LQG, strings and the IKKT matrix model

In summary, the conversation discusses the idea of formulating a fundamental theory of gravity in terms of functional states on a space of gauge connections, without the use of a background manifold. This is based on the concept of generalized Wilson lines, or networks, which can be described in terms of D-tuple valued 1-forms. This modified theory is then compared to the IKKT model, which identifies Wilson lines with fundamental strings and uses U(N) as the gauge group. It is shown that this theory can reproduce the equations of motion of string field theory. The potential implications and value of this approach are also discussed.
  • #1
Science Advisor
It is an old observation that both LQG and string theory are fundamentally built from 1-dimensional objects. This is no coincidence. In general, every gauge theory can be formulated in terms of Wilson loops and these can often be described by "a" theory of strings. This was discussed in detail in an old paper by John Baez
Strings, loops, knots, and gauge fields.

The natural question is: Which gauge theory has Wilson loops (or more generally: network states) that behave exactly like the fundamental strings of string theory. Surprisingly, an answer has been proposed already quite a while ago in the 90s by Ishibashi, H. Kawai, Y. Kitazawa and A. Tsuchiya: A review of their work is
hep-th/9908038 IIB Matrix Model .

I am trying to discuss this idea in the newsgoup sci.physics.research. But maybe here in this forum it will be interesting, too. This is what I originally wrote:

<quote from s.p.r (here is the full thread) >
"John Baez" <baez@galaxy.ucr.edu> schrieb I am Newsbeitrag
> In article <Pine.LNX.4.31.0309270749590.32151-100000@feynman.harvard.edu>,
> Lubos Motl <motl@feynman.harvard.edu> wrote:

> >Once an infinite number of incorrect terms is removed and
> >the theory [LQG] is made equivalent to string theory, then - of course -
> >will obtain a consistent theory of gravity.
> Yeah, yeah. Even if you're right and we're all mixed up
> and need to change the model drastically and when we do it
> turns into something like string theory, we'll still have
> something interesting, namely a manifestly background-free
> formulation of string theory as a state sum model. I would
> not mind this at all.

I find it really interesting that the following looks like a way to "change
the model" and indeed turn it into string theory:

So let's suppose we want to formulate our fundamental theory in terms of
functional states on a space of gauge connections A_mu taking values in some
Lie algebra. We want to be really background free. In LQG one does away with
background _fields_ on spacetime, but one still does need a manifold to set
up the theory. Let's do away with the assumption of a
(topological/differentiable) manifold, too.

Without a manifold it makes no longer sense to have the A_mu(x) be functions
of coordinates. Therefore let's assume, being very naive, that the
connection is _independent_ of any coordinates.

As we learn from LQG, functions (states) on the space of connections
are spanned by generalized Wilson lines ("networks", graphs - I'll avoid the
word "spin" for the moment). In ordinary LQG these are embedded into a
manifold. But since we have just done away with this manifold we now have to
evaluate our connections on abstract networks that are not embedded into any
a priori structure. The natural way to do that is to equip the network that
comes with a given state (function on the space of connections) with a
D-tuple valued (piecewise defined) 1-form k and compute the holonomy of

Sum_mu k^mu A_mu

along the edges of the network, intertwining at the vertices as desired and
finally tracing over the result.

In other words, in this manifold-independent formulation of "Ashtekar
geometry" a network state is given not just by a coloring of edges {e} by
representations {r} (and coloring of vertices by intertwiners {i}) but
instead by a coloring by representations _and_ D-tuple valued 1-forms k.
(The information that was previously contained in the coordinates of a given
edge has now moved into the extra piece of data k.) So a basis of states
should now be of the form

{ psi_{e,r,i,k} }

where each element psi_{e,r,i,k} is associated with an abstract
combinatorial graph e colored by r,i, and k.

This basis spans the kinematical Hilbert space. Now we need dynamics.
Personally I feel that in LQG kinematics is very beautiful but that as soon
as the ordinary dynamics enters the game things become rather awkward. A
fundamental theory is not supposed to look awkward, so let's slighly modify
the ordinary LQG dynamics. Instead of using the action of B^F theory on the
space of connections A we'd rather use the simplest action quadratic in the

S = Tr F^2 = Tr [A,A]^2 .

