Why I am REALLY disappointed about string theory

[tom.stoer]

I think I agree with suprised on this one. It seems implausible that the "theory of everything" will have a unique solution.

Where did you get the "unique solution" from?

About the specific list of questions.
My answer would be "too early to know" for most of them.

OK, fits to my impression.

To the first two, you should include the ads/cft part as well. If you can compute something on the cft side and it doesn't match, you've just falsified ads/cft.

Do you mean some generalized gauge/gravity duality? As we said AdS is unphysical / not realized in nature.

 

[negru]

Do you mean some generalized gauge/gravity duality? As we said AdS is unphysical / not realized in nature.

Like I said, no one said ads needs to exist for that to work. If we find a dS/cft duality or something I don't think the dS will be our universe. In the current formulation, you can think of the AdS space as the Hilbert space of QM. Plus, you only need AdS at the boundary. The bulk can be anything.

Or picture this. Say we extend the gauge/string duality to QCD, and it turns out to be C^2/QCD. Where C stands for camel. If we can predict stuff with it, what would that mean for the camel? Nothing, because in this case, the camel is just part of some internal machinery, which relates to the real world only through this particular formalism. It's an abstract camel.

Of course this depends on what you're trying to compute. If you want to compute a real world black hole entropy using the CFT, then yes AdS might not be suitable. If you want to compute gluon scattering amplitudes, it doesn't matter whether it's AdS space or camel^2 space. That's why some of the questions in this thread are not well-posed. You really need to be specific: at this moment, there are various string applications. The exact "physical" features of each of them might be different. You should refer to each specifically so we know what you're talking about.

As a futher example on the issue of ill defined questions. Consider the string/M theory duality. These two live in a different number of dimensions, but they are completely equivalent. So, wouldn't the question "just how many dimensions are there?", be pretty silly? Although it sounds very sensible and "physical", it just doesn't make sense. It depends very much on technical issues.

 

[tom.stoer]

As I said; I understand, but I prefer to talk about generalized gauge/gravity duality instead of AdS/CFT.

I think I agree with suprised on this one. It seems implausible that the "theory of everything" will have a unique solution.

Again my question: where did you get the "unique solution" from? I was talking about a "unique framework"; that means a mathematically well-defined set of axioms / rules / principles / theorems / equations serving as the basis of the theory.

As a futher example on the issue of ill defined questions. Consider the string/M theory duality. These two live in a different number of dimensions, but they are completely equivalent. So, wouldn't the question "just how many dimensions are there?", be pretty silly? Although it sounds very sensible and "physical", it just doesn't make sense. It depends very much on technical issues.

As I said before: this is exactly the kind of technical details of string theory internal issues I explicitly excluded when talking about physical questions (I did not exclude ordinary 3+1 dim. spacetime as this is something we observe).

Anyway: I would hesitate to insist on complete equivalence of two theories which are not yet defined (as we discussed above); or where the "definiton" is restricted to certain approximations.

Or let's turn it round: What is the definition of string and M-theory?

Btw.: similar questions have been asked by David Gross several times:

http://strings2009.roma2.infn.it/talks/Gross_Strings09.pdf, slide #16
http://www.ift.uam.es/strings07/040_scientific07_contents/transparences/gross.pdf , slide#22

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?

WHAT IS MISSING ?
Perhaps “String theory” is like quantum field theory - a framework and not a definitive theory.
Perhaps we are missing a fundamentally new principle of symmetry, of dynamics, of consistency, ... that leads to a unique solution --- not a “vacuum” but a space-time, a cosmology.
Emergent Space-Time

 

[atyy]

"Experimental discovery of Supersymmetry would certainly give string theory a big boost, and learning how Supersymmetry is broken might very well give string theorists crucial clues about how to proceed." http://conferences.fnal.gov/lp2003/program/papers/witten.pdf

Five Problems in Quantum Gravity
http://arxiv.org/abs/0906.1313

 

[negru]

Ok, unique framework. This definitely. There's just no chance you can have both a "holographic" type framework, and the "traditional" type, both with strings, with absolutely no connection between them. But if it turns out you can, then I'd have to conclude that strings are the most natural language humanity has found so far. I would advocate reinterpreting pythogoras's theorem in terms of string theory if that happens.

