Why I am REALLY disappointed about string theory

[marcus]

.
My wife recently read Brian Greene's first book, but she didn't like it and I have to agree with her criticism. The book makes many promises in the name of strings, predicting particle masses, being the single grand theory of gravity, etc., and none of these promises are fulfilled in the book. Indeed, string theory has still not fulfilled these promises. In fact, I would be comfortable betting that string theory will never predict the electron's mass. On the other hand, I think string theory has given us much we didn't expect, something far more interesting. And since our job in physics is not just to crank out numbers for the latest random experiment being done, but to understand the deep structure of nature, I think string theory will continue to play a role in our search for that structure.

How's he going to collect if he's right?

It's usual in these situations to remove infinities by introducing a cutoff.

From what Physics Monkey says, he would also be comfortable betting that the electron mass not be derived from Stringery by 2017, say. So then if anyone takes the bet it would be possible to collect on it.

 

[tom.stoer]

Yes.The mountain has given birth to the mouse.

any substantial contribution?

 

[Fra]

How's he going to collect if he's right?

How are we even going to establish if he is right, when the predictive systems (the theory) is merely solutions in another theory.

Would "finding ONE theory that predicts electron mass" count?

Or do we also need to explain this particular choice of solution to have made a complete prediction (or postdiction)?

/Fredrik

 

[Physics Monkey]

How much do you want to bet? i.e. please explain this opinion.

Haha, I don't know, I might be willing to bet quite a bit. I seriously doubt any prediction will be forthcoming. Basically, I think its crazy to think that structure of the world at a few GeV tells us much of anything about the structure of the world at 10^{18} GeV (and vice versa). I think such a situation is unprecedented in the history of science

This is because I suspect the landscape is a real thing. Does anyone really think that string theory, with all its incredible richness, can't accommodate a bit heavier of an electron, or an extra generation of very heavy particles, or any number of other minor (or even major) tweaks?

For example, suppose you have a vacuum that closely resembles our world.
1) I would imagine that there are hundreds more with slightly different low energy parameters.
2) And similarly, I would imagine that there are hundreds with the same low energy physics but with varying physics at higher energies.

1) is very familiar from condensed matter physics where we often have continuously variable parameters.
2) is an example of low energy universality i.e. the relative independence of low energy physics from the high energy details.

Holographic duality tells us that this intuition also applies to some part of the string landscape.

Another example from condensed matter physics. Suppose you were a tiny organism living inside a material in one of the labs down the hall. You discover through a series of ingenious high energy experiments your proposed theory of everything: electrons hopping on a lattice interacting electrostatically. You look for symmetry principles telling you that the lattice you found is the perfect lattice, that it had to be that way (haha, that actually sounds a lot like string theory). You write a popsci book declaring that you will soon compute all the interesting constants of nature: the speed of phonons, the mass of low energy quasiparticles, etc. But then you discover that far from giving a unique answer, the high energy theory predicts all kinds of worlds you've never seen: different lattices, different phonon speeds, different critical temperatures for the various phase transitions you've observed, different phases you've never seen. And you finally realize that knowing the high energy theory doesn't ultimately tell you that much about your low energy world. But that's ok because the high energy theory is still interesting and useful and it opens your eyes to wonderful new possibilities. That is what I understand string therory to be.

 

[Physics Monkey]

Haha, as a place to start, I would be willing to bet US $100 that no string construction will predict the electron's mass in the next 20 years. By predict I mean roughly the following: we may find a vacuum that has the electron's mass right (that wouldn't surprise me), but string theory should have given some reason why this vacuum should be preferred to any other. Or perhaps shown how the vacuum/landscape language I'm using is misleading.

I'm a reasonable monkey physicist , I would be happy to pay up if anything remotely predictive came out of string theory (regarding high energy particle physics).

 

[negru]

Haha, as a place to start, I would be willing to bet US $100 that no string construction will predict the electron's mass in the next 20 years. By predict I mean roughly the following: we may find a vacuum that has the electron's mass right (that wouldn't surprise me), but string theory should have given some reason why this vacuum should be preferred to any other. Or perhaps shown how the vacuum/landscape language I'm using is misleading.

I'm a reasonable monkey physicist , I would be happy to pay up if anything remotely predictive came out of string theory (regarding high energy particle physics).

