Why I am REALLY disappointed about string theory

[MTd2]

In this case, what people know is that there are extended objects in the nonperturbative spectrum of the supergravity theory (roughly acting like soliton states). We know what they are, and can write them down.

This is not a barrier to asymptotic safety. Even pure GR has a 2d surface at the asymptotic limit. 11d SUGRA maybe would have those on its asymptotic limit.

 

[MTd2]

BTW, LM answered me this:
"
Dear Daniel,

nope, nope. The compactification obviously doesn't preserve finiteness of field theories. Higher-dimensional field theories of the same kind are always more divergent - because of the higher-dimensional momentum integration. The whole counting has to be redone.

The maximal d=4 SUGRA is most likely perturbatively finite - because of the KLT relations and other things. The 11D SUGRA is not finite.

Cheers
LM"

 

[tom.stoer]

So it's basically power counting.

 

[suprised]

Ups, you are right, my bad. I got carried away into sloppy thinking. The argument fails because of KK modes arising the compactification. Power counting alone gives only superficial estimates of divergences, because amplitudes can be better behaved due to symmetries (or other, "miraculous" cancellations).

In particular, I had even forgotten that there are arguments that the N=8 supergravity in d=4 cannot be obtained by _any_ decoupling limit of 10d strings, so may be in fact separate from string/M-theory and thus be part of the swampland. see http://www-spires.dur.ac.uk/cgi-bin/spiface/hep/www?eprint=arXiv:0704.0777

 

[tom.stoer]

I would like to come back to an idea I had a couple of days ago: why not drawing a much more detailed map of the theory space?

• It should list all theories
• It should provide a notion for the relevant degrees of freedom (open/closed strings, M2/M5 branes, matrices - which one?,...) for each theory
• It should provide a small set of fundamental equations for each theory (action, ...)
• It should list all relations (the self-duality is missing in this picture)
• Each relation should be qualified like "S" with the appropriate couplings, "low energy", "classical limit" - with an explanation what that means, ...
• It should distinguish between different theories; it should especially not list one central M-theory as we know that this is stll a hope; instead it should show M2/M5-, matrix-theory etc. ...
• it should list what prevents a certain theory from being the mother-theory

I guess there isn't such a map - otherwise it would have been posted here. Is there a chance to construct it here in this forum? e.g. as a simple table?

 

[atyy]

http://arxiv.org/abs/1007.4001

"As a consequence of the last property, we consider such QFTs to be definitions of models of quantum gravity, with fixed asymptotic background. The idea that AdS/CFT defines a duality between two independently defined theories, is probably without merit."

"String perturbation theory is a non-convergent asymptotic expansion."

 

[mitchell porter]

I would like to come back to an idea I had a couple of days ago: why not drawing a much more detailed map of the theory space? ...
I guess there isn't such a map - otherwise it would have been posted here. Is there a chance to construct it here in this forum? e.g. as a simple table?

It won't be simple because the dualities get more and more complicated in lower dimensions. A full map of theory space would be an atlas. And it would be great for someone (or a dozen someones) to create an atlas of string theory, but it would have to be done by people working in the field, and it would have to be updated every few years, like one of those Particle Data Group publications.

What we can do here is work just to understand the basic dualities in 9, 10, and 11 dimensions which connect all the theories. Very briefly, M-theory compactified on S^1 is IIA, M-theory on T^2 is IIB, M-theory on S^1/Z_2 is heterotic E8xE8, M-theory on T^2/Z_2 is heterotic SO(32). (Simplest possible diagram of this, http://www.sukidog.com/jpierre/strings/duality.htm" .)

Many of the details are in Ashoke Sen's http://arxiv.org/abs/hep-th/9802051" . There must be a more orderly exposition somewhere, at the level of detail that Tom wants, but I haven't seen it.

 

[Lt_Dax]

Here is my view. Feel free to squash it if there's reason to.

Isn't the main criticism of String Theory nothing at all to do with falsifiability, use of abstract mathematical trickery, or usefulness?

(It is clear that an unfalsifiable idea may become falsifiable at technology improves. Weird mathematics is not a sign of overstretching ourselves, as was pointed out with regards to hilbert spaces in QM, for example. And nobody doubts the usefulness of string theory and AdS/CFT in pure mathematics for example).

Isn't the primary criticism of string theory that it introduces features uncalled for? What do I mean, well we usually formulate a quantum field theory with 4 space time dimensions and point particles because that is exactly what we observe in nature. Forgive me for being "old-fashioned", but usually we develop a scientific theory by making observations first, then drawing conclusions. We chastise literalist creationists for doing things the opposite way around.

Of course, sometimes theory matures so fast that it gets a little ahead of experiment. String theorists often cite Dirac's theory and the prediction of the positron as proof that it's acceptable to jump the gun. I disagree. While it is clear that Dirac was inspired and a genius to bring special relativity together with quantum mechanics, he didn't introduce anything exotic. No strings, bells, whistles, extra dimensions or any other peculiar entities. His theory assumes point particles and four dimensions, since there was no experimental evidence that anything more than this exists (and there still isn't, as far as I know).

