Will string theory ever be proven wrong?

[marcus]

Isn't the discovery of a spherical EDM electron and lack of SUSY particles disproving SUSY and there by falsifying string theory?

Also the lack of extra dimension, and things of such nature.

Eiyaz, I think I hear what you are trying to say but I wouldn't put it in those words. Maybe you could say that not finding evidence of SUSY would disfavor String. Increase the odds against it's coming up with a useful unified theory that actually describes nature.

Various observations or circumstances can reduce the odds in favor and increase the odds against. But I wouldn't talk about "falsifying" because that has a kind of technical meaning.
Things are not that clear cut. There are stringy schemes that don't need SUSY.
String is kind of amorphous with lots of different versions and no definitive formulation, say of "M-theory". M-theory is hoped for but not yet definitively written down. So it is a bit premature to talk about falsifying it.

I think it could happen that researchers don't ever prove stringy stuff "wrong", they might just gradually lose enthusiasm about it, and gradually shift interest over into other lines of research that are less and less explicitly stringy.

The program may never get to the point of producing one unique definitive formulation that predicts new physical phenomena that one can then look for (providing disproof if the predicted phenoms are not found.)

It might never be disproven in that sense, but it still might dwindle and fade some. It is something to watch for, in case it is happening.

You probably saw the report that this year the hiring season is over with essentially no offers of first-time faculty jobs (Usa and Canada) made to string theorists.

The first-time faculty job offers went to lattice gauge theorists, phenomenologists, and especially cosmologists as I recall. Google "physics jobs rumor mill" to find the URL, at UC Davis.

It is a sharp contrast with say 2003-2005 when there were lots of offers. Dozens every year as I recall.

This kind of thing is more practical than "disproof" or "falsification". It could represent a real change. Or it could just be a random fluctuation due to something else. Only time will tell.

 

[eiyaz]

Eiyaz, I think I hear what you are trying to say but I wouldn't put it in those words. Maybe you could say that not finding evidence of SUSY would disfavor String. Increase the odds against it's coming up with a useful unified theory that actually describes nature.

Various observations or circumstances can reduce the odds in favor and increase the odds against. But I wouldn't talk about "falsifying" because that has a kind of technical meaning.
Things are not that clear cut. There are stringy schemes that don't need SUSY.
String is kind of amorphous with lots of different versions and no definitive formulation, say of "M-theory". M-theory is hoped for but not yet definitively written down. So it is a bit premature to talk about falsifying it.

I think it could happen that researchers don't ever prove stringy stuff "wrong", they might just gradually lose enthusiasm about it, and gradually shift interest over into other lines of research that are less and less explicitly stringy.

The program may never get to the point of producing one unique definitive formulation that predicts new physical phenomena that one can then look for (providing disproof if the predicted phenoms are not found.)

It might never be disproven in that sense, but it still might dwindle and fade some. It is something to watch for, in case it is happening.

You probably saw the report that this year the hiring season is over with essentially no offers of first-time faculty jobs (Usa and Canada) made to string theorists.

The first-time faculty job offers went to lattice gauge theorists, phenomenologists, and especially cosmologists as I recall. Google "physics jobs rumor mill" to find the URL, at UC Davis.

It is a sharp contrast with say 2003-2005 when there were lots of offers. Dozens every year as I recall.

This kind of thing is more practical than "disproof" or "falsification". It could represent a real change. Or it could just be a random fluctuation due to something else. Only time will tell.

I have heard that string theory without SUSY require too much fine tuning and are generally not favored. Extra Dimension and SUSY are the backbone to the theory, but nonetheless am i correct in saying that string theory is in danger? If so why are there no articles on this yet?

 

[marcus]

I have heard that string theory without SUSY require too much fine tuning and are generally not favored. Extra Dimension and SUSY are the backbone to the theory, but nonetheless am i correct in saying that string theory is in danger? If so why are there no articles on this yet?

