# My talk with Brian Greene

So I was at Brian Greene's lecture at Iowa State University yesterday. I had the chance to meet with him and talk to him. I asked him what his viewpoints were on Loop Quantum Gravity. He said something to the extent of:"It's a great theory. Probably a mix between ST and LQG is where the actual answer lies. But, for reasons of just recent happenings, I think LQG is going in the wrong dirrection. I don't like the way things are going."

What was he talking about? Does anybody have any idea why he would feel this way?

## Answers and Replies

marcus
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PRodQuanta said:
What was he talking about? Does anybody have any idea why he would feel this way?

but good for you for getting in there with your question.

If you are in a similar situation again, and get such an answer to a question, if you then say---"can you give me a reference for that? maybe to something I can look up on arxiv?"----then there is some chance someone here will be able to read the paper and explain.

I know it's not too likely that you will be asking B.G. questions again soon , or writing him email, but as a rule I dont think it is impolite to ask for
a reference-----and you might get an author's name or a keyword in the title or both. With even skimpy information one can often track down a paper and get a concrete definite idea of what he's talking about. Otherwise it's sort of at the level of hearsay or gossip and after several repetitions may not mean very much.

Actually, it sounds a bit like the answer you might get from a Loop gravitist, but switched around to be about string theory, in the symmetric situation.

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congratulations ! for a getting a chance to meet Green !

PRodQuanta said:
So I was at Brian Greene's lecture at Iowa State University yesterday. I had the chance to meet with him and talk to him. I asked him what his viewpoints were on Loop Quantum Gravity. He said something to the extent of:"It's a great theory. Probably a mix between ST and LQG is where the actual answer lies. But, for reasons of just recent happenings, I think LQG is going in the wrong dirrection. I don't like the way things are going."

What was he talking about? Does anybody have any idea why he would feel this way?

Great!

You may have hit him with what is rather a question he(or string theorists) do not want to hear at the moment, the truth is that LQG is bringing ST to its knees, and as Brian and a great number of theorists have invested so much of their academic careers's into ST, its more than likely 'they' are feeling quite lonely and bemused at the obvious succes and elevated status of LQG.

as was said above about references, where do you get "the truth is the LQG is bringing ST to its knees"? any references or information on WHY LQG is bringing ST to its knees?

Staff Emeritus
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PRodQuanta said:
So I was at Brian Greene's lecture at Iowa State University yesterday. I had the chance to meet with him and talk to him. I asked him what his viewpoints were on Loop Quantum Gravity. He said something to the extent of:"It's a great theory. Probably a mix between ST and LQG is where the actual answer lies. But, for reasons of just recent happenings, I think LQG is going in the wrong dirrection. I don't like the way things are going."

What was he talking about? Does anybody have any idea why he would feel this way?

He was probably referring to the questions about the validity of LQG quantization. After Thiemann published his LQG-style quantization of the closed string, there was a pile-on by non-LQG theorists claiming that some of the technical methods the LQG school used to promote their classical functions to quantum operators were shaky and maybe unjustified. This has not so far been refuted by the LQG people.

marcus
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Gecko said:
as was said above about references, where do you get "the truth is the LQG is bringing ST to its knees"? any references or information on WHY LQG is bringing ST to its knees?

W.H.Particle may have used the wrong image---the athletic image of two developing theories as wrestlers doesnt work for me personally.

There can be a competition between rival researchers: for funding, for public recognition, for secure jobs in institutions, for the brightest grad students, etc. This may not always lead to the best science. And that is mere academic sociology----it doesnt necessarily reflect a struggle between the theories themselves.

And even in people terms, the image of a wrestling match doesnt seem quite right to me. I'd say there's a small-but-growing number of Loop gravitists---for whom the struggle is more to gain any recognition at all.
Worldwide they seem only about 10 percent as numerous, definitely a minority although growing.

If there is a germ of truth in what W.H.Particle says, it is more like this:
Loop, as a still-incomplete-theory, has not done any damage to still-unripe-String (except to challenge its claim to be the sole unique viable prospect for understanding gravity) and Loop theorists rarely waste time criticizing String or engaging in paper-warfare. However String has, since jan 2003, done rather much damage to itself because of the confusion about the Sandscape and Anthropery. Researchers seem to be getting out of String theory proper and moving into neighboring areas like astroparticle physics and cosmology. This is due to INTERNAL reasons, not to Loop competition.
Whatever troubles String is having, Loop didnt do it to it.

