# Why is string theory on the verge of collapse?

#### 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...

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#### marcus

Gold Member
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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.

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#### 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

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#### 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

Gold Member
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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

Gold Member
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.

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#### 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

mitchell porter;2806844 (...) it really is a unique "theory of everything". (...) [/QUOTE said:
(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?

#### mitchell porter

There are infinitely many possible field theories because you can choose fields, symmetries, interactions, parameters in infinitely many ways. But string theory has no free parameters. The multiplicity of vacua arises within the configuration space of a single theory. So string theory is unique in two ways: first, there's only one fundamental string theory; second, there's no other theory like it. That's what I meant.

#### element4

There are infinitely many possible field theories because you can choose fields, symmetries, interactions, parameters in infinitely many ways. But string theory has no free parameters. The multiplicity of vacua arises within the configuration space of a single theory. So string theory is unique in two ways: first, there's only one fundamental string theory; second, there's no other theory like it. That's what I meant.
Thank you for the clarification, this is very interesting. So string theory is not a unique theory of everything (whatever than means), there might be other candidates. But there is only one unique string theory! This is of course by itself quit interesting.

#### suprised

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.
In fact, gauge theories and string theory are deeply interwoven and one thing what ADS/CFT provides is a reconstruction of (background independet) string theory from gauge theory, in the large-N limit. So string theory will always come along the ride, no matter how little one likes it, whenever one talks about gauge theory. Thus most likely it will never go away.

And marcus, it is just futile to go through recent talks of people in order to fabricate a decline. String theory is a very wide field with many facets, it has always been that the focus shifts a bit from year to year, right now some more people than usual are interested in gauge amplitudes that can be studied by string and string inspired methods; one should see this a a framework for studying quantum field theories with and without quantum gravity, and no serious resaercher tries to draw a line between string theory and non-string theory, such as you like to do. I seriously think that this is paranoid what you do here, and all effect what it might have is to ruin some young "innocent" people for science. They would be better advised to listen to their professors rather than to clueless spin doctors.

#### BenTheMan

And marcus, it is just futile to go through recent talks of people in order to fabricate a decline. ... They would be better advised to listen to their professors rather than to clueless spin doctors.
These two sentences summarize the bulk of marcus' contributions to physics forums.

#### ensabah6

And marcus, it is just futile to go through recent talks of people in order to fabricate a decline. String theory is a very wide field with many facets, it has always been that the focus shifts a bit from year to year, right now some more people than usual are interested in gauge amplitudes that can be studied by string and string inspired methods; one should see this a a framework for studying quantum field theories with and without quantum gravity, and no serious resaercher tries to draw a line between string theory and non-string theory, such as you like to do. I seriously think that this is paranoid what you do here, and all effect what it might have is to ruin some young "innocent" people for science. They would be better advised to listen to their professors rather than to clueless spin doctors.
String theorists ranging from Lubos Motl and Jacques Distler, to Witten, Polanski, Kaku, have disparaged LQG.

How do you feel about loop gravity and are you suggesting no serious researcher draws the line between string theory and LQG?

Do you think LQG is promising and worthy of high-level investment, faculty hiring, research programs, post docs etc on par with strings?

#### MTd2

Gold Member
Well, coldly thinking, just the 1st one in your list disparaged LQG. The second one even hosts a blog for a person who does LQG...

But, the obvious conclusion of this thread it is that, due to personal attacks, smells like being locked...

#### Naty1

Seems like Marcus, and atyy in post #30, have summed up well...

string theory has already been "deemed dead" several times,

It was originally thought it applied to the strong force....that did not work out so well,
disappointment #1...and some physicsts returned to more traditional particle physics;

then someone discovered a spin 2 particle buried within...WOW the graviton!!! and revolution # 1 was underway.....but equations could not be solved and there were five theories that seemed different...with different answers.... disappointment #2,

Next, Ed Witten to the rescue with M theory!!!....and now the perturbative solutions and incomplete mathematical formulatios still apparently baffle scientists, disappointment # 3....still no real testable predictions..is it "too pretty to fail"????

who knows....but I would no rush to bury string theory....

