Are the LQG physicist less prepared than the ST physicist?

[Sardano]

I ask this question because some time ago I was attending some course in LQG, and I was very surprised of the lack of knowledge of the teacher in anything that was not completely related to his investigation. Even things that he knew, he only knew it in the particular context of his work and couldn't relate it to the same concept in more general context. I am talking about things that every ST physicist have used, like, fermion representations, arbitrary dimensional vielbiens, perturbative techniques, fiber bundles etc. It was also evident that all his education was coming from the same book: the thiemman QG book. Something that seems to be the usual among LQG people. For example, he used the triad (of course) which is nothing but the "three dimensional vielbien" of a riemannian metric. However, he didnt know anything about this, he just knew the specific definitio given in the LQG context (I guess from the Thiemman book, of course) that is based on the SU(2) group and his killing form. But he didnt know that this construction can be used in arbitrary dimensions, that is a non-coordinate change of basis, that this construction allow to introduce fermions in curved spacetime. In his own words: "I don't know what is a fermion, I have never seen one": he didnt know about spinorial Lorentz representations and anything related to fermions!. He was talking about holonomies in the LQG context, but he didnt know what the holonomy group of connection in a fibre bundle was!. These are just some examples of much more facts that have made me run into the conclusion of the question. It was obvious that he knew NOTHING about QFT, even the QFT's are the more tested theories (much more than GR, which only has a few tests). And I am pretty sure that he knew nothing about QFT because when we were talking about ST and LQG he commented:

"Well, is true that ST has a perturbative description of gravity that is finite order by order...but no one has showed that the whole sum is not divergent!"

I was astonished, and replied:

"The thing is that the sum IS divergent...otherwise everything would be wrong..."

And he said:

"What?, Why?"

My reply:

"Non perturbative physics...but this is something that happens already in QED..."

On the other hand, in ST we have to work with all this things in a mixed complicated set up, so we have to learn how to work with fermions, but not only in four dimensions but in arbitrary dimensions, we have to learn about huge gauge groups also in several dimensions, we have to learn also about gravity in several dimensions and we have to learn not only about pointlike actions but only about extenden objects and their gravitational an gauge interactions. And then we have to put everything together!. I think the background for ST is much wider than the background for LQG. I am not saying that is more difficult or something like that, but I think if you specialize on LQG, then you are not prepared to do antything else in Theoretical Physics. I got to say, I was going to start as PhD student in LQG when i attended this course, but the formation of the teachers and the stuff the were working in (not because it was LQG, but because other reasons) made me change mind. They seemed to be locked in his particular GR research and the have forgotten about all the others branchs and tools of theoretical physics.

 

[tom.stoer]

I think you shouldn't generalize this.

When I was a student about 20 years ago many relativists and particle physicists didn't have a common language, either. LQG is about qantizing gravity, not about other interactions and not about unification, so most research in LQG does not take particle physics and perturbative QFT into account. I bet you can also find string physicists who are not familiar with geometrical concepts well-known in the GR community.

And I think that both subjects are so complicated that some people do not have time and energy to invest (waste in their mind) in the other subject.

 

[f-h]

Given that many people in the community are collaborating with Rivasseau and other QFT experts you can rest assured that many are well familiar with things beyond. For example Rovelli and collaborators have looked at whether the fermion coupling to gravity through the vielbein is sensitive to the Immirzi parameter.

Also, the specific constructions used in LQG are tied to SU(2) and the 3 dimensional diffeo group, so as used here the concepts do not just generalise.

The Thiemann book also represents a school of LQG (which I'm not part of) that is based heavily in the ideas of axiomatic quantum field theory. That means the QFT language used there is extremely different from that used elsewhere in physics.
In the last school we organised in Zakopane the standard QFT picture was represented by Hollands who gave an extensive lecture series on QFT in curved space time. Finally, IMO the difficulty of making contact with other branches of physics is a valid concern.

 

[atyy]

QCD has a UV fixed point. There are calculations that are not practical perturbatively, but is this because the series does not converge, or because the series has many terms?

 

[atyy]

Finally, IMO the difficulty of making contact with other branches of physics is a valid concern.

I'm now reading the "old" spin foam literature, and it's very clear that it's tied to lattice gauge theory. With AdS/CFT, could one hope for a link via lattice gauge theory to string theory, especially since there are supposed to be supersymmetric spin foams?

Also, there seems to be overlapping interests between some in the LQG community and the tensor network community (who are practical condensed mattter people), eg.
http://arxiv.org/abs/1103.6264 , written by LQG people which references tensor network stuff in [123].

 

[Sardano]

Given that many people in the community are collaborating with Rivasseau and other QFT experts you can rest assured that many are well familiar with things beyond. For example Rovelli and collaborators have looked at whether the fermion coupling to gravity through the vielbein is sensitive to the Immirzi parameter.

Also, the specific constructions used in LQG are tied to SU(2) and the 3 dimensional diffeo group, so as used here the concepts do not just generalise.

The Thiemann book also represents a school of LQG (which I'm not part of) that is based heavily in the ideas of axiomatic quantum field theory. That means the QFT language used there is extremely different from that used elsewhere in physics.
In the last school we organised in Zakopane the standard QFT picture was represented by Hollands who gave an extensive lecture series on QFT in curved space time. Finally, IMO the difficulty of making contact with other branches of physics is a valid concern.

