What is the Current Status of Loop Quantum Gravity and Its Open Issues?

[suprised]

But in contrary to string theory the number of degrees of freedom is finte; no infinite tower of states; simply the final theory. And no additional degres of freedom but the correct degrees of freedom (QCD does not contain mesons as degrees of freedom).

Well how do you know what the correct degrees of freedom of QG actually are? Strings seem to tell that you need infinitely many in order to have consistent scattering. It also seems that strings do precisely have the necessary number of degrees of freedom in order to reproduce Bekenstein Hawking Entropy etc. So I see it the other way around, namely that LQG still needs to demonstrate that it can be a consistent approximation/realisation of gravity in the first place.

And certainly QCD has mesons as degrees of freedom, in the IR.

I do not see the problem of unitarity.

Well I do, as do my colleages. This problem can be addressed once one is able to describe scattering processes in LQG. We know that for string theory the intricate structure of the (moduli spaces) of Riemann surfaces is crucial for consistency, ie, unitary scattering. I wonder whether and if so, how, LQG would be able to reproduce this. It may well be, I have no opinion, I am just wondering.

Not on top; it has different problems. Most (technical) problems we know from QFT, SUSY, SUGRA, ST do not apply to LQG as the theory is formulated differently.

eg the hierarchy problem, which in a broader sense includes the cosmological constant, is certainly a problem for LQG as well, on top of its intrinsic problems.

Actually there is more to quantum gravity than UV problems, as certain problems do not depend on the UV completion at all. Moreover it is not even clear whether there are serious UV problems in the first place - due to the phenomenon of classicalization. Some of these issues are going to be discussed here:
http://ph-dep-th.web.cern.ch/ph-dep-th/content2/THInstitutes/2011/QG11/QG11.html

 

[Fra]

"Off-shell" is not a fundametal thing in a theory, it's created by (partiall inappropriate) approximations (chosing a background and doing perturbation theory). So by proving "off-shell finiteness" you do not validate your fundamental theory, you only validate the approximation - which is nice, but not fundametal.

Doesn't this reasoning a priori assume that there is a fixed observer independent theory that moreover does not need to be infered by any observers? Or equivalently a non-manifest set of theories that are related consistently by fixed objectively known transformation rules.

(1) Wherein lies the rationality and necessity of this assumption?

(2) In the quest for finding observer invariant physics, is it really appropriate to label choosing an observer an "approximation"? Isn't that in fact disrespecting the whole essence of measurement theory?

Yes, these are nontechnical but conceptual questions, but it seems quite clear that these things are what is the root cause of several technical issues as well.

/Fredrik

 

[tom.stoer]

And certainly QCD has mesons as degrees of freedom, in the IR.

No, just quarks and gluons as can be seen from lattice gauge theories; nobody forces you introduce mesons.

This problem can be addressed once one is able to describe scattering processes in LQG.

If you try to study scattering based on an approximation that may be the case - but you shouldn't. Again look at QFT: the problem of unitaritry arises in approximations. I would say that this contradicts the basis of LQG, namely background independence. Breaking background independence introduces new problems - so you should avoid it. But I agree that it's too early to answer this question b/c up to now graviton-graviton or graviton-matter scattering hasn't been derived from LQG. So the problem is entirely different: how to describe these scattering processes? It's like scattering in lattice gauge theory: you avoid a lot of problems - but you can't calculate the scattering amplitudes afaik.

eg the hierarchy problem, which in a broader sense includes the cosmological constant, is certainly a problem for LQG as well, on top of its intrinsic problems.

I agree; sooner or later this will arise.

 

[tom.stoer]

Yes, these are nontechnical but conceptual questions, but it seems quite clear that these things are what is the root cause of several technical issues as well.

Yes and no. Your conceptual questions do exist in QM already, but there seems to be no problem with off-shell closure. All I wanted to say is all problems we discuss should be categorized (roughly) as follows:
(1) conceptual problems posed by nature
(2) conceptual and technical problems posed by a specific approach or theory
(3) technical problems posed by a specific approcimation to a specific theory

Problems of category (3) are of no importance in a different theory and we should avoid waisting time to discuss problems (e.g.) raised or fixed in perturbative string theory in LQG.

 

[Fra]

(3) technical problems posed by a specific approcimation to a specific theory

Problems of category (3) are of no importance in a different theory and we should avoid waisting time to discuss

I certainly agree with the generic point.

But my definitive impression from reading both LQG papers and some ST reasoning is that sometimes a real confusion exists between "mathematical perturbation theory" and and inside observer trying to perform an inference on it's environment. There is also confusion between truncation in the context of regularization as mathematical methods, and natural truncation of information that is due to the observers limited information capacity.

In a way, you can think of an observers GUESSING or INFERENCES about it's own environment, as a kind of perturbation of what it KNOWS, to account for what can possibly be true and you can ORDER this in decreasing order of subjective probability. At some point, the expectations accounting for all possibilities (analogous to PI) stops to count possibilities because they are not distinguishable from the inside due to fallwing below some treshold. This kind of issue, does impact to the action of the system, and this is a conceptual and physical problem and different than mathematical perturbation theory.

For example. Given what you know, you form a prior. Then you can expand the possible distinguishable changs, and order them by falling probability (beeing related to information divergence) and any finite observer, neeeding to make a choice will either due to truncation of representation or due to finite time, truncate the possible considerations somewhere, and make a choice based upon incomplete information. And this is the most rational choice that is physically possible given the constraints.

I have a strong feeling that LQG thinking, exemplified by Rovelli', often treats the observer like an arbitrary choice almost like perturbation theory, for the very reason that he considers the observer invariants as what's physical. But this completely dismisses the inference perspective (beeing the essence of QM IMHO).

/Fredrik

 

[Fra]

Your conceptual questions do exist in QM already

Yes.

but there seems to be no problem with off-shell closure.

Yes, but there is IMHO a way to see why.

QM as we know it before we start to talk about gravity, is essentially all about scattering matrices. This mean you have an environment which is effenticely monitoring the very small sub-system you study. The shell notion is defined in the classical environment.

In QG, the above assymmetry does not hold. Scattering matrices in a cosmological theory simply makes no sense, because the observers is the small guy here, and is floating inside the "black box" rather than embracing and controlling it.

This assymmetry is IMO the root cause of why we do get away with things in ordinary QM, that will not do in a cosmological measurement theory.

I think the problem is two-fold.
1) Measurement theory as it stands with fixed hilbert spaces etc, doesn't make sense for a cosmological measurment theory.

2) The understanding of what on-shell or "equilibrium" means, is different in a mesurement theory than in calssical physics. GR is a realist theory, and on-shell is hard elements of reality. Such things is IMO not something we should have in a measuremnet theory. Instead the equilibrium must be infered from the inside.

/Fredrik

 

[tom.stoer]

My impression is that in the LQG field nobody discusses these topics.

My feeling is that one should try to develop a theory based on "boundary Hilbert spaces"only, where the boundary separates the "system" from the "observer" and possibly from the "environment". That would be in-line with the holographic principle.

 

[Fra]

My impression is that in the LQG field nobody discusses these topics.

If I could only figure out why.

My feeling is that one should try to develop a theory based on "boundary Hilbert spaces"only, where the boundary separates the "system" from the "observer" and possibly from the "environment". That would be in-line with the holographic principle.

I too think the boundary or communication channel between observer and the rest of the universe is a central starting point, but we really lack the framework for this. I have never seen anything near what I think is needed, but that's fine because it's a hard problem. What is more worrying is when the questions are avoided. I'd expect any so inclined theoretical physisitcs to wear these quesions on our forehead until we have answered them ;)

Most ideas like AdS/CFT end up with observers at infinity which effectively gives us just the scattering matrices, so there should exists a much more general framework in which these asymptotic observers positions are a special or limiting case.

Interestingly when you ask these questions unification becomes unavoidable, and suggests that the two things are related, contrary to the reasoning of Rovelli that suggests they are two different problems. The reason is that the logic of the action of a small subsystem (where roughly speaking) ordinary QFT works fine (not quite, but almost anyway) seen from the inside, MUST be a cosmological mesurement theory! So if we are ever to get away from "understanding" the SM by means of postulating more or less classical hamiltonians, and instead try to understand from first measurment principles the construction of the SM action (and thus unification) we need the cosmological measurement theory anyway, since this must be the correcty "inside view".