It's certainly a reasonable question to be asking. I think you mean specifically why no comment about the recent article you mentioned about the electron's EDM (electric dipole moment.)

I'll try to respond, but first here is an earlier comment by Haelfix that tends to agree with and expand on what you said. It's an intelligent concise summary of the situation, I think. If you didn't happen to see it, you might be interested:

Yes String Theory can output a small, nonzero CC. However it is highly fine tuned, and it exists only in toy models. So called Kachru Vacuua.

Worse, it is not unique, there is a huge landscape of such posibilities (see Susskind and Dines paper, etc) that all interact with each other in principle.

As for predictions made by String Theory. The theorists understand the theory kinda/sorta well in highly supersymetric 10 dimensional space. Its getting 4 dimensions that's hard, since there is in principle almost an infinite amount of different ways to compactify, and you end up with a large amount of parameters (added to the already enormous amount of degrees of freedom inherent in the theory).

If the LHC doesn't find SUSY, it becomes very hard for String Theory to continue in all innocence. Part of SUSY's original aim was to remove the fine tuning, and the most reasonable models put it firmly in the LHC's range. If it doesn't find the lightest superpartner there, unfortunately it doesn't falsify SuSY (it just breaks at a different scale the enthusiast will say). However much of the original niceness goes away.

If we do find SUSY at the LHC, String theorists will jump up and down and proclaim victory. However, the two theoriest are sufficiently disjoint, such that one need not imply the other. We could live in a world with SUSY, but no ST.

Brian Greene, and others are looking for ways to get a cosmological constraint on ST. There is some debate over whether Planck could in principle see a trace of quantum gravity. If it does find something interesting, it would be wonderful of course (it would say hey QG is in fact important), however that too won't falsify st. From what I gather, it will however add a few much needed constraints to the parameter space.

 

[Haelfix]

I wrote that statement (in 2004!) when I was still a first or second year grad student thinking mostly as a phenomenologist. Once upon a time the whole purpose of S.T at least for people in my neck of the woods was to be able to write down low energy MSSM models and ask questions about the sort of allowed or natural objects or schemes. Nowdays the applications are much more varied and far more subtle.

I think it is pretty clear that you can have interesting phenomenology without weak scale SUSY which wasn't particularly apparent or exploited as much at the time (alternatively I just didn't know about those models) and I also think I have a greater appreciation for gauge-gravity dualities which in a sense makes the whole premise moot (it would be akin to asking whether QFT can ever be proven wrong).

There you go evolution in thought at work. The former is more a pov for a straightforward unification paradigm, the latter is a much more subtle take on what it means to be a theory of nature.

 

[marcus]

...am i correct in saying that string theory is in danger? ...

I wouldn't disagree either with what you said, or with what Haelfix said (already back in 2004) that I just quoted. But I'd put it in my own words somewhat differently.

I think there is a String program, a well-populated and entrenched research enterprise, that is partly in some aspects in danger, but also in other respects not in danger.

I don't think there is anyone unique physical String theory that accomplishes unification and matches known low-energy particle data. More importantly there is no one useful theory, matching known particle physics, that theorists can actually calculate with.

So (as a non-expert, subject to correction) I would say that if there is no theory then it cannot be in danger.

However there is a well-populated busy research program, and this is PARTLY in danger of losing prestige, jobs, funding etc. It is partly in danger of being seen as NOT TERRIBLY USEFUL on the unification/basic theory front.

Now we are getting signs of "no SUSY"---there was what you mentioned but also in the past couple of months some preliminary notices from the LHC people. A quiet gradual revision of expectations, subtle change of mood. Then there was the unfavorable news about first-time faculty JOBS. We might also be seeing symptoms like an increased defensiveness, more excuses, more attacks on rival non-string QG, etc. I'm not sure about that, but it could happen.

On the other hand there is this big healthy body of stringy mathematical METHOD which can presumably be applied to a lot of other stuff besides unification. The String program can gradually change character so as to be less directly concerned with a fundamental theory of nature and more concerned with finding applications of method.

