Why I am REALLY disappointed about string theory

[suprised]

It's rather simple: I am referring to a statement "whatever ... it may be ... it may look like, we will still call it string theory" I know from Smolin's book ... I am not sure so I have to check ...

You don't need to look up, it doesn't matter what he said. I'd even refine it: everything what makes sense should be contained in the big blob of consistent theories, that one calls string theory. In particular the standard model, no matter in what parametrization! On the other hand, not Lisi's model, for example.

NCG is not related mathematically to string theory.

Ehm? Didn't you see papers on compactifications on non-commutative spaces? Eg http://arXiv.org/abs/hep-th/9711162
Non-commutativity is in fact a basic feature of any theory with open strings. Configurations of D-branes can generically be viewed as some kind of non-commutative manifolds, so called D-manifolds. And background "B"-fields automatically lead to non-commutativity as well.

If you have the QCD beta function which contains 11/3 and if I give you an 11-faced geometrical object with 3 yellow faces, that does not automatically mean that the QCD beta function is somehow related to this 11-faced object; it does not mean that this object appears somewhere in QCD; nor does it mean that Probability theory is dual to QCD.

Indeed so, but that's not claimed. Their model seems _nothing else_ than the (though incomplete, unquantized) standard model, isn't it? So why you insist it can't have a realization in terms of string theory?

Their only point, IMHO, is that this structure seems natural in their language, while not obviously so in string theory. But other choices would also be possible. I don't know what to conclude from this.

Let's present a Gedanken-teaser. Recall that the standard model gauge group, SU(3)xSU(2)xU(1), can be viewed as the exceptional group E4, and forms a member of the natural sequence E8,E7,E6,E5=SO(10),E4. So one could say: oh I "derive" the gauge group of the standard model incl its matter reps by postulating that the fundamental theory is based on E4! Sounds great, as exceptional structures fascinate everybody... but what does it buy? Another choice would also be possible.

This just boils down whether there is a mathematical principle that is free of any choices, that leads to the standard model. Surely everyone would dream about monster groups, leech lattes, division algebras...but so far nothing has ever be found! And there are good reasons to believe that it does not exist - for some arguments, read the article on the landscape I cited above.

 

[Haelfix]

Regarding NCG and string theory. Isn't there some sort of theorem that was found in the early days, whereby every low energy realization of NCG was consistent only if the high energy theory was also part of string theory (or a limit thereof).

I seem to recall a lecture about this, but I didn't understand it so I have now promptly forgotten.

 

[tom.stoer]

I do not know enough details regarding NCG a la Connes, but all what I have seen so far has nothing to do with strings; if this impression is wrong please forget about it; it's due to my limited understanding of NCG.

But be careful and please take my 11/3-example seriously. Yet another example: just because LQG and ST both use complex numbers, they need not be related.

As I don't think that you responded to the most important part of my post, so I'll re-post it here:

Even if both ST (for which it is not proven) and NCG (a la Connes) converge in some appropriate limit to the SM that does not automatically mean that NCG is related to ST mathematically. What you are saying here is rather dangerous - and I have the feeling that this applies to the web of dualities as well: the fact that certain theories or formulations match in some appropriate limit does not automatically mean that they match exactly. It is dangerous to think about theories like coordinate patches for which a small smooth overlapping region is sufficient. Low-energy effective theories (chiral perturbation theory, nonrelativistic quark model) are in some appropriate limit related to each other. But they are not identical; they can only be identified via QCD and w/o QCD the essential unifying structure is missing. For ST this means that the web of approximate dualities is perhaps too weak to be called a theory. It is perhaps not sufficient to stay with this web of approximate dualities w/o being able to identify the underlying unifying structure. The initial idea and program of M-theory is still waiting for completion!

 

[atyy]

There's NCFT which is a part of string theory, and NCG which is not a part of string theory (or at least not known to be).

 

[suprised]

Ill answer tom tomorrow, here just a remark on:

Regarding NCG and string theory. Isn't there some sort of theorem that was found in the early days, whereby every low energy realization of NCG was consistent only if the high energy theory was also part of string theory (or a limit thereof).
.

The NCG model of Connes et al focuses on the standard model coupled to gravity, at the classical level. Its virtue is in the derivation of these couplings from simple-to-state principles. It says nothing whatsoever on quantum gravity, and all problems with quantizing gravity will be there. In other words, the UV completion is not addressed, and either one assumes a string embedding, which means to be on the safe side, or one bets on a miracle by which this theory turns out to be consistent (for which there is not the slightest indication).

And I can't grasp why this model is listed here in other threads under quantum gravity...but never mind...

 

[Haelfix]

Hi Surprised,

Actually Connes' model wasn't what I had in mind. I was some statement that I heard in a colloquim regarding decoupling limits in string theory. Evidently the claim was something like a NC Yang Mills theory was only consistent if it had some sort of UV limit that also included extended states. The authors arrived at the result by carefuly analyzing the decoupling limits of nc backgrounds in string theory (which is probably where I lost it)

Something like that anyway, no matter.

 

[atyy]

Evidently the claim was something like a NC Yang Mills theory was only consistent if it had some sort of UV limit that also included extended states.

Are you thinking of something like Steinacker's work? He talks about the relationship to string theory a bit on p 18 of http://arxiv.org/abs/1003.4134

 

[Fra]

One point (still haven't read the entire paper properlt).

An overall impression I have is that he is closing his eyes for his own reasoning in certain respect.

At one point comments about the possible idea of (smolins) evolving law in the context of the ST landscape.

The answer to questions like these requires a proper understanding of the thorny issue of probabilities, and may well be beyond the edge of our current knowledge

I fully agree, but the point is that this is unavoidable anyway. He uses this implicitly all over the place in his inference.''

If different laws of physics are possible, we { or anything else of comparable intelligence { might not exist in some of those dierent circumstances.

Already here is is using precursors of inductive and probabilistic reasoning.

About the string landscape he talkes about his past conclusiosn

Observing that the number of possibilities was huge, in the approximation used at the time, was a relatively simple matter. But it was not obvious what to conclude from that.

In several places he still uses the notion which he admits is thorny.

It's clear that he is inconsistent in the sense that he seems to thinkg that probability is thorny when it comes to evolution, but uses it without problems in his mathematical sense.

My suggestion is merely that we need to be aware of our own reasoning, and try to reason consistently. I suggest here that the "thorny probability" issues has to be solved. He seems to reject evolution of law on this basis, yet he uses similar reasoning himself.

He also violates the use of observation, he sats he "observes" the probabilities in hte landscape.