This sounds overambitious for now. However, having a landscape doesn't mean one can't make predictions. People who say that really haven't thought about the topic too much or are just playing dumb. It doesn't matter if there are 10^500 solutions, or a continuum of them.

As long as the number of parameters needed to describe the vacuum is less than the current number of parameters in the Standard Model, it's more than enough. There's nothing bad about having a landscape. In my opinion, like in yours, having a landscape of solutions is the most sensible thing a theory of everything can have. We shouldn't we be able to have a theory in which the electron mass is not X, but X+0.0000000001 ? Like there is no sensible reason why the Earth's mass is X, but there is nevertheless a mechanism which can explain how it got that mass.

In fact. the improvement from 23 something parameters to 10^500 different solutions is quite big. The latter has measure zero in the former. It's infinitely better. Calling it a plague is really disingenuous. Assuming that some solutions come close to reality of course.

 

[A. Neumaier]

I'm a biologist. I spent years liking the underdog LQG from reading Scientific American articles in the gym without having any idea what the formalisms of LQG or strings were. But I finally decided to learn a little string and LQG a few years back after hearing all my friends talk about Smolin's book for weeks without my understanding a single word. Guess what? While being a string practioner requires real skill, the achievements of string theory were clear enough to me - a lay person - that strings are way ahead of LQG.

Please let us know according to which standards you measure being ''ahead''?

And which achievements show clearly enough that strings are way ahead of LQG?

 

[negru]

Please let us know according to which standards you measure being ''ahead''?

And which achievements show clearly enough that strings are way ahead of LQG?

Just to point out the first that comes to mind, string theory contains gravity while it remains to be shown that so does LQG.

 

[Fra]

It doesn't matter if there are 10^500 solutions, or a continuum of them.

Because 10^500 are still too many to be searchable in any rational manner?

Like there is no sensible reason why the Earth's mass is X, but there is nevertheless a mechanism which can explain how it got that mass.

What is the mechanism whereby the string action relative to a background or vacuum, couple to the action and evolution of of the background?

The problem isn't that there is some kind of landscape per see, the problem is that there is no measure on this landscape that allows evolution and learning.

What I would like to see is a fit learning algorithm that can make progress and converge in the landscape.

Say a rational search algorithm, and then show that this converges faster than the landscape grows further.

If ST can provide a mechanism for this, then there is notthing wrong per see with a landscape. To compare with evolution, the landscape itself should be observer dependent and be large enough to ensure variation, but not large enough to cause for the observer to get lost in his random walk.

I do not object to non-unique inferences at all, on the contrary do I embrace it. It's a part of the inferencial perspective I am into, but the algorithmic search is important. So important that I don't think it's reasonable to take the such a program conceptually serious until this gap is filled in. This isn't about beeing ambitous, it's just that before one engages in such sophisticated version of theories of theories, modifying the falsification criterias etc one must get it right, or it's better to stick with the old unique theory framework.

/Fredrik

 

[Fra]

Because 10^500 are still too many to be searchable in any rational manner?

Since I don't have the technical insight in exactly how these theories are counte in ST, I wonder this.

What structure does there exists in this set?

The structure, such are order relations etc, can be exploited to improve search and learning. In this sense, it's even POSSIBLE that it's faster to handle a continuum as one degree of freecom, rather than as one big SET with nor order relations defined. So cardinality alone doesn't settle wether a continuum is worse than a finite set when it comes to search times.

/Fredrik

 

[tom.stoer]

Just to point out the first that comes to mind, string theory contains gravity while it remains to be shown that so does LQG.

What about LQC which has FRW cosmology as a limiting case? What about deriving graviton propagators from spin foam? Of course not everything is completed in LQG, but afaik there is no fully developed string theory on dynamical backgrounds, either; so both programs are work in progress.

But this is not relevant here. I don't think we should compare LQG and string theory in this thread (we can do that, but not here, as we would immediately start to argue about LQG; this is a different subject). I started and continued this thread w/o mentioning LQG (hopefully :-), simply to avoid this never-ending discussion from the very beginning.

 

[atyy]

Please let us know according to which standards you measure being ''ahead''?

And which achievements show clearly enough that strings are way ahead of LQG?

