What caused the shift of interest in quantum cosmology?

[Sardano]

I don't think you comment fairly, Atyy. Some of what Sardano said about the change of focus in the String program (e.g. towards AdS/CFT) is quite true. But this is not central to the discussion of the shift of interest in QC which we are trying to understand in this thread.

Or if you think internal changes in String program are important to where the field of quantum cosmology is developing, and why, then please explain why you think that

On the String Theory side you are not making the right search. For example, virtually, all the supergravity papers are String Theory papers, because they are focused in the interest of Supergravity as a Strig Theory effective field description, and they include a lot of topics, like phenomenology, Black Holes in String Theory...However, you missed them in your searchs.

 

[marcus]

I think you were the first one to bring in strings.

Wait just a minute now! There is no taboo against mentioning the decline in the String program if it is relevant (although it may not be the whole cause of what has happened in QC). The decline in string cites that I mentioned includes all aspects of the String program! Don't take the easy denial route and seem to agree with Sardano that I was simply "looking at words in the title". I examined any papers in the Spires top 50 for each of these years that could possibly be stringy. I think if you wanted to take the time to check them out your classification would agree with mine.

Moreover the general decline in interest IS arguably a factor when we look at the specialized area of quantum cosmology. It is not the whole set of causes but it can be argued that it plays a role.

So if you think I could have misclassified, please look at the list and find some examples.
=====quote=====
It used to be that as many as twelve recent string papers would make the annual Spires top 50 list---the most cited papers during a particular year.

Here are top 50 lists for some past years with number of recent string papers making the list shown in parentheses.

http://www.slac.stanford.edu/spires/topcites/2001/annual.shtml (twelve)
http://www.slac.stanford.edu/spires/topcites/2003/annual.shtml (six)
http://www.slac.stanford.edu/spires/topcites/2005/annual.shtml (two)
http://www.slac.stanford.edu/spires/topcites/2007/annual.shtml (one)
http://www.slac.stanford.edu/spires/topcites/2009/annual.shtml (one)
http://www.slac.stanford.edu/spires/topcites/2010/annual.shtml (zero)

In this tally, papers are counted as recent if they appeared during the past five years. For instance in 2001 (recent meaning 1997-2001) twelve of the most highly cited fifty were recent string . Their ranks were 2,3,4,5,6,13,14,17,22,39,49, and 50.

By contrast in 2009 (recent being 2005-2009) only one of the fifty top-cited papers was recent string . It was number 33 on the list.
==endquote==

 

[marcus]

On the String Theory side you are not making the right search. For example, virtually, all the supergravity papers are String Theory papers,...

I think you may be living in a fantasy world where you attribute, say to Lance Dixon (an expert in SUGRA), a stringy motivation which he does not necessarily have.

But you are free to say this or that logically separate research line is "really string" and construct your own searches. If they have some bearing on the recent shift of interest in quantum cosmology (I use the DESY librarians definition of that category) then be my guest! You are welcome to post them.

Keep in mind that quantum cosmology as most people understand it is not primarily about Black Holes . It tends to be about the early universe, and moreover it is quantum rather than classical.

 

[marcus]

...Do you really think the new spin foam cosmology is the reason Bojowald's stuff is so nice?...

Aren't you ignoring the elephant in the room? Ashtekar is the central figure in Loop cosmology and his papers really define where the field is at present.

I'm not sure how you think "Bojowald's stuff" differs from the current mainstream of Loop cosmology. What is different or special about Bojowalds recent (say 2009-present) work?

 

[Sardano]

I think you may be living in a fantasy world where you attribute, say to Lance Dixon (an expert in SUGRA), a stringy motivation which he does not necessarily have.

.

If you are talking about the finitness of N=8 Sugra I agree, it is nowdays basically the only case in which a Supergravity Theory is studied "per se". However, it is related to string theory in the methods used and in the conclusions, because N=8 is the low energy field theory of type II compactified in torus down to 4 dimensiones, and much of the work made in the topic can be used to extract conclusions in String Theory. See the work of Kallosh for more details.

I all the other cases, the study of supergravity and lately, supergravity plus stringy corrections is always related to string theory.

 

[Haelfix]

It has been explained multiple times now, but the keyword 'quantum cosmology' is basically a buzzword for things like the no boundary proposal, loop quantum cosmology, the Wheeler De Witt equations and things like that.