So indeed IR and UV scales are related here in a sense, somehow the "UV action from an IR perspective is the inverse of the IR action from the UV perspective". Not sure if that makes sense but it seems even inconsistent to think that there is no relation.



/Fredrik

 

[marcus]

...
My feeling is that one should try to develop a theory based on "boundary Hilbert spaces"only, where the boundary separates the "system" from the "observer" and possibly from the "environment"...

That reminds me of Robert Oeckl's proposal of a "general boundary" formulation of quantum mechanics.
http://arxiv.org/abs/hep-th/0306025

Where does the formulation in 1102.3660 fall short of what you have in mind? What would you have to do to it to make it fit your idea?
What i mean is, the LQG Hilbert space of states is already entirely concerned with boundary geometry.

It is a projective limit of Hilbert H_Gamma all of which concern the boundary.
The limit is as Gamma -->∞. Gamma a finite graph serves as a truncation to finitely many degrees of freedom. When one computes one fixes a Gamma. So the boundary has only finitely many degrees of freedom.

One can think of the boundary (or the Gamma) as the "box" containing the system. It is however 3D because it persists in time. The experimenter can watch the box for a certain interval, making initial and final observations. There is a transition amplitude associated with the boundary state.

the 4D spin foam formalism is only used as a tool to compute the transition amplitude. What is real, so to speak, is the 3D boundary. And the LQG Hilbertspace is entirely based on that.

This is the theory as presented in 1102.3660. Is this the kind of theory which you say one should try to develop?

should try to develop a theory based on "boundary Hilbert spaces"only, where the boundary separates the "system" from the "observer" ...
​

If not, I'm curious to know how should it be different in order to match more closely?

 

[tom.stoer]

The first difference is the topology. My boundary Hilbert spaces would all be compact surfaces.

The second difference is that I would not calculate any transition amplitude between those boundaries as there is only one boundary, not two (as in the SF approach)

The third difference is that I would try to quantize the theory on the boundary, or to "represent" the volume on the boundary i.e. implement the holographic principle. This is similar to the LQG "isolated horizon" approach" for black holes with a topological surface theory.

The fourth difference is that the boundary represents something like "the system" as defined by the observer. That means the boundary is both something physical and something encoding the subjective perspective of an observer.

Now in order to get rid of the latter I think the theory as a whole must not look at one of these boundaries but at the infinite collection of all possible boundaries, i.e. at all splits of the universe into a "system" and an "environment" defined by an "observer".

I guess this could be a framework from which reduced density matrices could emerge, which would be a step forward to solve the measurement program, and to define the observer mathematically. In addition having this infinite collection of surface Hilbert spaces with its reduced density matrices, one could reconstruct the whole complete state from the this collection (at least mathematically, but not practically). That means that w.r.t. one observer there is a partial trace, decoherence, "wave function collaps" etc., but w.r.t. to all observers unitarity is conserved.

But this has nothing to do with the current status of LQG ...

 

[Fra]

That means the boundary is both something physical and something encoding the subjective perspective of an observer.

Now in order to get rid of the latter I think the theory as a whole must not look at one of these boundaries but at the infinite collection of all possible boundaries, i.e. at all splits of the universe into a "system" and an "environment" defined by an "observer".

The question is, is it really desirable to "get rid of the observer"?
This is the point where we probalby disagree.

Part of my point is even, I don't think it's POSSIBLE to get rid of the observer even if we wanted, because any attempt to do so is still bound to be confined to an inside view. The THEORY itself, must be observer dependent if we want the theory to be rationally inferred, because there is not way to even make an inference without an observer. And that latter is in my view at least at the cored of a "measurment theory" (give or take technical details such as fixed hilbert spaces).

Thus, while I agree that the boundary observer/environment is a central thing to elaborated around, I think what we a are seeking is interacting theories, but where the mechanism is evolutionary.

So the general concept of landscapes of theories appearing is not totally stupid. It's just that apparently essential ingredients are missing to make sense out of it (selection principles).

/Fredrik

 

[tom.stoer]

OK, let's rephrase this slightly.

We have "global theories", LQG in its current form is one, and we seem, to agree that we need a local theory which allows one to describe things relative to an observer. What I mean is that a "complete collection of local expressions, e.g. boundary Hilbert spaces" is sufficient to reconstruct the global theory. And in addition there should be a mechanism to reduce the global theory to a local one. This could e.g. be something like taking a partial trace in a reduced density matrix formalism.

In that sense we do not get rid of the observer but we are able to relate different observers.

It would be something like a "relativity principle", but not formulated in position space but in Hilbert space language (or in some other framework, I have no idea if Hilbert spaces will be the right stuff). Let's call it "Principle of relativity w.r.t. quantum-obervers" or something like that.

But as quantum theory seems to be the correct theory of nature I doubt that we need something new. It's perhaps only a re-interpretation of the formalism, just like decoherence. If this is correct then Hilbert spaces and LQG are still the correct framework for QG.

I don't think that we need landscapes or something like that.

 

[marcus]

Thanks for your response. It's a way of addressing the "current status" to say how it differs, as you see it, from your ideal. The first sentence might need editing, or a few extra words. I don't see how any Hilbert space can be compact.

The second sentence seems to contain a misconception about LQG.In the SF approach the boundary can consist of a single connected component. It is not necessarily "two" boundaries.

Intuitively the spin network state describes the geometry of a boundary which may be compact, connected, and surround the "system" before, during , and after. A kind of "box interval". I intended to suggest this in the preceding post when I was talking about the experiment being inside a box which has time-duration.

The first difference is the topology. My boundary Hilbert spaces would all be compact surfaces.

The second difference is that I would not calculate any transition amplitude between those boundaries as there is only one boundary, not two (as in the SF approach)

The third difference is that I would try to quantize the theory on the boundary, or to "represent" the volume on the boundary i.e. implement the holographic principle. This is similar to the LQG "isolated horizon" approach" for black holes with a topological surface theory.

The fourth difference is that the boundary represents something like "the system" as defined by the observer. That means the boundary is both something physical and something encoding the subjective perspective of an observer.

Now in order to get rid of the latter I think the theory as a whole must not look at one of these boundaries but at the infinite collection of all possible boundaries, i.e. at all splits of the universe into a "system" and an "environment" defined by an "observer".

I guess this could be a framework from which reduced density matrices could emerge, which would be a step forward to solve the measurement program, and to define the observer mathematically. In addition having this infinite collection of surface Hilbert spaces with its reduced density matrices, one could reconstruct the whole complete state from the this collection (at least mathematically, but not practically). That means that w.r.t. one observer there is a partial trace, decoherence, "wave function collaps" etc., but w.r.t. to all observers unitarity is conserved.

But this has nothing to do with the current status of LQG ...

It seems to me that your comments have VERY MUCH to do with the current status :-D
In some cases you are saying what you see as missing---to describe the shortcomings is part of a good status report. And also some of what you say is already achieved and so is already part of the current status of LQG.

The third difference is that I would try to quantize the theory on the boundary, or to "represent" the volume* on the boundary...​

*A common word for the spacetime volume inside the boundary is "bulk".

This is what LQG does. The standard formulation of LQG as given in 1102.3660 does, in fact, quantize the state of the boundary.

The fourth difference is that the boundary represents something like "the system" as defined by the observer. That means the boundary is both something physical and something encoding the subjective perspective of an observer.​

Well this is more philosophical and I'm less sure about it, but it seems to me to be "sort of kind of" or "so-wie-so" how I think about the standard formulation as in 1102.3660. The theory is primarily about the boundary. Which corresponds to what can be measured or observed. The H_Gamma hilbert spaces are about the boundary. Its quantum states.

The amplitudes that one calculates refer to the boundary H_Gamma. But in order to calculate them one sets up foams in the bulk. One sets up provisional histories in the bulk. However these are nothing but diagrammatic ways to calculate the boundary amplitudes!