Just a quick answer and I'll refrain after this, since following tom.stoer's note we shouldn't get into this discussion here.

From my lay point of view, strings are ahead becasue
1) Strings have gravity.
2) The AdS/CFT correspondence is a non-perturbative formulation of quantum gravity in some universes, and a concrete example of the emergence of space

LQG has not been shown to contain gravity. The theory is not well-defined ( http://arxiv.org/abs/1010.1939 , Eq 26). The "classical limit" addresses only kinematics, not dynamics ( http://arxiv.org/abs/1101.5061 ). My interest in LQG is that since string theory has shown that theories without gravity can contain gravity, maybe the LQG formalism does contain gravity, if it is interpreted differently.

 

[atyy]

Haha, as a place to start, I would be willing to bet US $100 that no string construction will predict the electron's mass in the next 20 years. By predict I mean roughly the following: we may find a vacuum that has the electron's mass right (that wouldn't surprise me), but string theory should have given some reason why this vacuum should be preferred to any other. Or perhaps shown how the vacuum/landscape language I'm using is misleading.

I'm a reasonable monkey physicist , I would be happy to pay up if anything remotely predictive came out of string theory (regarding high energy particle physics).

A tangent: I think condensed matter folks have long wanted a landscape (eg. Wen's manifesto at the start of his QFT text, if I'm reading it right). So from that point of view, the landscape is an achievement of strings?

Edit: I wrote the above before seeing these:

For example, suppose you have a vacuum that closely resembles our world.
1) I would imagine that there are hundreds more with slightly different low energy parameters.
2) And similarly, I would imagine that there are hundreds with the same low energy physics but with varying physics at higher energies.

1) is very familiar from condensed matter physics where we often have continuously variable parameters.
2) is an example of low energy universality i.e. the relative independence of low energy physics from the high energy details.

Holographic duality tells us that this intuition also applies to some part of the string landscape.

Another example from condensed matter physics. Suppose you were a tiny organism living inside a material in one of the labs down the hall. You discover through a series of ingenious high energy experiments your proposed theory of everything: electrons hopping on a lattice interacting electrostatically. You look for symmetry principles telling you that the lattice you found is the perfect lattice, that it had to be that way (haha, that actually sounds a lot like string theory). You write a popsci book declaring that you will soon compute all the interesting constants of nature: the speed of phonons, the mass of low energy quasiparticles, etc. But then you discover that far from giving a unique answer, the high energy theory predicts all kinds of worlds you've never seen: different lattices, different phonon speeds, different critical temperatures for the various phase transitions you've observed, different phases you've never seen. And you finally realize that knowing the high energy theory doesn't ultimately tell you that much about your low energy world. But that's ok because the high energy theory is still interesting and useful and it opens your eyes to wonderful new possibilities. That is what I understand string therory to be.

As long as the number of parameters needed to describe the vacuum is less than the current number of parameters in the Standard Model, it's more than enough. There's nothing bad about having a landscape. In my opinion, like in yours, having a landscape of solutions is the most sensible thing a theory of everything can have. We shouldn't we be able to have a theory in which the electron mass is not X, but X+0.0000000001 ? Like there is no sensible reason why the Earth's mass is X, but there is nevertheless a mechanism which can explain how it got that mass.

 

[qsa]

The reason why you can have a landscape problem in string(or any other similar in nature) is that the model is not designed from a fundamental entity. it is designed with a make blieve properties,just like point particles carrying charge, spinning(we don't know how) , has a mass! , it's a field and a particle and the list goes on and on. We do that by playing the diff equation game, the price you pay is that now you can twist things all the hell and create parameters and play with them as you please. These models are inherently free for all.

As a matter of fact if the universe is designed from a funamental entity, it will become clear that no much freedom exists in designing the universe. So, this entity is unique and whatever relations among these entities will be bounded, and that is how you get a finite orderly universe that is comprehensible. Sorry, Einstein.


.

 

[negru]

I agree that if we can't search the landscape it won't be very useful. But yeah, otherwise to me it's a good thing to have one. Having a unique solution might have generated nobel prizes more quickly, but I wouldn't have found that option more satisfying than our current predicament. There's no obvious reason why the parameters should have the unique values they have. We'd just like them to have specific values to test out theories - but what we want or need says nothing about how things actually work.