When we try to analyse the meaning of probability, we also come to the meaning of physical law? something which accoriding to hime as I see it is a forcing constraint (except of course there is a whole landscape of such POSSIBLE constraints) (*)

We certainly must aim to understand this. This in fact has nothing to do with stirng theory, but the fact that he touches on it, and manages to avoid digging into the problem is interesting and I think part of the core of SOME of the issues with string theory.

I think these things (the constructing principles and logic) are the most interesting parts that at this point - with the lack of experimental contant - are most WORTH discussing.

edit: more later...
(*) I could argue thta from the inference perspective this is irrational. Physical laws are better thought of as "expectations of the future" that constrains and guides the observers ACTION and interactions with the environmnt. In this way, several of the above things gets easier to understand. This indeed also gives us in a certain sense several different "laws" BUT the major difference is that they are interacting, and there is an evolution as a result of their negotiation. This is the projection I mentioned earler. But for some reason there is no hint of this association in the Schellekens paper. I think the rigid reasoning and the quest for deductive schemes, prevents him from seeing the beauty of a possible inductive scheme, which is related to evolution.

/Fredrik

 

[Fra]

I've skimmed Schellekens paper a couple of times, and while I agree on some things, some of his overall reasoning is objectionable, and I trying to figure how to make the point clear.

My point is quite the opposite: to show that even the greatest minds of their time can make the obvious anthropocentric mistake of thinking that what we see around us is all there is.

Again here is a difference between deduction and induction; absolute truth and rational expectations. A distinction I don't think Schellekens makes; he only sees truth and deduction.

Can we deduce that all we see is all there is and will ever be, or that we will ever see? Of course not, that would indeed be a fallacy.

This seems to be his conclusion.

I think the question he poses is wrong. The question is; is it possibly RATIONAL to base your actions on what you do see? I think the answer is yes. This how a rational player in a game acts. The players information about the rest of the world, largely rules his actions.

The question is not, to answer to eternal truth, what is or or not, or what we will or will not ever see. The rational question any information processing agent asks is: what is my next step ~ what is my optimally rational action?

For example a decision maker, makes a decistion based on available information and acts rationally from that. This can be rational, even if the final result proves to be wrong. There was long time ago a discussion about the notion of "false information" in the QM interpretaton, and the story is similar. A systems ACTION, given an initial state does not need a measure of false or true.

Similarly, we do not need to KNOW in some objective way, wether all we KNOW or see is all we will EVER know or so, or wether this will change in the future. All we need to do, is to evaluate and execute the next step. That's the only rational action.

This gets philosophical and it seems really hard to get this point through, but I'm trying to make a point here, about evolving model building in general, and suggest that it applies to the inference we physicists do, and that it also applies to any physical inferences.

Note that I don't object to lack of perfect uniqeness per see, I agree with part of what he says - that it's impossible to deduce from an obvious axiom system - the laws of physics in detail. The objection is that he seems to focus on the wrong question. He seems to focus on the ultimate state of law, rather than the process of inferring the law! Now that's to me as irrational.

The flawed focus is IMO what gives the gigantic landscape of possibilities. But I do not think these possibilities are physical, or relevant in the way they are devised.

Edit: implicit in my objection and reasoning here is also a different view of what "law" means, although I didn't write it explicitly. The law is rather a "rule" or "guide" for construction an action. A "tool" for interactions. This tool is itself evolving. This is quite different from the realist view of law as "forcing mathematical constraints".

/Fredrik

 

[suprised]

..
just because LQG and ST both use complex numbers, they need not be related…
..
Even if both ST (for which it is not proven) and NCG (a la Connes) converge in some appropriate limit to the SM that does not automatically mean that NCG is related to ST mathematically.

Of course, I completely agree. But what I was contesting was not this, but a statement that their model cannot be called string theory.

Actually I re-read what you wrote and found that you said it weaker than what I had in mind: "… it *may* be the first theory which cannot be called string theory :-)"
I won't argue against this ;-)

I also looked again at their paper, in order to find out to what extent it is an approximation to the standard model. In fact, it is an effective theory up to some scale L, and contains an arbitrary function f which becomes important at energy scales comparable to L. So without specifying f the theory is not fully defined and its consistency and the issue of UV completion is left open; as is the question whether there is some string construction into which it embeds and which would determine f.

This is not to play this nice construction down, rather to put it into perspective with string theory, which is the topic here. I would view its status as similar to what is called "local model building" in string phaenomenology. Here one also postulates a background geometry which reproduces the standard models as well as possible, as an effective field theory (containing corrections involving functions f that are not important at low energies). Again, this description breaks down at a scale above which the full theory becomes important.

In the string construction, one knows what the full theory is, and in principle one can compute the functions f.

For ST this means that the web of approximate dualities is perhaps too weak to be called a theory. It is perhaps not sufficient to stay with this web of approximate dualities w/o being able to identify the underlying unifying structure. The initial idea and program of M-theory is still waiting for completion!

Perfectly agreed. That's why I prefer to view it as a framework that generalizes QFT.

 

[tom.stoer]

So with my statement that

in ST ... the web of approximate dualities is perhaps too weak to be called a theory. It is perhaps not sufficient to stay with this web of approximate dualities w/o being able to identify the underlying unifying structure. The initial idea and program of M-theory is still waiting for completion!

and your agreement in the sense that you

prefer to view ST as a framework that generalizes QFT [to QFT+gravity]

we can now conclude this discussion, can't we?

 

[atyy]

An interesting review.

String Theory as a theory of quantum gravity: a status report
Matthias Blau and Stefan Theisen
http://www.blau.itp.unibe.ch/stqg.pdf

They conclude "String theory is a very promising (and fertile) framework for a consistent theory of quantum gravity. However, we still appear to be at a rather preliminary stage of our understanding of this theory. In particular a non-perturbative formulation of the theory and uncovering its symmetries are important open issues. Matrix models and AdS/CFT correspondence mark recent success on the former issue, while hyperbolic Kac–Moody algebras might be the right language for the latter. Thus, far-reaching statements on either side of the string theory debate, proclaiming either the imminent demise of string theory or the ultimate unavoidability (and virtue) of the anthropic/multiverse scenario, appear to be pre- (and im-) mature, and should not distract one from trying to better understand profound quantum gravitational issues to which string theory presumably holds the clue."

 

[marcus]

An interesting review.

String Theory as a theory of quantum gravity: a status report
Matthias Blau and Stefan Theisen
http://www.blau.itp.unibe.ch/stqg.pdf
...