This is how I think of the current status LQG formulation and I am not sure about the philosophical issue you mention. Is this subjective and observer dependent? Does this have to be "gotten rid of"? Remember that the boundary and bulk have no location in a fixed background spacetime. There is no background. Where could the observer be? Perhaps the boundary IS the observer and we just have to live with that. Maybe there is finally no ontology, no mathematical representation of the bulk reality, only a boundary of measurements related to other measurements. Nature is what responds to measurement in the way that she does and we don't know any more. I get dizzy here. don't feel philosophically adequate to discuss this. Provisionally then, I just accept the theory as it is.
As long as it let's us calculate amplitudes and eventually test.

Now in order to get rid of the latter I think the theory as a whole must not look at one of these boundaries but at the infinite collection of all possible boundaries, i.e. at all splits of the universe into a "system" and an "environment" defined by an "observer".
​

Well as I say, my philosophical grip is a bit too weak to proceed with this, but I note that in LQG there is an infinite collection of graphs Gamma, and they have no definite location since there is no background. Perhaps they could represent "all possible boundaries". (I was thinking of them as all possible truncations of a single boundary to finitely many geometrical degrees of freedom, but perhaps there is a better way to think.)

This is just a partial response to your post. I have to leave much unresolved. I am not sure about "getting rid of" observer dependence. Haven't resolved that in my own mind. But if you want to have an infinite collection of boundaries you might have the materials available to formulate that, given the infinite collection of boundary graphs.

Anyway, interesting post. Thanks!

 

[Fra]

Tom, I understand what you say. What you advocated is from my POV, very close to rovelli's view. Your analogy with global, local, "relativity" is well taken. They are good examples of your stance and I see what you mean but I do not agree.

I am not sure if you get my point or if you just disagree with it.

This gets slighltly off topic except to the extent that it "elaborates the logic in LQG" which was my original intention.

To address a single statement to again illustrate my key point.

In that sense we do not get rid of the observer but we are able to relate different observers.

My point is that "we" here is just another observer, infering or abducing the "theory" simply from it's interaction history.

My point would be that it's impossible for an arbitrary observer to DEDUCE from this "complete collection of local expressions" the global theory. It does not physically fit/encode in any part.

There is however possible for an inside observer to make this inference less strongly than a deduction in the sense that the observers is making guesses, and from feedback it can seem to encode more information than it in fact does. Some of the "information" is stored in the evolutionary selection process. But this information is not physically stored in the observer, it's stored in the form of an equilibrium in the ste of observers.

But in this view, your example of the relavitity symmetry, is NOT a hard fact or constraint - it's merely an equilibrium point. And to understand why it is what it is, we need to understand the equilibrium process. Somehow the difference is that symmetries are no longer "classical" they become "inferencial" in the lack of a better world. I wouldn't say quantum mechanical symmetries because QM as it stands is not the full inference I seek.

/Fredrik

 

[marcus]

Tom asked for a review article that sums up LQG current status and at first we couldn't offer anything really up to date. But several new papers have come out and the Zakopane lectures got updated in August 2011. So that makes a fairly compact current status report.

http://arxiv.org/abs/1105.2212
Cosmological Constant in LQG Vertex Amplitude
==quote Muxin Han conclusions==
To summarize, in this paper we propose a new q-deformation of the Euclidean EPRL/FK spinfoam vertex amplitude. The concrete construction uses the evaluation of the Vassiliev invariant from 4-simplex graph. We also show that the asymptotics of the q-deformed vertex amplitude gives the Regge gravity with a cosmological constant (from Regge calculus using flat 4-simplices) in the regime that the physical scale of the 4-simplex is much greater than the Planck scale l_p but much smaller than the cosmological length l_c.
==endquote==
For anyone not familiar with it, the cosmological length l_c, given by Λ = 1/l_c², is the length scale associated with the cosmo constant Λ.
...
That would make the combined essential "current status" review be
1102.3660+1108.2258+1105.2212
Zakopane lectures+Emergence of gravity+Cosmological constant
Rovelli +Magliaro Perini + Han
33 pages +6 pages +6 pages
On the basis of this overview, I'd sum up the essentials by saying loop is now a definite theory and evidently finite with the right limits. The loop research community has grown in size and shows an active interest in testing.

I will fetch the abstract for the Magliaro Perini article.
http://arxiv.org/abs/1108.2258
Emergence of gravity from spinfoams
Elena Magliaro, Claudio Perini
(Submitted on 10 Aug 2011)
We find a nontrivial regime of spinfoam quantum gravity that reproduces classical Einstein equations. This is the double scaling limit of small Immirzi parameter (gamma), large spins (j) with physical area (gamma times j) constant. In addition to quantum corrections in the Planck constant, we find new corrections in the Immirzi parameter due to the quantum discreteness of spacetime. The result is a strong evidence that the spinfoam covariant quantization of general relativity possesses the correct classical limit.
6 pages.

So basically, what this thread is about---getting a concise up to date review---boils down to these three papers with a total of 45 pages (33+6+6)

 

[marcus]

I should give some sources for what I said in the previous:

On the basis of this overview, I'd sum up the essentials by saying loop is now a definite theory and evidently finite with the right limits. The loop research community has grown in size and shows an active interest in testing.​

To show the growth, Inspire search using terms "quantum gravity: loop space" "quantum cosmology: loop space" and "spin:foam".

LOOP RESEARCH BY YEAR (Inspire beta)
2005 http://inspirebeta.net/search?ln=en...=&d2m=&d2y=2005&sf=&so=a&rm=&rg=10&sc=0&of=hb (42 found)
2006 http://inspirebeta.net/search?ln=en...=&d2m=&d2y=2006&sf=&so=a&rm=&rg=10&sc=0&of=hb (77 found)
2007 http://inspirebeta.net/search?ln=en...=&d2m=&d2y=2007&sf=&so=a&rm=&rg=10&sc=0&of=hb (120 found)
2008 http://inspirebeta.net/search?ln=en...=&d2m=&d2y=2008&sf=&so=a&rm=&rg=10&sc=0&of=hb (142 found)
2009 http://inspirebeta.net/search?ln=en...=&d2m=&d2y=2009&sf=&so=a&rm=&rg=10&sc=0&of=hb (145 found)
2010 http://inspirebeta.net/search?ln=en...=&d2m=&d2y=2010&sf=&so=a&rm=&rg=10&sc=0&of=hb (153 found)
2011 http://inspirebeta.net/search?ln=en...=&d2m=&d2y=2011&sf=&so=a&rm=&rg=10&sc=0&of=hb (124 as of 2 sept, annualized 180+)

To show the interest in testing (Lqg phenomenology) a Spires search for phenomenology papers 2009 and later.
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+LOOP+SPACE+AND+%28QUANTUM+GRAVITY+OR+QUANTUM+COSMOLOGY%29+%29+AND+%28GRAVITATIONAL+RADIATION+OR+PRIMORDIAL+OR+INFLATION+OR+POWER+SPECTRUM+OR+COSMIC+BACKGROUND+RADIATION%29+AND+DATE%3E2008&FORMAT=www&SEQUENCE=citecount%28d%29
Currently it gets 44 papers.
None of these have to do with "fuzziness of distant quasars" produced by "graininess of spacetime foam" or dispersion (energy dependence of speed of light.) AFAICS they all have to do with features of the cosmic background radiaton (power spectrum, polarization.) You can examine the papers--sample them and see for yourself.

As for the definiteness of the theory, see a definitive formulation in the August 2011 version of http://arxiv.org/abs/1102.3660 .
The theory has changed greatly since 2007. It seems curiously similar to what Tom was describing. I talked about that in post #63 of this thread:
https://www.physicsforums.com/showthread.php?p=3481820#post3481820

It is a quantum theory of the 3d boundary surrounding a spacetime region ("bulk") that holds the system or experiment. The boundary represents what the experimenter knows. It can consist of a single connected component encompassing initial-during-final. Or several separate components.
The hilberts of the theory are quantum states of the boundary. H_Gamma where Gamma is a graph. Think of Gamma as the basic graph on which a spin network could live, this being a quantum state of the 3d geometry (potentially with matter as well) of the boundary.

