misleading intuition:
...
Tom's most recent long one was:
Quote Quote by tom.stoer View Post
I would like to come back to suprised's list regarding possibly wrong turns...
===quote post #554 ===
I would like to come back to suprised's list regarding possibly wrong turns.
  1. - That geometric compactification of a higher dimensional theory is a good way to think about the string parameter space
  2. - That perturbative quantum and supergravity approximations are a good way to understand string theory
  3. - That strings predict susy, or have an intrinsic relation to it (in space-time)
  4. - That strings need to compactify first on a CY space and then susy is further broken. That's basically a toy model but tends to be confused with the real thing
  5. - That there should be a selection principle somehow favoring "our" vacuum
  6. - That a landscape of vacua would be a disaster
  7. - That there exists a unique underlying theory
  8. - That things like electron mass should be computable from first principles
Let's look at this list again: there is a deep connection between some topics; that's why I was mentioning background independence. I would like to comment on this once more.

String theory walked - for a rather long time - on the trail of particle physics and quantum field theory. Of course there was a graviton, but after recognizing this particle one immediately focussed on QFT-like reasoning (background, strings on top of this background, perturbative quantization, ...). I would say that the first few topics are essentially due to this perception of string theory.

Looking at the field today most researchers are convinced that non-perturbative approaches are required. Thousands of backgrounds / vacua have been identified, but still they are mostly perceived as reasonable backgrounds on which standard particle- or QFT-like theories can be formulated. This is OK for model building an phenomenology (it is not only OK but of course heavily required in order to achieve a closer relation to reality).

But using intuition to find such backgrounds and doing "ordinary physics" on top of these backgrounds does not help in order to understand the relation between these backgrounds and to identify the "unique" and deeper origin of these backgrounds, which I would call the underlying theory.

I think another wrong turn - perhaps the most serious one - would be to turn a bug (the missing unique underlying theory) into a feature (we do not need a unique underlying theory). It would be same as looking at the periodic system and stating that happily there is no underlying theory required as we have a collection of relations between different chemical elements.

I think we do not need to look for a selection principle ("why is it iron instead of copper?"), we do not need to condemn the landscape ("iron, copper, mercury, oxigen, ... is too much; we need a single solution"), we do not need to look for a way to calculate the mass of the electron ("how do we calculate the mass of the mercury atom in a theory which does not explain why there is a mercury atom?"). All what we have to do is to understand what string theory really is. My impression is that we still do not know, we are scratching at the surface, we see some "effective models", not more (and not less).

So 1. - 4. may have been wrong turns - but were overcome somehow over the last years. 5., 6. and 8. are perhaps wrong turns which are in the spotlight today. 7. is not a wrong turn but the essential driving force of progress in physics. I would not abandon it w/o having a worthy successor.

I am still with David Gross (and others - like Weinberg I guess) who asked exactly these questions:
  • 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. It is hoped that the recent efforts to construct covariant second quantized string field theories will shed light on this crucial question.
  • We still do not understand what string theory is.
    We do not have a formulation of the dynamical principle behind ST. All we have is a vast array of dual formulations, most of which are defined by methods for constructing consistent semiclassical (perturbative) expansions about a given background (classical solution).
  • What is the fundamental formulation of string theory?

Denying the relevance of these questions is - in my opinion - the "wrongest turn ever".
==endquote from Tom's post #554 ==

===quote Suprised reply, post #555===
Nicely said, Tom.

Though I think I should explain what I meant with 7) "there exists a unique underlying theory".
Much could be said here. For the time being, let me provocative and say the following:

Strings seem to be the natural generalization of gauge theory, actually closely related to it by dualities, such as AdS/CFT; in the latter context, strings are indeed reconstructed from gauge theory. So let's view strings as analogous to gauge theory; and then re-ask the same question: "what is the underlying unique theory of gauge theory" ?

Clearly this is a not very fruitful question to ask, because it presupposes something which does not exist, at least in the sense of the question. All there is with gauge theory, are various degreses of freedom that are exposed depending on the energy scale (gluons, quarks, mesons...)