Meanwhile Loop has been having a growth-surge, with a run of interesting results published in just the last few years, and a rapid growth in the number of papers published. Citations to Loop papers are up. So even though it may be small and nearly invisible in the public media, in academic terms the field is having a heyday. This may not be bringing anybody "to their knees" but it could be causing some envy (among those not participating) and a need to deny the value of the Loop approach.

Gecko has called for references, which is usually a good idea, and i will try to supply some for what I'm saying.

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marcus
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Gecko said:
as was said above about references, where do you get "the truth is the LQG is bringing ST to its knees"? any references or information on WHY LQG is bringing ST to its knees?

You have called for references to back up statements, and i think that is a good thing. My view is that basically that Loop is a small-but-growing approach. so I will back that up with the most basic data. virtually everybody posts on the arxiv (usually even before the paper is accepted for publication) and the annual paper-counts gotten with the arxiv search engine are an index of research activity. The search engine is not perfect and it gets some wrong and misses some, but it is still a rough indicator.
This is the same search, for Loop-and-allied approaches to quantum gravity, run over the course of 10 years.

-----these are from 5 November----

Year 1994:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/1994/0/1
Year 1995:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/1995/0/1
Year 1996:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/1996/0/1
Year 1997:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/1997/0/1
Year 1998:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/1998/0/1
Year 1999:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/1999/0/1
Year 2000:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/2000/0/1
Year 2001:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/2001/0/1
Year 2002:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/2002/0/1
Year 2003:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/2003/0/1
Last twelve months (e.g. 5 November 2003 to 5 November 2004):
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/past/0/1

Code:
1994    61
1995    83
1996    72
1997    70
1998    67
1999    76
2000    89
2001    98
2002   121
2003   140
LTM    162
---------

It is obvious that Loop, or anyway non-String, QG research is only about a tenth (rough order of magnitude) of String. And also obvious it is growing.
there's also been a slight increase in visibility in popular and non-specialist media, like Rudi Vaas articles in BdW, Smolin's Scientific American article, Rovelli's Physics World article. But that is a separate issue. String is incomparably strong in popular media. Here i am just talking about technical journal-type articles.

It's clear that nobody is bringing anybody to their knees!
At most one might say that in the past couple of years String has lost a bit of its self-image of being the only game in town or the sole best hope for progress in theoretical physics. And things are different if one compares the situation in US with non-US (like Canada, Mexico, UK, Germany, France, Italy, India, Netherlands, and now Edgar1813 has reminded us, Uruguay!)

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wow, thanks alot. great explainition, and thanks for the references, i'll try to read them.

marcus
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Gecko said:
as was said above about references, where do you get "the truth is the LQG is bringing ST to its knees"? any references or information on WHY LQG is bringing ST to its knees?

Gecko you have asked for "references or information", I may be giving you more than you want. I hope some of this is interesting. I find it fascinating that most of the recent progress in Loop is happening either at Penn State, or at centers outside the US. here is some info about that.

If I look at the papers that appeared this year, in Loop and allied research.
I see Penn State of course where Abhay Ashtekar has his Institute, and Louisiana State (where Jorge Pullin is) but besides that, look:

Berlin (Potsdam and Gölm are on Berlin's outskirts)
Cambridge
Chennai India
Imperial College London
Marseille
Mexico City
Montevideo
Pune India
Rome
Utrecht Netherlands
U. Western Ontario
U. British Columbia
U. New Brunswick
Waterloo (Perimeter Institute)
Wroclaw Poland

Something seems to be stifling non-String quantum gravity research at the major US universities----but letting it grow rapidly elsewhere. You may not find this interesting but it fascinates me. I am used to seeing US institutions at the leading edge of important theoretical physics developments.

Still no one has brought anyone "to their knees"-----the athletic analogy just does not work. And I think one group trying to discredit the other's research is bad, and usually based on out-of-date unrepresentative information too.

And I think "triumphalism" is always a mistake no matter on whose part.
It is actually counterproductive IMO.

marcus
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Gecko said:
wow, thanks alot. great explainition, and thanks for the references, i'll try to read them.

Gecko, thanks for the kind words! for my part I still haven't given you any references to things to read! Others may have. Up to now in this thread, my links here are just to search engine paper-counts to give an overall rough idea of how things are going.