The spinfoams are diagrammatic tools for calculating the amplitude associated with the boundary, more precisely the amplitude associated with a quantum state of the boundary. So the formulation of the theory is very boundary-focused, curiously like what Tom described as desirable!

I think of the boundary as what the observer can control and read, in a sense the boundary IS THE OBSERVER. But this is getting a little philosophical. We should try to stick with the here-and-now.

Probably most people realize how important it is for a theory to be TIMELY. Theory should progress incrementally step-by-step. It concerns what is PRACTICAL and appropriate to theorize and test at any given time. Frank Wilczek made that point several times in his talk at Uppsala.

 

[tom.stoer]

Thanks for your response. It's a way of addressing the "current status" to say how it differs, as you see it, from your ideal. The first sentence might need editing, or a few extra words. I don't see how any Hilbert space can be compact.

I agree; it should read my boundary Hilbert spaces would all life on compact surfaces.

The second sentence seems to contain a misconception about LQG. In the SF approach the boundary can consist of a single connected component. It is not necessarily "two" boundaries.

Please explain!

It seems to me that your comments have VERY MUCH to do with the current status :-D
In some cases you are saying what you see as missing---to describe the shortcomings is part of a good status report. And also some of what you say is already achieved and so is already part of the current status of LQG.

It would be rather nice if LQG is that close to what I have in mind; but my idea goes far beyond the formalism of LQG. It's an idea how to re-interpret quantum mechanics in terms of boundary Hilbert spaces representing "cuts" and "systems" introduced by "observers". It would be the first time since Heisenberg, Schrödinger and Dirac that one can pointing the finger at a mathematical entity and say "this is the oberver!" If LQG is compatible with that idea - fine. But of course it was not the intention of LQG.

This is what LQG does. The standard formulation of LQG as given in 1102.3660 does, in fact, quantize the state of the boundary.

But my intention is to have a theory which relates all boundary Hilbert spaces and which allows to explain "systems", "observers", "collaps of the wave function", i.e.to re-interpret QM. Again: LQG may be compatible with that idea, but it is by means complete in that sense.

This is how I think of the current status LQG formulation and I am not sure about the philosophical issue you mention. Is this subjective and observer dependent? Does this have to be "gotten rid of"? Remember that the boundary and bulk have no location in a fixed background spacetime. There is no background. Where could the observer be? Perhaps the boundary IS the observer and we just have to live with that. Maybe there is finally no ontology, no mathematical representation of the bulk reality, only a boundary of measurements related to other measurements. Nature is what responds to measurement in the way that she does and we don't know any more. I get dizzy here. don't feel philosophically adequate to discuss this. Provisionally then, I just accept the theory as it is.

These are questions to be asked.

As long as it let's us calculate amplitudes and eventually test.

I agree that currently LQG is in the "shut-up-and-calculate" phase and that my ideas are ot really open issues of LQG but of quantum physics in general. So somehow we lost track. Anyway, thanks for the three references.

 

[Fra]

It would be rather nice if LQG is that close to what I have in mind; but my idea goes far beyond the formalism of LQG. It's an idea how to re-interpret quantum mechanics in terms of boundary Hilbert spaces representing "cuts" and "systems" introduced by "observers". It would be the first time since Heisenberg, Schrödinger and Dirac that one can pointing the finger at a mathematical entity and say "this is the oberver!" If LQG is compatible with that idea - fine. But of course it was not the intention of LQG.

But my intention is to have a theory which relates all boundary Hilbert spaces and which allows to explain "systems", "observers", "collaps of the wave function", i.e.to re-interpret QM. Again: LQG may be compatible with that idea, but it is by means complete in that sense.

I am not sure exactly what you mean, but I assume you are well aware of Rovelli's Relational Quantum Mechanics paper (http://arxiv.org/abs/quant-ph/9609002)? Even when I started to read his LQG book, some things of his views was much more cleanly explain in the RQM paper. Rovelli's specific view of QM, does IMO not have much specifically to do with other details of LQG. So to understand his QM interpretation I think the RQM paper might be good to reference. I'm not aware of that he has written any updates on this?

What is interesting, and the reason why I don't just ignore rovelli's reasoning, is that the initial reasonign of Rovelli is also pretty much well in line with what I said. It's so close, this is why the slight difference is the more annoying.

Rovelli's point as per the RQM paper, is that there are no outside observers, all you have are inside observers. Moreover you have relations between difference observers. But again Rovellis acknowledges that there is no "absolute" relation, the only way for observer A or make statemetns of relations between B and C is by interacting with the system.

First of all, one may ask what is the “actual”, “absolute” relation between the description of the world relative to O and the one relative to P. This is a ques tion debated in the context of “perspectival” interpretations of quantum mechanics. I think that the question is ill-posed. The absolute state of affairs of the world is a meaningless notion; asking about the absolute relation between two descriptions is precisely asking about such an absolute state of affairs of the world. Therefore there is no meaning in the “absolute” relation between the views of different observers. In particular, there is no way of deducing the view of one from the view of the other

Does this mean that there is no relation whatsoever between views of different observers? Certainly not

Up until this point, this is exactly my point as well. I think Rovelli phrase this conceptual point clearly.

The big difference lies in what the relations is!

Rovelli goes on to say

it means that the relation itself must be understood quantum mechanically rather than classically. Namely the issue of the relation between views must be addressed within the view of one of the two observers (or of a third one). In other words, we may investigate the view of the world of O, as seen by P. Still in other words: the fact that a certain quantity q has a value with respect to O is a physical fact; as a physical fact, its being true, or not true, must be understood as relative to an observer, say P. Thus, the relation between O’s and P’s views is not absolute either, but it can be described in the framework of, say, P’s view.

There is an important physical reason behind this fact: It is possible to compare different views, but the process of comparison is always a physical interaction, and all physical interactions are quantum mechanical in nature.

Rovelli essentially here says that the relations can only be "communicated" between observers as a physical intreaction, and this is described by QM (which he has no ambition to change). Here I claim that his analysis is insufficient.

I fully agree that the communication is an interaction and that it can be thought of as them performing measurments on each other, the only problem is that THIS "extended" usage of QM really takes ot BEYOND the testable domain of QM. I am convinced that to implement what Rovelli clearly wants here... QM needs revision.

It would be the first time since Heisenberg, Schrödinger and Dirac that one can pointing the finger at a mathematical entity and say "this is the oberver!"

The exten to which I propose to continue Rovellis' reasoning, with a modified QM, means that even the THEORY is observer dependent. Even theories are not absolute. In my view, the theory IS the observer. The structure of the theory should be one to one with an observes "inference machniery".

Which was what I referred to here

In that sense we do not get rid of the observer but we are able to relate different observers.

My point is that "we" here is just another observer, infering or abducing the "theory" simply from it's interaction history.

/Fredrik

 

[Fra]

I fully agree that the communication is an interaction and that it can be thought of as them performing measurments on each other, the only problem is that THIS "extended" usage of QM really takes ot BEYOND the testable domain of QM. I am convinced that to implement what Rovelli clearly wants here... QM needs revision.
...
The exten to which I propose to continue Rovellis' reasoning, with a modified QM, means that even the THEORY is observer dependent. Even theories are not absolute. In my view, the theory IS the observer. The structure of the theory should be one to one with an observes "inference machniery".

If one starts looking at the conceptual problem, from the above reasoning of Rovelli, I can see two modes of critique:

1) Either you can say that, QM as it stands is correct, and that Rovelli is applying QM in the wrong way. Ie. it's wrong to try to apply quantm mechanics to observer-observer relations (in the extension spacetime/gravity). Instead we should try to recover the observer-observer relations from observer dependent quantum mechanics. I think this is closely the critique I would expect from string theorists.

2) You think that Rovellis is essentially correct, up to the point mentioned, Rovelli's is correct that it should be the measurement theory that explains the observer-observer relations, and this would bound to be an intrinsic measurement theory. But the problem is that Rovelli while having the right idea, tires to be also "conservative" and assume that QM as it stands will be this intrinsic measurement theory.