As for strings, the situation is unclear but it may be similar - there may be no further "unique underlying theory". All there might be is the complicated web of perturbative approximations related by dualities, but there is no regime where "universal, more fundamental" degrees of freedom would be liberated.

The real question is whether there is an encompassing, "off-shell" mother theory which would contain all the known theories as "critical points", and describe transitions between them, etc. This may, or may not exist (analogous to gauge theory). So this question is a potential blind ally as well!
==endquote Suprised==

==quote Atyy==
So we don't obviously need Calabi-Yau compactifications?
==endquote==

==Suprised reply to Atyy, post #556==
They are just special examples of vacua, their main advantage is being relatively well under technical control. That's why there has been so much focus on them, unfortunately thereby creating the impression that they would be somehow essential. But there are zillions of other constructions (generalized geometries with fluxes, non-geometric vacua, brane backgrounds, non-perturbative F-theory vacua, M-Theory vacua,.... ).

Of course, many of such vacua are equivalent via dualities, and this shows, again, that there is no objective, unambiguous meaning of a compactification geometry.
==endquote==
marcus
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Mar30-11, 12:37 PM
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This discussion is plowing deep and turning up the soil in a potentially fertile way. I reflect on on the title of the thread: "Why I am REALLY disappointed about string theory."

What has just now come up, interestingly, are not faults/limitations of theory (as I see it) but deficiencies of "program management". As I hear it, the leadership (funding committees, conference organizers) may have allowed too many "wrong turns"---so that creative talent was wasted on "blind alleys".

So a kind of meta-question would be does Tom's question matter: "Does it matter why experts are disappointed about the string program?"

Or if "disappointed" is too specific, be more general and say experts show a loss of interest, loss of energy, tendency to go off into borderline areas or spend more time in other fields, loss of focus on the hard core problems---some or all these things.

If loss of focus by the best people matters to you, and if it is real, then that looks like a program management problem. Is the string leadership listening enough to what David Gross says, or for that matter, what some people in this thread are saying? Just a thought.
tom.stoer
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Mar31-11, 01:14 AM
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Marcus, thanks for the summary.

Quote Quote by marcus View Post
... not faults/limitations of theory ... but deficiencies of "program management". As I hear it, the leadership ... may have allowed too many "wrong turns"
Hindsight is always wiser; I was listening to a talk of a great QCD guy 20 years ago. His reply to my question how to find the best way to proceed was "how shall I do the calculation if I don't know the result?" Unfortunately this approach is not available in string theory :-(

Quote Quote by marcus View Post
Is the string leadership listening enough to what David Gross says, or for that matter, what some people in this thread are saying? Just a thought.
They should definitly listen to Gross. The problem may simply be to identify a blind spot. In order to achieve that new questions and perspectives are required.

String theory is (in my opinion) in a situation like the strong interaction with all its hadron multiplets but w/o the fundamental representation = w/o quarks. Nice relationships, but no fundamental building block.

My guess is that strings, branes, dimensions and spacetime are only "effective" descriptions valid in certain regimes.
Haelfix
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Quote Quote by tom.stoer View Post
My guess is that strings, branes, dimensions and spacetime are only "effective" descriptions valid in certain regimes.
That is definitely the state of the art right now. The very real possibility (which I believe Surprised has hinted at) is that this might *always* be the case. It might be that's simply how nature has made her mathematics! Actually, it might be the case for low energy QCD as well. There might simply not be a simple analytic result that humans can package up in a simple way and pretend like it covers the entire energy range perfectly.

Certainly, most of the discoveries about dualities as well as insight into the nonperturbative physics in the last 15 years has followed this road.