Maybe it's analogous to asking "If quantum theory predicts a cosmological constant 120 orders of magnitude greater than the observed value, is it on the verge of collapse?."

#### MTd2

Gold Member
Maybe it's analogous to asking "If quantum theory predicts a cosmological constant 120 orders of magnitude greater than the observed value, is it on the verge of collapse?."
No, that's because you've got asymptotic safety!

Gold Member
Dearly Missed

#### BenTheMan

Seems like Marcus, and atyy in post #30, have summed up well...

string theory has already been "deemed dead" several times,

It was originally thought it applied to the strong force....that did not work out so well,
disappointment #1...and some physicsts returned to more traditional particle physics;

then someone discovered a spin 2 particle buried within...WOW the graviton!!! and revolution # 1 was underway.....but equations could not be solved and there were five theories that seemed different...with different answers.... disappointment #2,

Next, Ed Witten to the rescue with M theory!!!....and now the perturbative solutions and incomplete mathematical formulatios still apparently baffle scientists, disappointment # 3....still no real testable predictions..is it "too pretty to fail"????

who knows....but I would no rush to bury string theory....

Maybe it's analogous to asking "If quantum theory predicts a cosmological constant 120 orders of magnitude greater than the observed value, is it on the verge of collapse?."

Fail.

#### Dav333

Just curious, I have heard some on Discovery type documentaries call ST science fiction. Isn't this an insult???

#### suprised

What are professors telling students re: string theory?
In my institution, we tell them: be prepared to work hard for many years, and there are more students willing to do so than can be supervised.

How do you feel about loop gravity and are you suggesting no serious researcher draws the line between string theory and LQG?

Do you think LQG is promising and worthy of high-level investment, faculty hiring, research programs, post docs etc on par with strings?

Yes, a serious reseacher would draw no line between strings and LQG, in fact strings as we know them today, are likely not the complete story, ie an off-shell or backgrond-independent “topological phase” might be underneath and ordinary backgrounds would emerge from an analog of spontaneous symmetry breaking. This underlying theory might be similar in spirit to what the LQG people aim for.

So I think the viewpoint of strings and LQG as competing alternatives is artificial, created by people who like to polarize the field and grab more attention to their work than it deserves. My bet, based on the history of the last 15 years, would be that all what makes sense and is physically consistent
would ultimately fit together in a big picture. String theory will certainly be a part of it, at least since it can be reconstructed from gauge theory as said above.

As for LQG in the strict sense, it didn’t get very far after 20+ years of research; there is not just one emergent theory but many different attempts, which just shows that not even a good starting point has been identified; none of which works convincly so far, most of it are hopes and promises (for example, that it describes 4d gravity). Actually we are at odds why LQG is mentioned in the same sentence as string theory, as if it would be in any way an alternative program, either in scope or in achievements. It is a field with much less ambition to begin with, namely to describe gravity, and as far as unification with particle physcis is concerned, it seems still in its early infancy, to say it politely. That’s why most colleagues don’t find it appealing and promising to work on it. If they would find otherwise, they’d work on it, it is as simple as that.

Well all of this and more has been repeated many times over and over, it doesn’t make any sense to repeat it again, especially in front of people who dont want to hear it.

#### MTd2

Gold Member
One has many paths leading to theories that cannot remotely possibly shown to describe the world during their lifetimes. This awfully looks like knowing which interpretation of a holy text is correct. So, it is a subjective choice of what path feels like more *real*. Just like new converts to a religion, people will go to wherever they feel right and go to whatever looks more stable, will fight and for land (fundings) and even more converts.

But for the sake of the novelty, I really enjoy new stories. It is easy to get tired with Star(String) Wars sequels, cartoons, expanded universe etc, if you really are no fan of it.

#### BenTheMan

It is a field with much less ambition to begin with, namely to describe gravity, and as far as unification with particle physcis is concerned, it seems still in its early infancy, to say it politely. That’s why most colleagues don’t find it appealing and promising to work on it. If they would find otherwise, they’d work on it, it is as simple as that.
This is the overwhelming sentiment of every grad student/post doc I have ever talked to about this issue.

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