My stance is (2). I think the problem with current QM is exactly that is NOT an intrinsic measujrement theory. IT is an extrinsic measurement theory. This is why it's so hard to make sense out of bulk observables. It seems to only be possible to get S-matrix style observables.

/Fredrik

 

[marcus]

...
My feeling is that one should try to develop a theory based on "boundary Hilbert spaces"only, where the boundary separates the "system" from the "observer" and possibly from the "environment". That would be in-line with the holographic principle.

Where does the formulation in 1102.3660 fall short of what you have in mind? What would you have to do to it to make it fit your idea?
What i mean is, the LQG Hilbert space of states is already entirely concerned with boundary geometry.

One can think of the boundary (or the Gamma) as the "box" containing the system. It is however 3D because it persists in time. The experimenter can watch the box for a certain interval, making initial and final observations. There is a transition amplitude associated with the boundary state.

the 4D spin foam formalism is only used as a tool to compute the transition amplitude. What is real, so to speak, is the 3D boundary. And the LQG Hilbertspace is entirely based on that.

This is the theory as presented in 1102.3660. Is this the kind of theory which you say one should try to develop?

should try to develop a theory based on "boundary Hilbert spaces"only, where the boundary separates the "system" from the "observer" ...
​

If not, I'm curious to know how should it be different in order to match more closely?

The first difference is the topology. My boundary Hilbert spaces would all be compact surfaces.

The second difference is that I would not calculate any transition amplitude between those boundaries as there is only one boundary, not two (as in the SF approach)

The third difference is that I would try to quantize the theory on the boundary, or to "represent" the volume on the boundary i.e. implement the holographic principle. This is similar to the LQG "isolated horizon" approach" for black holes with a topological surface theory.

..

The second sentence seems to contain a misconception about LQG.In the SF approach the boundary can consist of a single connected component. It is not necessarily "two" boundaries.

Intuitively the spin network state describes the geometry of a boundary which may be compact, connected, and surround the "system" before, during , and after. A kind of "box interval". I intended to suggest this in the preceding post when I was talking about the experiment being inside a box which has time-duration.

... my boundary Hilbert spaces would all live on compact surfaces.

The second sentence seems to contain a misconception about LQG.In the SF approach the boundary can consist of a single connected component. It is not necessarily "two" boundaries.

Please explain!

Page 10 of http://arxiv.org/abs/1102.3660.
Especially Figure 6.

The states in H can be viewed as describing quantum space at some given coordinate time. A more useful interpretation, however, and the one I adopt here, is to take them to describe the quantum space surrounding a given 4-dimensional finite region R of spacetime. This second interpretation is more covariant and will be used below to define the dynamics. That is, a state in H is not interpreted as “state at some time”, but rather as a “boundary state”. See Figure 6.​

Notice that there is a potential confusion on the part of readers connected with the word "transition". The dynamics will be defined by showing how to calculate a "transition" amplitude (there is no other word in general use). But this transition can be the transition within a single compact connected boundary as shown in the figure 6.

We can be talking about the amplitude of transition within and along a boundary, rather than a transition between two disconnected initial and final.

But this is just a confusion due weakness in the English language. We only have one word so we are not able to make the distinction between transition-between two separates and transition-within channeled by the confines of a boundary.

 

[suprised]

No, just quarks and gluons as can be seen from lattice gauge theories; nobody forces you introduce mesons.

I thought we had discussed this before. Of course you need to introduce mesons etc at low energies as the relevant degrees of freedom, since quarks and gluons don't even exist as asymptotic states!

But that discussion gets more and more off topic. I wanted to illustrate a certain point but it didnt get trough.

If you try to study scattering based on an approximation that may be the case - but you shouldn't. Again look at QFT: the problem of unitaritry arises in approximations. I would say that this contradicts the basis of LQG, namely background independence. Breaking background independence introduces new problems - so you should avoid it.

No we should keep the fingers at the trouble points and avoid obfuscation. In order to reproduce the classical limit, and compare to what we call Einstein gravity, you need to introduce a background, ie a metric. Otherwise how could you claim to describe gravity in the first place? And that's exactly where the problem lies; namely when doing so, the problems of continuum quantum gravity tend to come back and the question is how does LQG manage to get around them.

Actually I found that Nicolai in his critical assessments writes much more clearly what I wanted to say. So let me cite it (http://arxiv.org/pdf/hep-th/0601129v2).:

Regarding the non-renormalisable UV divergences of perturbative quantum gravity, many spin foam practitioners seem to hold the view that there is no need to worry about short distance singularities and the like because the divergences are simply ‘not there’ in spin foam models, due to the existence of an intrinsic cut-off at the Planck scale. However, the same statement applies to any regulated quantum field theory (such as lattice gauge theory) before the regulator is removed, and on the basis of this more traditional understanding, one would therefore expect the ‘correct’ theory to require some kind of refinement (continuum) limit, or a sum ‘over all spin foams’ (corresponding to the ‘sum over all metrics’ in a formal path integral). If one accepts this point of view, a key question is whether it is possible to obtain results which do not depend on the specific way in which the discretisation and the continuum limit are performed (this is also a main question in other discrete approaches which work with reparametrisation invariant quantities, such as in Regge calculus). On the other hand, the very need to take such a limit is often called into question by LQG proponents, who claim that the discrete (regulated) model correctly describes physics at the Planck scale. However, it is then difficult to see (and, for gravity in (3+1) dimensions has not been demonstrated all the way in a single example) how a classical theory with all the requisite properties, and in particular full space-time covariance, can emerge at large distances. Furthermore, without considering such limits, and in the absence of some other unifying principle, one may well remain stuck with a multitude of possible models, whose lack of uniqueness simply mirrors the lack of uniqueness that comes with the need to fix infinitely many coupling parameters in the conventional perturbative approach to quantum gravity.

Actually in some other review he more concretely shows that these is a multitude of ambiguities of that sort, in accordance with expectations. It's difficult to have a free lunch!

 

[marcus]

Suprised, you might be interested in the Magliaro Perini article I mentioned just a few posts back.

Tom asked for a review article that sums up LQG current status and at first we couldn't offer anything really up to date. But several new papers have come out and the Zakopane lectures got updated in August 2011. So that makes a fairly compact current status report.

I will fetch the abstract for the Magliaro Perini article.
http://arxiv.org/abs/1108.2258
Emergence of gravity from spinfoams
Elena Magliaro, Claudio Perini
(Submitted on 10 Aug 2011)
We find a nontrivial regime of spinfoam quantum gravity that reproduces classical Einstein equations. This is the double scaling limit of small Immirzi parameter (gamma), large spins (j) with physical area (gamma times j) constant. In addition to quantum corrections in the Planck constant, we find new corrections in the Immirzi parameter due to the quantum discreteness of spacetime. The result is a strong evidence that the spinfoam covariant quantization of general relativity possesses the correct classical limit.
6 pages.

So basically, what this thread is about---getting a concise up to date review---boils down to these three papers with a total of 45 pages (33+6+6)

The 45 page up to date review referred to is this

 

[marcus]

Suprised, you might be interested in the Magliaro Perini article I mentioned just a few posts back.

Tom asked for a review article that sums up LQG current status and at first we couldn't offer anything really up to date. But several new papers have come out and the Zakopane lectures got updated in August 2011. So that makes a fairly compact current status report.

I will fetch the abstract for the Magliaro Perini article.
http://arxiv.org/abs/1108.2258
Emergence of gravity from spinfoams
Elena Magliaro, Claudio Perini
(Submitted on 10 Aug 2011)
We find a nontrivial regime of spinfoam quantum gravity that reproduces classical Einstein equations. This is the double scaling limit of small Immirzi parameter (gamma), large spins (j) with physical area (gamma times j) constant. In addition to quantum corrections in the Planck constant, we find new corrections in the Immirzi parameter due to the quantum discreteness of spacetime. The result is a strong evidence that the spinfoam covariant quantization of general relativity possesses the correct classical limit.
6 pages.

So basically, what this thread is about---getting a concise up to date review---boils down to these three papers with a total of 45 pages (33+6+6)

The 45 page up to date review referred to is this
1102.3660+1108.2258+1105.2212
Zakopane lectures+Emergence of gravity+Cosmological constant
Rovelli +Magliaro Perini + Han
33 pages +6 pages +6 pages
On the basis of this overview, I'd sum up the essentials by saying loop is now a definite theory and one has good grounds to suspect finite with the right limits.

