Europe's most influential string theorist has an overview piece in Nature

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  • #1
marcus
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The current (18 October) issue of Nature
http://www.nature.com/nature/journal/v449/n7164/index.html
in the "News and Views" section has this piece by Hermann Nicolai

String theory: Back to basics p797
Hermann Nicolai
Summary: "Long touted as a theory of everything, it seems that string theory may at last succeed as a theory of something very specific — the interactions of particles under the strong nuclear force."

Hermann Nicolai strikes me as a modest person who would find it awkward to think of himself as Europe's most influential string theorist. :smile: But I think that is simply how it is. He is the Director of the Unified Theories wing of the Albert Einstein Institute at MPI Potsdam.
http://www.aei.mpg.de/english/contemporaryIssues/home/index.html[/URL]
He and Roy Maartens are chief co-editors of the journal GRG. He partnered with Abhay Ashtekar in organizing the October 2004 [B]Strings Meet Loops[/B] conference at the Potsdam Max Planck Institute (AEI).
He is a key person on the directorate of the European Science Foundation Quantum Geometry/Gravity support network. And I'm sure I'm just scratching the surface.

The fate of a lot of research depends on his judgment---so his vision of how things are going is part of the fundamental physics research climate.

I remember Urs Schreiber reporting from some conference in Germany in 2004 and it was clear that for him (the smartest postdoc string theorist in the String Coffetable lineup) his conversation with Nicolai was the most important chat he had at the conference.

If anybody can think of a Euro string-person of equal leadership standing, or wants to correct what I said, please say. Tell me somebody else. This is just my impression from observing the scene for a few years.

I can't read Nicolai's opinion/perspective piece in this week's Nature, because i haven't a subscription. I'd be glad to see exerpts from it, if anybody has a subscription and wants to share. It could very well not be earthshaking at all, just a measured balanced mainstream Euro-string viewpoint. I don't care if there is something new in it or not--I'm interested to hear anything overview H.N. says.
 
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  • #2
josh1
...string theory may at last succeed as a theory of...the interactions of particles under the strong nuclear force."
What you're referring to is known as the AdS/QCD correspondence. The idea is to find a dual description of QCD. Just another great reason to pursue string theory. It’s tentacles are everywhere.
 
  • #3
arivero
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. The idea is to find a dual description of QCD. Just another great reason to pursue string theory. It’s tentacles are everywhere.
well, the problem is that it sounds as "lets use general relativity to explain particle theory". I can understand that a mathematical theory can be applied in different fields, and I can understand that a physical framework as Lagrangian Mechanics or Fluid Dinamics or, even, Quantum Field Theory, can be applied in different problems. But this upgrade of string theory towars a basic mathphys framework, it is strange. See, nobody says "Lagrange Mechanics is the true theory of planetary trajectories". They say "we can use Langrange Mechanics to describe planetary trajectories".

The point is that string theory is very unique: 10 dim, some aspects of a unique M-theory, etc... it is very surprising to have a theory claimed to be so rigid and so flexible at the same time.
 
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well, the problem is that it sounds as "lets use general relativity to explain particle theory".
That's what's so great about it.
 
  • #5
marcus
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Has anybody read the paper?
 
  • #6
arivero
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That's what's so great about it.
Really? Most newgroups are filled of posts with the same claim.
 
  • #7
josh1
well, the problem is that it sounds as "lets use general relativity to explain particle theory".
If you accept that a purely gravitational theory can have a holographic description in terms of a pure Yang-Mills theory, the possibility of an AdS/QCD correspondence shouldn't be too shocking.

The point is that string theory is very unique: 10 dim, some aspects of a unique M-theory, etc... it is very surprising to have a theory claimed to be so rigid and so flexible at the same time.
Not “flexible”, “powerful”: A theory of everything virtually by definition should be both rigid and comprehensive.
 
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  • #8
arivero
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Marcus, I read the paid content, and it does not go beyond the abstract: it is basically a reference list of recent results.

If you accept that a purely gravitational theory can have a holographic description in terms of a pure Yang-Mills theory, the possibility of aAnd n AdS/QCD correspondence shouldn't be too shocking.
And if you accept that Hooke law relates distance to force, it is not shocking to notice that it generates elliptical orbits. But it fails to meet Kepler laws.

String theory uses the mathematics of fields and forces, with an extra emphasis on unique or peculiar structures, so it keeps finding some results relevant to fields and forces and peculiar structures. It is not shocking, it is so elementary that it is almost irrelevant. Following along the lines of Hooke laws, if you expand any classical potential and it happens that the linear term can be ignored, then you have the quadratic term and it produces Hooke law and then the harmonic oscillator. But we do not start crying the harmonic oscilator is a theory of everything.
 