My comment about the unique solution was related to the landscape issue, which most string critics are focusing on and which I find completely irrelevant at this stage of development.

 

[Haelfix]

Ben mentioned it in this thread, but the landscape problem of quantum gravity (really high energy physics) is not going to go away and will be a generic problem for any approach, even if they haven't studied or appreciated it yet.

String theory does better than any other current alternative on the market in this regard since it restricts what can come out of the low energy physics (the swampland).

Whereas a high energy theory of gravity that can arbitrarily couple any matter without constraints, will automatically have an (infinitely) worse landscape problem.

 

[marcus]

Whereas a high energy theory of gravity that can arbitrarily couple any matter without constraints, will automatically have an (infinitely) worse landscape problem.

Haelfix, I have often found statements similar to this to be unreliable/misleading and I suspect the reason may be as follows:
They depend on a particular person's necessarily limited individual way of imagining future research.
You could have a very specific format in mind for "coupling matter without constraints" so that what you are saying actually has necessarily limited applicability. Another reader might not realize this and might think that what you are saying actually applies broadly to all theoretical approaches to gravity and matter.
As a check you might want to consider the following hypothetical example and explain why you think it would automatically lead to an infinitely worse landscape problem, if it should happen to work.

You probably know something of Caltech prof. Matilda Marcolli (co-author with Alain Connes on several occasions). Marcolli et al posted this in May 2010. See especially section 8 "Spin foams with matter"
https://www.physicsforums.com/showthread.php?t=402234

If you wish, you could explain why an infinitely worse landscape problem would necessarily arise over in that thread. That way it would not distract from this thread's focus on string theory.

The title of the paper is Spin Foams and Noncommutative Geometry http://arxiv.org/abs/1005.1057

 

[marcus]

I guess what worries me is two things. One is the presumption and tone of omniscience. You present yourself as someone who can imagine every approach to quantum gravity and matter and who is able to foresee a landscape for every possible approach.

... the landscape problem of quantum gravity (really high energy physics) is not going to go away and will be a generic problem for any approach, even if they haven't studied or appreciated it yet...

The other worrisome thing is that this claim seems to be motivated as a defense of string.
The defensiveness is disappointing. You seem to be trying to deflect comment on the string landscape difficulty by the questionable suggestion that all other possible approaches would inevitably suffer the same or worse. It sounds suspiciously like part of some rhetorical battle you think you are engaged in, rather than an objective scientific observation.

 

[negru]

Well, part of the issue is that string theory IS already a theory of quantum gravity. The problem so far is that we cannot determine all the parameters uniquely. Other than that it's pretty ok. Suppose the compactification which leads to the dreaded landscape is shown to contain the SM (and some solution exists with the right constants). Then the problem is that whatever other theory you find (with say a unique solution), would itself just be a particular solution to the more general string theory.
To see if the theories are actually incompatible you'd need to go to higher energy and make a prediction there. This higher energy would likely be around the Planck scale, so good luck with that.

[this is of course assuming that no further progress is made in string theory, in terms of sub-planckian predictions]

So this problem will remain whatever other theories we find.

 

[marcus]

Well, part of the issue is that string theory IS already a theory of quantum gravity...

With all cordial respect, Negru, that is not the issue at all.
I am objecting to Haelfix's unsupported assertion that any QG+M approach which might possibly be invented must encounter a landscape dilemma similar to string's, or worse.

We cannot be sure that what you call "string theory" is the final and only theory of gravity, so we cannot infer that any theory of gravity must encounter the same sorts of problems, as you seem to be suggesting.

What I would like to see Haelfix try to explain concerns the example of Marcolli's joining Spinfoam and Noncommutative Geometry (SF+NCG). That is an approach. (Not an approach I especially favor or know a lot about, but one I've followed over the past 3 or 4 years as it gradually got started.) As an example, I want some plausible argument why THAT approach, just to be specific, must encounter a landscape dilemma.