=========================

I mention the Magliaro Perini article because they explicitly take a continuum limit. You indicated several times that you were skeptical of the UV finiteness because, according to you, it requires the [Planck scale] cutoff. You said a continuum limit, removing the cutoff had NOT been taken. You were suspicious that UV finite would fail if the cutoff were removed.
Therefore I think you could be interested in this paper where they keep everything the same overall size and shrink away the discreteness to zero.

gamma is the minimal area expressed in terms of Planck area (to be dimensionless) and j is the quantum number for area.
So gamma*j is a handle on the overall size. What they do is let j-->∞ while keeping
gamma*j = constant

 

[atyy]

Rovelli's Zakopane lectures, p21, shows explicitly that the continuum limit has not been taken.

 

[tom.stoer]

Rovelli's Zakopane lectures, p21, shows explicitly that the continuum limit has not been taken.

To which sentence or equation are you referring to?

In http://arxiv.org/abs/1108.2258v1 it seems that they know how to define the classical limit.

 

[suprised]

If this limit, if it exists, unique? Some people, incl Nicolai, suspect that the burden of non-renormalizability of QG is translated into an inherent ambiguity in defining hamiltonians, and if this is the case, then the theory is as unpredictive as old QG, because one would need to specify an infinite amount of data. LQG better adds "more" to QG rathen than just being some kind of discretization of it.

 

[atyy]

To which sentence or equation are you referring to?

In http://arxiv.org/abs/1108.2258v1 it seems that they know how to define the classical limit.

The figure on p21 of http://arxiv.org/abs/1102.3660. "Continuous limit" is not the classical limit.

 

[atyy]

If this limit, if it exists, unique? Some people, incl Nicolai, suspect that the burden of non-renormalizability of QG is translated into an inherent ambiguity in defining hamiltonians, and if this is the case, then the theory is as unpredictive as old QG, because one would need to specify an infinite amount of data. LQG better adds "more" to QG rathen than just being some kind of discretization of it.

I think the idea is that if the "continuous limit", as Rovelli calls it in the figure on p21 of his Zakopane lectures, exists, then the theory is triangulation independent and unique in that sense. As I understand, the existence of such a limit is unknown at the moment.

 

[atyy]

In asymptotic safety, theories which have a continuum limit lie on a critical manifold, and are attracted to a fixed point on that manifold. Parameters must be tuned so that the theory is one which is on the critical manifold. The predictivity of the theory depends on the dimension of the critical manifold. As http://www.percacci.it/roberto/physics/as/faq.html" explains "we want to use the condition of having a good UV limit as a way of selecting physically acceptable trajectories. From this point of view the ideal case would be that in which a single trajectory reached the fixed point. This would pin down the theory uniquely."

However, http://arxiv.org/abs/1107.2310" envisages that his notion of a continuum limit is different and "does not require tuning a parameter in the action to a critical value".

 

[suprised]

In conventional QG, the continuum limit is asymptotic safety.

That's a conjecture! Another one of many.

 

[atyy]

That's a conjecture! Another one of many.

Yes, of course (ie. the existence of asymptotic safety is unknown).

 

[atyy]

Hmm, Rovelli seems to make different conjectures in the figure of of p21 of http://arxiv.org/abs/1102.3660 and in http://arxiv.org/abs/1107.2310. In the Zakopane lectures, it seems that the continuum and classical limits must commute, but in Ditt-invariance, maybe not, at least not for recovering some regime of GR.

 

[marcus]

Hmm, Rovelli seems to make different conjectures in the figure of of p21 of http://arxiv.org/abs/1102.3660 and in http://arxiv.org/abs/1107.2310. In the Zakopane lectures, it seems that the continuum and classical limits must commute, but in Ditt-invariance, maybe not, at least not for recovering some regime of GR.

Atyy I think you are over-straining yourself in putting your own interpretations on the figure on page 21. At that point there little or no substantive discussion of the figure to support any interpretation. Discussion of the results on limits are found elsewhere in the paper.

As you point out, the same diagram occurs in 1107.2310. And it is accompanied there by a bit more discussion, however I woud not call that conclusive either.

The same illustration occurs with considerably more discussion on page 5 of a new August 2011 paper. I would suggest you have a look.
http://arxiv.org/abs/1108.0832
On the structure of a background independent quantum theory: Hamilton function, transition amplitudes, classical limit and continuous limit
Carlo Rovelli
(Submitted on 3 Aug 2011)
The Hamilton function is a powerful tool for studying the classical limit of quantum systems, which remains meaningful in background-independent systems. In quantum gravity, it clarifies the physical interpretation of the transitions amplitudes and their truncations.
7 pages

The caption there is:"TABLE II. Continuous and classical limits in quantum gravity."
A diagrammatic framework or table like that does not say anything by itself, it serves as a focus for investigation and topic of discussion. A nucleus around which ideas and understanding develop.

 

[atyy]

Atyy I think you are over-straining yourself in putting your own interpretations on the figure on page 21. At that point there little or no substantive discussion of the figure to support any interpretation. Discussion of the results on limits are found elsewhere in the paper.

The same figure occurs in 1107.2310, as you point out, with more discussion, however I woud not call that conclusive either.

The same figure occurs as "Table II" with considerably more discussion on page 5 of a new August 2011 paper. I would suggest you have a look.
http://arxiv.org/abs/1108.0832
On the structure of a background independent quantum theory: Hamilton function, transition amplitudes, classical limit and continuous limit
Carlo Rovelli
(Submitted on 3 Aug 2011)
The Hamilton function is a powerful tool for studying the classical limit of quantum systems, which remains meaningful in background-independent systems. In quantum gravity, it clarifies the physical interpretation of the transitions amplitudes and their truncations.
7 pages

A diagrammatic framework or table like that does not say anything by itself, it serves as a focus for investigation and topic of discussion. A nucleus around which ideas and understanding develop.

Don't Eq 7,8,9 require the limits commute?

 

[marcus]

Don't Eq 7,8,9 require the limits commute?

It's risky for you (or anybody) to put your own non-expert spin on stuff when it is not actually spelled out. You are pointing our attention at this diagram on page 21 of Zako Lectures.
Equations 7,8,9 are somewhere else, page 4, so 17 pages away, with other discussion. If you mean Equations 7,8,9 in Zako. But they don't have anything to do with the topic.

Or maybe you mean equations 7,8,9 in the new paper that goes into much more detailed discussion of that topic, with that picture of the continuous and classical limits.
It would make sense to be focusing on the new paper http://arxiv.org/abs/1108.0832

But if you mean the new paper, then equations 7,8,9 do not refer to Table II at all!

So I am left without the slightest idea of what you are talking about.

 

[atyy]

Rovelli:

"I have given a tentative overall picture of the structure of the theory, the observables, and the form of the continuous and classical limits."

"Finally, not much is known about the effect of the radiative corrections on this structure (for partial results, see [31, 54–56]). These are finite in the deformed version of (27) [17, 18] but this does not make them irrelevant. The main open problem in quantum gravity, I think, is to study their effect on the convergence of the continuous limit."

 

[marcus]

Rovelli:

"I have given a tentative overall picture of the structure of the theory, the observables, and the form of the continuous and classical limits."

"Finally, not much is known about the effect of the radiative corrections on this structure (for partial results, see [31, 54–56]). These are finite in the deformed version of (27) [17, 18] but this does not make them irrelevant. The main open problem in quantum gravity, I think, is to study their effect on the convergence of the continuous limit."

Yes! I was just about to quote that myself. I think it is a good explicit indicator of where the program is at present on the important issue of continuous limit.

