Why is string theory on the verge of collapse?

[zewpals]

The question I want to ask is pretty much in the title: Why is string theory becoming less of an excitement amongst the scientific community?

I don't want to sound picky, but my background in physics is pretty inferior. I understand that the explanation is mainly in the mathematics, so, just to give a quick background, I've taken first year physics in college and have a pretty minor background in cosmology. In mathematics I've gotten no further than multivariable calculus so far. Please answer accordingly =D. Thanks!

EDIT:

Marcus, your post was EXTREMELY helpful. Thank you very much for your input :D.

For the rest of you: When I posted my minor background in science and math I did not intend for you guys to treat me like a half-brained nitwit. Some of you directly criticized me for asking a question and some of you provided responses that were nothing more than "do some research". Inducting that I am (and I quote) "brainwashed" and that I asked this as if I was "stating fact" is absurd logic. I am not here to be flamed at, I am here to learn. Simply stating "only dumb people think this" is naive and uncalled for. Modern application of string theory is substantially smaller than Newtonian physics and Einstein's Relativity, so it is very viable to question the theory.

I really am looking for input that is at the very least honorable, not to mention insightful, which is what I was originally going for.

I refuse to address each insufficient response (and sorry for the few of you that were helpful) out of sheer laziness.

Instead, I will rephrase the question:

Why have I observed many respectable scientists with PhD's in physics or astronomy personally telling me that they see string theory as an improbable theory? Why does a portion of the scientific community not invest their beliefs in string theory?

What is the science behind this rebuttal? I am not looking for a history lecture. If you cannot think of a scientific answer, then I ask that you do not contribute. Any posts that scientifically explain why string theory is valid or invalid is fine by me. The topic is, though, why are strings seen as an scientific improbability by some scientists?

 

[atyy]

It is not. Anyone interested in quantum gravity must study string theory.

 

[Demystifier]

Why is string theory becoming less of an excitement amongst the scientific community?

Because it failed to make a mayor breakthrough around 2005.

Let me explain:
- Around 1975, it was realized that string theory has o potential of unifying all interactins, including gravity. It was really a breakthrough.
- Around 1985, the first string-theory revolution happened by realizing that certain superstring theories are anomaly free.
- Around 1995, the second string-theory revolution happened by realizing that all these anomaly free superstring theories seem to be special cases of a single theory (M-theory).
- By induction, it was naturally expected that a new breakthrough should happen around 2005. Unfortunately (or fortunately, for those who do not like string theory) this has not happened.

 

[bapowell]

I agree with atyy that string theory is an integral part of modern attempts to quantize gravity, and should be studied accordingly. The most common criticisms of string theory stem from its lack of falsifiable predictions, and the related lack of a unique solution to the string theory equations of motion. The lack of a unique solution stems from the fact that string theory predicts a 10-dimensional universe, but has not yet yielded much insight into the form of these extra dimensions (notably their geometry). Different geometries give different physics -- different particle species with different properties -- with our universe just one of the many possible realizations. Just how our universe is 'selected' out of this (very very large) set is currently unknown, and has been the target of much criticism in recent times.

 

[Demystifier]

It is not. Anyone interested in quantum gravity must study string theory.

Tell it to the LQG people. :tongue2:

 

[bapowell]

Because it failed to make a mayor breakthrough around 2005.

Let me explain:
- Around 1975, it was realized that string theory has o potential of unifying all interactins, including gravity. It was really a breakthrough.
- Around 1985, the first string-theory revolution happened by realizing that certain superstring theories are anomaly free.
- Around 1995, the second string-theory revolution happened by realizing that all these anomaly free superstring theories seem to be special cases of a single theory (M-theory).
- By induction, it was naturally expected that a new breakthrough should happen around 2005. Unfortunately (or fortunately, for those who do not like string theory) this has not happened.

By induction? I didn't realize that scientific breakthroughs in the field of string theory followed a 10 year cycle.

 

[Finbar]

It is not. Anyone interested in quantum gravity must study string theory.

why?