Haelfix may just wish to retract the broad claim he made and say something more narrowly constructed. Which would be fine! I, for one, would certainly be interested in some revised form of the idea.
Besides Marcolli, there are further possible examples/test cases that one could consider.

If he decides to tackle Marcolli's idea, then I hope he goes over to the Marcolli thread to do it, so it won't distract here. I think the focus here should be on string. And forget about trying to point out presumed flaws with other approaches as a defense mechanism. I gave the link to the Marcolli thread earlier---https://www.physicsforums.com/showthread.php?t=402234

 

[tom.stoer]

OK, I think it's time to focus on the unique mathematical framework.

I guess we agree that old-fashioned string theory + 11 dim. SUGRA is not unique in that sense. It requires different formulations depending on the energy / strong-weak regime.

If you look at QCD there are some low-energy effective theories which are just that (and which cannot be derived but only motivated from high-energy QCD); nobody would doubt that even in the low-energy domain QCD (quarks, gluons) are the fundamental degrees of freedom, even if it's hard to calculate photo-pion production w/o chiral perturbation theory.
Not so in string theory. It is required to change the description once you move to a different regime. It is not just that the calculations are hard if you don't, It seems impossible not to do so. One central issue is that dualities between these different theories are not always established rigorously but only in certain approximations (s-duality). I would accept a framework as unique even if you have different calculational tools in different regimes once you have defined the theory in one regime and once you have shown the equivalence between the two regimes.
It's simpler in QCD as you do not need to derive a low-energy effective theory rigorously as soon as you a) can plug in the phenomenologically correct (experimentally well-known) degrees of freedom which you already know and b) derive some experimentally testable results. As b) is missing in most regimes of string theory one has to go the hard way and stick to more rigorous math.

The discussion regarding AdS/CFT, twistor strings etc. shows that it may be necessary to find new formulations to answer new questions. Still no reliable unique framework exists from which you can always start (again: compare it to QCD).

Let me comment on a few ideas, research programs etc.:

String perturbation theory is not defined beyond 2 loops (3, 4 loops?). Some time ago I studied a paper in which they tried to define the genus 3 and 4 superspace measure. I don't know if this has been achieved; but it is clear that a consistent definition of string perturbation theory including proof of finiteness order by order and convergence of the whole perturbation series is still not available.

Then what is non-perturbative string theory? Or what are quantum mechanical completions of effective solutions / vacua? In most cases one throws away all the high-energy string modes and uses the low-energy effective theory, e.g. MSSM-like models. As this is fine from the phenomenological point of view it is not useful in defining the theory. It can serve only as a specific expansion aroung a specific vacuum ( as an example: it cannot describe picking one vacuum neither tunneling between vacua).

I haven't seen much about second quantized string field theory. Could this be one unique framework with descents to other more specific formulations?

Regarding M-theory: What is the current state regarding infinite-dimensional matrix theories, emergent gravity and/or tri-linear algebras? Of course this is work in progress but are these really candidates to define the theory uniquely? to identify the "fundamental" degrees of freedom?

Is there a framework available in which fully dynamical spacetime (beyond the pertubative graviton limit) can be studied? (this is related with the usual question regarding background independence). Afaik this is more or less settled as soon as a framework regarding gauge/gravity is available. But as we saw this is realized only in certain specific cases like AdS/CFT. Would the uniqueness issue go away once one can establish such a duality, being able to translate all questions regarding fully dynamical spacetime into more tractable field theory stuff?

Again let me conclude with David Gross:

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.

 

[tom.stoer]

@marcus, Haelfix: the discussion you started is certainly interesting, but why should we care about a landscape problem in theories X, Y, Z, ... if we know that they are not related to strings - but strings are the unique theory (including gravity)?

My guess is that what we call landscape problem is due to the fact that we have one great achievement in string theory, namely that it turned theories into solutions / vacua. That is nice, but it does not make all the theories go away. There seems to be not so much difference whether there is a landscape of theories or a landscape of vacua :-)

 

[suprised]

That is nice, but it does not make all the theories go away. There seems to be not so much difference whether there is a landscape of theories or a landscape of vacua :-)

Yes indeed, however some people like Tom Banks have an interesting take on that, see his recent TASI lectures.