Turning to reading material, in case you might want it, the three best online references to get an idea of what Loop gravitists are doing IMO are these:

Rovelli's Physics World (November 2003) article
http://cgpg.gravity.psu.edu/people/Ashtekar/articles/rovelli03.pdf
Ashtekar Gravity and the Quantum
http://arxiv.org/hep-th/0410054 [Broken]
Smolin An Invitation to Loop Quantum Gravity
http://arxiv.org/hep-th/0408048 [Broken]

These are some links of introductory stuff to read, but, you know, I dont think you should feel obliged to read up on Loop because as i understand it your personal interest is with String theory. There is no real need to compare and argue about what's best if you like String then maybe you should just go with it! It is most efficient when everybody focuses their energy on what appeals most to them. Well that is one viewpoint. I will get other links too if i think of some.

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Haelfix
I've talked with Brian a lot, he is a big fan of geometry in physics in general. Much of his work in ST has been on the geometry side of things (b/c of dualities its often easier to switch perspectives).

He very much likes background independance (moreso than most), but I think he is like most string theorists in feeling that without matter fields it will be very hard to ultimately make sense of things.

marcus
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Haelfix said:
I've talked with Brian a lot, he is a big fan of geometry in physics in general. Much of his work in ST has been on the geometry side of things (b/c of dualities its often easier to switch perspectives).

He very much likes background independance (moreso than most), but I think he is like most string theorists in feeling that without matter fields it will be very hard to ultimately make sense of things.

Thanks, and great to have your comment, Haelfix! You earlier described the argument many String theorists give that background independence isn't needed ("Background Independence, who needs it? Just derive gravitons in a sufficient range of fixed geometries. The world is basically flat anyway, curvature is just a second or third order correction.") which I found enlightening and probably others here did too.

I want to amplify your comment here because it shows from your direct contact with him why Brian Greene may be more open to Loop's possibilities than are many of his String colleagues.
To repeat the obvious, 1915 Gen Rel is a geometric approach to gravity---gravity IS the changing geometry of the world----and as such it starts with no preconceived fixed geometry on which to build. It is "background independent"---having no fixed background geometry or framework of space and time---in that sense. A lot of people seem to think this is a mistake. But 1915 Gen Rel continues to be verified to high precision and so far has no competition for accuracy where it applicable. So this is a dilemma: Its dynamic geometry does not fit well with other physics, but there is currently no substitute.

LQG is distinguished by its attempt to quantize Gen Rel while keeping the fundamental principles intact---that gravity is geometry and that geometry is dynamic instead of being a fixed background.

So you are saying Brian Greene likes background independence and geometric-style physical models----which would tend to make him more understanding of LQG, other things things being equal. Potentially sympatico.

You are also saying that his main objection is that LQG does not include matter. That is also of interest because a number of recent LQG papers have begun exploring ways to include matter.

I should get some references.
There is one by Gambini and Pullin IIRC---Edgar1813, who I think is a grad student doing related research, would know about that.
There is a recent talk by Hanno Sahlmann at the end-October conference at perimeter. There is something by Laurent Freidel and David Louapre.
Also current work by Bojowald and Ashtekar has bearing on this---discussed by Ashtekar in his 20 September talk at Penn State.
This is not to say that I am aware of any progress being made, only that there are signs of people working about bringing matter into the picture.
Matter is already important in Loop Quantum Cosmology---in how LQC explains inflation. However LQC is different from the full LQG, so it
provides at best a guide for what may reasonably be expected in the full theory.

It's good to have a sense of where Brian Greene is on these basic issues
since he is such an opinion-leader.

Gecko said:
as was said above about references, where do you get "the truth is the LQG is bringing ST to its knees"? any references or information on WHY LQG is bringing ST to its knees?

Well firstly I am not Brian Greene, but I was making comments on this:It's a great theory. Probably a mix between ST and LQG is where the actual answer lies. ..which I agree on in principle. Thus:But, for reasons of just recent happenings, I think LQG is going in the wrong dirrection. I don't like the way things are going." I take this to mean that Brian Greene does not like the fact that LQG seems to be in a better position to explain, with testable predictions of Gravity, while string theory cannot get anyway near to explaining basic GR concepts such as Space-times.

It is this context of testable predictions that I was highlighting, there is not a lot of direct pointing out specific meanings in Brians comments, so forgive me for 'reading-between-the-lines'.

The really interesting experimention for LQG is going to be in Cosmology, for instance the recent amount of workings on 'Space singularities' disconnected from 'Spacetime Singularities', shows that the reworking of certain paramiters withing SR and GR is giving realistic understanding.

What I find more of a puzzle is how 'Paden' managed to ask the Question?..I mean I could understand if he asked Brian Greene:What do you think aboutGirls

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