Then again, there are so many very intricate mathematical relations and surprises going on within String theory, that I think the original belief that there is some as yet unknown 'super structure' that controls it all is not entirely without merit either.
tom.stoer
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Mar31-11, 04:00 PM
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Now that we have consensus (OK, not really, Suprised will not agree) the interesting question is how to identify the underlying theory from which all these effective string models do emerge.
atyy
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Quote Quote by tom.stoer View Post
Now that we have consensus (OK, not really, Suprised will not agree) the interesting question is how to identify the underlying theory from which all these effective string models do emerge.
As a layman, the main line so far seems to have been that AdS/CFT is the sector in which this underlying theory exists, so let's study it better. The main results in recent years seem to have been about integrability and the ABJM case. In here, there is also the hope that twistors may be a reformulation of the gauge theory which will generalize - Arkani-Hamed even talks about emergent unitarity.

The other line, which is a minority, but very pretty, is the West/Damour, Henneaux, Kleinschmidt, Nicolai work on E10,E11.

I remember Mitchell Porter some time ago pointed to http://arxiv.org/abs/1008.1763 as yet another line trying to formulate the underlying theory.

Naturally, I don't know the relationship between these, or if there are in fact other more important approaches, would be interested to hear from the pros.
Physics Monkey
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Mar31-11, 09:11 PM
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It seems to me that holographic duality suggests that there is no simple metatheory. Of course, many of those terms are undefined so who knows if it means anything. I don't usually think of there being some kind of metaframework for gauge theory beyond the basic structures inherent in any quantum field theory, but the existence of a string metatheory along with holographic duality would seem to imply that there is such a metaframework for gauge theory. That would be cool but also surprising in my opinion.
atyy
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But could one hope for non-perturbative definitions of other sectors of the theory in the same spirit?
Calrid
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Quote Quote by tom.stoer View Post
Now that we have consensus (OK, not really, Suprised will not agree) the interesting question is how to identify the underlying theory from which all these effective string models do emerge.
How about trying an experiment?

Ok just kidding.

That is probably the most salient reason to be disappointed though. No evidence. Disagreements about meta questions are after all just philosophical objections atm.

Nice try but no cigar is the best thing we can say atm.

Peturbative or non peturbative, back ground dependant or not, one thing science is dependent on is discernible reality.
Fra
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If we take a theory to - rather than be some objective description of outcomes of all possible measurements - be one observers inferred expectations of possible measurements it can do - than it seems plausible that two interacting observers is the same thing as two interacting theories, and in addition to that that there is no objective meta theory of how the theories interact. All there can be, is a holographic connection between theories. And that the theories that we do see in nature are somehow the result of some evolutionary selection, just like one can imagine all kinds of crazy by physicall consistent orgnisms on earth, yet the organisms we do see are many but constrained.

There can't be an *inferrable* fixed super meta space of theories. If it exists, it's only in the sense of structural realism.

So my projection of string theory, I think surprised hunch that there may not exists unique timeless eternal mother theory makes perfect sense.

But that doesn't mean it can't exists an evolving meta theory that solves our problems. This evolving meta theory then IS the same thing as what we usually call effectiv theories. I mean it could be that all there is are effective theories. But what is wrongwith that? I see nothing wrong with that. On the contrary; the search beyond effective teories is the search for realism! I was hoping that after a couple of scientific revolusions we was done with that ;) But I was wrong.

Seens as inference, this is just the same thing as acknowledging that there is no ultimate eternal truth. Ie. from the point of view of LEARNING, its' wrong to FOCUS on some ultimate truth. Doing this may in fact inhibit progress. The focus should I think be on learning, without bias of some ultimate truth.

It's the description of this process, I seek. This is exactly what interacting theories is about. So I definitely defend som of these weird things of ST, MY question is merely where the methodology of string research is optimal. Ie. is future string theory the ultimate theory of theory, or do we need to rethink the entire business from scratch?

If I understand this summary right....

Loosely speaking? Many people here except surprised, at least hopes that there will be found some unique mother theory (in order to ST ot make sense)? Is that fair?

/Fredrik
suprised
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Quote Quote by tom.stoer View Post
Now that we have consensus (OK, not really, Suprised will not agree) the interesting question is how to identify the underlying theory from which all these effective string models do emerge.
Well I am playing advocatus diaboli here, in raising awareness that certain views that are taken for granted by most, may potentially be wrong or at least based on misleading preconceptions.