But I should clarify some aspects that are often confused/deliberately misrepresented, concerning landscape, predicitibilty and so on. It has been often stated that string is "unique", on the other hand critics claim it makes "no predictions" - as if string theory would be a completely structureless, floppy, ambiguous, ad hoc mathematical construct which leads to arbitrary results.

The point is of course, as has been said before, that a unique theory can have arbitrarily many solutions (like electromagnetism that governs all possible DNA molecules for example), which is a fact probably understood by anybody.

But what is often overlooked, is that once a solution is chosen, ie. a background around which to expand, there are infinitely many scattering processes one can do, in principle, which leads to infinitely many predictions. That is, string theory is quite the opposite of a random arbitrary theory, rather the infinite mass spectrum and all the interactions between these infinitely many states are finely tuned and completely determined, __once the background has been fixed__. In other words, there are infinitely many counter terms in the effective field theory that arise from integrating out the massive spectrum around a given background, and in principle each one could be tested by scattering experiments and so this leads to infinitely many "predictions". And if just a single one of such terms would come out wrong experimentaly, or one predicted massive state would be missing, this would disprove string theory; as the missing of any single state, say at the one millionth mass level, would render the string theory immediately inconsistent.

I am not claiming, though, that we humans of today would be able to do such computations, nor do the experiments, rather this is a matter of principle. This is to illustrate that string theory is an exceedingly determined theory without any room to adjust -- except for the ground state (it seems). The unfortunate thing is that we humans with our limited capabilities are able are able to access only the physics of the ground state and not the excited string spectrum; but in a sense this is our own fault and not the one of the theory ;-)

Things may however need not be as bad, as there is the (remote) possibility that the string scale is very low after all, and if it is near the weak scale then one might be able to see massive string states and KK states even at the LHC. There are a some papers out analyzing this situation, including plots of cross sections, so very clearly string theory can make specific predictions; even if measuring a finite number of such resonances would not logically prove that the theory is right (as the one-millionth might be missing, eg), it nevertheless would amount to a physicists’ proof of string theory (well, the armcair critics would never accept this that but who cares?)

I hasten to add, though, that finding such a scenario is extremely unlikely, as there is no real reason why the string scale should be so low. We certainly expect that “something” must happen at the weak scale, and finding susy or extra dimensions or string states are just less or more remote possibilites/wishful thinking.

Finally some remarks on predictibility; so far we talked about massive states and counter terms in the effective action at low energies; what about predicting the structure of the standard model, eg.? Indeed IMHO there is no way that this can be predicted from string theory as of today. It always amounts to make certain choices, eg about a brane configurations, fluxes, manifolds, etc, and one could make other choices as well. This is not a specific failure of string theory, but of any other theory I know - let’s mention Connes’ setup (or Lisi’s construct, which I wouldn’t call a theory); in each case one makes some choice (eg of the non-commutative geometry, the gauge group, the space-time dimension, etc), and then one goes from there. This is morally the same “problem” that string theory has, and there is no answer why just this structure is chosen as compared to any other possible one. In string theory, those ad hoc choices are interpreted as choices of background, in other theories these choices just define the theory. I would view the situation from the string point of view, namely as solutions of one single theory, as more satisfying, though in practice it doesn’t make much of a difference - this relates to the first line and is a good place to stop for now!

 

[tom.stoer]

surprised!

Thanks for the long and well-elaborated statement. In the light of our discussion I agree with most of your ideas (except for minor details which I don't want to discuss - just to stay focussed). As I already indicated: accepting that string theory's major achievement is to turn fundamental (gauge) theories into low-energy effective theories or ground states goes hand in hand with accepting some kind of landscape.

Of course I expect that there will be additional discoveries restricting possible vacua, telling us more about dynamical selection principles, stability, tunneling, etc. This will perhaps reduce the number of vacua, lift some degeneracies, ... but in the end we will still face a reduced landscape (unless a third superstring revolution will throw over everything).