It is interesting! Thanks for posting the link. When one sees a review that is largely non-mathematical reportage, interpretation, and opinion its helpful to know who are the reporters. So I checked here for Prof. Theisen. I don't know of him but it speaks well that he is at the Einstein Institute (MPI Potsdam) which also has LQG and Group Field Theory teams.
http://www.aei.mpg.de/english/php-Skripte/quMembPage/index.php?%20personKey=theisen
I also looked for Stefan Theisen's publications
http://arxiv.org/find/grp_physics/1/au:+Theisen_S/0/1/0/all/0/1
His most heavily cited papers are from the 1990s, especially mid-90s
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+THEISEN%2C+S&FORMAT=www&SEQUENCE=citecount%28d%29

I must also try to find Prof. Blau...yes, here!
http://www.blau.itp.unibe.ch/blau.html
http://arxiv.org/find/grp_physics/1/au:+Blau_M/0/1/0/all/0/1
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+a+Blau%2C+M&FORMAT=www&SEQUENCE=
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+BLAU%2C+M&FORMAT=www&SEQUENCE=citecount%28d%29
Very respectable. Obviously these people are quite well-known (just not to me).

Likewise Blau's most highly cited papers are from 1990-2002. Now at Bern, but then he was mostly at Trieste ITP, part of SISSA (?), I think for instance Percacci is at Trieste. Both Blau and Theisen started publishing in the mid-1980s, like 1984-1986.

 

[Fra]

I have to conclude that I dislike A.N. Schellekens's reasoning and his reasoning with regards to the gedanken experiments of constructing a anthropic measure that would yield "viable" windows of life in this "theoretical theory space" - which he defends, but he rejects entropic selection and compares the computation of unique statistical distributions with unique theories and thinks it would be a mystery why the statistical prediction would happen to be inside his viable windows of "theory space".

I agree with one thing and that is that it's obvious that the notion of "statisics" needs to be defined, in such a contex, as it's obvious that there is no observer, or scientist that could ever collect and verify this statistics in "theory space".

My opinon is that this, including his insisting on a mathematically obvious "theory space" (gauge theory place) is founding this problem.

If we instead constrict ourself to only realizable measures, the only way is to let an inside observer physicall construct these new measures that would eventually answer his question of "what does statistics and probability mean in this case". That would even apply to any antrhophic measure or viable theory paramteers as even this measure needs to be constructed from the inside (and not in external mathematical spaces); then these two measures should conincide - this there is no "conicidence" that entropic predictions match the viable windows, because they are the same thing if you accept that they must be intrinsically constructed.

I think the main problem is the idea that there exists a unquestionable an observer independent and static theory space. This in itself IMHO an unscientific notion that I find hare to make sense out of. It comes out to me as a mathematical idea; having no clear relation to phyiscs.

/Fredrik

 

[mitchell porter]

Just on a very basic level, I now think the most exciting thing happening is the development of worldvolume theories for M2-branes and M5-branes - the fundamental membranes in M theory. There was a "membrane minirevolution" for the M2-branes a few years ago, and work on the M5-brane is already leading to spinoffs like Witten's "Fivebranes and knots". In the end I think this should give us something like a new fundamental equation for M theory.

 

[tom.stoer]

In the end I think this should give us something like a new fundamental equation for M theory.

I hope so.

I thought about restarting this discussion for a while but concluded that this is not possible. There is one problem when discussing string theory, namely that (as we agreed) there is currently no fundamental formulation available. But w/o such a formulation I do not know which questions I should ask.

 

[MTd2]

The only thing I found with "Fivebranes and knots" was this August 17th talk:

http://online.kitp.ucsb.edu/online/duallang_m10/witten2/

Is there anything else?

 

[Fra]

In the end I think this should give us something like a new fundamental equation for M theory.

Can someone explain what and if such equation imply for the landscape and ambigousness problem? Does it improve the predictability issue?

Is the idea that we count the solution space to M-theory so that we get at best a probability measure? or would ST still have to resort to strage anthropic reasoning?

/Fredrik

 

[tom.stoer]

Can someone explain what and if such equation imply for the landscape and ambigousness problem? Does it improve the predictability issue?

Is the idea that we count the solution space to M-theory so that we get at best a probability measure? or would ST still have to resort to strage anthropic reasoning? /Fredrik

This is not the issue. The main problem today is that you cannot say what string theory really IS; you cannot describe the whole solution space with one fundamental formulation. Having such a formulation at hand you may be able to address new or currently unsolved problems, you may find new symmetry principles, you may be able to identify a selection principle = something that restricts the vacuum degeneracy etc.

 

[Fra]

fundamental formulation. Having such a formulation at hand you may be able to
...
identify a selection principle = something that restricts the vacuum degeneracy etc.

That _was_ my main issue here, thanks.

But I'm not sure I understand the rationality of the method; since one can not "find symmetries" in the observational sense this this is all speculated abstractions. So, let's suppose we CAN describe the full solution space. The question still remains, what methodology to use to find the right constraints. In principle one can imagine that up to "experiments" using random or linear search given that you can count the solution space, but that sounds unlikely to be a good searching method.

(To me "string theory" certainly isn't a theory in the sense of there beeing an equation where you set the input and get the output, in my eyes it's a way of approaching an open problem (ie. a strategy or inference construction), that starts by postulating the FORM of the microstructure where information is coded; and then attempts to exhaust all possible constructible actions and tries to find in that the actions of the SM. The question is HOW this leads to increased predictability rather than disproportionally inflating the set of possibilities. I see this from the pure inference perspective. But these was also already concluded in the thread; that it's a 2framework etc". I actually have not "problem with this" - ie the fact that it's a framework and not a definite theory; MY concern is wether it's the RIGHT framework at all. )

Edit: As far as I understand, ST theorist find motivation for this framework in part by an ad hoc original idea (STRINGS instead of points) and then combining it with QFT and assuming it makes sense to extrapolate QM to cases where it's nto verified). Then I guess it's possible to be comitted to the way of reasoning, even though the goal isn't in sight, if it seems like the only rational option.

/Fredrik

 

[tom.stoer]

Look at QM. In became a fundamentally well-defined theory by changing the formalism from ad-hoc quantization a la Bohr-Sommerfeld to canonical quantization in Hilbert spaces a la Dirac etc. In doing that it was NOT the case that all questions were answered, INSTEAD some of them simply disappeared. I guess something like that must happen in string theory as well.

 

[marcus]

...
(To me "string theory" certainly isn't a theory in the sense of there being an equation where you set the input and get the output, in my eyes it's a way of approaching an open problem (ie. a strategy or inference construction), that starts by postulating the FORM of the microstructure where information is coded; and then attempts to exhaust all possible constructible actions ...

It also has another aspect---not so much a "way of approaching" as a directionless floundering in differential geometry. The mathematics of differential manifolds is astonishingly rich---a fertile womb of abstraction that brings forth wonders of every kind.