It's also tempting to claim that Planck introduced, seemingly as a random fudge, the quantum in order to explain the unexplainable. "See, there you have it, that's what string theorists do!" they claim. Except that Planck did what he did because there was no other way to explain the data. He knew it felt like a fudge and he felt bad doing it, but it worked well because there was no alternative, and experiment quickly supported the idea (in fact, the experimental proof for the existence of the photon already existed, it was just languishing without explanation).

Then there is the claim that Einstein developed special and general relativity "just by thinking about it", divorced from experiment. String theorists seem to use this idea (which is highly questionable anyway) to validate the claim that they're actually "discovering" new physics all the time. Ed Witten is particularly fond of making this claim, suggesting that by writing equations down we are actually discovering things. But you don't discover things with theory, you explain things with theory. You discover things with experiment. Surely experiment (observation) is the arbiter of truth.

In any case the comparison with Einstein's approach is flawed. Special relativity follows from the requirement that electromagnetism produces invariant results in different reference frames coupled with the constancy of the speed of light, an experimental observation. General relativity follows from these things but obviously not straightforwardly otherwise it wouldn't have taken him another ten years, but general relativity is as devoid of exotic concepts as it can be. Curved space time is a necessity, and was quickly confirmed experimentally, just as Planck's hypothesis was.

Another claim I've noticed that string theorists make is that it does in fact originate from experiment, because it was proposed to explain the strong force (another attempt to parallel the introduction of the quantum, for example). However, this hypothesis was rejected in favour of QCD which (unsurprisingly in my view) in based on point particles and four dimensions, nothing uncalled for and nothing unobserved. So what is the continuing motivation for studying string theory as a candidate for explaining nature (as opposed to just mathematical interest)?

It may finally be tempting to say that unification is the motivation, but although it looks compelling, there is not yet any proof that all the forces are unified. Making the coupling strengths match at the appropriate energy scale required altering the standard model so I remain unconvinced. Physics may or may not be unified. It is my firm belief that experiment will tell us whether or not all of the forces are unified together, and how they are (we don't even have a grand unified theory minus gravity yet!).

Isn't it also clear that Einstein, if we wish to follow his career with interest (which string theorists understandably like to), was far more successful in his early years. What distinguished his early career? In his younger years, he studied experimental results, unsolved problems and he reaped large results from listening to nature. In his later life, he tried to dictate to nature, and he made little progress. The rest of the physics community viewed him as an old timer who'd lost touch. And yet the string theorist community choose to imitate the later half of his career. The fact that Einstein tried to find a unified theory seems to provide some sort of validation for thousands of physicists to engage in a global effort to do so, when in my view his failure actually teaches the opposite lesson.

Isn't this the key, then: experiment is king. Theory is (or should be) subservient, in my view. The predictability of theory is surely meaningless unless the theory has its origins in observation in the first place; an experiment sandwich with theory in the middle. Why do I say that? Because without some form of experimental constraint both in the formulation and confirmation of a theory, there are literally an infinite number of ways to solve the same problem (isn't that the fundamental reason why there are so many string vacua?) There are probably an infinite number of (unconstrained) uses for a paper clip.

In my view, because I view the origin of a theory as just as crucial as its later validation, string theory doesn't even fit the usual definition of an empirical theory, which originates from experimental observations, by definition. The standard model fits this description well, and contains within its parts some of the most spectacularly verified predictions ever produced. Isn't it rather unfortunate that the Standard "Model" is a true theory, but String "Theory" is actually just a model? (Isn't that also a source of enormous confusion when discussing it?).

It's worth noting that although it's acceptable for a (real) theory to get ahead of experiment for a little while, this uncomfortable no-man's land does not typically last long. Never has it lasted for 30 years! Dirac's theory would have been discarded long before 30 years had passed. Before anyone claims that our technology has become so limited that we've reached an impasse, consider that there is currently an enormous list of unsolved problems in physics that are far more deserving of our attention. If our technology (ability to make experimental observations) is so limited/limiting, then how did it produce such an enormous list of unsolved observations?

Far fewer unsolved problems led to two huge revisions in our understanding of physics in the early 20th century, so we could probably learn a lot about the universe just by taking the approach of the young Einstein, to sit down and try to explain them by developing or revising a theory. We may even learn some things about high energy physics. If string theory is pure mathematics then put it in the mathematics department, surely. This is not a trivial issue; it actually takes up lots of PhDs and funding which the funding councils assume is used to advance physics, not mathematics.

Am I old-fashioned, behind the times and hopelessly naive? I'm not being belligerent, I genuinely want to know why I have the view I do. Did the entire physics community decide in the 1980s that we could change the way we discover new science, and I haven't realized the new "sophisticated" way of doing things? Or is the claim of sophistication and beauty just a cover to play in a sand box?

 

[mitchell porter]

Reality has fermions, gauge fields, and gravity. String theory has all those things. It also offers a framework in which masses, coupling constants, and symmetry groups have a deeper explanation. It would be crazy to ignore it.