I do not agree with haelfix that all theories will face a landscape issue b/c nobody knows all theories - not even within string theory, let alone other theories like LQG, NCG or XYZ

But again this brings us back to Gross' question and to my last two topics (slightly adjusted)

• what string theory really is
• what the final theory will look like (in terms of strings or other fundamental degrees of freedom?)
• what the major obstacles (inherent to string theory) are preventing us from constructing this unique mathematical framework

Anybody out there who wants to comment on these questions?

 

[Haelfix]

Another reader might not realize this and might think that what you are saying actually applies broadly to all theoretical approaches to gravity and matter.

I would hope that a reader would realize that what I am saying is that in fact it DOES apply broadly to any tentative theory of matter.

What we now know, which wasn't necessarily appreciated a few years ago, is that a landscape of meta stable vacua is not just a truism about KKLT in string theory, but also a *generic* property of quantum field theory (and not just supersymmetric ones) including the standard model.

That is to say, any theory (under a reasonable set of conditions like having a reasonable scalar spectrum) that wishes to include gravity, and that contains a small cosmological constant and that possesses the standard model as an effective field theory at low energies also necessarily has a large landscape of metastable vacua.

Nima, Michael Dine and a few others have convincingly shown this, and I emphasize again that it is completely independant of the nature or properties of any tentative UV completion at high energies.

See for instance arXiv:hep-th/0703067

 

[suprised]

[*]what string theory really is

A quick provocative claim: what we have constructed so far is nothing but a portion of the space of the consistent theories that include gravity.
(This could be refuted by presenting a theory that is consistent but is not contained in this framework; in a sense this is the question whether the "swampland is hospitable" or not.)

If it is just that, then what we call "string" theory (including 11dim M-Theory that is not a string theory) is a framework that is useful to describe/parametrize this space of consistent theories, in certain regimes at least. But which does not out of itself determine the ground state etc.

I would view this analogous to gauge theory. Much like N=4 supersymmetric Yang-Mills theory is an interesting model for real world QCD (with important differences and simplifications), the ten dimensional string theories and their compactications with many supersymmetries are models for gravitational theories. They are useful for studying certain aspects like non-perturbative phyisics and black holes. But in the same spirit in which one should not view N=4 gauge theory as being underlying (or more fundamental than) QCD, one should not view those string theories as underlying the real world.

 

[mitchell porter]

Anybody out there who wants to comment on these questions?

At any time, there are dozens of papers proposing to be the next big thing, saying we need to think about "BPS preons", or "Azumaya noncommutative geometry" of D-branes, or "representation theory of higher categories of cobordisms", or E_n with n>8... But I'm still learning the basic duality web of the mid-1990s, so I certainly can't tell you if any or all of these is relevant to the future. I can only mention a few statements I've gleaned from the literature.

(1) The string coupling can vary dynamically. But in M-theory, the 11-dimensional corner of the duality web, this quantity turns into a new spatial dimension, so you can't take strong or weak coupling limits of M-theory proper. This has something to do with the difficulty of writing down an equation for M-theory. Ashoke Sen and others say that M-theory won't be understood until the whole theory is understood. But we could still turn this around and say that understanding the M-theory corner should help us understand the whole theory. The tri-algebras you mention provided incremental progress here because they give the worldvolume theory for a stack of M2-branes.

(2) I believe you need a different matrix model for each distinct spacetime background. (There is more about this in Lubos Motl's thesis.) So matrix models, while important, aren't The Answer.

(3) String field theory also isn't The Answer. Edward Witten, who invented it, long ago dismissed it as "too messy" to be fundamental. The one important role it played in recent times, that I know about, is Sen's work on tachyon condensation and brane-antibrane annihilation. The tachyon is usually unwanted because it has negative energy. But here it just balances out the positive energy of the brane tensions.

 

[tom.stoer]

A quick provocative claim: what we have constructed so far is nothing but a portion of the space of the consistent theories that include gravity.

If it is just that, then what we call "string" theory (including 11dim M-Theory that is not a string theory) is a framework that is useful to describe/parametrize this space of consistent theories, in certain regimes at least. But which does not out of itself determine the ground state etc.