 

[tom.stoer]

I agree that transforming a landscape of theories into a landscape of solutions is a big success of string theory. But there are several problems:

1) up to know you do not have a unique theory from which you can derive one landscape and its vacua; instead you have different approximate theories (related via exact or approximate) dualities which by themeselves define a (small) landscape of theories
2) up to now you are not able to calculate (not even approximately) any of the physical parameters we are interested in; you cannot derive the low-energy mass spectrum and the coupling constants in the (nearly realistic) vacua
3) once a set of vacua is known, there should also be some selection principles forcing the theory to adopt some vacuum states

In condensed matter physics there is one single theory (1) from which - via certain approximations - one can derive effective theories describing condensed matter states. One is able to calculate (approximately) certain physical parameters like density, heat capacity, ...(2). And one is able to specify certain conditions (temperature, pressure, ...) that force the system under consideration to be in a specific state (liquid, magnetized, ...) or show certain excitations or effective degrees of freedom (phonons, polarons, ...). In string theory the unique theory is not known, the calculations are not possible, the effective degrees of freedom are partially known but the selction of specific vacua, the transition between vacua etc. are not known.

 

[negru]

Yes I completely agree, and I also agree with the ordering of your points. We should first understand what the unique framework would be, if there is one, or what else dualities can teach us, before we even attempt to ask whether a selection principle exists.

Which is why I'd personally consider any work on selection principles or computing parameters to be way premature, and likely a waste of time. And this is why we still need to focus on more subtle issues, as well as ads/cft. Again it is my opinion that we won't find out too much from strings before fully understanding ads/cft - which is why many string theorists are working SYM for the moment. This however is something that most critics will complain about.

 

[tom.stoer]

Yes I completely agree, and I also agree with the ordering of your points. We should first understand what the unique framework would be, if there is one, or what else dualities can teach us, before we even attempt to ask whether a selection principle exists.

Thanks. This is one of the posts which distribute the disappointment I felt when starting the whole discussion.

 

[Fra]

We'd just like them to have specific values to test out theories - but what we want or need says nothing about how things actually work.

What I tried to convey is that AFAIC if you attempt something as bold as a theory of theories, then you are doing inference. It's more than just physics, and with this I don't mean it's mathematics, I mean it's a general theory of rational learning to describe any scientific process. And in this perspective the theory is an interaction tool for learning and interactions.

Here, how things "actually work" refers to how inference works, not how an electron behaves.

(although with a separate conjecture, that I personally make, you can make this even more radical by thinking that any physical interaction IS an inference. In this perspective, it is not the case that our inference says nothing about how things "actually work". The very radical point here is that it does)

I've expressed this view before, but my point is that if you take ST to really by the grand thing that at least some people think of; like the theory of theories... in a deeper sense. Then one must also take the inferencial perspective serious, or it simply doesn't stick together.

For me, this "vision" of ST as theory of theories, is not bad at all, it's just that it was NOT how string theory was started. I think it's at best something that may be some conclusion from failing to find a unique theory. But still, most arguments I have read from string people, does not seem to acknowledge these points.

In short, there is a lack of understanding what the landscape really means in terms of inference. That's my firm opinon. Paradoxally, string theory may hint this, and some other camps critique it, but it seems that string theory isn't the right "theory of theory", becuase some basic traits are missing.

Fill in those gaps, and I would be prepared to take it more seriously as an inferencial theory.

/Fredrik

 

[Fra]

1) up to know you do not have a unique theory from which you can derive one landscape and its vacua; instead you have different approximate theories (related via exact or approximate) dualities which by themeselves define a (small) landscape of theories

I have to admit that from my perspective, I do not think the lack of a unique meta theory of theories is a problem because I don't see how such a meta theory could be inferred as independent of the inference system used to infer it. It just doesn't make sense.

The only way it could be realized is as a conjecture, or element of structural realist reality. But those kind of things is exactly what I think are non-inferencial.

I think the landscape itself must evolve and can only be described from the point of view of an observer; and this evolution can not be predicted by the same observer. But it can be predicted by other observers.