When string theorists study cosmology, it typically involves models like string gas cosmology or membrane cosmology and the idea is to generate suitable conditions to start inflation and/or to explain the inflaton field as well as other questions (like why 4 dimensions). This is just as testable (in fact testable in the exact same way) as anything from LQC, with the caveat that probing the power spectrum suffers from an enormous inverse problem.

As far as quantum cosmology proper, string theorists lack the mathematical technology to really ask questions about how the quantum foam behaves or if it even exists (that is a question in the strong coupling limit of string theory that is still very opaque even in contexts like AdS/CFT). The few papers on the subject that exist, typically go back to the more established textbook treatments like trying to derive a Hawking-Hartle state.

So there has been no 'shift of interest', instead there hasn't been much interest at all in the same way that there wasnt interest in studying classical fluid mechanics with string theory (up until recently) since the mathematical connections weren't there yet.

However if you want to ask 'what does string theory have to say about cosmology' then by all means, write 'cosmology' or 'inflation' or 'compactification' as keywords and let us know what you get.

 

[marcus]

...As far as quantum cosmology proper, string theorists lack the mathematical technology to really ask questions about how the quantum foam behaves or if it even exists (that is a question in the strong coupling limit of string theory that is still very opaque even in contexts like AdS/CFT). The few papers on the subject that exist, typically go back to the more established textbook treatments like trying to derive a Hawking-Hartle state.

So there has been no 'shift of interest', instead there hasn't been much interest at all in the same way that there wasnt interest in studying classical fluid mechanics with string theory (up until recently) since the mathematical connections weren't there yet.
...

Haelfix that is an interesting point of view. Thanks for sharing your ideas!

It could be that the main change I am noticing here is primarily the growth of interest in Loop quantum cosmology.

I am definitely focusing on quantum cosmology proper, as you call it. I think that is primarily the resolution of the classical singularity ("big bang") by quantum means.

 

[fzero]

I think you may be living in a fantasy world where you attribute, say to Lance Dixon (an expert in SUGRA), a stringy motivation which he does not necessarily have.

Have you ever read one of his papers? A huge part of the program that Bern, Dixon and Kosower have championed for the past 15-20 years is based on string inspired relations between open string amplitudes (which give gauge theory amplitudes) and closed string amplitudes (which give (super)gravity amplitudes). These are referred to as Kawai-Lewellen-Tye (KLT) relations in Dixon's most recent papers. Twistor methods (again string inspired) have also been a huge part, but the most recent papers on N=8 sugra amplitudes rely almost entirely on the KLT relations between N=8 sugra and N=4 SYM amplitudes. This is an entire, extremely fruitful, branch of research that would not exist in the absence of string theory.

But you are free to say this or that logically separate research line is "really string" and construct your own searches. If they have some bearing on the recent shift of interest in quantum cosmology (I use the DESY librarians definition of that category) then be my guest! You are welcome to post them.

Again, these are the same librarians that labeled a single paper with the term "quark model"
in the years 1995-1999 (http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+quark+model%29++AND+DATE+%3E+1995+and+date+%3C+2000&FORMAT=www&SEQUENCE=citecount%28d%29 ), which is incidentally the 4 1/2 year period after the discovery of the top quark. They may have well-intentioned motivations behind their labeling system, but it's pretty obvious that it's not useful for any reliable statistics.

 

[atyy]

Aren't you ignoring the elephant in the room? Ashtekar is the central figure in Loop cosmology and his papers really define where the field is at present.

I'm not sure how you think "Bojowald's stuff" differs from the current mainstream of Loop cosmology. What is different or special about Bojowalds recent (say 2009-present) work?

No, I include Ashtekar automatically when I say Bojowald (like I automatically include Wegner if I say Wilson loop). The distinction I was drawing is between the canonical approach and the new spin foam dipole calculations. The former, which includes Bojowald, Ashtekar, Henderson, and others I'm not remembering off the top of my head, seems beautifully controlled, if under somewhat artificial conditions. The latter drops many higher order terms, whose unimportance has not been demonstrated.