I don't see why I must study string theory.

Part of the problem is that not enough young string theorists are studying quantum field theory. Studying established physics is far more useful to understand quantum gravity than studying strings.

 

[atyy]

Tell it to the LQG people. :tongue2:

The best of them do

http://math.ucr.edu/home/baez/numbers/24.pdf

 

[Demystifier]

By induction? I didn't realize that scientific breakthroughs in the field of string theory followed a 10 year cycle.

It was a joke, of course. But like for any good joke, there is some truth in it.

 

[Demystifier]

The best of them do

http://math.ucr.edu/home/baez/numbers/24.pdf

Maybe some guys (such as Rovelli) would not agree that Baez is the best, but I think it is safe to say that Baez has the best sense for scientific humor.

 

[MTd2]

John Baez doesn't work on LQG anymore or any quantum gravity. After week 300 (the actual is 299) he will shift his interests to environmental issues. He will have a blog for that.

But he still assists people interested on QG. His student John Huerta has been publishing nice things on this.

 

[George Jones]

It was a joke, of course. But like for any good joke, there is some truth in it.

Something I posted a few years ago:

https://www.physicsforums.com/showthread.php?p=998390#post998390.

 

[suprised]

The question I want to ask is pretty much in the title: Why is string theory becoming less of an excitement amongst the scientific community?

How come you state this as a fact? Perhaps because you are brainwashed by the ideological climate here, a truly amazing case of group think and wishful/hateful thinking? Where sense and nonsense is mixed up beyond any recognition? It represents amateurs and few, if at all, serious professional scientists.

Just have a look at hep-th where the current research keeps to be focused on for very good reasons; there is no collapse in sight (I remember that already years ago people counted here papers in order to fabricate evidence for such a decline... nope. Now they count talks at conferences to painstakingly identify some decline...they want it sooo bad... it is nothing but silly. It just won't happen).

Or do you seriously believe that amateurs and armchair scientists would have any better insight into things that need many years of hard work to learn? There is no way that you can form a qualified opinion by reading those opinions expressed here. It is a pure waste of time.

 

[BenTheMan]

- By induction, it was naturally expected that a new breakthrough should happen around 2005. Unfortunately (or fortunately, for those who do not like string theory) this has not happened.

AdS/CFT just came too early.

 

[BenTheMan]

The question I want to ask is pretty much in the title: Why is string theory becoming less of an excitement amongst the scientific community?

I think you should stop getting all of your physics news from Marcus and pop science magazines.

Science forums cranks (i.e. he-who-will-not-be-named) and pop science magazine editors who realize that ``underdog'' stories sell more are the only people that think string theory is on the ``verge of collapse''.

 

[humanino]

Demystifier, there was at least one major breakthrough in 2003 with the twistor string. In case you missed it, Witten provided momentum which ranged all the way to QCD background calculation for LHC.

 

[Demystifier]

AdS/CFT just came too early.

Actually, maybe it is AdS/CFT that destroyed string theory (from the inside), because it turned out that AdS/CFT makes sense even without string theory, so many string theorists suddenly started to do AdS/CFT without doing string theory.

But then again, some say that whatever they do, they call it string theory. For example, strings do not play a fundamental role in M-theory, but they still call it string theory. In this way, string theory cannot collapse; it can only change its form.

 

[marcus]

The question I want to ask is pretty much in the title: Why is string theory becoming less of an excitement amongst the scientific community?

Well, you asked two quite different questions:

a) Why is string theory on the verge of collapse?
b) Why is string theory becoming less of an excitement?

There still are lots of stringy papers being written, although they tend now to be by the less prominent people and they tend to be less cited. Current papers don't get as much notice by fellow string researchers, and don't get cited as references as much as, say, pre-2005 papers.

In terms of raw numbers, to gauge the research output and get an idea of the string workforce actively writing papers, here is a link:
https://www.physicsforums.com/showthread.php?p=2802820#post2802820
Part of that post discusses the changed rate of peer-reviewed research publication. Another part of the post tracks popular book sales over several years.