So the landscape of theories defining the first layer of landscapes is then just corresponding to the observers. IMHO the landscape problem should thus related to the population of inference system in nature and is thus perfectly analogous to evolution. But this is not the same as the antrophic principles.

A landscape ~ diversity of inferencial systems

Not all points in the landscape are viable for the same reason we do not find one-legged lions in nature - even though such a lion would be perfectly "consistent". This is why consistency is an insufficient selector here, we need to account also for the fitness.

Unfortunately, i don't see that string theory as it's formulated, starting with the continuum etc, is able to provide the analysis needed to push this to the next level. Too many things needs to be reworked that it probably wouldnt' be string teory anymore except as a limiting inferencial model where the continuum is realized. But I am convinced that the resolution of the problems here lie at the level prior to the continuum.

/Fredrik

 

[Physics Monkey]

This sounds overambitious for now.

Sorry, I didn't understand you. This means that you also think strings won't make a prediction for the electron mass anytime soon?

However, having a landscape doesn't mean one can't make predictions. People who say that really haven't thought about the topic too much or are just playing dumb. It doesn't matter if there are 10^500 solutions, or a continuum of them.

Of course, I agree that just the existence of a landscape does not imply lack of predictivity. However, I also think landscape issues tend to shift thinking, forcing one to ask different types of questions and make different types of predictions.

In fact. the improvement from 23 something parameters to 10^500 different solutions is quite big. The latter has measure zero in the former. It's infinitely better. Calling it a plague is really disingenuous. Assuming that some solutions come close to reality of course.

While I agree in a certain mathematical sense that a finite set is better than a continuum, 10^500 is still far beyond tractable if the landscape is "rugged". And besides, unless I missed something, we do not know that the solution space is discrete just as we have poor control over non-susy vacua. By the way, who calls it a plague?

 

[fzero]

While I agree in a certain mathematical sense that a finite set is better than a continuum, 10^500 is still far beyond tractable if the landscape is "rugged". And besides, unless I missed something, we do not know that the solution space is discrete just as we have poor control over non-susy vacua. By the way, who calls it a plague?

I believe that the argument for a discrete space of solutions is the following. A point in the landscape is particular background where the scalar fields (moduli) have been fixed to their minima in some potential. One way to generate this potential is to add fluxes through compact cycles of the internal geometry of the background. But these fluxes are quantized, so in turn the moduli vevs depend on discrete parameters.

Non-SUSY vacua could be considered (I'm not implying control), but it depends on what question you want to ask and what scale you are working at. Usually one looks for theories with low-energy SUSY and the presence of a suitable Higgs sector. If SUSY is found at the LHC, it would at least confirm that such solutions are the ones to look for. It would be much harder to try to determine a landscape of nonSUSY theories at 1 GeV.

 

[Fra]

I know this thread already arrived at several summarized conclusions from Tom, but due to this last thing I'd like to add one thing that is important for me, but not necessarily for someone who doesn't get the point of lack of uniqe theory.

The logic of the critique against ST can be seen originating from two views.

1. A certain amount of critique can be traced to the understanding that a theory must be unique as to be cleanly falsified. Ie. it's a critique AGAINST the general concept of theory or theory (=framework), and it's lack of unique predictivity.

2. Another crituque, where I belong, is those that arent string theorist, but still appreciate the concept of theory of theory in the inferencial sense. This type fo critique is very different from those jumping on ST failure to confirm to the old style definite theory, and falsification scheme. Instead the argument here is that ST doesn't seem to have all te right properties that seemse necessary from such a framework.

Some evidence of the confusion is that as far as my impression goes from listengin to string theorists, the exact meaning and handling of the landscape in ST is of some debate even within ST - suggesting that this is somthing ST stumbled upon, rather than been constructing principle. The defense of the landscape seems to be of debate.

/Fredrik

 

[tom.stoer]

I am not sure if I get this point. Perhaps there is the problem that it's not so clear what "the landscape" really is and if this ladscape s uniquely defined (I do not mean "defined" in the sense of a complete set of vacua, but in the sense that I can give you a short definition which summarizes all abstract properties of the landscape).

I have the impression that we talk about different "levels" of landscapes.