I had expressed my personal views on "the underlying theory" already here:

http://www.physicsforums.com/showpos...&postcount=251

http://www.physicsforums.com/showpos...91&postcount=9
tom.stoer
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Quote Quote by suprised View Post
Well I am playing advocatus diaboli here, in raising awareness that certain views that are taken for granted by most, may potentially be wrong or at least based on misleading preconceptions.
I fully understand.

My question is this: dropping uniqueness as guiding principle, do you have a something new?
Calrid
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Quote Quote by tom.stoer View Post
I fully understand.

My question is this: dropping uniqueness as guiding principle, do you have a something new?
Wouldn't you have to establish uniqueness as being more coherent as an axiom than not being unique first at least inductively. Philosophically it would be difficult to do I think. I don't understand the meat and potatoes of the math, but that at least is logically sound. Something new or something better in terms of an overarching meta theory. I think differences that converge are ok, but differences that don't are forbidden. So with appropriate terms you could say that the same way of describing the same thing is better than something different, but I'm not sure how you would justify that beyond an axiom without something else to weight the consequences.

Sometimes agreeing is probably not the best way to explore things perhaps is putting it too simply.

Perhaps I am misreading your point?
tom.stoer
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Apr1-11, 09:48 AM
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I don't really understand what you are saying.

My idea is rather simple: up to know physics collected experimental phenomena and theoretical descriptions and tried to "unify" them via a few axioms, principles, formulas etc. QED is based on a single Lagrangian and a common understanding how to quantize it and how to extract physics. So in some way we agree on a "unique fundamental formulation" of QED. It's not one single formula, but a few formulas plus a few principes how to use them. The same applies to QCD, etc.

All what I am saying is that this was always one guiding principle in physics. If one drops this guiding principle (there is no unique formulation, there are no fundamental degrees of freedom, ... could be 5-branes, could be E8 heterotic strings, ...) there should be some replacement, a new guideline for a research program.

My question to suprised is whether he has something to offer.
suprised
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Quote Quote by tom.stoer View Post
My question is this: dropping uniqueness as guiding principle, do you have a something new?
Well that was a loaded remark... let's not open another Pandora's box. Just a brief comment. Think about a conformal field theory. One and the same CFT can arise in the IR limit of many different microscopic theories; a CFT can be viewed in this sense as a universality class of theories, with the defining property that they lead to the same IR physics.

In perturbative string theory, CFTs appear as part of the world-sheet theory, and each choice corresponds to a classical background, and defines some on-shell physics (because the equations of motion are equivalent to requiring conformal invariance). One may speculate that going away from conformality is like going off-shell, and a priori it is unclear whether doing this is unique or not. In fact, it is known that a given on-shell theory may have different off-shell completions. So it may be that there is a bunch of "different" underlying theories that lead all lead to the same on-shell physics.

Essentially, this boils down to semantics and what one means by "unique" underlying theory. Eg., is lattice QCD a "different" theory as compared to the usual perturbative lagrangian formulation of QCD? No, because when performing the proper limits it lies in the same universality class. A similar phenomenon could happen eg for LQG and strings, etc.
atyy
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Quote Quote by suprised View Post
Well that was a loaded remark... let's not open another Pandora's box. Just a brief comment. Think about a conformal field theory. One and the same CFT can arise in the IR limit of many different microscopic theories; a CFT can be viewed in this sense as a universality class of theories, with the defining property that they lead to the same IR physics.

In perturbative string theory, CFTs appear as part of the world-sheet theory, and each choice corresponds to a classical background, and defines some on-shell physics (because the equations of motion are equivalent to requiring conformal invariance). One may speculate that going away from conformality is like going off-shell, and a priori it is unclear whether doing this is unique or not. In fact, it is known that a given on-shell theory may have different off-shell completions. So it may be that there is a bunch of "different" underlying theories that lead all lead to the same on-shell physics.
That would be really nice. Is there any construction that does give the off shell theory from a QFT, or something in that direction? (I assume this is in a completely different direction from AdS/CFT?)


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