  • #9
josh1
And if you accept that Hooke law relates distance to force, it is not shocking to notice that it generates elliptical orbits. But it fails to meet Kepler laws.
This is nonsense. Hooke's and Keplers laws are really just two different applications of Newtonian physics to two completely different physical systems: There is no Hooke/Kepler duality. The gauge/gravity correspondence says that a purely gravitational system can have a precise holographic dual in the form of a system without gravity. They are thus different descriptions of the same system.
 
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  • #10
arivero
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  • #11
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Has anybody read the paper?
Some excerpts follow.

Whether string theory can live up to its claim
of being a ‘theory of everything’, and whether
it will ever produce a falsifiable prediction as
such, remain hotly debated questions. Mean-
while, developments in a quieter side-alley1–8
indicate that the theory might be about to
deliver something of its original promise: help-
ing us to understand the physics of interactions
mediated by the strong nuclear force.
String theory was born in the 1960s, (...)
But initial attempts to describe the
forces between the quarks, and why they form
the bound states they do, failed miserably.
So particle physicists started casting around
for other ways of attacking the problem. In
1968, the Italian theoretician Gabriele Vene-
ziano made a brilliant guess9 and wrote down
a concrete mathematical expression, the Vene-
ziano amplitude, that explained some impor-
tant features of high-energy scattering. But
his formula could not be understood in terms
of point-like particles; instead, it required the
existence of extended objects — strings. (...)
The arrival in the early 1970s of quantum
chromodynamics (QCD), the quantum-field
theory of the strong interaction, dealt the final
blow to these early attempts to understand
nuclear physics in terms of string theory. But,
unfortunately, QCD is incredibly complex. (...)
In this ‘perturbative’ regime, we understand (at least in principle)
how to work with QCD. But for the strong cou-
pling that occurs over larger distances, one has
to resort to computer-simulation techniques,
known as lattice QCD. (...)
The new approach that revives the link to
string theory first suggested itself in 1998,
when Juan Martín Maldacena conjectured12
a link between a close relative of QCD and a
‘superstring’ living in a ten-dimensional curved
space-time. (...)
The Maldacena conjecture raised a lot
of interest, but seemed for a long time to be
quantitatively unverifiable. (...)
Help came from an entirely unexpected
direction. Following a prescient observa-
tion13, the spectrum of the N = 4 theory has
been found1,2 to be equivalently described
by a quantum-mechanical spin chain of a
type discovered by Hans Bethe in 1931 when
modelling certain metallic systems. (...)
Indeed, even though the
mathematical description of the duality on
the string-theory side is completely differ-
ent from that on the condensed-matter side,
a very similar, exactly solvable structure has
been identified here as well3–5.
Puzzling out the details of the exact solution
is currently an active field of research. (...)
Just recently, Beisert, Eden and Staudacher8 have
extracted the analogue of this observable on
the field-theory side, and have been able to
write down an equation valid at any strength of
the coupling. Since then, work has established
that their ‘BES equation’ does indeed seem, for
the first time, to offer a means of reformulating
theories such as QCD as string theories.
Much still needs to be learned from this
one exactly solvable case. There is justifiable
hope that this solution will teach us how to go
back to the physically relevant case of QCD
and finally arrive at the long-sought dual
description by a string theory. It may even
take us closer to realizing the quantum-field
theorist’s ultimate dream, unfulfilled for more
than 50 years: completely understanding an
interacting relativistic quantum-field theory
in the four space-time dimensions that we are
familiar with. Progress towards this goal can
be judged independently of loftier attempts to
use strings in the construction of a theory of
everything.
Hermann Nicolai is at the Max-Planck-Institut
für Gravitationsphysik (Albert-Einstein-Institut),
Mühlenberg 1, D-14476 Potsdam, Germany.