As I said: forget about the ground state, I can live with the landscape, but I (and Gross :-)still miss a well-defined mathematical framework and underlying physical principles.

So the provocative question of Gross does not mean what string theory currently is - here I agree with your claim - but what string theory fundamentally is? (compare it to the situation in the early days of quantum mechanics).

 

[tom.stoer]

Hello Mitchell,

thanks for your ideas. My conclusion is that M-theory seems to be nothing else but a new domain of the full space of theories with its own mathematical framework, technicalities and their specific limitations.

So it's not the mother-theory :-)

But in the very end we are all expecting one unique defining equation, one mother of all theories, don't we?

I do not know who had the idea here in the beyond-forum; it goes like that: think about a huge complicated n-dim. manifold for which you need a huge number of coordinate charts. You have to construct each chart and each mapping between two overlapping charts seperately. Whenever one inspects a new region of the manifold one finds new topological structures, new coordinates, new maps etc. There is no global view! Now compare this to string- / M-theory: perhaps all the different pieces (theories) we have so far are nothing else but these different charts, the dualities are the maps between the theories. Once we investigate a new domain of string- / M-theory we eventually find new structures, new dualities etc. So again there is no global view for this manifold or space of theories (and therefore no single defining equation).

Is it just that? Or is there anything around the corner - a third superstring revolution - which harmonizes all these theories into a single defining framework from which they can be derived - at least in principle?

 

[suprised]

I do not know who had the idea here in the beyond-forum; it goes like that: think about a huge complicated n-dim. manifold for which you need a huge number of coordinate charts. You have to construct each chart and each mapping between two overlapping charts seperately. Whenever one inspects a new region of the manifold one finds new topological structures, new coordinates, new maps etc. There is no global view! Now compare this to string- / M-theory: perhaps all the different pieces (theories) we have so far are nothing else but these different charts, the dualities are the maps between the theories. Once we investigate a new domain of string- / M-theory we eventually find new structures, new dualities etc. So again there is no global view for this manifold or space of theories (and therefore no single defining equation).

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.

 

[tom.stoer]

Not all statements are worth to be remembered :-)

 

[atyy]

In fact, Haelfix's claim is self-evidently correct.

 

[tom.stoer]

In fact, Haelfix's claim is self-evidently correct.

which one? regarding the landscape issue?

 

[atyy]

which one? regarding the landscape issue?

Yes, the one about a theory of gravity which can couple to any form of matter.

 

[tom.stoer]

I don't care in this context. We have a candidate theory (string theory) which has the landscape problem. Regardless if it's right or wrong, it is obvious that nobody will find a new candidate theory w/o any connection to strings but as deeply investigated as strings within the next couple of weeks. But as soon as this new theory is published (I am checking arxiv daily), I will open a new thread.

 

[BenTheMan]

I guess what worries me is two things. One is the presumption and tone of omniscience. You present yourself as someone who can imagine every approach to quantum gravity and matter and who is able to foresee a landscape for every possible approach.

It's very possible that some, as of yet uninvented, approach to quantum gravity may evade these problems. Of course, the burden of proof is on that community to actually come up with a workable theory first. Surely it is possible that such a theory exists, but it is also possible that the LHC will produce fire-breathing dragons which will ravage Europe. It's also possible that some bright graduate student will find a selection principle on the landscape and predict the electron's mass from string theory. It's just not likely.

The point is, you have to follow your nose. When you build a model or a theory, you have to first start with what has worked for you in the past, and then build from there. And at every stage you have to make sure that you effective field theory matches the standard model or the MSSM.

So Haelfix's comment is right, given what we currently know about the theory of quantum gravity, based on 100 years of research of some very smart people. Is it possible that 4 generations of work has led to the wrong conclusions? Sure---it's happened before.

So you can wring your hands and say ``We just don't KNOW!'', or you can shut up and calculate something, and try to advance the state of knowledge to the best of your abilities. Naval gazing works for some, but not for people who are interested in science.

 

[BenTheMan]

A quick provocative claim: what we have constructed so far is nothing but a portion of the space of the consistent theories that include gravity.
(This could be refuted by presenting a theory that is consistent but is not contained in this framework; in a sense this is the question whether the "swampland is hospitable" or not.)