Let me expand a bit. I don't believe spin foams is on the right track. I think the formalism has potential, but they are ignoring AdS/CFT as to its correct interpretation. However, there are two nice results from non-string approaches that at present are in isolation. The first is CDT's, the second is Bojowaldian LQC. What is the correct context to place them in? Vidotto and Rovelli are trying to place it within the Rovellian interpretation of spin foams, which I suspect will not work (I could of course be wrong), and which is undeniably kludgey at the moment, so I am wondering if there are any othe ropenings known to you for explaining Bojowaldian LQC.

Incidentally, I wonder what Loll's thinking of the underlying theory for CDT is at the moment. Asymptotic Safety is no longer seen to be so probably the reason. I wonder if she has strings at the back of her mind.

 

[Haelfix]

It could be that the main change I am noticing here is primarily the growth of interest in Loop quantum cosmology.

Yep!

I am definitely focusing on quantum cosmology proper, as you call it. I think that is primarily the resolution of the classical singularity ("big bang") by quantum means.

Maybe, maybe not. Again it depends really what the correct degrees of freedom are in the quantum gravity world.

Historically, the fundamental object of study in quantum cosmology was the space of connections modulo gauge transformations. This was then inserted into a formal generalization of a path integral (with appropriately illdefined measure), and then various approximations or boundary conditions were imposed on the object (mini/midi superspace, etc).

But the key thing to note is that the ensuing wave function of the universe, was still characterized by the starting point, which was still all about the metric (or really the connection symbols).

If string theory is correct, such a thing will probably be only yet another crude approximation to the fundamental underlying physics, which contains much more information than merely the classical geometry.

 

[marcus]

I think Haelfix's point (if valid) would lead to the conclusion that the String numbers in this table are irrelevant. The String contribution to what we would consider quantum cosmology proper (quantum resolution of the classical bb singularity, basically) is not ready to be counted because the math is not yet there.

To get this table, I went to the Stanford-SLAC Inspire search engine and use the keyword "quantum cosmology", ranking by citation count. Then counted the number of papers of each kind which made the top 25. It would be interesting to know if anyone gets a different count.

Papers in the QC top 25
Years   1996-1998  1999-2001  2002-2004  2005-2007  2008-2010
String       5          6          3          5          1
Loop         0          7         16         16         16

Perhaps I should just erase the first row because, if Haelfix is right, it is not meaningful. There was no significant "shift away" (in that case) but only a "shift towards". This could be true! and some readers may want to look down the "top 25" search result listings to see what kinds of research is actually involved. It could very well corroborate what Haelfix just said:
1996-1998
http://inspirebeta.net/search?ln=en...2y=1998&sf=&so=a&rm=citation&rg=25&sc=0&of=hb
1999-2001
http://inspirebeta.net/search?ln=en...2y=2001&sf=&so=a&rm=citation&rg=25&sc=0&of=hb
2002-2004
http://inspirebeta.net/search?ln=en...2y=2004&sf=&so=a&rm=citation&rg=25&sc=0&of=hb
2005-2007
http://inspirebeta.net/search?ln=en...2y=2007&sf=&so=a&rm=citation&rg=25&sc=0&of=hb
2008-2010
http://inspirebeta.net/search?ln=en...2y=2010&sf=&so=a&rm=citation&rg=25&sc=0&of=hb

In the last interval, 2008-2010, the one String paper I found in the top 25 QC was actually a review of bounce cosmologies some part of which was about stringy bounces. It was not what you normally think of as stringy research but it did discuss string QC as part of a general review. The paper was
Bouncing Cosmologies.
M. Novello, S.E.Perez Bergliaffa
and it has 76 citations.

 

[Sardano]

Anyway, Marcus, what are you traying to say?

Are you traying to say that string theorist are are changing their fields to LQG?

Are you traying to say that the new researchers are choosing LQC in front of String Theory?


Because if there is really a growth in the number of LQC papers is because the people that used to do LQG is now doing LQC, since LQG is obviously a dead end. You should compare your numbers with the number of LQG papers among the most cited ones. You will see that if LQC is going up, LQG is going down in more or less the same amount. I haven't checked, its a prediction ;) .

 

[marcus]

...

Because if there is really a growth in the number of LQC papers is because the people that used to do LQG is now doing LQC, since LQG is obviously a dead end. You should compare your numbers with the number of LQG papers among the most cited ones. You will see that if LQC is going up, LQG is going down in more or less the same amount. I haven't checked, its a prediction ;) .

Thanks for your prediction.