 

[tom.stoer]

We had all these issues during our discussion; so I would say that everything has been discussed carefully, but I understand that some arguments are hidden in 400 posts so it is clear that you come up with some of them again.

I'll try to answer rather quickly and gope you will study the other posts, too.

The problem of falsifiability (in practice, not in principle - we discussed this difference) is not specific for string theory but applies to all theories including quantum gravity. Therefore either you accept this paradigm shift (that your guidelines are more mathematical then experimental) or you have to stop doing physics at all.

Yes, it's true that string theory introduces more new and abstract theoretical concepts then every other theory we had so far (but that was true even for quantum mechanics). The difference is again testability, not additional assumptions. Once proved experimentally nobody will care about.

A theory does not necessarily originate from experimental observations only. This was a famous discussion between Einstein and Heisenberg.I thin with QM it became clear that the basic entities are not necessarily directly accessible via experiment.

Regarding the 30 years: the Higgs mechanism has been discussed and used in the sixties to explain gauge invariant mass generation. Still the Higgs remains unobserved and is probably only a low energy effective degree of freedom, i.e. the GSW theory may fail to me UV complete.

Last sentence regarding what string theory "is". String theory seems to be more a framework than a single theory which allows one to consider all theories including quantum gravity consistently; the main difference to gauge theories (which do the same for theories w/o gravity) is that in string theory the different theories are solutions (SU(N) + SUSY + ... theories emerge as solutions) instead of isolated theories.

I agree with most of your skepticism (this was the reason why I started this thread) but I have to admit that there are rather convincing arguments that string theory is a unique framework which has a lot of potential - I haven't seen any other approach which has the same richness.

 

[Lt_Dax]

Thanks for your views Tom. You're right it is difficult to find these points in 400 post, so I especially appreciate your response all the more. I've been trying to develop a more mature viewpoint, so this thread has been useful to read (if not in its entirety).

 

[marcus]

...
The problem of falsifiability (in practice, not in principle - we discussed this difference) is not specific for string theory but applies to all theories including quantum gravity. Therefore either you accept this paradigm shift (that your guidelines are more mathematical then experimental) or you have to stop doing physics at all...

... I've been trying to develop a more mature viewpoint, so this thread has been useful to read (if not in its entirety).

Tom, I don't think this is as fairly balanced as many of your posts, so I will suggest a different viewpoint on testing. Loop has become a coherent theory of quantum gravity (QG) which stands to be falsified by observation of the cosmic microwave background (CMB) by proposed next generation missions such as B-Pol.

There are certainly details of the combined theory (canonical+path integral+cosmology) that still have to be worked out. But there has been a remarkable convergence and the prediction of a cosmic bounce is robust. Repeatedly, under varying assumptions, Loop applied to cosmology replaces the singularity with a bounce and a natural inflation episode.
This has consequences for practical B-mode polarization maps of the CMB. The B-Pol mission has been proposed for the 2015-2025 timeframe. If it gets funded (a big "if") it could effectively falsify Loop.

So that QG theory predicts new phenomena and bets its life on the prediction. This is the customary behavior of scientific theories that we expect since Bacon set out the empirical philosophy 400 years ago.

It's not granted yet that we discard the Baconian paradigm or "stop doing physics".

======================

A Loop-related conjecture by Smolin that had stood since 1993 was falsified this year by the discovery of a neutron star with mass > 1.9 solar. Smolin had conjectured that the parameters of the standard particle model were at a local optimum for black hole production and had derived from this the prediction that no neutron star would be seen with mass > 1.6 solar. This conjecture was not a prediction of QG theory, but it assumed that classic black hole singularities were replaced by bounces. That is still possible, but the optimality conjecture has been falsified.

======================

QG theory itself, and QG-related conjectures, can be tested.

=======================

A relevant philosophical point is that fundamental physical theory is inferential. As Bohr told us, physics is not about how nature "is". It is about how it responds to measurement. What we can detect, measure, infer, and check by further measurement.

As Newton said "Hypotheses non fingo". We do not pretend that little wiggling strings exist. Or that spin networks exist--with nodes of quantum volume and links of quantum area--flickering into and out of existence. Or that little triangles of quantum triangulation exist. So theory-testing does not require a big "magnifying glass" to see these human imaginings.
There is no fundamental ontology.

These imagined things represent nothing but ways of calculating and relating measurements to other measurements.

Theories can be roughly graded according to how fundamental since from a deeper theory one can sometimes derive a more coarse-grain theory.

So in order to test comparatively fundamental theories such as Loop (I think everybody here realizes) instead of building an impossibly powerful "magnifying glass" what one needs to do is use the theory to predict new phenomena (which distinguish one theory from another) and look for the phenomena.

A theory must be predictive about the universe we actually live in, including the ancient light which we observe, or it is useless. And so it must be possible to test.

Apart from that, it can be as mathematical as anyone could wish there is no contradiction between empirical testing and mathematical elegance as guides.

That's my contribution to help balance the view you expressed. Hope it does not repeat too much from the previous 400 posts of this great thread!