One can be so surprised by a few uncanny coincidences that one becomes superstitious.

 

[tom.stoer]

In the meantime I became confused about the fact that string / M-theory deals with strings and/or certain branes.

My feeling is that if we assume that spacetime is somehow quantized on a fundamental level, it does not make sense to use smooth objects as basic buildung blocks. If one looks at LQG then the lesson is that smooth objects are useful at an intermediate stage but that in the very end they will disappear from the theory. I guess that the same applies to string theory. In that case strings or branes would emerge as effective degrees of freedom valid in a certain domane, energy range or something like that. The question then is which principles derived for world sheet geometry, conformal invariance etc. could survice this reformulation and become principles of the fundamental theory.

 

[marcus]

... it does not make sense to use smooth objects as basic building blocks...

this sounds like it could be the verdict of History.

 

[suprised]

In that case strings or branes would emerge as effective degrees of freedom valid in a certain domane, energy range or something like that.

Exactly - that's how many people look at it. It is not at all clear whether there must be any "more fundamental" theory behind, perhaps all there is are effective theories (in the sense that they make sense only in certain regions, never globally).

 

[atyy]

Exactly - that's how many people look at it. It is not at all clear whether there must be any "more fundamental" theory behind, perhaps all there is are effective theories (in the sense that they make sense only in certain regions, never globally).

If string theory is only effective, then does one need 10 dimensions still?

 

[suprised]

If string theory is only effective, then does one need 10 dimensions still?

Translate 10 (rather 6 of them) to "internal, non-gravitational degrees of freedom", and then the answer is yes. And obviously this is a desirable feature.

The meaning of "effective" might have been confusing here - it was not in the usual sense of "low energy effective action" which arises from integrating out massive states; and which is incomplete and ill-defined above some energy scale, so that some other, more fundamental theory would take over above this scale.

I meant "effective" in the spirit described earlier: namely as analog of coordinate patches on some abstract manifold, each of which captures only part of it and there isn't a set of globally defined "master" coordinates what would be well-defined everywhere. In other words, there would not be any "master" theory that would display more fundamental degrees of freedom. On the contrary, by all what we know so far string theory seems complete, there is no indication that any states would be "missing" (for example, to account for the entropy of black holes).

To what extent "emergent gravity" is an indication to a more fundamental theory, or not, is unclear to me and certainly a matter of debate.

 

[tom.stoer]

Are there attempts to identify these fundamental (discrete) building blocks?

A couple of years ago people focussed on matrix theory, but in the meantime it became quiet about that topic. What about an algebraic approach, e.g. hyperbolic Kac-Moody algebras or 3-algebras? Do they have the potential to provide a more fundamental formalism or are they just another couple of "coordinate patches"?

To be honest: I understand the idea of these "patches", but I dislike it. It sounds like surrender.

 

[suprised]

Are there attempts to identify these fundamental (discrete) building blocks?

Of course, in the corner where strings are well tractable (10/11 dimensional theories), this is a solved problem. This is where these ideas came from, they didn't come out just out from the blue.

A couple of years ago people focussed on matrix theory, but in the meantime it became quiet about that topic. What about an algebraic approach, e.g. hyperbolic Kac-Moody algebras or 3-algebras? Do they have the potential to provide a more fundamental formalism or are they just another couple of "coordinate patches"?

Same thing - coordinate patches. Matrix theory deals well with the high-dimensional theories, but not with the low-dimensional ones. Algebraic approaches never lead to anything concrete so far.

To be honest: I understand the idea of these "patches", but I dislike it. It sounds like surrender.

Well.. it's not a matter of liking or not. This is simply the situation as it appears for the higher dimensional theories, as a result of definite computations. The question is whether this picture applies generally, by extrapolation, to all string theoeries. And this again, in principle, is a matter of computation.

This is what I counter-criticise here. Often people have opinions about what a fundamental theory of nature should or should not be like. Whether emergent, relational, based on "inference", fundamentally discrete, background independent, etc. All this is fine, as are political or religious opinions. But what should be realized that theoretical physics is about actual computations and their results, rather than ideological beliefs and wishful thinking. Every fool can cook up some half-baked ideas, but to make it actually _work_ (or at least make some concrete sense), is almost infinitely more difficult.

 

[tom.stoer]

This is what I counter-criticise here. Often people have opinions about what a fundamental theory of nature should or should not be like. Whether emergent, relational, based on "inference", fundamentally discrete, background independent, etc. All this is fine, as are political or religious opinions. But what should be realized that theoretical physics is about actual computations and their results, rather than ideological beliefs and wishful thinking. Every fool can cook up some half-baked ideas, but to make it actually _work_ (or at least make some concrete sense), is almost infinitely more difficult.

Look, the problem is simply this: string theory claims to be the fundamental, unified theory of nature (and I agree that it is the only candidate as I see no other theory which has comparable same unifying ambitions). But unfortunately string theory is less developed regarding its fundamental formulation (degrees of freedom) compared to other, less ambitious theories (LQG for example has identified its fundamental degrees of freedom, QCD succeeded as well; of course there are a lot of open questions in LQG - just as in ST; of course QCD is focussed on string interactions only; but that's not relevant for my example). So there is a theory which claims to be the fundamental theory of nature, but at the same time is not able to explain what its fundamental buildung blocks ARE.

Several conclusions are possible:
- there are no fundamental building blocks, but coordinate patches and "effective" degrees of freedom only
- string theory is "work in progress" with one outstanding problem - what are its fundamental degrees of freedom
-...

I cited Gross here several times; and I think this is again a good point to come back to his central questions WHAT IS STRING THEORY? What is the fundamental formulation of string theory?

 

[suprised]

I cited Gross here several times; and I think this is again a good point to come back to his central questions WHAT IS STRING THEORY? What is the fundamental formulation of string theory?

Well clearly no one has an answer for this, all that I am doing is playing with the idea that a more fundamental set of degrees of freedom might not exist. Only if there were a regime (say at ultra-high energies) where those new degrees of freedom would "liberate", it would make sense to talk about them; non-oberservable quantities should be avoided. But there are indications that when going up in engergy, beyond the Planck scale in particular, nothing of that sort happens (see eg the recent papers by Dvali & Co).

 

[tom.stoer]

I understand you speculation, but my conclusion is different simply because of the rather immature status of the theory; or worded positively because of its huge but still latent potential. In a rather well-developed theory I would agree, but as there are continuously new and surprising (and hopefully promising) research directions I think it's too early to give up.

As this thread originated in my disappointment regarding (the status of) string theory I have to admit that abandoning they idea of identifying one fundamental formulation would be another disillusion.

 

[suprised]

Well it is probably obvious that things are not developed enough to draw a conclusion on these matters, time will hopefully tell.