So in raw numbers there may be some decline but it is not a "collapse". So Atyy is right about that.

I'm not sure that someone who wants to be an expert in Quantum Gravity and get a faculty job actually needs to learn string though. I've noticed lots of hires in LQG recently, compared with the (small) size of the existing loop research community. There may be some math ideas which are not the sole property of string theorists which it is good to pick up---techniques which string folks might think of as stringy but which aren't especially.

But I agree with Atyy in not seeing an imminent "collapse". For one thing there are too many people with careers invested in the field for it to collapse.

================================
I think the real question you are asking is b).
Why is string theory becoming less of an excitement?

I think that question is quite interesting. There are several factors. I can't say what the single most important factor is. Maybe some are not causes but symptoms. I will mention a few.

1. Leaders like Witten, Horava, Verlinde, Arkani-Hamed, Steinhardt... getting interested in other stuff, or wavering about whether string is the right way to go.
2. The "landscape"---huge number of string vacua. Unsuccessful search since 2003 for selection principle. Exclusion of "anthropic landscape" people from giving talks at String 2008. Rejection of landscape-ology by important sectors of string community.
3. Tendency of talks at annual String conference to be about application of string-inspired math tools to other physics---like modeling nuclear/condensed matter physics---not fundamental/unification. Less enthusiasm for the Grail and more straying around looking for something to do with the techniques.
4. New astro instruments put the focus on cosmology---which is a Loop strength. Loop quantum cosmology (Lqc) has models of the big bang from which predictions can be derived and tested with instruments either planned or already in orbit. Cosmology has drawn attention to Lqc, where one can calculate stuff, run computer models of the universe with various simple sorts of matter etc, and there has been rapid growth in the number of Lqc papers.
5. Judgement by prominent string theorists such as Hermann Nicolai that "string phenomenology has become increasingly Baroque, if you follow the literature." Said with some distaste at the XXV Max Born conference in 2009.

These are just ideas, often involving a subjective element, and people will no doubt disagree---or give different reasons.
You asked a rather subjective, but significant, questions: "why has excitement diminished?"
It has diminished. The excitement, conviction, dedication, energy is very different now from what it was in 2000 or even as recently as 2005. I can only offer some guesses. Maybe others can say something more definite.

 

[MTd2]

I don't know about a decline of a string theory. But if anyone take a look at the non QG papers, at least the ones listed on this thread https://www.physicsforums.com/showthread.php?t=7245&page=62

will see that the number of papers is increasingly fast. We usually cannot follow the number of papers posted daily and miss many.

 

[Demystifier]

Demystifier, there was at least one major breakthrough in 2003 with the twistor string. In case you missed it, Witten provided momentum which ranged all the way to QCD background calculation for LHC.

I must admit, I never heard about that. But isn't it an indication that this is not SUCH a big breakthrough, as compared with those I mentioned?

Anyway, I would appreciate a link to the mayor publication on this stuff. (Not only to see what is this about, but also to see the number of citations it received, which can be taken as a measure of breakthroughness.)

 

[MTd2]

Actually, maybe it is AdS/CFT that destroyed string theory (from the inside), because it turned out that AdS/CFT makes sense even without string theory, so many string theorists suddenly started to do AdS/CFT without doing string theory.

AdS/CFT is basically due t'Hooft from the late 70's. The actual approach is also due to t'Hooft, published a few months before Maldacena, but it didn't include supegravity as an example, if I am not mistaken.

 

[MTd2]

Demystifier, there was at least one major breakthrough in 2003 with the twistor string. In case you missed it, Witten provided momentum which ranged all the way to QCD background calculation for LHC.

Twistor string is a weird thing, given that it comes from topological strings, not the string theory itself, but it got its usefulness to calculate SYM N=4 and SUGRA N=8 (possibly) in 4d. But didn't have an important impact on the mainstream string theory, so it is a bit of an outcast.