Looking at one specific string theory X which one can write down in the sense of an action integral one can derive a certain set of vacua (and one can expect other vacua not constructed so far) and call this set of vacua "landscape L[X] of theory X". I am not sure if this definition via "vacua" is exhaustive, because there may be "wrong" vacua, tunneling, etc.

Then there is the meta-landscape ML generated by different theories X, Y, ... where the problem of defining a theory enters the stage. Here I would expect (from a traditional point of view) that one can construct a unique theory (or meta-theory) from which all other theories can be deduced (at least in principle). The starting point is the set {M, SUGRA, I, IIA, IIB, E(8), SO(32)}. I mention M simply as a member of this set as I don't think that M-theory in it's current stage is the unique mother-theory. It's nothing else but a new limiting case of "something". Now looking at the dualities between these theories I still do not see that this set is fixed once an for all. The discovery of M was a kind of surprise and I guess there may be more surpises waiting for us.

Last but not least I am not sure whether the two levels of landscapes L and ML are not intertwined somehow.

Things are rather simply in condensed matter physics. One has QED as a fundamental theory and one can derive a landscape of vacua (ice, iron, ...) with certain effective description (phonons, spin waves, ...). I think there is no "mix of levels of landscapes". I do not have this clear picture in string theory - but perhaps this is simply due to my limited expertise on this subject.

 

[tom.stoer]

Last but not least my feeling is that at a rather early stage there was a wrong turn (I cannot tell exactly which one) which prevents us from asking the right questions. This is our blind spot.

Think about condensed matter physics and classical electrodynamics. You can do a lot based on continuous approximations like electrodynamics in media using polarizability, susceptibility, ...; you can use effective theories like navier-stokes equations; you can study London equations, Ginzburg–Landau theory, ... I would say that collecting those effective theories one can study a huge amount of condensed matter physics. Perhaps one can even use a kind of construction principle, I would say this could be Maxwell plus Schroedinger equations.

Unfortunately based on this construction principle one is not able to ask questions based on photons. They simply do not exist in this framework. So the framework allows us to construct a nearly exhaustive description of low-energy phenomena is therefore certainly "right". But at the same time it's incomplete as it is unable to ask the right questions about photons. Now in this case you have experiments at hand which force you to think about potons (photo-electric effect), but in string theory these experiments are missing. Therefore we must find the correct theory (theories) simply by matehmatics, logics and intuition. No experimental guideline! Even worse we are not even able to say which experiments are missing. We are not ableto ask these questions in the string theory framework.

String theory (as any other theory) limits our ability to ask questions. w/o further experimental input we are stuck. In the standad model we can ask questions regarding the Higgs boson. We can even ask questions regarding alternative mechanisms and we are not stuck once the LHC shows that there is no Higgs boson.

Now the problem is that I can only say that at a very early stage in string theory we may have chosen the wrong direction. From that point onwards we lost the ability to ask questions which would enable us to overcome the blind spot of string theory.

Now let's talk about other theories, like LQG. I don't want to promote LQG as the alternative theory to string theory in sthe sense that it has the ability to achieve unification of forces. I don't think so. I am simply saying that LQG is able to ask different questions. LQG is able to ask questions regarding an algebraic spacetime structure. This question is (afaik) not pronounceable in the language of string theory (maybe I am wrong; I am not an expert on matrix models).

So an alternative theory X may have some value because it enables us to ask different questions. If these questions seem to be "wrong" in the context of string theory this is not a problem of theory X, but a step forward for string theory - provided one accepts that this question could make sense in general and that one should try to find out what prevents string theory from asking this question.

Perhaps there are string theorists here able to tell us what could have been this wrong turn in the very beginning.

 

[atyy]

Think about condensed matter physics and classical electrodynamics. You can do a lot based on continuous approximations like electrodynamics in media using polarizability, susceptibility, ...; you can use effective theories like navier-stokes equations; you can study London equations, Ginzburg–Landau theory, ... I would say that collecting those effective theories one can study a huge amount of condensed matter physics. Perhaps one can even use a kind of construction principle, I would say this could be Maxwell plus Schroedinger equations.

Unfortunately based on this construction principle one is not able to ask questions based on photons.

What about http://arxiv.org/abs/cond-mat/0407140 ?