 

[Fra]

In that case strings or branes would emerge as effective degrees of freedom valid in a certain domane, energy range or something like that.

Exactly - that's how many people look at it. It is not at all clear whether there must be any "more fundamental" theory behind, perhaps all there is are effective theories (in the sense that they make sense only in certain regions, never globally).

As someone have a different approach and strategy to understanding physics, I still can agree that that understanding the various more or less dual "choice of microstructure" of ST (ie different strings, branes etc) as an effective or "relative" in a different context is the one way where some of ST can make sense to me as well. But then, as far as I imagine this (for me strings would then be coninuum limits of more combinatorically discrete microstructures) the new constructing principles from which a continuum string or brane might be "emergent" in the high complexity limit (with high complexity I refer to the AMOUNT of information thta is in fact encoded in a continuous memory strucutre; even if there is redundancy there, the apparent information is still massive) would be so different from the constructing principles of ST that I'm not sure if it would make sense to associate it with string theory as we know it today? With this I mean that it is equally possible that other programs; say LQG with their spin-networks ideas, can expand this to encode also internal information and matter and that it can be developed to the same limit.

It is not at all clear whether there must be any "more fundamental" theory behind, perhaps all there is are effective theories (in the sense that they make sense only in certain regions, never globally).

I symphatize with this and I think this is the case. I think the idea of fundamental (fixed observer independent) microstructure for information encoding is an idea that isn't verifiable. I think too that all there is are effective degrees of freedom (except I prefer to think in terms of discrete complexions, not continuum structures). Ultimately each observer, "sees" not only different states, but different state spaces. This is also why I insist interpreting the string background choices as choices of observers, and to me the non-uniquess is simply due to the non-uniquess of observer. Thisi s notrhing strange, but I just object to how ST treats this. To me this also applies to tha laws of nature, there are not objetive laws that a give observer can infer with certainty. Only effective laws are inferrable. To infer that the effective law is in fact fundamental is not possible as far as understand. Wether that's politics I don't know, but for me it's a quite clear point.

/Fredrik

 

[tom.stoer]

Frederik,

I think you are mixing up two different issues.

I roughly understand your ideas regarding observer-dependent descriptions, but I think we should keep them separate from the discussion regarding a unique formulation of string theory.

One can have a unique framework which allows for observer-dependent descriptions. In quantum mechanics you have on Hilbert space on which you can select different sets ob observables, related by unitary transformations, reflecting observer-dependece; e.g. a two-particle system as seen from the the center-of-mass frame. In quantum gravity with the holographic principle one expects something like boundary Hilbert spaces defined by observer-dependend Dirac observables, nevertheless there is one unique mathematical framework (certain aspects are visible in LQG when introducing horizons and horizon-degrees-of-freedom from Chern-Simons theory). This is what I expect to be incorporated in a unique formulation of string theory as well.

What you are saying is that in addition the mathematical framework itself becomes observer-dependend. I understand your idea but I would like to stress that this is a second step beyond the more basic discussion we have here. It depends how you interpret the situation in string theory as of today. Does it reflect the impossibility of an observer-neutral unique framework? Or is it simply due to the limited understanding of the theory? I guess it's the latter.

Look a quantum mechanics: wave mechanics (Schrödinger) and matrix mechanics (Heisenberg) have been proven to be strictly equivalent in a broader picture provided by fully developed quantum mechanics. Whereas in the early days of QM it appeared mysterious how two different formalism could generate the same predictions, today it is clear that they can be derived from a fundamental theory using the notation of abstract Hilbert spaces; today you have the choice which description to use - depending on the problem you are trying to solve. I expect something like that to happen in string theory as well.

So for me me the different "coordinate patches" or "theories" are not fundamental entities due to some deep principle but simply due to the limited understanding of what string theory is. So instead of accepting these losely coupled patches I would like to insist on their unification.

-------- edit --------

@Fra: let me ask the other why round: what is the reason that in string theory it is not possible to identify a unique fundamental formulation whereas in other (less ambitious) theories like QCD, LQG it is possible?

 

[Fra]

I think you are mixing up two different issues.

Yes you're right, I do, but it's not due to my ignorance. I'm making it into a point.

From my perspective at least (which is of course as biased as anyones) consistency of reasoning implies that they do mix. The physical process of one observer inferring the interaction rules for two other observers or subsystems are subject to the same rules as the process whereby the two first observer interaction from their inside perspectives. What I am suggesting is the deeper idea that they are mixed by nature, rather ME "mixing them up" due to confusion. I'm just trying to acknowledge this "mix" as a fact of nature, and try to use it to understand it.

One can have a unique framework which allows for observer-dependent descriptions.

Alot of people, includiding rovelli, would agree with you on this, and I certainly know what you mean.

But your "point" here is what I call a form of structural realism. I'm suggesting that this realism is not something we need, and that this form of realism still is a source of confusion.

Let me put it like this; I am not saying this form of objective descriptions are impossible, I'm just saying that it's not possible for a physical inside observer to DEDUCE the observer-independent status beyond the effective level. An inside observer can and DO infer EXPECTATIONS of such observer invariance - this is the way I explain the current objective descriptions or QM, GR and SR. But in the perspective I suggest this is special cases.

This is also part of similar to part oF smolin/ungers argument against timeless laws. Smolin makes similar argument but my understanding is that unger takes this to a deeper level than smolin currently appreciates.

What you are saying is that in addition the mathematical framework itself becomes observer-dependend.

Yes.

I understand your idea but I would like to stress that this is a second step beyond the more basic discussion we have here.

This is probably true, but I do not see the value in try to aim for an obviously flawed intermediate next step. I do agree with you, that improvements probably is possible, that is in line with the common structural realism. But to have this as a target, when it seems clear that it's still flawed is not rational.

If the reason is that it would be "easier" to make this smaller step first, then I am not so sure I agree about that. I think we have reached the point with regards to the nature of several open problems that a more radical new thinking is needed.

Does it reflect the impossibility of an observer-neutral unique framework? Or is it simply due to the limited understanding of the theory? I guess it's the latter.

I would say that string theory as it stands now, does NOT imply this. So I think you are right that it's an incomplete understanding of ST. BUT, my reasons for believe in impossibility of observer independent fixed framework comes from a totally different direction! I just project an understanding on the string landscape from a different direction.

So if we eventually understand that such objectivity is not possible, I think it is certainly not thanks to ST as I see it. ST seems more due to a conincidence due to, like marcus also notes, it's nature of mathematical fiddling (but guided by some questinonal extrapolations of QFT principles and extra ad hoc assumptions)

@Fra: let me ask the other why round: what is the reason that in string theory it is not possible to identify a unique fundamental formulation whereas in other (less ambitious) theories like QCD, LQG it is possible?