===some references===
B-Pol mission proposal:
http://www.b-pol.org/index.php
Bee Hossenfelder's post:
http://www.math.columbia.edu/~woit/wordpress/?p=3262&cpage=1#comment-67988
Recent paper by Julien Grain et al:
http://arxiv.org/abs/1011.1811
Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters
Julien Grain, A. Barrau, T. Cailleteau, J. Mielczarek
12 pages, 5 figures
(Submitted on 8 Nov 2010)
“Cosmological models where the standard Big Bang is replaced by a bounce have been studied for decades. The situation has however dramatically changed in the last years for two reasons. First, because new ways to probe the early Universe have emerged, in particular thanks to the Cosmic Microwave Background (CMB). Second, because some well grounded theories — especially Loop Quantum Cosmology — unambiguously predict a bounce, at least for homogeneous models. In this article, we investigate into the details the phenomenological parameters that could be constrained or measured by next-generation B-mode CMB experiments. We point out that an important observational window could be opened. We then show that those constraints can be converted into very meaningful limits on the fundamental Loop Quantum Cosmology (LQC) parameters. This establishes the early universe as an invaluable quantum gravity laboratory.”

 

[atyy]

LQC is not a theory of QG until it can be connected with LQG.

When it is connected with LQG in its present form, it will make no predictions that can falsify LQG, because (i) canonical LQG is not defined - no hamiltonian constraint (ii) Rovellian LQG in its latest form is probably divergent.

 

[marcus]

Gravity/gaugefield/Higgs unification bid by Lisi-Smolin-Speziale

LQC is not a theory of QG until it can be connected with LQG..

Connected.

http://arxiv.org/abs/1010.0502
Local spinfoam expansion in loop quantum cosmology

http://arxiv.org/abs/1010.1258
Big Bounce in Dipole Cosmology

Your objection about Hamiltonian does not make sense to me, since the current leading formulation of LQG does not employ a Hamiltonian.
We will just have to see if "Rovellian LQG", the current formulation of the theory, is divergent--and how that plays out. Are you making a prediction?

 

[marcus]

This thread is about string theory---it would be nice to keep it that way.

I only want to point out that the customary apology for string: that other programs (in QG) have equivalent problems does not hold water.

It should not be made. And it is not a very good apology in the first place.

There is no need to "change the paradigm" of empirical science simply to give one elderly overgrown late-20th century research program a break.

One doesn't have to talk about Loop to make that point, either. You can for example ask Matilde Marcolli, the hot noncommutative babe at Caltech (this is a joke, I am not being a male chauvenist or something, she is a first rate mathematician: Oberwolfach grade.)

Anyway it is a general point. We do not change the paradigm of empirical science for light causes.

 

[Careful]

Just out of curiosity, I thought these non-commutative approaches were not quantized yet. So, it wouldn't be fair to compare it with something like string theory...

This thread is about string theory---it would be nice to keep it that way.

I only want to point out that the customary apology for string: that other programs (in QG) have equivalent problems does not hold water.

It should not be made. And it is not a very good apology in the first place.

There is no need to "change the paradigm" of empirical science simply to give one elderly overgrown late-20th century research program a break.

One doesn't have to talk about Loop to make that point, either. You can for example ask Matilda Marcolli, the hot noncommutative babe at Caltech (this is a joke, I am not being a male chauvenist or something, she is a first rate mathematician: Oberwolfach grade.)

Anyway it is a general point. We do not change the paradigm of empirical science for light causes.

 

[marcus]

Just out of curiosity, I thought these non-commutative approaches were not quantized yet. So, it wouldn't be fair to compare it with something like string theory...

Careful! Nice to see you after a 2 year absence! I saw you thanked in the acknowledgments section of a QG paper recently for helpful discussions with the authors---glad to see that indication of your continued interest and activity in QG.
I forget what type it was, Noncommutative, GFT, AsymSafe, maybe even spinfoams--it's gone out of my head.

Anyway I was not talking merely about specific rivals for some specific research goal but about the bid to abandon the empirical science paradigm. If you asked Marcolli I don't think she would say there is any pressing need to do that.

She recently organized a 2010 workshop at Oberwolfach on "Spinfoam+Noncommutative Geometry". Krajewski participated. You may know him. Indeed you may know more about that workshop than I do.

And Marcolli recently posted a "Spinfoam+NCG" paper.
http://arxiv.org/abs/1005.1057

So there is QG stuff brewing with NCG. Also NC field theory is considered one of the new "paths" to quantum gravity. There was a conference in Denmark on that exact theme funded by Barrett's QGQG branch of the ESF (Euro. Sci. Foundn.) I forget who organized it. Richard Nest probably.

Nest has collaborated on some "LQG+NCG papers". It is not a fully formed research gambit but some kind of initiative is taking shape. Just another reason not to be scared into abandoning the 400-year old Baconian tradition.

I'll get some links to introduce people to Marcolli, Krajewski, Nest etc, in case they are unfamiliar.