 

[marcus]

BTW after weighing some of the well-considered points in this thread and thinking some more about it, I am leaning toward the view that what we are seeing in quantum cosmology could simply be described as growth of interest in the bounce picture---mainly the Loop cosmology version perhaps (judging from the make-up of the most highly-cited QC papers---but perhaps not even that specific.

Maybe the key thing here is growth of research interest in bounce cosmology. Haelfix may have concurred with a "yep" to something like that, no time to check right now.

The physics features that could explain that might be (you may want to suggest others):

a) testability
b) simplicity (no extra baggage)
c) concreteness (you can calculate and derive observable predictions leading to a.)
d) natural compatibility with inflation.

Part of the concreteness is that bounce provides a platform for stating initial conditions, since it replaces singularity with a describable event. Probability distributons on initial conditions can be defined, and it then offers a mechanism for triggering inflation, and for juicing up an inflaton (if one is included in the picture) to render the desired number of e-folds.

The bounce gives something concrete and testable that fits together with inflation rather well. It does not logically need to be followed by an inflation episode (the bounce has its own brief inflation without any extra assumptions) but if you want inflation it can eliminate some of the fine-tuning.

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

Here I am just trying to guess what might be physical reasons that researcher interest has grown in the direction of bounce cosmology. That may be what we are looking at here, and I'd like some idea why it has happened.

 

[atyy]

The physical reason is that Bojowaldian LQC is the first place anywhere in LQG-related stuff that something like the Einstein equations have appeared in a principled way - I can make a theory with one fairy so it's simple, and have it predict a bounce so it's testable, but it's unprincipled so no one will be interested in it.

Does anyone find it ironic that background independence was discarded for a hint of the Einstein equations to appear?

 

[Sardano]

The physical reason is that Bojowaldian LQC is the first place anywhere in LQG-related stuff that something like the Einstein equations have appeared in a principled way - I can make a theory with one fairy so it's simple, and have it predict a bounce so it's testable, but it's unprincipled so no one will be interested in it.

Does anyone find it ironic that background independence was discarded for a hint of the Einstein equations to appear?

Great comments. I had never though about it, but now that you mention it, yes it is ironic. It evenis more ironic if you recall that "background independence" is one of the criticism to String Theory that the LQG people uses. It is useless to explain them that String Theory is background independient, but we just know how to "do String Theory" in some particular consistent backgrounds. Its like saying that General Relativity is not background independient because I am working with a concrete solution and trying to obtain physical information from it.

 

[Sardano]

Anyway, the tendency in both fields is the same: try to work in something feasible with as direct as possible physical implications. As a result the LQG people goes to LQC and the String Theorist tend to work in things like ADS/CFT and related.

 

[marcus]

...
Because if there is really a growth in the number of LQC papers is because the people that used to do LQG is now doing LQC, since LQG is obviously a dead end. You should compare your numbers with the number of LQG papers among the most cited ones. You will see that if LQC is going up, LQG is going down in more or less the same amount. I haven't checked, its a prediction ;) .

I think you must be joking, because this is so obviously wrong. Both sets of numbers are going up: Loop cosmology and also the other Loop gravity research (that is not about cosmology).

If you seriously meant that as a prediction, you can falsify it easily with a Spires search.

To get you started here is a search for Loop overall (including cosmo). Type or paste this into the search box at Spires:

FIND DK SPIN,FOAM OR QUANTUM GRAVITY, LOOP SPACE OR QUANTUM COSMOLOGY, LOOP SPACE AND DATE = 2005

and then repeat, each time increasing the date. You will get

Loop gravity research overall
2005    41
2006    81
2007   121
2008   142
2009   141
2010   154

You draw a false analogy between trends in the two communities. People have not "discarded" background independence and moved OUT of developing the main LQG theory in favor of working in the cosmo application. Both lines of research (main and applied) are growing and attracting new people.

I would say that Atyy is wrong if he means that the main evidence that the theory recovers Gen Rel is in "Bojowaldian" LQC. The bulk of the evidence concerns the main (background independent) theory, not the application.

No one in the Loop community, that I've seen, talks about "Bojowaldian" LQC and I'm not sure what Atyy means. That word is his alone. Bojowald's original formulation of LQC was
flawed and was replaced by other people's in 2006. If he really means Bojowaldian then most of what he says seems to be wrong or misleading because it is not the current (post 2006) mainstream.