 

[Haelfix]

Certain very special vacua do allow you to calculate certain low energy quantities exactly. Like for instance the infamous prediction of the top quark mass by stringy methods before it was discovered. This of course was a bit hokey and presumptous at the time, and I think it has been understood that those particular subclasses of vacua are ruled out, but well it illustrates the point.

So for certain classes of vacua, it is often the case that you will have fixed values for certain low energy quantities (or at least ratios or differences thereof), and these won't change upon continuation deformation of the geometry (at least 'quasi locally' in the moduli space). However at the same time, you might have other parameters that have large continuum like spacings. So it might be possible one day to find some appropriate selection mechanism that reduces things down to a subclass where you can specify the electron mass exactly, but need experiment to say figure out what the neutrino masses are (b/c they might take a discretum of values +/- N * .00000000000001 ev where N is an integer).

Yet another case one finds in the phenomenology literature is where you have some vacua that you know in principle gives a unique value for some parameter, but the calculation is so horrendous that you end up having to impose parametrizations by hand anyway!

My personal belief is that I suspect that there is likely myriad selection mechanisms out there (both microscopic and cosmological) and its just a question of time and research before we start finding vacua that are in some sense truly priviledged, and I do think that if any theory has a chance of doing this, it would be string theory (b/c it is so tightly constrained and has such large symmetry and duality groups acting on it).

Moreover, I also think it to be fairly likely that the KKLT like constructions will go away, b/c we haven't entirely understood what's going on with the cosmological constant properly.

That hasn't been talked about much in this thread, and its ashame b/c imo its the single biggest theoretical knock on string theory (or any theory of quantum gravity). Namely the complete lack of a prediction or explanation for this value which on dimensional grounds it ought to be able to predict.

Taken at face value, the existence of a tiny but positive cosmological constant implies several really ridiculous things about the nature of our universe.

1) That we live in a universe that admits finetuning to one part in 10^120, 10^60, or 10^32 depending on how you count or if you admit supersymmetry or not.
2) That we live in a universe that steadily approaches DeSitter asymptotically. Now for various reasons, its likely that asymptotic DS space doesn't exist as a full quantum theory, and so we are reduced to invoking really vague bubble nucleation events to get us out of that embarrasment! Highly unsatisfying I might add.

 

[tom.stoer]

I agree, the cc is a big mistery. In QG there are attemps to predict its low-energy value dynamically based on renormalization group approaches (asymptotic safety). In LQG there are attempts to introduce it kinematically via quantum deformations of the underlying SU(2) which does not fix its value and which does not allow for any "flow". So at first glance both approaches cannot be reasonable at the same time.

Question: why do you think that dS space does not exist as a full quantum theory? (what does that mean exactly?) Is it based on string theoretic reasing, or are there more general ideas?

 

[Fra]

I don't have much time to write a lot atm but I think this kind of discussion is good. Hopefully some of the pro strings may contribute too. I see myself as commenting from my own inferencial perspective only.

My personal belief is that I suspect that there is likely myriad selection mechanisms out there (both microscopic and cosmological) and its just a question of time and research before we start finding vacua that are in some sense truly priviledged, and I do think that if any theory has a chance of doing this, it would be string theory

I think we can distinguish between two kinds of uniqeness here, that are easy to confuse when we are talking about theories of theories.

I do think that human scientists will be able to come to an agreement about the inferencial framework, but this is just to the extent that human based science constitutes a certain class of observers.

I still think it's necessary for understanding unification of interactions exactly how theories as well as frameworks changes with the observer. In this sense two observers/systems interaction can be abstractly seen from the inferencial perspective as an "interaction between two theories". In this interaction both theories excerts selective evolutionary pressure on each other to establish objectivity.

So even though I do think that there will be from the point of view of human science a unique framework (at some level) I think it's a conceptual mistake to think of this as eternally true timeless properties of the universe, which singles out a unique observer independent theory. If one assumes that, it at the same time becomes impossible to understand it. If a theory is an interaction tool, this is always observer dependent. This is why I think there is still plenty of things yet to understand around this.

/Fredrik

 

[A. Neumaier]

Last but not least my feeling is that at a rather early stage there was a wrong turn (I cannot tell exactly which one) which prevents us from asking the right questions. This is our blind spot.

Since you seem to be aware of there being a blind spot, what would be the right questions to ask, from your perspective?