 

[marcus]

Here Matilde's Caltech page
http://www.its.caltech.edu/~matilde/
with a nice picture of her and a description of her research interests.
Here's a sample paper:
http://arxiv.org/abs/1005.1057
Spin Foams and Noncommutative Geometry
Domenic Denicola (Caltech), Matilde Marcolli (Caltech), Ahmad Zainy al-Yasry (ICTP)
48 pages, 30 figures
(Submitted on 6 May 2010)
"We extend the formalism of embedded spin networks and spin foams ... We end by sketching a possible approach to combining the spin network and spin foam formalism with matter within the framework of spectral triples in noncommutative geometry."

Here's some pictures from the 2010 Oberwolfach workshop she co-organized.
http://owpdb.mfo.de/show_workshop?id=783
There is a shot of Nest and Krajewski there, just so folks here can attach faces to the names. My intuitive feeling is that what these people are doing is potentially significant because NCG has already in a sense reproduced the Standard Model of particle physics at least in a rudimentary way, and spinfoam LQG has a pretty good model of evolving quantum geometry.

The point is all these people's theories can make predictions testable with available technical means--given the funding.
Spinfoam has been merged with Loop Cosmology (and appears to predict a bounce) so observational astrophysics is available to test it.
NCG is capable of generating predictions about particle masses, respectable despite some initial reckless shooting from the hip , and NCG is on track to supply matter to Spinfoams. Another possible merger.

This is just in answer to Careful's question. The main thing is this should be a string discussion thread, and I'm generally not interested in participating (I've kept out almost totally until now). But the point should be made that the apologetic argument that other approaches have equivalent problems, and that we need to relax empirical standards, should not be made.

String has to stand scrutiny on its own in the usual light.

 

[arivero]

It won't be simple because the dualities get more and more complicated in lower dimensions. A full map of theory space would be an atlas. And it would be great for someone (or a dozen someones) to create an atlas of string theory, but it would have to be done by people working in the field, and it would have to be updated every few years, like one of those Particle Data Group publications.

What we can do here is work just to understand the basic dualities in 9, 10, and 11 dimensions which connect all the theories. Very briefly, M-theory compactified on S^1 is IIA, M-theory on T^2 is IIB, M-theory on S^1/Z_2 is heterotic E8xE8, M-theory on T^2/Z_2 is heterotic SO(32). (Simplest possible diagram of this, http://www.sukidog.com/jpierre/strings/duality.htm" .)

Many of the details are in Ashoke Sen's http://arxiv.org/abs/hep-th/9802051" . There must be a more orderly exposition somewhere, at the level of detail that Tom wants, but I haven't seen it.

Lets assume that such atlas exists, very much as a new version of Slansky's group atlas for unified theories. In Slansky, if a given group happens to be the GUT group, it will be easy to give meaning to the subgroup descendants of such theory, and probably also to the nonexistence of bigger groups above the GUT one; it is a pretty trivial relationship, a directed tree or network in any case.

But with the "string atlas", we have the network of dualities, going up and down and around. If we locate a specific theory (and remember I have my guess ) as the real one, what are we supposed to do of all the network of dualities relating such theories with a lot of other ones. Should all the theories of the network be observed? Or perhaps should we look for a theory with a minimal quantity of dualities?

 

[atyy]

The main thing is this should be a string discussion thread, and I'm generally not interested in participating (I've kept out almost totally until now). But the point should be made that the apologetic argument that other approaches have equivalent problems, and that we need to relax empirical standards, should not be made.

Other approaches have similar problems, no question about that. No one is saying that physics is not about experiments any more. Only that the lack of immediate testability should not prevent one from exploring. In this spirit "But let us finish by a quote from Erwin Schroedinger, ... , ' ... or else, one might seriously worry that just where we forbid further questions there could be still quite a bit worth knowing about.'" http://arxiv.org/abs/1005.2471

 

[marcus]

I'm glad you said "similar" Atyy An equilateral triangle that is 2 inches on a side is similar to an equilateral triangle that is one mile on each side. But I would not call them "equivalent".

Where is Lieutenant Dax? Methinks she was well-spoken and made interesting points. I hope she decides to rejoin us.

 

[mitchell porter]

If we locate a specific theory (and remember I have my guess ) as the real one, what are we supposed to do of all the network of dualities relating such theories with a lot of other ones. Should all the theories of the network be observed? Or perhaps should we look for a theory with a minimal quantity of dualities?

If you look at compactifications of M-theory on a torus, the more compact dimensions there are, the bigger the U-duality group. Since for a torus with dimension n \geq 6, the U-duality group is just a form of E_n, and since it is speculated that the master symmetry of M-theory is E₁₀ or E₁₁, and also that the dualities may be derived from M-brane worldvolume symmetries... this growth of U-duality seems to be a migration from explicit string/brane symmetries you could see on a fixed background, to implicit symmetries which require a change of variables and/or background to be demonstrated. So you might say that any phenomenologically relevant solution of string theory is going to be low-dimensional, therefore, it will have a large U-duality group, because most of the symmetries will have become "implicit". (Though remember we might also be on a braneworld in a high-dimensional bulk space.)