1. Minahan, J. A. & Zarembo, K. J. High Energy Phys. 0303,
013 (2003).
2. Beisert, N., Kristjansen, C. & Staudacher, M. Nucl. Phys. B
664, 131–184 (2003).
3. Bena, I., Polchinski, J. & Roiban, R. Phys. Rev. D 69, 046002
(2004).
4. Kazakov, V. A., Marshakov, A., Minahan, J. A. & Zarembo, K.
J. High Energy Phys. 0405, 024 (2004).
5. Arutyunov, G., Frolov, S. & Staudacher, M. J. High Energy
Phys. 0410, 016 (2004).
6. Gubser, S. S., Klebanov, I. R. & Polyakov, A. M. Nucl. Phys. B
636, 99–114 (2002).
7. Frolov, S. & Tseytlin, A. A. J. High Energy Phys. 0206, 007
(2002).
8. Beisert, N., Eden, B. & Staudacher, M. J. Stat. Mech. P01021
(2007).
9. Veneziano, G. Nuovo Cimento 57A, 190 (1968).
10. Ramond, P. Phys. Rev. D 3, 2415–2418 (1971).
11. Neveu, A. & Schwarz, J. H. Nucl. Phys. B 31, 86–112
(1971).
12. Maldacena, J. M. Adv. Theor. Math. Phys. 2, 231–252
(1998).
13. Lipatov, L. N. preprint available at www.arxiv.org/abs/ [Broken]
hep-th/9311037 (1993).
14. Zaanen, J. Nature 448, 1000–1001 (2007).
NATURE|Vol 449|18 October 2007NEWS & VIEWS
 
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  • #12
marcus
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Thank you Christine, that was kind of you!
With someone like Nicolai, I really want to see exactly what he says and what he does NOT say---get a sense of the wind and current (as he may feel them).
 
  • #13
josh1
That was the idea.
What I meant was - and I think you knew this - that your analogy was nonsense. My point still stands.
 
  • #14
josh1
With someone like Nicolai, I really want to see exactly what he says and what he does NOT say---get a sense of the wind and current (as he may feel them).
It's great to see you're changing your tune about string theory. Am I misinterpreting your remarks? Perhaps for you, the royal road to strings as a theory of gravity is through what it says about the other interactions. Does lqg say anything interesting about QCD?
 
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  • #15
marcus
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I'm actually a fan of Nicolai's, for reasons I won't enumerate at the moment. I am going to do some manicure on the exerpt that Christine gave us, so as to make it more readable.

==exerpts from 18 October issue of Nature==

Whether string theory can live up to its claim of being a ‘theory of everything’, and whether it will ever produce a falsifiable prediction as such, remain hotly debated questions. Meanwhile, developments in a quieter side-alley[1–8] indicate that the theory might be about to deliver something of its original promise: helping us to understand the physics of interactions mediated by the strong nuclear force. String theory was born in the 1960s, (...)

But initial attempts to describe the forces between the quarks, and why they form the bound states they do, failed miserably. So particle physicists started casting around for other ways of attacking the problem. In 1968, the Italian theoretician Gabriele Veneziano made a brilliant guess [9] and wrote down a concrete mathematical expression, the Veneziano amplitude, that explained some important features of high-energy scattering. But his formula could not be understood in terms of point-like particles; instead, it required the existence of extended objects — strings. (...)

The arrival in the early 1970s of quantum chromodynamics (QCD), the quantum-field theory of the strong interaction, dealt the final blow to these early attempts to understand nuclear physics in terms of string theory. But, unfortunately, QCD is incredibly complex. (...) In this ‘perturbative’ regime, we understand (at least in principle) how to work with QCD. But for the strong coupling that occurs over larger distances, one has to resort to computer-simulation techniques, known as lattice QCD. (...)

The new approach that revives the link to string theory first suggested itself in 1998, when Juan Martín Maldacena conjectured[12] a link between a close relative of QCD and a ‘superstring’ living in a ten-dimensional curved space-time. (...) The Maldacena conjecture raised a lot of interest, but seemed for a long time to be quantitatively unverifiable. (...)

Help came from an entirely unexpected direction. Following a prescient observation[13], the spectrum of the N = 4 theory has been found[1,2] to be equivalently described by a quantum-mechanical spin chain of a type discovered by Hans Bethe in 1931 when modelling certain metallic systems. (...) Indeed, even though the mathematical description of the duality on the string-theory side is completely different from that on the condensed-matter side, a very similar, exactly solvable structure has been identified here as well[3–5]. Puzzling out the details of the exact solution is currently an active field of research. (...)

Just recently, Beisert, Eden and Staudacher[8] have extracted the analogue of this observable on the field-theory side, and have been able to write down an equation valid at any strength of the coupling. Since then, work has established that their ‘BES equation’ does indeed seem, for the first time, to offer a means of reformulating theories such as QCD as string theories. Much still needs to be learned from this one exactly solvable case. There is justifiable hope that this solution will teach us how to go back to the physically relevant case of QCD and finally arrive at the long-sought dual description by a string theory. It may even take us closer to realizing the quantum-field theorist’s ultimate dream, unfulfilled for more than 50 years: completely understanding an interacting relativistic quantum-field theory in the four space-time dimensions that we are familiar with. Progress towards this goal can be judged independently of loftier attempts to use strings in the construction of a theory of everything.