There are several of this small mathematical hits along the years, with a similar impact of twistor strings, but nothing like the other "string revolutions".

 

[Demystifier]

3. Tendency of talks at annual String conference to be about application of string-inspired math tools to other physics---like modeling nuclear/condensed matter physics---not fundamental/unification. Less enthusiasm for the Grail and more straying around looking for something to do with the techniques.

Yes, this is similar to my first suggestion in #17.

 

[marcus]

Demystifier, I wrote without seeing intervening posts such as your #17, which makes the my general point but with detailed insightfulness.

I also think there is some substance to your partly humorous post about the disappointed expectation of revolution.

Because it failed to make a mayor breakthrough around 2005.

Let me explain:
- Around 1975, it was realized that string theory has o potential of unifying all interactins, including gravity. It was really a breakthrough.
- Around 1985, the first string-theory revolution happened by realizing that certain superstring theories are anomaly free.
- Around 1995, the second string-theory revolution happened by realizing that all these anomaly free superstring theories seem to be special cases of a single theory (M-theory).
- By induction, it was naturally expected that a new breakthrough should happen around 2005. Unfortunately (or fortunately, for those who do not like string theory) this has not happened.

By induction? I didn't realize that scientific breakthroughs in the field of string theory followed a 10 year cycle.

A good window on this dynamic at the level of expectations is the Toronto Strings 2005 Panel Discussion
which was called THE NEXT SUPERSTRING REVOLUTION.
http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/panel.html
Moderator:
Steve Shenker (Stanford)
Panelists:
Raphael Bousso (UC Berkeley)
Shamit Kachru (SLAC & Stanford)
Ashok Sen (Harish-Chandra Research Institute)
Juan Maldacena (IAS, Princeton)
Andrew Strominger (Harvard)
Joseph Polchinski (KITP & UC Santa Barbara)
Eva Silverstein (SLAC & Stanford)
Nathan Seiberg (IAS, Princeton)

What a list of STARS. The rising generation of young leaders as of 2005.
We can watch the video and see what this brilliant and creative group of young people had to say about what could be the revolution.

So we see that the panel discussion of the revolution came exactly according to Demystifier's schedule, in 2005. But the revolution itself did not arrive on time.

 

[BenTheMan]

Actually, maybe it is AdS/CFT that destroyed string theory (from the inside), because it turned out that AdS/CFT makes sense even without string theory, so many string theorists suddenly started to do AdS/CFT without doing string theory.

I don't know...I think ``AdS/CFT makes sense without string theory'' is kind of a misleading statement. It makes sense if you assume the presence of a certain set of internal symmetries.

If you're willing to accept[\I] the presence of internal symmetries, then why are you doing string theory?

Where else do the internal symmetries come from, other than by ansatz?

 

[rod_worth]

My more emotionally-driven analogy is that it's like a modern theory of epicycles; just modify it until it works!

 

[murray92]

Demystifier, I wrote without seeing intervening posts such as your #17, which makes the my general point but with detailed insightfulness.

I also think there is some substance to your partly humorous post about the disappointed expectation of revolution.



A good window on this dynamic at the level of expectations is the Toronto Strings 2005 Panel Discussion
which was called THE NEXT SUPERSTRING REVOLUTION.
http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/panel.html
Moderator:
Steve Shenker (Stanford)
Panelists:
Raphael Bousso (UC Berkeley)
Shamit Kachru (SLAC & Stanford)
Ashok Sen (Harish-Chandra Research Institute)
Juan Maldacena (IAS, Princeton)
Andrew Strominger (Harvard)
Joseph Polchinski (KITP & UC Santa Barbara)
Eva Silverstein (SLAC & Stanford)
Nathan Seiberg (IAS, Princeton)

What a list of STARS. The rising generation of young leaders as of 2005.
We can watch the video and see what this brilliant and creative group of young people had to say about what could be the revolution.

So we see that the panel discussion of the revolution came exactly according to Demystifier's schedule, in 2005. But the revolution itself did not arrive on time.