In any case several recent Atyy comments don't make very much sense. And yours, Sardano, seem based on a fairly general deficiency of information.

I think we are, however, getting closer to understanding why there has been a growth of interest in Loop cosmology. There are physical reasons for this surge of growth.*

There also seems to have been a decline of interest (activity and citations) in the String program proper---actual work with string and brane models---but these two things may very well be unrelated. They may have occurred for a separate set of physics reasons.

*simple, concrete, testable, based on spinfoam QG (a background independent theory.)

No time to edit, I'll get back to this later today.

 

[Sardano]

I think you must be joking, because this is so obviously wrong. Both sets of numbers are going up: Loop cosmology and also the other Loop gravity research (that is not about cosmology).

If you seriously meant that as a prediction, you can falsify it easily with a Spires search.

To get you started here is a search for Loop overall (including cosmo). Type or paste this into the search box at Spires:

FIND DK SPIN,FOAM OR QUANTUM GRAVITY, LOOP SPACE OR QUANTUM COSMOLOGY, LOOP SPACE AND DATE = 2005

and then repeat, each time increasing the date. You will get

Loop gravity research overall
2005    41
2006    81
2007   121
2008   142
2009   141
2010   154

Marcus, you do not have to choose the words in the search in order to obtain the values at your will: you only will obtain fake results. It is very simple. If you are looking for LQG related papers, you write like this (in Inspire, of course):

LOOP QUANTUM GRAVITY AND DATE=YEAR

If you want to look for LQC papers, you have to write like this:

LOOP QUANTUM COSMOLOGY AND DATE=YEAR


Results:

LQG

2006 104

2007 106

2008 134

2009 128

2010 132

As you can see, in the last years the number of papers has no significatively changed.


LQC

2006 33

2007 58

2008 62

2009 72

2010 76

In the last years the number of LQC related papers has growth in a few number of papers. Not significant. ;). Speak about "more general interest" in LQC is therefore false, since if that were true, I should expect an increase in the number of papers of a few hundred. In fact, the number of LQC papers is ridiculous.

STRING THEORY

2006 1513

2007 1506

2008 1545

2009 1412

2010 1453

No significant changes.

 

[marcus]

I tend to suspect you don't have valid DESY keyword searches there, Sardano.
Let's see the links you are using for, say, 2007.

Oh! I see you are using Inspire. You may have forgotten to set it to do a keyword search, so it is just looking for occurrences of words in various fields, like the title, abstract etc.

I've had a lot of experience watching the Loop literature over the years and find that at least after 2006 or so the DESY librarians' classification is pretty reliable. I trust the librarians cataloging rather than a computer just picking out occurrences of words.

You may just be clowning. Your numbers for String Theory are way way off (much too low).
I don't know if I should take you seriously. The others numbers you get I would imagine are meaningless. (I expect I could immediately spot false positives, and if I wished to take more time could discover omissions as well, I imagine).

 

[Sardano]

Oh! I see you are using Inspire. You may have forgotten to set it to do a keyword search, so it is just looking for occurrences of words in various fields, like the title, abstract etc.

No, I havent. And the numbers are the real results. You can check yourself.

 

[marcus]

No, I havent. And the numbers are the real results. You can check yourself.

Then please provide the links that get your searches for, say year 2008.

I may have figured out which you used, I get 134, the same number you listed for 2008, with this search:
http://inspirebeta.net/search?ln=en...Search&sf=&so=d&rm=citation&rg=100&sc=0&of=hb

It gets this string theory paper:
http://inspirebeta.net/record/796131
and this philosophy paper:
http://arXiv.org/abs/arXiv:0804.3742
Maybe that's all right, not too bad. My Spires searches may pick up a few false positives too.

Now I do exactly the same search (using the beta version of Inspire, as you did) but for 2005:
http://inspirebeta.net/search?ln=en...Search&sf=&so=d&rm=citation&rg=100&sc=0&of=hb
I get 79.

Again, same search but for 2006, I get 104. Same number you got. Very likely these are the settings you used, then.
I noticed this false positive:
http://arXiv.org/pdf/hep-th/0602256
but again it doesn't seem way off.

 

[atyy]

See post #114 for what I mean by "Bojowaldian".