I can't provide the correct string theory answers (ie what the reason is from the point of view of a string insider); this is what I'd like to hear the insiders view, and it's why I appreciate surprised contribution here.

But as I understand it (ie. projecting it onto my reasoning) is that, loosely speaking:

The original starting point of string theory, where one postulates the microstructure of where information is encoded (ie the continuum STRING, the background space) and the string ACTION which encodes the inference machinery contains too many degrees of freedom that are unknown, and the "consistency" principles ST uses from QFT and so on, to constrain the possibilities are simply too weak and you still end up with an extremely large set of "possibilites" that simple originate from the massive amount of information that is encoded in the continuum strings and the choice of background spaces. There is also an unclear understanding of WHY the correct action correspond to quantized classical string.

Alot of this is IMO traced to the ambigoty if starting with using continuum structures as coding structures, beause it's ambigous howto count and quantify information in these structures. This ambigouty itself yields a landscape of possible "limiting procedures".

From my inference perspective, it is not sensible to start out with an uncountalbe set, and try to apply reasoning since the only way to make any sense of that requires to view the uncountalbe infinite set as a limiting case of something if not finite, at minimum countable.

So the "gigantic landscape" of string theory, in my view corresponds (If at all!) to a highly evolved very complex and massive observer. And the evolution history would hold the answer to WHICH structure is the "right one".

/Fredrik

 

[Fra]

The physical process of one observer inferring the interaction rules for two other observers or subsystems are subject to the same rules as the process whereby the two first observer interaction from their inside perspectives. What I am suggesting is the deeper idea that they are mixed by nature, rather ME "mixing them up" due to confusion. I'm just trying to acknowledge this "mix" as a fact of nature, and try to use it to understand

An illustrative point of disagreement here with Rovelli is his RQM paper, he first makes some excellent point, such as there beeing no objective states and that there are only relative states or relations between observer and observed, and no absolute relations. The only way for two observer to compare their observations is by means of interaction = communication.

So far, it's all good. But the coherence of his reasoning when he flatly assumes that all communiation follows QM structure. He further explicitly avoids discussing the meaning of probability etc.

Here rovelli resorts to structural realism and faith in the structure of QM, in a way that I find inconsistent. His reasoning is not consistent.

Instead of assuming that observer independent structures are forcing, I'm considering observer democracy where the observer indepdepent laws are evolving constraints, where this evolution contains darwinian and unpredictable elements.

The string landscape somehow seems to attempt to describe the complete statespace, instead of considering it to be evolving. It's both too small and too large! An evolving statspace in evolution is always sufficiently large to be adaptive, but not too large to stall progress.

/Fredrik

 

[tom.stoer]

Thanks for the long reply. I think we mostly agree, especially about the difference between the radical change in science you are proposing, namely to abandon structural realism in which all scientific theories are deeply rooted, and the more technical details regarding string theory.

I disagree with you that it makes no sense to "aim for an obviously flawed intermediate next step". String theory isn't fully developed ad there is a huge potential regardless if one agrees to your reasoning or not. In addition I do not agree that your conclusion is "obvious". And I think that regarding string theory it is not true that "we have reached the point with regards to the nature of several open problems that a more radical new thinking is needed".

I am perfectly aware of the fact that I am limiting myself to standard reasoning, structural realism etc - at least here in this discussion regarding string theory.

Your argument against string theory using smooth structures and eventually deriving fundamental discrete ones is not valid. LQG did exactly this: start with smooth structures and derive discrete ones. You need not love LQG but you should agree that reducing smooth structures by large symmetries to descrete structures seems physically reasonable. Anyway - we agree on the basic problem.

You argument against the landscape isn't fundamentally enough. The landscape is only the space of solutions of a set of theories related by dualities. The argument should be against the missing uniqueness of the (formulation of the) theory itself, not against the missing uniqueness of the solutions (look at the standard model: the sun, iron, my laptop, a pint of beer, ... all solutions of the SM - by no means unique).

My conclusion is that I am still willing to rely on structural realism and that therefore the major issue for me is to identify objective, structurally simple and phenomenologically viable laws of nature. As long as this seems to be possible I am willing to accept string theory as a (candidate of a) physical theory.

 

[Fra]

Thanks. Yes I think we understand each other and have a partial agreement.

Given that you're willing to accept strucutral realism at least for the moment, your other arguments may fit in with that. I could comment on your arguments but those arguments again go back to the general reasoning, because in my perspective "solutions" and theories are a result of similar proceses; the reason for missing FUNDAMENTAL uniquesss in both cases are to me similar, but living at different levels of the inference hierarchy.

The inference has hierarchies and parts. A given theory + initial conditions may give a prediction, or more correctly an expectation - which usually is in the form of a proability distrubution of possible results. But even the theory is also a result of an inference at a depper level, the history of interaction has made us EXPECT these laws, and it is thus determining our actions and expectations.

What should in my view give the uniquess you seek; is when you single out an observer. Only once you consider a specific observer, should be expect unique theory (ie an inference machinery with produces expectations of the future from the present); the theory itself encodes somehow the history, and the theory itself is nothing more than an expectations as well. IT's just a far more confident one, than the specific outcomes OF the theory + initial conditions.

The fact that two different observers may end up with inconsistent predictions is in fact not really a physical inconsistency. It merely reflects that their expectations are different. The normal way to "restore" this is to consider a second observer that can find transformations between the observer, to restore consistency - but this consistency is restore relative to a third and usually MORE complex observer. So from the inside view a lot of the big symmetries remain broken. I know this is a different way of thinking than strucutral realism, and does present some own novel problems, but I find it easy to convince yourself that these are features of nature we should understand. I used to have faith in structural realism as well, but have come to abandon it. My main argument used to be that if the laws of physics aren't observer invariant, then what's they worth? That may sound like a reasonable point - it did to me, but once you see that laws are in a different light, there are different ways to see this.

/Fredrik

 

[Fra]

Your argument against string theory using smooth structures and eventually deriving fundamental discrete ones is not valid. LQG did exactly this: start with smooth structures and derive discrete ones. You need not love LQG but you should agree that reducing smooth structures by large symmetries to descrete structures seems physically reasonable.

Maybe I'd need to expand on this to explain. In my view, which is inference, the choice of microstructure and the choice of action is connected. But I don't want to destroy this good thread with expanding too much on this. I'll just note that yes continuum models can produce unique discrete stuff, but that's not enough for me. LQG is not constructed from intrinsic inference (like I seek) it also uses actions that are either ad hoc or inspired, borrowed from GR.