However, these results apply to solutions with a large or even maximal number of supersymmetries, and something else about the real world is that you expect supersymmetry to be broken, probably completely. In terms of the explicit+implicit framework I just gave, this might mean that the total number of "remaining explicit symmetries + dualities" is small. Actually I'm just confused about the relationship between supersymmetry and the dualities. Solutions with maximal supersymmetry have been useful in understanding the nonperturbative dualities, but that's because they're easier to analyse. I simply don't know whether breaking supersymmetry also means reducing the "total amount of non-susy M symmetry", whatever that means, or whether the "susy and non-susy parts of M symmetry" can be broken independently.

If you have a preferred GUT model, you could also just look for string models which contain it, and find out afterwards how they look from the fundamental perspective.

 

[suprised]

As if precisely these points haven't been spelled out here N times... there seems again a confusion about the meaning of dualities. In fact there are two main meanings that need to be separated.

In the strict sense, duality refers to different descriptions or parametrizations of the very same theory. Since there is just one theory, there is no issue of "observing or not the others".

In a wider sense, duality refers to different descriptions or parametrizations of the same theory, but related to different parameter values of the same theory. Again, observing "other" theories is an ill-defined question.

 

[arivero]

In the strict sense, duality refers to different descriptions or parametrizations of the very same theory. Since there is just one theory, there is no issue of "observing or not the others".

You are right! But it is very easy to forget the point, as I did, because each of these different desctiptions have a different set of massless states. In a duality, the fundamental states of a "parametrization" are mapped nonperturbatibely to massive, excited or solitonic states of the other, and reciprocally.

I can imagine one or two such maps in our particle spectrum, particularly for the top, which is massive while all their cousins are massless, and for the neutrinos, which have a light Dirac mass and a heavy "majorana" mass for the see-saw. But this should allow for a couple of different "parametrizations", not a whole web of them.

 

[arivero]

Addendum, let me put an extra bit in the stack of experimental evidence: it is well known (or well neglected) that in the experimental spectrum there are 84 almost massless fermionic states. They should be protected by some symmetry. Opening at random the Slansky report, I can see 84 in SU(4) (with triality!), SU(6), SO(9),... and I could also look for 42 (hattip Douglas Adams) or 21. So it does not seem a big clue. But the source of the 2-brane of M-theory is the antisymmetric tensor of 84 components, the complement of the 11D graviton (44+84=128) in the N=1 sugra fundamental multiplet. Thus I'd say that the M-theory brane is a candidate to protect the Yukawa couplings of the fermions, in some yet unknown parametrisation of a yet unknown compactification.

 

[Lt_Dax]

Where is Lieutenant Dax? Methinks she was well-spoken and made interesting points. I hope she decides to rejoin us.

She made some interesting points, but is still too inexperienced to really get into the thick of it (some of the mathematical terminology others use makes it tough for me to extract the arguments people are making).

One thing I did pick up on from a response to my original post: I'm still skeptical that it is possible to actually learn things about the universe without "interrogating it".

It is probably possible to learn new physics by developing mathematics in ways which are consistent with the rest of mathematics (which is more a definition of mathematical proof than physical proof), but I can't avoid the feeling that the parts of mathematics which tell us real things about the universe actually originate from observation in the first place. For example, we can develop lots of new physics starting from the assumption that space is Euclidean, but someone made that assumption from observing the behaviour of lines and triangles, real objects (of course we often have to revise the assumption as our knowledge becomes more sophisticated).

String theory could be described like this (having the lofty goal of being self consistent, consistent with the rest of mathematics, and in principle at least, predictive), but my problem is that its fundamental premise hasn't been established. It is probably possible to "predict" the entire standard model by using any fundamental object as a starting point. The string idea still relies on the assumption that the only alternative to a point object is a quantized oscillating string (the wave "paradigm"). Is this a failure of imagination?

So even if string theory can predict, say, the entire known particle spectrum, or anything else we already know about, I'm unmoved by it. Does what I'm saying make sense?

Some string people say that string theory solves the problem of unification, but that assumes that unification is a problem (it might not be). When they say that string theory "predicts gravity", I assume they mean the same thing - that gravity must be part of a completely unified theory which satisfies our own biases about what a unified model would look like. Even if we could develop a quantum theory of gravity, there could be ten other fundamental forces we don't know about. Will string theory predict those? Can we unlock deep secrets about the universe with a pen and paper? I'm highly skeptical.

 

[marcus]

Lieutenant, one thing we could do is start a new thread, with copies of several of your posts, called "The Dax Discussions" and let people reply to you in that thread, and comment on the issues you raise.

That would allow Tom, Arivero, Mitchell, Surprised, and the others to stay focused on the ground they've been plowing so far.

I'm not PF staff, so I can't make a judgement call and move posts and split off a piece of a thread. But my personal inclination is to let them have uninterrupted technical discussion and reply to you in a separate thread. I think your opening posts are engaging.

Maybe what I'll do as a private "on spec" initiative is start such a thread, then if you don't like that you can simply not participate and the thread will die, if Tom does not like that and wants to keep Dax discussions here, he simply has to say and we can abandon the split-off thread.
I'll wait a few hours and see if there is any immediate reaction to that idea.