Interesting - Polchinski seamed the least enthusiastic...
1:17:18 is this leonard Susskind? saying that MWI is the same as ethernal inflation?

Edit: I also like what Strominger says at 1:27:00 about (not) advocating string theory...

 

[marcus]

Interesting - Polchinski seamed the least enthusiastic...
1:17:18 is this leonard Susskind? saying that MWI is the same as ethernal inflation?

Edit: I also like what Strominger says at 1:27:00 about (not) advocating string theory...

I came away from that admiring and respecting Strominger. He seemed to have integrity and no appetite for hype. It was long ago i watched the video. You are probably right in identifying Susskind as one of the voices speaking from the floor, but I can't check right now.
I know he did speak up but I can't remember what he said.

Maybe someone who has watched it more recently can say.

For anyone who hasn't seen the video: they give something like 8 minutes per, for each of the young panelists to give their vision of the stringy future, and then after about 1 hour they open it up to comment from the audience (which is several hundred string researchers who participated in the Strings 2005 conference.) Witten spoke from the floor. Susskind. Djordje Minic. many others. Shenker moderated. It was a decisive moment in a sense. At one point they had a vote on whether physics was going to be an accidental random choice from among a large landscape of possible versions of physics or whether they wanted to keep trying to find the key to why it was this way. Was the vacuum merely 'environmental' like the accident that there are 9 planets in the solar system instead of 5 or 13. Or did it have to be this way for some reason. Some 400 people had a show of hands on this. Shenker was surprised by the outcome and said *expletive*. Kind of wonderful.

 

[murray92]

I came away from that admiring and respecting Strominger. He seemed to have integrity and no appetite for hype. It was long ago i watched the video. You are probably right in identifying Susskind as one of the voices speaking from the floor, but I can't check right now.
I know he did speak up but I can't remember what he said.

Maybe someone who has watched it more recently can say.

For anyone who hasn't seen the video: they give something like 8-10 minutes to each of the young panelists (to give their vision of the future of string) and then after about 1 hour they open it up to comment from the audience (which is several hundred string researchers who participated in the Strings 2005 conference.) Witten spoke from the floor. Susskind. Djordje Minic. many others. Shenker moderated.

Thanks for your replay...

 

[atyy]

I came away from that admiring and respecting Strominger.

I haven't watched that video, but I like Strominger's point of view very much. To me string theory is important, because even if it eventually turns out to be wrong for our universe, it has given pointers to what the correct theory of quantum gravity is - Strominger and Vafa's derivation of black hole entropy from microscopic degrees of freedom, and Maldacena's AdS/CFT is the best understood version of holography at the moment - both came from string theory, but perhaps neither needs it in general, and this will perhaps lead us toward an understandimng of general properties of quantum gravity, stringy and not.

"One of the deepest discoveries in modern theoretical physics is that of holographic dualities, which relate a quantum theory of gravity to a quantum field theory without gravity in fewer dimensions. These dualities become especially powerful when combined with string theory [1]. It is an occasional misconception, however, that the existence of holographic dualities is contingent on the validity of string theory. This is not the case." http://arxiv.org/abs/0809.4266

"Ever since the seminal work of Bekenstein and Hawking, it has been clear that there is a deep and fundamental relation between gravity, thermodynamics and quantum mechanics, while its detailed form and origin was and is largely mysterious. Much light has been shed on this triangle in the specific context of string theory following the microscopic description of black holes as a finite temperature two dimensional CFT [41] and its higher dimensional generalization to AdS/CFT [42]. It seems likely that the basic triangular relationships transcend string theory and AdS/CFT, although lessons from string theory are likely useful guides for unraveling the more general picture. It is our hope that the attempt here to generalize fluid/gravity duality away from the stringy context to its most essential ingredients may be useful in understanding this triangle." http://arxiv.org/abs/1006.1902

 

[humanino]

"Perturbative gauge theory as a string theory in twistor space"
Commun.Math.Phys.252:189-258,2004
is Witten's most cited article since it was written (~500 citations)

It is also related to "Gravity as the Square of Gauge Theory"

If you ask me what is the most important direction of research in QG, I would have no doubt. After all, this thread can only be speculative.