I'm thinking of a pure natural inference (essential that ALL interactions are entropic), and there computing gets important, in evaluating hte action. If both the action form and the microstructure contains continuum ghosts then it sure is possible that they somehoe match and cancel. But everything gets far more complex and you have to spent a lot of work to make the expectations cancel properly. This simply won't happen in the program I envision. On the contrarcy do I expect thta continum models may "fit" as extrapolations on the discrete structure, but I think it's easier to understand the logic of the dynamics and emergence of these continuum models if you look at the distinguishable structure.

/Fredrik

 

[tom.stoer]

The discussion was interesting, but I think we somehow lost focus.

I asked whether there are candidates towards a reformulation of string theory from which other "effective descriptions" can be obtained. Unfortunately both the standard string theories and SUGRA, as well as matrix models and other algebraic approaches were classified as "patches valid in certain regimes" but not as candidates based on fundamental degress of freedom.

Some weeks ago I asked regarding string field theory. Something more to say about that?

 

[marcus]

I don't wish to interrupt and so will only comment briefly that I find the discussion instructive and this last question quite interesting. Structural realism has several times been mentioned as a key idea here. I understand your last question to express that concern: Are there any mathematical models which a structural realist could take seriously? If I understand your meaning, a structural realist stance requires models of how really how the world is, not mere "patches" with limited applicability, or ad hoc schemes for calculating.

I may not understand this position or criterion, in which case you would need to spell it out.

 

[atyy]

The discussion was interesting, but I think we somehow lost focus.

I asked whether there are candidates towards a reformulation of string theory from which other "effective descriptions" can be obtained. Unfortunately both the standard string theories and SUGRA, as well as matrix models and other algebraic approaches were classified as "patches valid in certain regimes" but not as candidates based on fundamental degress of freedom.

Some weeks ago I asked regarding string field theory. Something more to say about that?

These guys http://www.blau.itp.unibe.ch/stqg.pdf hardly mention it. They point to http://arxiv.org/abs/0710.3017 . But they seem to think AdS/CFT and E11 stuff more promising for trying to figure out a non-perturbative formulation, and what the symmetries of string theory might be.

Incidentally, what if the question is analogous to asking for a single coordinate system to cover a sphere?

 

[tom.stoer]

Marcus,

I am not sure if I understand you correctly.

The discussion regarding structural realism was a detour only.

Simply speaking my expectation is that different observers may not agree on the details of their observations or experiments (different locations, different reference frames, different energy or length scale, ...) but they should agree on a basic framework (= the "theory") which allows them to translate or transform these different observations into each other.

Fra insists on abandoning this structural realism and allow even the framework i.e. the theory to become observer-dependent. That's why he is not worried about the "patches" one observes in string theory (as of today).

I do not ask these question because of the discussion regarding structural realism but simply because it always was and still is my expectation (since I started to study physics): that physics is able to unveil parts of "ethernal truth of nature". Physics over the last centuries identified phenomena on different scales, energy ranges etc. At the same time scientists always insistet on the same paradigm, namey that even so there are a plethora of phenomena, behind them always is a small set of eternal laws. So I simply stick with this approach and insist on the existence of eternal laws which we should partially uncover.

That's why I find it puzzling to be comfortable with a situation where there is a plethora of different formalism suitable for specific scenarios, domains, energy or coupling ranges etc.

I tried to find examples where (looking backwards in time) one may find a similar situation. E.g. in physics regarding the strong interaction one finds S-matrix, current algebra, chiral perturbation theory, non-relativistic quark models xyz bag models etc. Now we know that the existence of these theories is NOT due to observer-dependent theories, but simply due to limited knowledge regarding the strong interaction. Today we know that we can to a certain extend derive these theories as effective theories of QCD. We observe their limitations not only based on phenomenological considerations but we are able to understand both their successes and their failures based on QCD. Even so QCD is still not able to allow for the calculation for soft pion-proton scattering (which is a simple task within the Skyrme model) one is able to understand to what extend one can derive or motivate the Skyrme model from QCD. That's why I am absoluetly sure that QCD is "more true" than the collection of low energy effective theories I just mentioned.

As I am not an expert in string theory I follow the real experts' reasoning explaining to me that the theory is still in an early stage of development. If this is true (and it's not up to me to question this) then the natural conclusion is that we should insist on finding this "eternal law behind string theory". I want to make clear that "eternal law" need not mean that string theory is true. An SU(4) gauge theory of the strong interaction is physically wrong, even so we know its eternal law :-)

Regarding my first post where I tried to explain why I am REALLY disappointed about string theory: it is exactly this, namely that up to now nobody is able to explain what string theory fundamentally IS.

 

[Fra]

Just a last note on this

Now we know that the existence of these theories is NOT due to observer-dependent theories, but simply due to limited knowledge regarding the strong interaction.

From my perspective, there is no difference between the two possibilities you describe.

I like to quote Zurek here (although I take the meaning further than I think Zurek did):

"What the observer knows, is indistinguishable from what hte observer IS"

My point of intrinsic inference, means that there are constraints of what a given observer CAN know or CAN optimally infer; and this relates to the complexity and information capacity in my view. This means that it doesn't make sense to just dismiss it as ignorance in the ordinary sense, as this type of ignorance is due to how nature works. And this "ignorance" will influence the action of this observer. Because theories are also information and they neeed representation.

I know I'm comparing human level inference with physical inference between particles in the analogy here but I'm suggesting there is a deeper connection and the analogy is just to illustrate the principles. So in a sense there is a common framework, but it's not a theory like we know it. The framework is rather some constructing principles behind rational inference and action.

The distinction between "ignorance" or just observer dependence makes sense in the structural realism view. Different observers generally make difference inferences and thus end up with different information.

/Fredrik

 

[mitchell porter]

The only thing I found with "Fivebranes and knots" was this August 17th talk:

http://online.kitp.ucsb.edu/online/duallang_m10/witten2/

Is there anything else?

It's work in progress by Witten. Partly it descends from hep-th/9912123 but there are many other ingredients. For the 6D (2,0) SCFT which is a worldvolume theory for parallel M5-branes (and which only shows up at about 88 minutes in the talk), you could try hep-th/0608014, section 4.2, for an introduction.

An interesting basic fact about M-branes is that you can have an M2-brane in the shape of a cylinder stretched between two M5-branes. It's analogous to an open string stretched between D-branes in string theory. But the M2 cylinder ends on a loop in the M5-brane, so from within the M5-brane, the ends of interacting M2-branes look like closed strings inside the M5!

Some weeks ago I asked regarding string field theory. Something more to say about that?

Last month, in comment #87 in this thread, I said Witten invented string field theory, which is wrong. It goes back to Kaku and Kikkawa in 1974.