 

[Lt_Dax]

I'd support that idea marcus. If you start a new thread, I'll participate when I can. It doesn't matter if the thread dies, it's worth it just to find out if anyone else is interested in discussing my points.

 

[mitchell porter]

Speaking of new threads, I'm going to start one for Alejandro Rivero's idea in comment #425. I don't think the number of degrees of freedom in 11 dimensions is much of a clue for phenomenology, because moving to lower dimensions creates so many new states and relationships. But it would be a good exercise for interested parties to really think this through, and the technicalities might interfere with the discussion here.

edit: https://www.physicsforums.com/showthread.php?p=2983996"

 

[friend]

One of the complaints about string theory is that we really don't know what the theory is, what it's basic premises are. Let me offer a suggestion.

Could the basic premise of String Theory/ M-Theory be that, "the concepts of the Lagrangian and the Action are applicable to sets as well as points"? Then those sets might come in the form of open and closed strings and higher dimensional branes.

Just like scalars and vectors and matrices are generalized by tensors, perhaps branes are generalizations of strings and points. It would be an interesting mathematical study to prove that the Feynman path integral also applies to "sets" as well as to point particles, whatever that means. Perhaps it also applies to sets of discrete point as well as to continuous lines and branes. Or has this already be proposed?

 

[tom.stoer]

This is true from the perspective of "first quantization" where the action of world lines of point particles / of world sheets of strings is promoted to the path integral.
But we know that in quantum field theory we have to describe nature not via first quantized point particles but via "second quantized" fields.

I am confused b/c there is still no understanding of string field theory which would do the same with strings.

 

[arivero]

Other question is if the string theory we are dissapointed about, is the same string theory that people is studying. Check for instance this list of recent seminars.

http://www.talks.cam.ac.uk/show/archive/18332

 

[tom.stoer]

Can you give me a hint what exactly I should look for?

 

[arivero]

Nothing particular, just the feeling, arguable I expect, that the topics studied there doesn't seem to fit with the topics usually discussed in this subforum.

 

[elivil]

@tom.stoer

I've been following this thread on an on-and-off basis for some time but it's really hard to navigate in the space of >400 posts. In your post #333 you suggested writing a short summary of identified problems and questions. I think it would be indeed great to have such a summary and it would help young physics students to cut through the hype from both sides if there were a technical summary dealing with actual physical questions that are investigated at the moment.

From my part, I can add that there is a substantial interest in AdS/CFT correspondence as applied to different problems (including outside of particle physics). At my university recently there was a short introductory course into AdS/CFT (or, to be more precise, better to call it "gauge/gravity duality" since after Maldacena's first paper other instances of similar dualities to AdS/CFT have been identified) and this course was well attended by the members of the condensed matter groups and even some senior undergrads (have to 'fess up:). So that offshoot of string theory is growing strongly and with results from LHC (q-g plasma etc) will have at least some hope of its predictions being experimentally verified.

 

[Careful]

Careful! Nice to see you after a 2 year absence! I saw you thanked in the acknowledgments section of a QG paper recently for helpful discussions with the authors---glad to see that indication of your continued interest and activity in QG.

Well, I have been busy... :-) Ah, if you have a good idea how to solve a not so trivial problem and you are not interested in publishing it yourself (because you think the general idea fails in a deeper way), then you just give it away in case the author happens to be a nice, intelligent and open minded person. That's how you make friends. I am not interested in publications, but in solving the problem (and I happen to disagree with almost everyone on rather substantial things). So, if I would publish in the (near) future, you know the content of the paper

 

[Careful]

Here Matilde's Caltech page
http://www.its.caltech.edu/~matilde/
with a nice picture of her and a description of her research interests.
Here's a sample paper:
http://arxiv.org/abs/1005.1057
Spin Foams and Noncommutative Geometry
Domenic Denicola (Caltech), Matilde Marcolli (Caltech), Ahmad Zainy al-Yasry (ICTP)
48 pages, 30 figures
(Submitted on 6 May 2010)
".

Ok, to actually answer the content of your message. My understanding here is that some kind of landscape problem is unavoidable in *any* approach to quantum gravity. NCG isn't quantized yet, so God knows how many free parameters, new particles, symmetries will be necessary to make the whole thing consistent. It is always like that with unification ... the number of possibilities goes up and you will have to figure out new types of boundary conditions on your theory restricting the number of solutions/kind of physics drastically. So, in that sense, string theorists are correct beyond reasonable doubt that any approach will face a landscape problem. This will weaken 'predictivity' of your theory: the same thing happened already with for example general relativity. It is never said in this way, but you could easily hold the point of view that for example the precession of mercury is *not* a prediction of GR. It is just a possiblity ... if I were to turn on some gravitational waves so strong that they would precisely knock mercury out of its regular orbit (at some point in space and time), that would be a false prediction of GR! The same goes with quantum mechanics, we really never know the state of the system and have to make lot's of assumptions about (a) decoupling of the system under study from the evironment (b) the precise form of the wavepackages and so on... There are so many possible choices that eventually everything can be fit. The question of course is, are these assumptions 'natural' ? What do we mean with that? For example is it natural to assume in GR that post Newtonian corrections are the good thing to calculate? Isn't it just psychology because we think Newton must be valid everywhere in the universe (which it isn't because of the 'dark matter' puzzle) to some high degree of accuracy? From the point of GR, this doesn't *explain* Newton unless you somehow find a criterion why the universe must almost be spatially flat.