Hermann Nicolai is at the Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Mühlenberg 1, D-14476 Potsdam, Germany.

1. Minahan, J. A. & Zarembo, K. J. High Energy Phys. 0303, 013 (2003).

2. Beisert, N., Kristjansen, C. & Staudacher, M. Nucl. Phys. B 664, 131–184 (2003).

3. Bena, I., Polchinski, J. & Roiban, R. Phys. Rev. D 69, 046002 (2004).

4. Kazakov, V. A., Marshakov, A., Minahan, J. A. & Zarembo, K. J. High Energy Phys. 0405, 024 (2004).

5. Arutyunov, G., Frolov, S. & Staudacher, M. J. High Energy Phys. 0410, 016 (2004).

6. Gubser, S. S., Klebanov, I. R. & Polyakov, A. M. Nucl. Phys. B 636, 99–114 (2002).

7. Frolov, S. & Tseytlin, A. A. J. High Energy Phys. 0206, 007 (2002).

8. Beisert, N., Eden, B. & Staudacher, M. J. Stat. Mech. P01021 (2007).

9. Veneziano, G. Nuovo Cimento 57A, 190 (1968).

10. Ramond, P. Phys. Rev. D 3, 2415–2418 (1971).

11. Neveu, A. & Schwarz, J. H. Nucl. Phys. B 31, 86–112 (1971).

12. Maldacena, J. M. Adv. Theor. Math. Phys. 2, 231–252 (1998).

13. Lipatov, L. N. preprint available at www.arxiv.org/abs/hep-th/9311037 (1993).

14. Zaanen, J. Nature 448, 1000–1001 (2007).

NATURE|Vol 449|18 October 2007NEWS & VIEWS
==endquote==

Most of Nicolai's references are to papers already several years old. There is one 2007 paper that plays a pivotal role in what he has to say, the BES. I will put it here for convenience of anyone who wants to check it out as well:

http://arxiv.org/abs/hep-th/0610251
Transcendentality and Crossing
Niklas Beisert, Burkhard Eden, Matthias Staudacher
31 pages
(Submitted on 23 Oct 2006 (v1), last revised 14 Nov 2006 (this version, v2))

"We discuss possible phase factors for the S-matrix of planar N=4 gauge theory, leading to modifications at four-loop order as compared to an earlier proposal. While these result in a four-loop breakdown of perturbative BMN-scaling, Kotikov-Lipatov transcendentality in the universal scaling function for large-spin twist operators may be preserved. One particularly natural choice, unique up to one constant, modifies the overall contribution of all terms containing odd zeta functions in the earlier proposed scaling function based on a trivial phase. Excitingly, we present evidence that this choice is non-perturbatively related to a recently conjectured crossing-symmetric phase factor for perturbative string theory on AdS5xS5 once the constant is fixed to a particular value. Our proposal, if true, might therefore resolve the long-standing AdS/CFT discrepancies between gauge and string theory."

Notice where the guys are from:

Niklas Beisert and Matthias Staudacher
Max-Planck-Institut für Gravitationsphysik
Albert-Einstein-Institut
Potsdam, Germany

Burkhard Eden
Institute for Theoretical Physics and Spinoza Institute
Utrecht University
Utrecht, The Netherlands

In Nicolai's perspective, it happens that research from Potsdam (AEI) and Utrecht plays an important role. I find it an interesting coincidence that Potsdam and Utrecht are like Perimeter Institute in being strong in non-string QG as well as string research. They are places where string and non-string QG researchers work in neighboring offices, chat in the coffeeroom and can easily attend each other's seminars. One group is not frozen out by the other. Grad students have a choice. That is not how it typically is in the US. Another thing I notice is the relative modesty of the claims that Nicolai starts with. I may be talking here about a kind of external style that connects with an internal way of thinking. Here's an illustration:

==sample==
String theory: Back to basics
Long touted as a theory of everything, it seems that string theory may at last succeed as a theory of something very specific — the interactions of particles under the strong nuclear force.

Whether string theory can live up to its claim of being a ‘theory of everything’, and whether it will ever produce a falsifiable prediction as such, remain hotly debated questions. Meanwhile, developments in a quieter side-alley...
==endquote==
 
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  • #16
arivero
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What I meant was - and I think you knew this - that your analogy was nonsense. My point still stands.
Yes I know, but had you resisted the pun, in my position?
 
  • #17
arivero
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In any case, there is not such a thing as gauge/gravity correspondence in a general setting; it is only that you can hope to recover N=4 sQCD or N=2 or some similar beast, and probably it is because it already shares a lot of mathematical structure with string theory.

Still, suppose it holds. Would you like to claim that N=4sQCD is the right theory of quantum gravity? Duality is a double edged sword.
 