I think we have to add this August paper "On the structure" to our list. I hate to increase the number of pages of the "review of current status" but it is only 7 pages so here we are

That would make the combined essential "current status" review be
1102.3660 + 1108.2258 +1105.2212 + 1108.0832
Zakopane lectures+Emergence of gravity+Cosmological constant + On the structure
Rovelli + Magliaro Perini + Han + Rovelli
33 pages +6 pages +6 pages + 7 pages

That brings us up to 52 pages. I have been thinking for some time that I should include "On the structure" in our current status review. But Tom said at the outset that he did not want a LIST of papers, he wanted something like a single review paper compact package. So I was reluctant to include this one. Four papers begins to look like a list

But maybe we are forced to include this because it is the up to date discussion of work in progress on understanding and defining the continuum limit (considerable evidence now supporting the conclusion that the classical limit is right.)

 

[Fra]

Don't Eq 7,8,9 require the limits commute?

Rovelli:
...
The main open problem in quantum gravity, I think, is to study their effect on the convergence of the continuous limit."

Istarted to skim that paper lat night but only got a some of pages before I fell asleep, but IMHO, what Rovelli has done is converting the conceptual problem (that he IMHO previously ignored) into technical issue where in the end the same problem comes back.

It's easy to get the feeling reading rovelli's paper, looking at "classical limits" and "continuum limit" of refinement as a technical or mathematical problems. But I don't think it is just that.

For the discussion, to add my personal conceptual interpretation of those limits, from my own biased view:

The continuum limit corresponds to the infinite observer mass limit (ready also asymptotic observables). Because in my logic, no finite observer, can count infinitely many "possibilities". This is why I think that the "continuum limit" corresponds just to a limiting case of observables. The reason is that Rovelli transformed the "complexity of an observer" into a technical think where treat it just as a mathematics with no physical meaning.

The classical limit OTOH, corresponds also to the system complexity (ie. the Observed) going to infinitiy.

Conceptually, then both the observer and the observed are increasing in complexity. From the point of view of counting, it seems the results is entirely dependent on exactly how the limit is taken. And the way rovelli presents this (as a technical issue) there seems to be no physics in this choice.

I'd rather like to see a reworking here, where the complexity of the observer imposes truncation of C, that is physical. If you want to study C -> infinity, fine, but then it means that you are looking from a large and larger observer. But I'd say the more interesting perspective is to see if from the perspective of a finite observer.

Somehow, I have a decent feeling that the problem Rovelli ends up with isn't much difference from the original one that I got the impression he ignored?

/Fredrik

 

[Fra]

Isn't it already clear, given how LQG is constructed (as finding a specific stance to GR, where it's easier to defined the PI without running into infinites) that the way limits are taken do matter? Isn't that somehow the whole point?

If so, shouldn't there in fact be physics in the choice of ordering the coupling of the theory?

And if this ordering of couplings, is made dependend on the observers complexity, then it seems we have a quite intersting candicate for explaning interactions since the coupling between A-B intercating seen from C, naturally depends on te mass of A and B as well as C.

I think some of the things that looks like technical issues here, due to the analysis of choice here, might better be understood as having physical significance.

Edit: A hunch from this perspective is that the above limiting procedures can't be understood properly without seeing it together with theory scaling and mass generation. Scaling the observer and the system up, IMO corresponds to considering how their masses are scaled up; and how that affects their interactions. In there, I think there is interesting physics having to do with mass generation.

/Fredrik

 

[atyy]

Yes! I was just about to quote that myself. I think it is a good explicit indicator of where the program is at present on the important issue of continuous limit.

I think we have to add this August paper "On the structure" to our list. I hate to increase the number of pages of the "review of current status" but it is only 7 pages so here we are

That would make the combined essential "current status" review be
1102.3660 + 1108.2258 +1105.2212 + 1108.0832
Zakopane lectures+Emergence of gravity+Cosmological constant + On the structure
Rovelli + Magliaro Perini + Han + Rovelli
33 pages +6 pages +6 pages + 7 pages

That brings us up to 52 pages. I have been thinking for some time that I should include "On the structure" in our current status review. But Tom said at the outset that he did not want a LIST of papers, he wanted something like a single review paper compact package. So I was reluctant to include this one. Four papers begins to look like a list

But maybe we are forced to include this because it is the up to date discussion of work in progress on understanding and defining the continuum limit (considerable evidence now supporting the conclusion that the classical limit is right.)

I think Rovelli's http://arxiv.org/abs/1108.0832 is a good concise summary of the present position of LQG. Two things that I think are important are also noted by him. First, the Immirzi to zero for recovery of GR is a kludge, although certainly reasonable (footnote 7). Second, the existence of the continuum limit, or the full theory in fig 2 is now the key question (Section VI).

Personally, I'd say the continuum limit is even more important than getting GR, since string theory shows we don't need gravity to get gravity.

Also, I believe the key points in this latest review were in remarks made 3 years ago by Conrady and Freidel. I think the key advance since then is the proposal for the continuum limit made by Rovelli, and further expanded on by Rovelli and Smerlak. I'm also very partial to the work from Lewandowksi and colleagues, but I don't know how that relates to Rovelli and Smerlak's proposal. Tantalizingly, Lewandowski says in his latest paper that he thinks someone (not necessarily himself) knows the answer!

 

[marcus]

Another aspect of the current status of the program is the rate of research publication. New researchers have been getting in and the rate increasing:
Here's another index we've been tracking:
LOOP RESEARCH BY YEAR (loop quantum gravity, loop quantum cosmology, spin foam)

2005 http://inspirebeta.net/search?ln=en...2y=2005&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (42 found)
2006 http://inspirebeta.net/search?ln=en...2y=2006&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (77 found)
2007 http://inspirebeta.net/search?ln=en...2y=2007&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (120 found)
2008 http://inspirebeta.net/search?ln=en...2y=2008&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (142 found)
2009 http://inspirebeta.net/search?ln=en...2y=2009&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (145 found)
2010 http://inspirebeta.net/search?ln=en...2y=2010&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (152 found)
2011 http://inspirebeta.net/search?ln=en...2y=2011&sf=&so=a&rm=citation&rg=25&sc=0&of=hb (139 as of 26 Sept, annualized > 180)

To illustrate, http://howlonguntil.net/ 268 days, so in this case annualized rate is 139*365/268 = 189

 

[marcus]

...

I think we have to add this August paper "On the structure" to our list. I hate to increase the number of pages of the "review of current status" but it is only 7 pages so here we are

That would make the combined essential "current status" review be
1102.3660 + 1108.2258 +1105.2212 + 1108.0832
Zakopane lectures+Emergence of gravity+Cosmological constant + On the structure
Rovelli + Magliaro Perini + Han + Rovelli
33 pages +6 pages +6 pages + 7 pages

That brings us up to 52 pages. I have been thinking for some time that I should include "On the structure" in our current status review. But Tom said at the outset that he did not want a LIST of papers, he wanted something like a single review paper compact package. So I was reluctant to include this one. Four papers begins to look like a list
...

In September two more papers appeared which significantly advance the current status. Now we have a problem. Are there any of the original four that we can drop because now superseded? How to keep this current status picture compact?

The two important new papers are by Dittrich et al and by Bianchi Ding.

Bianchi Ding seems to pretty much take care of the Regge limit of LQG. Could it maybe replace the Magliaro Perini "Emergence of gravity" 1108.2258 that we have here? Let's not do that. Let's include ALL these papers for the time being and then maybe edit the list down later.

Dittrich et al pursues the ideas of coarse-graining and numerical analysis---basically understanding how to do extensive lattice computations with spin nets and foams.
http://arxiv.org/abs/1109.4927" Coarse graining methods for spin net and spin foam models
It is a beautiful paper: thoughtful and lucid, and at the same time driving forward. Or so I think anyway---just a bystander's impression.

First let's look at Bianchi Ding http://arxiv.org/abs/1109.6538" Lorentzian spinfoam propagator. A key paragraph is at the bottom of page 1:
"Our main result is the following. We consider the limit, introduced in [13] and discussed in [9, 10], where the Barbero-Immirzi parameter is taken to zero γ → 0, and the spin of the boundary state is taken to infinity j → ∞, keeping the size of the quantum geometry A ∼ γj finite and fixed. This limit corresponds to neglecting Planck scale discreteness and twisting effects, at large finite distances. In this limit, the two-point function we obtain exactly matches the one obtained from Lorentzian Regge calculus [38]. We therefore extend to Lorentzian signature the results of [13]."
Reference [13] is a 2009 paper by Bianchi Magliaro Perini.