The picture is certainly not all bad!

If I were involved, what I would probably be most concerned about is the quality of incoming string PhD students. Are they as bright, independent-minded, talented as those who were entering string say 10 years ago?

My intuitive feeling is that a smart young person entering grad school now might prefer to go into cosmology (with just a dash of stringy seasoning) or into condensed matter (with perhaps some string-related method). Those who are now making a full commitment say to string phenomenology might be among the less talented or less alert. It is hard to gauge but nevertheless the kind of thing one ought to watch out for.

You brought up the topic of dangers---I guess my view would be that the interesting and possibly significant ones are dangers to the program (its academic prestige and the other things that go along with that, like ability to recruit talent etc). We just have to wait and see.

I think you are definitely right about the declining odds of SUSY (and regardless of whether logically necessary it does impact the perceived usefulness.)
==EDIT==
Haelfix, thanks for the update on your earlier POV. Always interesting. I will copy so it doesn't get covered up by this post:

I wrote that statement (in 2004!) when I was still a first or second year grad student thinking mostly as a phenomenologist. Once upon a time the whole purpose of S.T at least for people in my neck of the woods was to be able to write down low energy MSSM models and ask questions about the sort of allowed or natural objects or schemes. Nowdays the applications are much more varied and far more subtle.

I think it is pretty clear that you can have interesting phenomenology without weak scale SUSY which wasn't particularly apparent or exploited as much at the time (alternatively I just didn't know about those models) and I also think I have a greater appreciation for gauge-gravity dualities which in a sense makes the whole premise moot (it would be akin to asking whether QFT can ever be proven wrong).

There you go evolution in thought at work. The former is more a pov for a straightforward unification paradigm, the latter is a much more subtle take on what it means to be a theory of nature.

 

[smoit]

... If we do find SUSY at the LHC, String theorists will jump up and down and proclaim victory. However, the two theoriest are sufficiently disjoint, such that one need not imply the other. We could live in a world with SUSY, but no ST...

Just to clarify this point: in the real world, any supersymmetric extension of the Poincare symmetry is bound to be local because after all, we already know from GR that the Poincare symmetry is only realized as a local symmetry. Therefore, a discovery of superpartners at the LHC, if it happens, would imply that the low energy world is effectively described by some type of N=1 D=4 supergravity coupled to some SUSY extension of the SM. It could even happen that the LSP is the gravitino - the superpartner of the graviton, if the sparticle spectrum happens to be consistent with gauge mediated susy breaking. Now, it is well known that the effective N=1 D=4 supergravity naturally arises as a low energy limit of many realistic compactifications of string theory. Suppose that we are lucky and will be able to quickly establish that SUSY breaking is gravity-mediated. It is known that in models with gravity mediated SUSY breaking, there generically exist Plank-suppressed operators in the UV theory, which can directly affect the physics at the electroweak scale by generating flavor-changing neutral currents. These effects may give us a direct connection to the UV physics and this is precisely where string-theoretic computational techniques may prove to be very useful.

 

[med17k]

At least string theory can reproduce the standard model . can LQG do this?

 

[marcus]

You might like this little movie

 

[martinbn]

At least string theory can reproduce the standard model . can LQG do this?

Actually can string theory do that?

 

[med17k]

As far as I know heterotic E_8 x E_8 string theory can lead to supersymmetric SM gauge group in the context of a procedure called orbifold compactification.Actually many string models may be able to reproduce the usual standard model physics.

 

[martinbn]

As far as I know heterotic E_8 x E_8 string theory can lead to supersymmetric SM gauge group in the context of a procedure called orbifold compactification.Actually many string models may be able to reproduce the usual standard model physics.

So, can it reproduce the standard model, not supersymmetric extensions, or it _might_ be able to?

 

[tom.stoer]

I haven't seen any paper claiming that string theory (using a specific compactification) exactly reproduces the MSSM or the SM. But I guess we would all have seen such a paper - think about the excitement it would have caused.