See Wati Taylor's recent work regarding the swampland in 10 and 6 dimensions.

 

[MTd2]

So Haelfix's comment is right, given what we currently know about the theory of quantum gravity, based on 100 years of research of some very smart people.

What is *the* theory of quantum gravity?

Are you single?

 

[marcus]

In fact, Haelfix's claim is self-evidently correct.

https://www.physicsforums.com/showthread.php?p=2826555#post2826555

 

[petergreat]

Regardless if it's right or wrong, it is obvious that nobody will find a new candidate theory w/o any connection to strings but as deeply investigated as strings within the next couple of weeks. But as soon as this new theory is published (I am checking arxiv daily), I will open a new thread.

In the 19th century, no amount of ingenuity from theorists could've explained Mendeleev's periodic table before J J Thompson actually discovered the electron and before the development of quantum mechanics! The SM is better than the periodic table but still has 20+ free parameters. After so many years of efforts in vain, is there any reason for believing that such an absolute mess can be explained from theory alone, without some revolutionary experimental discovery? I don't believe yet another speculative theory posted to arxiv (which will reach you on the same day) will magically solve the world.

P.S. Maybe people should simply give up before seeing new experimental hints?

 

[tom.stoer]

Thanks Marcus!

So let's come back to Gross' question - and to my last two topics (again slightly modified) -

• what string theory really is
• what the fundamental principles are and how the final theory will look like (in terms of strings or other fundamental degrees of freedom)
• what the major obstacles (inherent to string theory) are preventing us from identifying these underlying principles and constructing this unique framework or theory

 

[MTd2]

After so many years of efforts in vain, is there any reason for believing that such an absolute mess can be explained from theory alone, without some revolutionary experimental discovery? I don't believe yet another speculative theory posted to arxiv (which will reach you on the same day) will magically solve the world.

P.S. Maybe people should simply give up before seeing new experimental hints?

Very good point. There is a new preprint today on arxiv.org, about History of Physics, that gives a very insightful view on how we are clueless even with experiments:

http://arxiv.org/abs/1008.0447

Failed theories of superconductivity

Joerg Schmalian
(Submitted on 3 Aug 2010)
Almost half a century passed between the discovery of superconductivity by Kammerlingh Onnes and the theoretical explanation of the phenomenon by Bardeen, Cooper and Schrieffer. During the intervening years the brightest minds in theoretical physics tried and failed to develop a microscopic understanding of the effect. A summary of some of those unsuccessful attempts to understand superconductivity not only demonstrates the extraordinary achievement made by formulating the BCS theory, but also illustrates that mistakes are a natural and healthy part of the scientific discourse, and that inapplicable, even incorrect theories can turn out to be interesting and inspiring.
Comments: 14 pages, 3 figures, to appear in: Bardeen Cooper and Schrieffer: 50 YEARS, edited by Leon N Cooper & Dmitri Feldman

****

It is worthless to just shut up and calculate if you don't turn screws and vice versa.

 

[tom.stoer]

Yes and No.

Of course theoretical physics lacks new fundamental experimental insights from HEP since 2-3 decades (all these experiments either confirmed the SM - W- and Z-bosons, tau, ... or disproved simple GUTs - proton decay).

But there are other experimental data available (for a rather long time) waiting for an explanation (# of dimensions, global and local symmetries, particle spectrum, cosmological constant). The problem is that all these data may be consistent with some version (or vacuum) of string theory, so they do not provide new insight how to proceed.

Look at high-temperature superconductors. It does not help to find a new material with an even higher T_c; we do not understand how it works.

If we agree that string theory is a "sketch of a theory" which could be consistent with all these observations (but that these observations do not help to understand the fundamental principles, "only" the vacua) then we must look for a different modus operandi how to identify or construct the fundamental theory.

That's why I am asking these questions.

 

[MTd2]

Proton decay was not disproved, unless for GUTs with the shortest proton half-life.

 

[tom.stoer]

Proton decay was not disproved, unless for GUTs with the shortest proton half-life.

Agreed