 

[marcus]

@Sardano,
I see what your trouble was! You only put in "loop quantum gravity" and you left out the keyword "spin foam"! After 2008 or so much LQG is formulated with spin foam dynamics (instead of the old canonical/hamiltonian formulation)

I still don't fully trust the beta version of Inspire. It was unstable and buggy when I tried it a month or so ago. But I see what you were doing and I can correct the numbers!

Put in "Loop quantum gravity AND spin foam AND year = xxxx"
and now let's see what numbers we get starting 2005

and then repeat, each time increasing the date. You will get

Loop gravity research (using Inspire beta)
2005    96
2006   118
2007   125
2008   159
2009   164
2010   173

Here for example is the link for the Inspire 2009 search in case anyone wants to see the settings or run down the list.

 

[marcus]

@Sardano,
I was able to reproduce your LQG numbers just by leaving out the "spin foam" key word. So I think I understand how you are doing Inspire searches.

I will now do the same thing for LQC---just put in "Loop quantum cosmology AND year = xxxx"
and now let's see what numbers we get starting 2006 when the Bojowald version was discarded and replaced by new dynamics.

Loop cosmology research (using Inspire beta)
2006    33
2007    58
2008    62
2009    72
2010    76

Your "prediction" was that LQG would decline to balance increase in LQC---that is no net increase in total activity. But actually both LQC and total LQG were increasing. However it does seem that the non-cosmo level stayed roughly flat! If we go by what Inspire says, then the growth was mostly in the cosmology part.

Loop gravity research (using Inspire beta) minus the LQC contribution
2006   118-33    85
2007   125-58    67
2008   159-62    97
2009   164-72    92
2010   173-76    97

That is not exactly what you predicted, but it kind of parallels it and so I have to give credit for a good hunch.

 

[marcus]

Anyway I wish people would stop trying to talk away the growth of interest in Loop quantum cosmology and try to offer some physics reasons for it!

Sardano, I appreciate your showing how to put Inspire into use (I was still using Spires and still consider it more reliable.)

But in any case the growth in interest should be obvious and it is not so interesting just to tabulate the numbers. What do you think are the causes?

Do you really think that it is merely because it is easy, or as you say "feasible"?

Anyway, the tendency in both fields is the same: try to work in something feasible with as direct as possible physical implications. As a result the LQG people goes to LQC and the String Theorist tend to work in things like ADS/CFT and related.

Is drawing an analogy with the shift in String program to "things like AdS/CFT an adequate explanation?

Personally I suspect it has a lot to do with the prospect of testing, and the masses of early universe data which are coming in. If that is what you mean (I don't see the analogy with AdS/CFT but) you might be right, or we could be in agreement.

Also to be explained is the upsurge in citation numbers. I gave a table of those earlier, a few posts back. They multiply the effect, one might say, of the increasing numbers of Loop papers, and represent additional demand for explanation.

 

[atyy]

BTW, the physics reason I gave in post #120 is exactly what this advertisement says too:
http://www.aei.mpg.de/english/research/teams/quantumGravity/index.html

 

[marcus]

BTW, the physics reason I gave in post #120 is exactly what this advertisement says too:
http://www.aei.mpg.de/english/research/teams/quantumGravity/index.html

Then it should be interesting to quote the relevant part of that---it's the philosophy and mission statement of the Quantum Gravity and Unification division at Potsdam MPI Albert Einstein Institute, led by Hermann Nicolai. As you know, the division hosts LQG and Group Field Theory/Spin Foam as well as String research. It could help me understand your view of LQG better if we see what we can glean about Nicolai's view of it from this statement.
===quote===
Quantum Gravity & Unified Theories

Director: Prof. Dr. Hermann Nicolai
This division is concerned with the unification of general relativity and quantum mechanics into a theory of quantum gravity, which also provides a consistent framework for incorporating the other fundamental forces in nature.

Despite intense efforts over the last years it is far from clear at this time what a consistent theory of quantum gravity will look like and what its main features will be. In view of these uncertainties, the best strategy appears to be one which is both diversified and "interdisciplinary". For this reason, the division aims to represent the major current approaches to quantum gravity, in particular string theory and loop quantum gravity.