In other words, the slight modification of LQG that I am proposing here is a
theory whose configurations are given by _constant_ gauge connections A and
whose observables (correlation functions) are

<psi_{e1,r1,i1,k1} psi_{e2,r2,i2,k2}...psi_{en,rn,in,kn}>
int DA psi_{e1,r1,i1,k1}(A) ...psi_{en,rn,in,kn}(A) exp(-S(A))

with S and psi_{...} as defined above. The two major modifacations as
compared to ordinary LQG are the absence of the manifold background and the
switching from an action linear in the curvature to the simplest one
quadratic in the curvature.

The point of all this is the following: In 1996 the authors N. Ishibashi, H.
Kawai, Y. Kitazawa and A. Tsuchiya have proved for us (see hep-th/9908038
and references given there) that

if we identify Wilson lines (network edges) in the above theory with
fundamental strings

and if we use U(N>>>1) as the gauge group then the above action for the
connection induces on these Wilson line, which are now also regarded as
functionals (states) on the configuration space of the fundamental string,
the equations of motion of string field theory!


I spent some time at the "Strings meet loops" symposium trying to find LQG
people who would find this as interesting as I do. That's because my
impression is that maybe the true value of this IKKT model is not properly
appreciated in the string community, which might have to do with its radical
background independence. Lubos mentioned that timelike T-duality which does
away with time (!) looks suspicious. But maybe it is just what we need, I
wonder. In any case, when looked at it from the proper perspective (as I
have tried to demonstrate above) the IKKT model looks much more like LQG
than like string theory. Of course, looked at it from another perspective it
completely looks like string theory. Great.

Luckily, I found several very friendly and open minded LQGists who did find
this interesting. I am looking forward to hearing what they come up with
when studying this in detail.
<end quote>
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  • #2
Welcome to the PF Urs. Your idea looks extremely interesting and I hope some workers pick up on it. Wish I could myself, but I'm way to low on the knowledge ladder to qualify.
  • #3
Originally posted by selfAdjoint
Welcome to the PF Urs. Your idea looks extremely interesting and I hope some workers pick up on it. Wish I could myself, but I'm way to low on the knowledge ladder to qualify.

I agree. welcome Urs!
At first sight the part of your idea that involves doing away with the manifold reminds me of the paper by Laurent Freidel and Etera Livine called "Spin Networks for Non-Compact Groups"


they find they can throw away the underlying manifold (where originally the connections lived) and use a collection of oriented graphs &Gamma;
and define a space "discrete connections" on each graph


and then define a measure, and an L2 hilbertspace, on each of
these A&Gamma; spaces of discrete connections
one such hilbertspace for each graph

(but these graphs do not have to be embedded in a specific manifold,
or, if they are, one must consider diffeomorphism equivalence classes of graphs---seems better to let them be abstract)

finally they put together all these hilbertspaces (one for each &Gamma;) into one big one---reminiscent of fock space
and this then is the quantum states of geometry

I'm giving you only an unreliable first impression because I'm just now trying to read the paper, but it also (like your idea) seems
to get away from the original manifold

would like to hear more about what you propose. happy if you post
here since easier for me to read and reply to than SPR is
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1. What is LQG?

LQG, or Loop Quantum Gravity, is a theory that attempts to reconcile general relativity and quantum mechanics by describing the fabric of space-time as a network of interconnected loops. It is a theoretical framework for understanding the nature of space and time on a quantum level.

2. What are strings in string theory?

In string theory, strings are tiny, one-dimensional objects that vibrate at different frequencies. These vibrations give rise to the particles and forces that make up our universe. String theory suggests that these strings are the fundamental building blocks of the universe.

3. What is the IKKT matrix model?

The IKKT matrix model is a mathematical framework that combines elements of both string theory and loop quantum gravity. It proposes that the fundamental structure of space-time is made up of matrices, or arrays of numbers, which interact with each other to create the universe.

4. How do LQG, strings, and the IKKT matrix model relate to each other?

LQG, strings, and the IKKT matrix model are all attempts to understand the nature of space and time on a quantum level. They each have different approaches and focus on different aspects of this problem, but they all share the common goal of reconciling general relativity and quantum mechanics.

5. What are some current challenges or criticisms of LQG, strings, and the IKKT matrix model?

Some challenges and criticisms of these theories include their lack of experimental evidence, their complex mathematical nature, and the fact that they have not yet been able to fully resolve the conflict between general relativity and quantum mechanics. There are also ongoing debates about whether these theories can ever be fully unified or if they will always remain separate approaches to understanding the universe.

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