 

[tom.stoer]

Just a last note on this


From my perspective, there is no difference between the two possibilities you describe.

...

"What the observer knows, is indistinguishable from what hte observer IS"

Just to clarify: inventing QCD means (to us humans) to learn more about the strong interaction; so the perspective of the observer changes. But it does not mean that the strong interaction itself changes. It's only that we understand more about it. Strong interaction is the same before and after the appearance of QCD in our physical theories.

It's like prime numbers: A number which is prime is prime even if we do not know that this number is prime. Calculating its prime factors changes our knowledge, but not the number itself.

What does that mean to string theory? It means that we are still looking for one unifying framework which harmonizes the different patches of the theory (just like the different low-energy effective theories for QCD). The framework of string theory (still to be identified) will then reflect our current knowledge regarding the different interactions we observe in nature. If this knowledge increases it may become necessary to change or enlarge this framework again. Therefore it is of course necessary to go through this mess of different formulations, symmetry breaking, vacua, low-energy descriptions, discussing different patches within M-theory etc. Nevertheless one framework to address all these questions is preferred over the situation as of today.

So again: for me there is no (known) PHYSICAL reason which prevents us from identifying this framework; it's our limited knowledge, or limited mathematical capabilities, or perhaps missing genius (a la Einstein). Not being able to identify QCD over a couple of decades was due to the limited knowledge, not due to PHYSICAL principles of the strong interactions.

 

[Fra]

Just to clarify: inventing QCD means (to us humans) to learn more about the strong interaction; so the perspective of the observer changes. But it does not mean that the strong interaction itself changes. It's only that we understand more about it. Strong interaction is the same before and after the appearance of QCD in our physical theories.

I understand and I agree you are right of course! I suspected you'd respond with this, it's hard to describe properly and be brief at the same time.

What I mean is this: The implication and difference "law" makes, is only when you consider the action of the system that encodes it.

I do not suggest that human understanding (information coded in human brain) causes the strong or weak or any other interaction to change! What I DO suggest is that the "knowledge" a quark, proton or electron have (ie what's encoded in the microstructure of the particle with given mass/energy etc) about physical interactions DOES actually change the interactions it participates in. This is the type of reasoning I expect to explain why certain interactions are indistinguishable at say different energy scales or, why only charged particles are deflected in an electrial field without beeing decomposed or destabilised.

The human analogy would take into account the action of the human. And indeed, this differes. When the scientists has a certain view of law; it surely reveals itself in the questions he asks, the experiments he designs. When his understanding has changed, so does his further questions and experimental designs.

So again: for me there is no (known) PHYSICAL reason which prevents us from identifying this framework; it's our limited knowledge, or limited mathematical capabilities, or perhaps missing genius (a la Einstein).

I see two domains where this makes a different. Cosomology - here human based knowledge is still tiny and physical truncations of information are unavoidable. So we really need to ask ourselves what we MEAN by say probabilities on cosmo level? Or states of the universe?

The other thing does affec the action of the microstructure of matter,and it's unification to large scale physics is that the scaling of interactions might be hard to understand if we insist timeless eternal fixed laws. After all, we still lack a GUT - this is one aread where I think this does matter. The external view that we get, comes with a lot of distinctions that are PROBABLY non-physical to the insiders in extreme high energy itneractions, and it we can understand how "physical law" scales properly, I think it will be easier to understand!

Not being able to identify QCD over a couple of decades was due to the limited knowledge, not due to PHYSICAL principles of the strong interactions.

Yes fully agreed of course. Maybe I overstated the implications of my view to the practical things. I certainly think that we humans can find such a framework for ST. I'm not really defending ST. I think the diversity in ST, is non-physical anyway. I just defended some traits of it.

I just think it's a guiding principle in general to seek for these structural realist eternal laws is wrong. I fully agree that we can and will find such things, I just say that they are merely effective and evolving. I think our learning will be more efficient if we have the right guiding principles.

/Fredrik

 

[tom.stoer]

OK, so let's come back to the main issue.

Suppose I am a brilliant, young physicists with the potential to identify the very foundations of string theory, the unifying framework (honestly: I am neither young nor brilliant). Suppose I have the chance to ask other physicists (including you) regarding the most promising research direction within string theory and regarding my future work.

What is your advice?

 

[Fra]

OK, so let's come back to the main issue.

Suppose I am a brilliant, young physicists with the potential to identify the very foundations of string theory, the unifying framework (honestly: I am neither young nor brilliant). Suppose I have the chance to ask other physicists (including you) regarding the most promising research direction within string theory and regarding my future work.

What is your advice?

I'm not at all qualified to answer to specifics of ST, if the premise of the question is to stick with ST.

I guess I would just encourage critical thinking and making your own assessements, and ask yourself wether ST really IS the only option?

What I personally think is the most interesting reaserch direction is neither within string theory, nor necessarily good advice if you want someone else to pay for your work, then you also need to look at the commercial and political aspects. (Carreer advice is a completely different question, and I don't think that's what we dicsuss here).

The only link I personally see to ST; is a reconstruction which is a completely different way of thinking - but it has similarities - where the "strings" may emerge as the simplest possible continuum structures in the large complexity limit. I actually associate the "string" to a "probability distribution" in the continuum limit of a discretely indexed string with discrete amplitudes, where each such distribution has a defined complexity. The full continuum string is not existing, it's rather discrete. IF that would work, it might in the end either provide an explanation and understanding to some of the assumptions of ST. Why strings, why the string action (should be a form of minimum divergence), but it requires non-commutative structures, so it would rather have to be different sets of discrete distributions that are related, what about the landscape etc. But it might as well show that string theory (seens as an inference model; which is what I think is the way to do) is simply wrong, maybe the string action is wrong, maybe something else is wrong - but then the right form should be found. That's impossible to tell at this point. In any way, it seems to me that IF ST is to make sense to ME, then it will also just be an effective model. In particular are the strings and branes not fundamental. The more general case would I envision be so different that it would be just silly to even call it string theory or m-theory.

My confidence in this direction I've just acquired over time. I really do listen and read what the experts think and there are some excellent ideas that tangent to this. But there are only some papers that sniff this. It's still highly undeveloped thinking and controversial. There simply is no group to join I'm aware of.

Edit: the "scaling of law" I mention is not just some regular renormalization. It's different, as it also contains evolutionary elements. The scaling is not cleanly separable from evolution. Therefore, do I not believe in objective deterministic renormalization rules. That still has too much strucutral realism in it. I think renormalization still await another revolution where it can be understood at an even deeper level. The scaling of an observer and thus law is not just mathematics, I think it's a physical process.

/Fredrik