Unification will make the possible worlds even crazier and string theory is so far the only candidate which did manage to even adress this issue. I think we will have to learn to live with theories where we cannot really predict the future anymore unless we fix lots of boundary conditions. If you want to have a theory of boundary values, fine! That's the next step. But Connes and co did not *predict* the standard model either, they almost have put it in by hand.

 

[rdjesch]

I'm quoting Leonard Suskind from his book.
"Elegant theories have more beauty depending on the lowest number of defining equations. Therefore String theory is the most beautiful theory because it has exactly zero defining equations."

 

[tom.stoer]

wow!

 

[rdjesch]

I guess Mr Susskind may have been joking in his book "The Cosmic Landscape."

Quoted correctly "A beautiful theory is one with a few elegant defining equations. By these standards, String theory is the most beautiful. String theory still does not have a single defining equation."
I think I understand a little about the too many landscapes problem but the book was very sparse on explanations or mathematics.
But seriously, Is there some way of using string theory to predict something?
How does it work?
I'm trying to decide if I should devote more energy into understanding this.

Please help.
Where would you start?

 

[suprised]

Please help.
Where would you start?

A good idea would actually be to start reading this thread, rather than posting meaningless stuff!

 

[tom.stoer]

@tom.stoer

I've been following this thread on an on-and-off basis for some time but it's really hard to navigate in the space of >400 posts. In your post #333 you suggested writing a short summary of identified problems and questions. I think it would be indeed great to have such a summary and it would help young physics students to cut through the hype from both sides if there were a technical summary dealing with actual physical questions that are investigated at the moment.

I think the only thing we can do here is to write a summary regarding the discussion and the conclusions in this thread. For a full review regarding string theory (current status, research directions, open issues, ...) we have to find independent review articles.

 

[Careful]

I guess Mr Susskind may have been joking in his book "The Cosmic Landscape."

Quoted correctly "A beautiful theory is one with a few elegant defining equations. By these standards, String theory is the most beautiful. String theory still does not have a single defining equation."
I think I understand a little about the too many landscapes problem but the book was very sparse on explanations or mathematics.
But seriously, Is there some way of using string theory to predict something?
How does it work?
I'm trying to decide if I should devote more energy into understanding this.

Please help.
Where would you start?

Of course, Susskind was joking: a theory without defining equations simply does not exist, hence string theory doesn't exist - whether the theoretical vacuum is a beautiful thing or not, I leave that to the philosophers I think if you read my post about the more modern meaning of what it means to ''predict something'' you should get a clearer idea. The discussion is not whether there will be a landscape or not, just how big the f*cking thing has to be.

 

[lumidek]

The problem with the theory is that it thinks the universe does calculus every time. In genetics there are only four base units and that creates all living things. What if there is something simple that creates all of matter from energy?

Hasn't someone already told you that it's not your problem with string theory but rather your problem with all of physics? It was Isaac Newton who first figured out that the Universe does calculus every time. To show that it was the case, he had to single-handedly invent or discover the calculus, too. ;-)

 

[rdjesch]

A good idea would actually be to start reading this thread, rather than posting meaningless stuff!

Thank you, and I do mean that sincerely. I guess somebody let me out of my cage. I don't know what came over me. For sure I'm a mega junior compared to most of you on this list. I apologize. There must be something done about this mess and this thread seemed to have some potential to do it.

Meaningless... Hmmmm ... do you actually read most of these posts?
Some out of context quotes may surprise you because of how much you can ignore.

Just trying to focus on the headliner of this thread:
"Why I am REALLY disappointed about string theory"

Have you contributed today?

 

[suprised]

I guess I have contributed my fair share. The first part of my remark was about your question: "But seriously, Is there some way of using string theory to predict something?"
This had been discussed at length here; and not just once! We can't repeat this again and again simply because people don't want to spend some effort in reading. Though I admit that a forum is not a suitable medium to confer this, as information gets incoherently presented, diluted and mixed with desinformation.

The second part was aimed at your Susskind quote; for what was this good for?

 

[MTd2]

The second part was aimed at your Susskind quote; for what was this good for?

To show that string theorists are getting the increasing impression by many people outside this field of being paranoids, having delusions of grandeur and being cranks.

 

[suprised]

Sigh... I was hoping to have this thread concentrate on science and avoid sociology. Seems impossible.

 

[Greg Bernhardt]

first call to get this thread back on track

 

[tom.stoer]

As I wrote in #333:

I am afraid I can't do more than indicate what the central problems and questions are which have been identified throughout the discussion (to be honest, I don't think that we found out something new; we only collected facts and questions well-known to the experts). ... It could make sense to write a short summary and conclude this thread instead of reiterate and spin in circles.

Anybody there to summarize from a string theory perspective what the essential conclusions are?