  • #18
josh1
In any case, there is not such a thing as gauge/gravity correspondence in a general setting; it is only that you can hope to recover N=4 sQCD or N=2 or some similar beast, and probably it is because it already shares a lot of mathematical structure with string theory.

Still, suppose it holds. Would you like to claim that N=4sQCD is the right theory of quantum gravity? Duality is a double edged sword.
I too doubt what nicolai is doing will turn out to have anything to do directly with gravity. But who knows? It's just an interesting application of some of the ideas that came out of string theory research.
 
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  • #19
arivero
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It's just an interesting application of some of the ideas that came out of string theory research.
Well, large N QCD came along string theory, not out of string theory research.

Perhaps most of the theological problem come from the continuous interaction between a theory of extended relativistic objects (call it Brane theory) and a theory of quantized relativistic superstrings (string theory). The later is a very exclusive theory, with objects in 10 dimensions and with only a few uncompactifyed models. The later is not exclusive, it includes non quantized objects (eg 1-branes in 3,4,6 dimensions) and even can be extended to be understood as an approximation.
 
  • #20
josh1
Well, large N QCD came along string theory, not out of string theory research.
Yes. But what did come out of string theory was a way to deal with general supersymmetric SU(N) gauge theories for large N in terms of low energy supergravity.

Perhaps most of the theological problem come from the continuous interaction between a theory of extended relativistic objects (call it Brane theory) and a theory of quantized relativistic superstrings (string theory). The later is a very exclusive theory, with objects in 10 dimensions and with only a few uncompactifyed models. The later is not exclusive, it includes non quantized objects (eg 1-branes in 3,4,6 dimensions) and even can be extended to be understood as an approximation.
I'm unsure what you mean here. It sounds as if you think that there are different aspects of string theory which are difficult to reconcile with each other. If this is the case, let me just mention one fact about the relation between strings and D-branes which is that D-brane states can be described mathematically in terms of open string states. More specifically, these two descriptions are T-dual to each other. In fact, this was how D-branes were discovered in the first place.
 
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  • #21
Chronos
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Mathematical discoveries that lack observational support are not easily defended.
 
  • #22
arivero
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I'm unsure what you mean here. It sounds as if you think that there are different aspects of string theory which are difficult to reconcile with each other. If this is the case, let me just mention one fact about the relation between strings and D-branes which is that D-brane states can be described mathematically in terms of open string states. More specifically, these two descriptions are T-dual to each other. In fact, this was how D-branes were discovered in the first place.
No, I mean that there is a "wider" string theory and a "strict" string theory, the later being the theory of quantum ten-dimensional superstrings. For instance a lot of the work on Duff on Branes is about unquantized supersymmetric branes and everybody considers it to be a work on string theory.
 
  • #23
josh1
No, I mean that there is a "wider" string theory and a "strict" string theory, the later being the theory of quantum ten-dimensional superstrings. For instance a lot of the work on Duff on Branes is about unquantized supersymmetric branes and everybody considers it to be a work on string theory.
I know that Duff’s branes aren’t D-branes. So what’s the “theoretical” problem you refer to? Are you saying that Nicolai’s work has nothing to do with string theory and that part of the reason why I and other people think it does is a mistaken view that Duff’s work is part of string theory? Are you saying that Nicolai would tell you that in no way did string theory play any role in his decision to study the idea of some kind of AdS/QCD correspondence?
 
  • #24
josh1
Mathematical discoveries that lack observational support are not easily defended.
Maybe we should just shut-down research in quantum gravity and high energy theory altogether and just wait until the correct theory or a theory that you like somehow miraculously appears in it's entirety and of it's own accord.
 
  • #25
arivero
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I know that Duff’s branes aren’t D-branes. So what’s the “theoretical” problem you refer to? Are you saying that Nicolai’s work has nothing to do with string theory and that part of the reason why I and other people think it does is a mistaken view that Duff’s work is part of string theory? Are you saying that Nicolai would tell you that in no way did string theory play any role in his decision to study the idea of some kind of AdS/QCD correspondence?
Well what I was saying is that there are two aspects in the defense of string theory: "broad string theory", which includes things as AdS/QCD and Duff Brane theory, and "unique string theory", which is the D=10 quantum stuff. Some people can defend an aspect without defending the other, some people can defend both.

Now, if AdS/QCD were related to "unique string theory", I would think it vindicates, somehow, old Veneziano et family approaches but it diminishes the interpretation of strings as fundamental blocks at Planck scale. If on the contrary AdS/QCD is about "broad string theory", it does not add any bit about quantum gravity.
 

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