The canonical approaches to quantum gravity emphasize the geometrical aspects and appear well suited to deal with unsolved conceptual issues of quantum gravity, such as e.g. the "problem of time" or the interpretation of the "wave function of the universe". Important new insights have been gained over the past decade in the framework of loop quantum gravity, which is one of the division's main research directions. This approach, which complements and extends the old "geometrodynamics" approach, employs a non-perturbative and background independent framework allowing to describe the fluctuations of geometry itself, and leading to a discrete structure at the Planck scale. On this basis, it is now possible to study the full quantum dynamics of gravity. Most recently, these concepts have been successfully applied to the study of cosmological or black hole singularities, where classical general relativity breaks down, spawning an entirely new field of research, loop quantum cosmology.
===endquote===

BTW Nicolai's division of the AEI has a number of people working on Spin Foam QG although he does not stress that in the statement you linked us to.

I see a 2009 paper from them, a 2010 paper, and two spin foam/path integral papers already in 2011. The spin foam approach is just as "principled" as canonical and the two are closely linked. In the lexicon of Nicolai's outfit spin foam QG is one of the "covariant" as contrasted with "canonical". So you see these people listed at AEI:

Banisch, Ralf Canonical and Covariant Dynamics of Quantum Gravity

Benedetti, Dr. Dario Canonical and Covariant Dynamics of Quantum Gravity

Dittrich, Dr. Bianca Canonical and Covariant Dynamics of Quantum Gravity

Eckert, Frank Canonical and Covariant Dynamics of Quantum Gravity

He, Dr. Song Canonical and Covariant Dynamics of Quantum Gravity

Martin-Benito, Dr. Mercedes Canonical and Covariant Dynamics of Quantum Gravity

Ryan, Dr. James Canonical and Covariant Dynamics of Quantum Gravity

E.g. two 2011 papers on the "covariant" side, and another from 2010:
1. arXiv:1103.6264 [pdf, ps, other]
Spin foam models with finite groups
Benjamin Bahr, Bianca Dittrich, James P. Ryan
47 pages, 6 figures

2. arXiv:1101.4775 [pdf, ps, other]
Perfect discretization of reparametrization invariant path integrals
Benjamin Bahr, Bianca Dittrich, Sebastian Steinhaus
22+8 pages

4. arXiv:1006.4295 [pdf, ps, other]
Simplicity in simplicial phase space
Bianca Dittrich, James P. Ryan
29 pages, 1 figure

Incidental info:
http://www.iem.csic.es/departamentos/qft/CV/CV_Martin-Benito.html

 

[marcus]

See post #114 for what I mean by "Bojowaldian".

Then I suggest you find a better word, like Hamiltonian LQC or canonical LQC.

Bojowald's formulation of LQC was seriously flawed. Bill Unruh among others pointed this out around 2004. Bojowald LQC was finally abandoned in 2006 when Ashtekar (with Corichi and Singh) came out with the new equations. The socalled LQC "new dynamics".

By now the Ashtekar et al version is just what people mean by canonical LQC.

You indicated you approved of that Hermann Nicolai statement. It uses the term "canonical", which says it clearly and is less likely to cause confusion.

 

[atyy]

Then I suggest you find a better word, like Hamiltonian LQC or canonical LQC.

Bojowald's formulation of LQC was seriously flawed. Bill Unruh among others pointed this out around 2004. Bojowald LQC was finally abandoned in 2006 when Ashtekar (with Corichi and Singh) came out with the new equations. The socalled LQC "new dynamics".

By now the Ashtekar et al version is just what people mean by canonical LQC.

You indicated you approved of that Hermann Nicolai statement. It uses the term "canonical", which says it clearly and is less likely to cause confusion.

I suppose you will not approve of the "Kadanoff-Wilson" renormalization group!

I used to say "Wilsonian", but every friend of mine who's in condensed matter (a small sample admittedly, perhaps those who hang around these fora can correct me) said well, Kadanoff had almost all of it.

Anyway, I bet it is agreed even outside of the LQG community that canonical LQC is neat and intriguing. It's a cute result, like CDT, that is looking for an explanation. I doubt there's similarly broad agreement on EPRL or FK.

If you recall, AS was thought to be the probable foundation for CDT some years ago. Now, other guesses have become prominent, although AS is still the leading guess. Similarly, I feel (yes, just a feeling) that EPRL and FK are not the way to go, and so they will not be the explanations for canonical LQC (I suspect some other spinfoams could do it).

 

[atyy]

BTW Nicolai's division of the AEI has a number of people working on Spin Foam QG although he does not stress that in the statement you linked us to.