But think about the following: what would happen after the publication of such a paper?

@med17k: LQG does not aim for doing particle physics; the LQG approach is restricted to quantizing gravity; whether this restriction is physically viable and whether LQG does indeed provide a theory both viable in the IR and at Planck-scale is still under investigation.

 

[martinbn]

I haven't seen any paper claiming that string theory (using a specific compactification) exactly reproduces the MSSM or the SM. But I guess we would all have seen such a paper - think about the excitement it would have caused.

But think about the following: what would happen after the publication of such a paper?

When one reads popular books and articles on string theory, one gets the impression that SM is in ST and that this is a classical result. So, I am puzzled, if it is not the case why is it being said so often!

 

[Fra]

Often similar statements are conjectural to it's nature. String theory isn't yet a well defined theory, it's best understood as a research program. So any statement about what a theory that is supposed to be the result of that research program can or can't to is bound to be conjectural. It can even be a constructing principle to say that the "unkonwn theory we are looking for" have this or that trait (unless we are ALL wrong, but that's your potential loss in the game). When comitting to the constructing principles you have on one side all the promises, in the other side you have the loss of investments if you're wrong.

String is by construction aimed to be a unified theory. So maybe you can say that - given that this "string theory" exists - it must explain the SM.

Since no theory of unification or QG is yet meature producing clear testable predictions allowing discrimination, the only measure of the presumed theories are to rate the plausability and rationality of their founding/constructing principles. Its seems that String theory certanly have the POTENTIAL to describe the SM.

Another question to the title, how do you falsify a research program? I'd say failure to make progress relative to competing reserach programs. What I find remarkable is that some of the - IMHO most basic but most important questions - have seen minimal progress. I personally think it's because ot misdirection of resources into very complex "technical issues" which highly uncertain connection to physics.

/Fredrik

 

[tom.stoer]

... one gets the impression that SM is in ST and that this is a classical result

it is - with high proability, but not proven.

... if it is not the case why is it being said so often!

marketing? hope? b/c they are nearly their?

 

[mitchell porter]

When one reads popular books and articles on string theory, one gets the impression that SM is in ST and that this is a classical result. So, I am puzzled, if it is not the case why is it being said so often!

Here is a paper claiming http://arxiv.org/abs/1106.4804" . What they mean are compactifications which have the "minimal supersymmetric standard model" as a low-energy limit. The MSSM in turn has the standard model as a low-energy limit. However, this is the MSSM and SM without particular parameter values. That is, the SM has those two dozen parameters like particle masses, coupling constants, and mixing matrices; and the MSSM, when SUSY-breaking potential is considered, has over 100. The theory, at a certain level of abstaction, is defined by equations in which all those parameters are just unspecified constants. So then you could say that the version of the SM which applies to the real world is the one with all the measured values for those parameters.

Roughly speaking, a stringy standard model is one which reduces to the 'abstract' SM or MSSM at low energies. There isn't any model which is known to give the right values for all the various parameters, though in some cases there are arguments that the parameters in the model will have the right order of magnitude or the right hierarchy of ratios. The situation is like this because such parameters are typically very hard to calculate (i.e. in many cases we just don't know how to do so). String phenomenology proceeds through the discovery of new classes of string model which might match reality (e.g. the industry, beginning a few years ago, in "F-theory GUTs"), and with incremental improvement to the calculability and qualitative phenomenological viability of known classes of models. The paper that I linked above is state of the art in heterotic phenomenology, the oldest type of superstring phenomenology (dating back to 1985).

 

[yoda jedi]

can string theory ever be proven wrong? will or do we have the equipment to find out?

maybe, easily, string theory is constructed on standard quantum mechanics (a linear theory) if quantum mechanics is proved to be nonlinear string theory it will fall so quickly, same thing for loop theory.

read:
posts 4 and 12
https://www.physicsforums.com/showthread.php?t=452912

and

https://www.physicsforums.com/showthread.php?p=3006265#post3006265
post 3-5-7-11
.