 

[marcus]

If you ask me what is the most important direction of research in QG, I would have no doubt. After all, this thread can only be speculative.

In that case, let's put up the abstract. Some may be encouraged to look out for new work in that direction, or to investigate for themselves.

http://arxiv.org/abs/hep-th/0312171
http://arxiv.org/cits/hep-th/0312171
Perturbative Gauge Theory As A String Theory In Twistor Space
Edward Witten
(Submitted on 15 Dec 2003)
"Perturbative scattering amplitudes in Yang-Mills theory have many unexpected properties, such as holomorphy of the maximally helicity violating amplitudes. To interpret these results, we Fourier transform the scattering amplitudes from momentum space to twistor space, and argue that the transformed amplitudes are supported on certain holomorphic curves. This in turn is apparently a consequence of an equivalence between the perturbative expansion of $${\cal N}=4$$ super Yang-Mills theory and the D-instanton expansion of a certain string theory, namely the topological B model whose target space is the Calabi-Yau supermanifold $$\Bbb{CP}^{3|4}$$."

 

[MTd2]

Just an observation to the above abstarct: CP^{3|N} is the twistor space, with a string defined with N supercharges. N=4 is the 4 dimensional SYM. But what you see above, gave birth to what is considered by most not really as a physical theory, but as a mathematical tool. It is possible to define Twistors as a kind of fundamental string within a twistor set up, or so it seems, by writing directly a open twistor worldsheet, like berkovits did. Setting N=8, it is conjectured that one gets N=8 supergravity in 4d, that one that may be perturbatively renormalizable and that Marcus likes.

 

[mitchell porter]

zewpals, in string theory, it's the geometry of the extra dimensions which determines how the strings behave. String theorists used to expect or hope that there would be one unique, stable geometry, which would provide specific predictions for the particle masses and interactions, and that in this way the theory would be confirmed or falsified. But in the past decade, the view has been growing that this was naive, and that there are enormous numbers of stable geometries.

This is the one real and consequential change that is relevant to your question, because it weakens the predictive power of string theory. We may end up with one or more string models which match experiment perfectly, but with no deeper explanation as to why the geometry is like that.

Particle physics for a long time was just about keeping up with experiment - it was a triumph just to squeeze all the facts into a single theory, the "standard model". But after that, the quest became to explain the standard model itself - why those particles and not some other particles. A series of deeper and deeper theories were developed and string theory was the culmination. The high point for string theory itself was its internal unification in the 1990s, when the various competing versions of string theory were proven to be just a single super-theory from different perspectives.

But if string theory does not lead to a uniquely preferred geometry, then that takes us back to the way things used to work in particle physics: we are just trying to match the experimental facts. That used to mean, finding a field theory like the standard model that contains all the observed particles. Now it means finding a stable geometry where the strings behave like the observed particles.

There is a huge advance in understanding when you pass from field physics to string physics: it's a whole new physical picture, it explains gravity and the other forces in a very distinctive way, it really is a unique "theory of everything". But the very highest ambition - to explain *everything* about the standard model as somehow inevitable - is in doubt.

The particular string geometry which produces the standard model may be how it is just in this region of the universe, with other very distant regions (far beyond all the observed galaxies) having a completely different geometry and a different effective physics. It is still a very controversial idea. But to me it does sound like the future of physics.

 

[element4]

(Sorry if the question is a little of topic).
I always hear string theorists claim that String Theory is a "unique theory of everything". How should this "unique" be understood?
1) Is this meant poetically (as in unique in its beauty)?
2) Is it a dream of being able to prove that string theory is a unique extension of current theories (under some assumptions)?
3) Some other meaning?

Does this "uniqueness" claim have any rigorous meaning?