Yes, my point is not that spin foams are uninteresting (they are very interesting). My point is that you asked why did interest in LQC increase. I think everyone except you has said that it's due to what's been achieved in canonical LQC itself. It has nothing to do with the non-existent decrease of interest in string, and little to do with spin foams. And what has been achieved? Well, canonical LQC is the first place, and only place so far in LQG that anything like Einstein's equations have popped out of a theory that is completely well-defined and thus deserves to be called QG. And this was achieved by discarding background independence.

So to use words from MPI's advertisement, I would say "LQC made it possible to study the full dynamics of quantum gravity, even near black hole and cosmological singularities".

 

[marcus]

Yes, my point is not that spin foams are uninteresting (they are very interesting). My point is that you asked why did interest in LQC increase. I think everyone except you has said that it's due to what's been achieved in canonical LQC itself. It has nothing to do with the non-existent decrease of interest in string, and little to do with spin foams. And what has been achieved? Well, canonical LQC is the first place, and only place so far in LQG that anything like Einstein's equations have popped out of a theory that is completely well-defined and thus deserves to be called QG. And this was achieved by discarding background independence.

That sounds like a fairly reasonable perspective on things, though I think you underemphasize (possibly ignore) some parts of the picture.

I don't recall saying that the decline of the String program was a cause of increased interest in LQC. I imagine that is something you thought I said. On the other hand I think the decline in citations in several areas is real---it is just an objective fact, not "non-existent", and helps to put things in perspective without being alleged as a cause.

The Einstein equations have not popped out of canonical LQC. The Friedmann equation is a radical simplification, so I look at that "popping" simply as one of several bits of evidence which encourage optimism that Loop is turning out to be right. I think you are mistaken to give heavy emphasis to the evidence from canonical LQC and ignore other types of evidence. It gives an impression of distorted vision. But that's how you see things, according to what you say. So be it.

I have been suggesting here that the increase of interest in canonical LQC can be attributed to intrinsic physics features (eg simplicity, concreteness, testability) which have been "achieved" in canonical LQC, to use your word.

So the main message in your post agrees with what I have been saying---and then says that I have not been saying it

Now the same features or "achievements" are beginning to be carried over to the full theory, but so far, as you know, there are only a few papers. These are the spin foam cosmology papers which show the possibility that the results from canonical LQC (radical simplification which boils the universe down to a couple of numbers) may carry over to a formalism which is NOT such a simplification and which is in fact background independent.

In that sense background independence was never discarded. No theory can properly be said to recover GR that doesn't explicitly have BI, so it was always on the agenda to work out a connection between LQC and the full theory. Now that is beginning to happen and I expect voices of belittlement and denial will simply call attention to the fact.

 

[atyy]

In that sense background independence was never discarded. No theory can properly be said to recover GR that doesn't explicitly have BI, so it was always on the agenda to work out a connection between LQC and the full theory. Now that is beginning to happen and I expect voices of belittlement and denial will simply call attention to the fact.

Very prescient. Let me be the first to fulfill your prediction! The full theory doesn't even exist. But if it does, it won't yield the Einstein equations.

 

[marcus]

The full theory doesn't even exist. But if it does, it won't yield the Einstein equations.

This question of the existence of theories is interesting. Partly it simply has to do with conventional expectations that one has in a mathematical-type (empirical) science. A theory is a human artifact that therefore does not exist until the equations are written down.
And you have to be able to derive predictions from the equations so the theory can be tested (or it has no content).

Also we've come to expect, at least in this area of physics, that a theory should rest on some basic principles, e.g. like background independence, general covariance, the finiteness of geometric information,...etc. Hard to be precise about that, but I think it's a case of "you know it when you see it": Open-minded people of good faith occasionally reach agreement on whether something is or is not based on principles, even though there might be no formulaic definition of what a principle is

There are some nebulous pie-in-sky things which don't yet exist as theories (and may never exist in the sense of some definite formulation.) Such a thing can even be named this or that "theory"---but that's just a hopeful misnomer.

 

[atyy]

Very prescient. Let me be the first to fulfill your prediction! The full theory doesn't even exist. But if it does, it won't yield the Einstein equations.

I see I forgot to add that by full theory I mean EPRL.

Actually, I was just looking at Vidotto and Rovelli's first spin foam cosmology paper. They seem to use KKL, so I guess they already agree with me;)