Why I am REALLY disappointed about string theory

marcus

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

Marcus, thanks for the summary.

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 :-(

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

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

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

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

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

But could one hope for non-perturbative definitions of other sectors of the theory in the same spirit?

Calrid

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

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

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

I fully understand.

My question is this: dropping uniqueness as guiding principle, do you have a something new?

Calrid

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

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

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

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?)

marcus

The new formulation of LQG has analogies with both lattice QCD and diagram QED. The way the combinatorial structures (spin networks, spinfoams) are defined makes them analogous to both generalized lattice and Feynman diagram. 1102.3660 talks about this if I remember right.

The main difference from lattice QCD, I think, is that the lattice is irregular and has no "metric" content, no fixed edgelengths etc. The main difference from Feynman diagrams is that you have a 2-cell complex instead of a 1-cell (graph). The vertices are still places where an interaction occurs.

"...phenomenon could happen eg for LQG and strings,.." I take to suggest that some type of LQG spinfoam model could turn out to be the combinatorial cousin of some type of String model. I'm not sure what I mean by that. Perhaps there is no definite meaning at this point. Just a vague notion.

Do you watch NCG? Are you familiar with the "almost commutative space" C(M) x F where F is the finite noncommutative piece? Any ideas about this mysterious F entity?

tom.stoer

@suprised: very good point, the "off-shell d.o.f." do not matter as long as the on-shell results are correct. [but in QCD there seem to be a very good reasons to identify quarks and gluons as the fundamental d.o.f. - even if they cannot be identified as real on-shell particles]