 

[marcus]

For the moment I'm finding it hard to cover the current LQG status in a concise compact way. Given the potential importance of the September paper by Eugenio Bianchi and You Ding, I want to add it to our short list of papers. We may be able to edit the list down later but at present I do not see how.

That makes the combined essential "current status" review consist of:
1102.3660 + 1108.2258 + 1109.6538 + 1105.2212 + 1108.0832
http://arxiv.org/abs/1102.3660"
Rovelli + Magliaro Perini + Bianchi Ding + Han + Rovelli
33 pages +6 pages +13 pages +6 pages +7 pages

and brings us up to 65 pages.

 

[tom.stoer]

marcus, thanks for the time spent for discussing and compiling this list.

You are right, when looking at you September poll it becomes clear that it's hard to vote for one specific LQG paper (in the past it was hard to vote for one single QG paper, now even for one specific approach there are many interesting new aspects).

Besides the papers you already have in your list I would add Coarse graining methods (which is the first attempt towards Kadanoff's block spin approach in the LQG context; I was waiting for something like that for years), Emergent Braided Matter (which is still an active but unfortunately small and slow research project) and of course Thiemann's papers trying to link spin foams and the canonical approach.

And of course Han's paper on the cc - especially b/c it shows that even the basic algebraic structure to be used is still under discussion.

 

[marcus]

hurrah! It's good to have your perspective. It is 12:30 here and I am falling asleep, so I will not try to respond. I'd like to ask for some help imagining what sort of calculations might arise using Kadanoff method in LqG context. What might people be calculating, or proving analytically. I am looking forward to re-reading this in the morning.

 

[tom.stoer]

marcus, honestly: do you really think that this is the Current status of LQG? It seems that it is a very active research program, but at the same time the big picture is (partially) missing. I think we don't know (yet) how to fit these puzzle pieces together:
- canonical and covariant formulation
- renormalization in the canonical approach (what is H?), renormalization a la Kadanoff, ...
- asymptotic safety
- cc as running parameter in the asymptotic safety approach, cc as a quantum deformation
- matter on top of LQG vs. emerging braided matter ...
- ...

I am afraid that the situation becomes comparable to string theory: plenty of indications, little hard evidence, no experimental facts. Maybe we are simpy not able to do physics w/o experiments!

 

[atyy]

I am afraid that the situation becomes comparable to string theory: plenty of indications, little hard evidence, no experimental facts. Maybe we are simpy not able to do physics w/o experiments!

I think there has to be an exploration of language so that when new data does come we'll be more ready to describe it.

 

[atyy]

hurrah! It's good to have your perspective. It is 12:30 here and I am falling asleep, so I will not try to respond. I'd like to ask for some help imagining what sort of calculations might arise using Kadanoff method in LqG context. What might people be calculating, or proving analytically. I am looking forward to re-reading this in the morning.

http://arxiv.org/abs/1109.4927" which describes the link between the two: "We describe a simple real space renormalization group technique for two dimensional classical lattice models. The approach is similar in spirit to block spin methods, but at the same time it is fundamentally based on the theory of quantum entanglement." Dittrich et al explicitly say "In this work we will therefore apply the Migdal-Kadanoff scheme [59, 60] and the tensor network renormalization (TNR) method [61, 62]."

 

[marcus]

marcus, thanks for the time spent for discussing and compiling this list.

You are right, when looking at you September poll it becomes clear that it's hard to vote for one specific LQG paper (in the past it was hard to vote for one single QG paper, now even for one specific approach there are many interesting new aspects).

Besides the papers you already have in your list I would add Coarse graining methods (which is the first attempt towards Kadanoff's block spin approach in the LQG context; I was waiting for something like that for years), Emergent Braided Matter (which is still an active but unfortunately small and slow research project) and of course Thiemann's papers trying to link spin foams and the canonical approach.

And of course Han's paper on the cc - especially b/c it shows that even the basic algebraic structure to be used is still under discussion.

marcus, honestly: do you really think that this is the Current status of LQG? It seems that it is a very active research program, but at the same time the big picture is (partially) missing. I think we don't know (yet) how to fit these puzzle pieces together:
- canonical and covariant formulation
- renormalization in the canonical approach (what is H?), renormalization a la Kadanoff, ...
- asymptotic safety
- cc as running parameter in the asymptotic safety approach, cc as a quantum deformation
- matter on top of LQG vs. emerging braided matter ...
- ...

I am afraid that the situation becomes comparable to string theory: plenty of indications, little hard evidence, no experimental facts. Maybe we are simpy not able to do physics w/o experiments!

There is a lot of truth in what you say. First of all, as you indicate, LQG is a research program. When we try to describe the current status of LQG we are talking about the status of that program

That covers a number of different initiatives, some more active than others. Some approaches can drop out of sight for a while---seem hardly to exist---and then regain prominence and importance.

For me, the picture goes through periods when it looks focussed and coherent, and then other times when it seems more fragmented and in flux.

I can't serve as anything more than an onlooker with my own personal impressions, so you mustn't take it too seriously when I say that I don't see much future for some things that both you and other smart informed people see as interesting. But I see OTHER new formulations that I think could challenge the dominant "Zakopane" version.

I think the expression is "dark horse"----the contender nobody noticed was even in the race.
This alternative formulation intrigues me right now: http://arxiv.org/abs/0907.4388 . it may be something that you noticed and commented on two years ago, but I did not realize at the time was interesting.

What sparked my interest in this alternative Lqg formulation is this recent talk:
http://pirsa.org/11090125
given 21 September, titled Loop Gravity as the Dynamics of Topological Defects

...Loop states measure the flux of the gravitational magnetic field through a defect line. A feature of this reformulation is that the space of states of Loop Gravity can be derived from an ordinary QFT quantization of a classical diffeomorphism-invariant theory defined on a manifold. I'll discuss the role quantum geometry operators play in this picture, and the [prospect] of formulating the Spin Foam dynamics as the local interaction of topological defects.​

This talk is based on 0907.4388 and has 24 very good slides (in fact my computer cannot get the Flash video so all I can see are the slides!) Slide 22 has an interesting point:

Appealing scenario for Quantum Gravity
No trans-Planckian d.o.f. because topological (and therefore finite) at small scales
at larger scales, finitely many d.o.f. which can be described effectively in terms of a local quantum field theory.​

In the Perimeter pdf, slides are repeated so there are 48 pdf pages and this slide #22 does not appear until page 41. So you have to scroll down a lot to get there.

I suppose that this Topological Defect approach is not automatically unfriendly to braids. But one could also see them as two rival approaches, both very much on the periphery with only a few people currently attending to them. Bad luck that PIRSA, out of all its great collection of video lectures, just managed to lose or spoil this one, or that it just happens to be the one that my computer cannot read.

 

[marcus]

I think there has to be an exploration of language so that when new data does come we'll be more ready to describe it.

http://arxiv.org/abs/1109.4927" are in the spirit of Kadanoff's "block spin renormalization"...

I think you are right about being more ready. It does not matter if the LQG program has several different approaches being worked on. Not all the pieces need to be connected all times. What matters is that somebody has an approach which they are willing to say "This is the theory." and which they can calculate with and confront with observation.

In the case of Loop Gravity, I expect modeling of the cosmological bounce, calculating features of the CMB ancient light, and confrontation with polarization data from whatever mission comes after Planck. (Or possibly even with data from the current Planck mission.)

I know that Dittrich et al is largely about Migdal-Kadanoff method and they say a lot about the ability to calculate. What I am curious about is calculating WHAT? Can you or anyone help me imagine what kind of massive lattice calculation might be in view? Can you see how this might connect up with CMB observations, for instance, or with some other data?

Maybe it could lead to simulations of the bounce? Or or of black hole collapse?

I'm convinced that Dittrich's work will play a critical role, I just wonder what that role might be, more specifically. How do you picture it?

BTW Atyy, Eugenio Bianchi in his 21 September PIRSA talk referred to something of XG Wen.
It is on one of those slides. I checked: slide 23/24 on page 46/48 of the PDF