The thing is that, if you are working in LQG, the tools you are going to use are much more specific and concrete and has less applications in other areas of physics. What I mean is that, if I am working in String Theory I have to master a lot of very different tools that are used in different branches of theoretical physics, which gives me a rich background in order to decide (for example after the PhD), where I want to work. I am not completely sure that working in LQG you are going to end in the same situation.

 

[Sardano]

I think you shouldn't generalize this.

When I was a student about 20 years ago many relativists and particle physicists didn't have a common language, either. LQG is about qantizing gravity, not about other interactions and not about unification, so most research in LQG does not take particle physics and perturbative QFT into account. I bet you can also find string physicists who are not familiar with geometrical concepts well-known in the GR community.

And I think that both subjects are so complicated that some people do not have time and energy to invest (waste in their mind) in the other subject.

Of course the LQG people master their techniques and procedures. But I got the feeling that the had deeply studied the Thiemman book, with the specific tools it gives, and then they had started to perform the LQG calculations forgeting about the rest of the physics!. I don't know, of course it is not right to generalize and I understand that if you don't need to work with fermions, and the only "gauge" group that you are going to use is SU(2), then you forget about other things and focus on your work. But for me, that means that you are worse prepared: okey, you master your specific tools that are useful in LQG, but what about the rest of the physics?, are you condemned to stay all your life in LQG? where am I going if I don't know how to work with fermions, fibre bundles, gauge groups etc?

 

[element4]

I don't know anything about the LQG community, but my experience is that it is impossible to generalize such things. This sounds more like a "good theoretical physicist VS bad theoretical physicist" question, rather than a, say, "LQG'sts VS String theorists".

In my opinion any good theoretical physicist should have a very broad interest in physics (and maybe math) beyond his/hers own research interests. For example I know several good condensed matter physicists which have good understanding of things like classical hydrodynamics, particle physics, non-equilibrium physics, quantum optics, lattice gauge theory, differential geometry, fiber bundles (characteristic classes), algebraic topology (homology, cohomology, twisted K-theory, index theory), differential topology (mapping class group, knot theory, morse theory), non-commutative geometry, supersymmetry, CFT (Wess-Zumino-Witten, quantum groups, Kac-Moody algebraes), category theory (modular tensor categories, modular functors), few aspects of string theory and so on! Much of this is even farther away from their original research, than it would be for LQG'sts.

On the other hand I also know a few string theorists which have very superficial understanding of most of the subjects they work on, even important and basic aspects of QFT! All they do is brainless calculations. (I must add that most string theorists I know are very bright.)

My point is that you might have been unlucky to meet a bad example of a LQG'st, and most other people might have much broader and deeper understanding of physics and math. But again, I don't know anything about LQG and might be wrong.

 

[atyy]

For example I know several good condensed matter physicists which have good understanding of things like classical hydrodynamics, particle physics, non-equilibrium physics, quantum optics, lattice gauge theory, differential geometry, fiber bundles (characteristic classes), algebraic topology (homology, cohomology, twisted K-theory, index theory), differential topology (mapping class group, knot theory, morse theory), non-commutative geometry, supersymmetry, CFT (Wess-Zumino-Witten, quantum groups, Kac-Moody algebraes), category theory (modular tensor categories, modular functors), few aspects of string theory and so on! Much of this is even farther away from their original research, than it would be for LQG'sts.

Turns out the condensed matter guys need LQG too! http://books.google.com/books?id=VgO0dbjJchUC&dq=condensed+matter+solvay&source=gbs_navlinks_s , p230

 

[element4]

Turns out the condensed matter guys need LQG too! http://books.google.com/books?id=VgO0dbjJchUC&dq=condensed+matter+solvay&source=gbs_navlinks_s , p230

Yeah, there have been some interest in LQG by some condensed matter physicists in recent years. In particular people working on various applications of tensor networks and non-chiral topological lattice models such as the Levin-Wen models (also called string-net models). People claim that there are interesting connections to LQG, but I don't know how useful this has been so far.

EDIT: for condensed matter tensor networks <-> LQG see http://golem.ph.utexas.edu/category/2010/09/jacob_biamonte_on_tensor_netwo.html" post by John Baez.

 

[arivero]

The split between GR and QFT is traditional, and undeniable. It even permeates physicsforums, where BSM audience is more "about quantum gravity" that "about GUT".

 

[smoit]

I got to say, I was going to start as PhD student in LQG when i attended this course, but the formation of the teachers and the stuff the were working in (not because it was LQG, but because other reasons) made me change mind. They seemed to be locked in his particular GR research and the have forgotten about all the others branchs and tools of theoretical physics.

Dude, you definitely missed out on a great opportunity to get a Ph.D. much faster. An expert opinion clearly states https://www.physicsforums.com/showthread.php?t=477171" that you can just as well stick to studying the theory of angular momentum, work out some asymptotic properties of 15j symbols and get a Ph. D. in LQG from Berkeley!

 

[f-h]

The thing is that, if you are working in LQG, the tools you are going to use are much more specific and concrete and has less applications in other areas of physics. What I mean is that, if I am working in String Theory I have to master a lot of very different tools that are used in different branches of theoretical physics, which gives me a rich background in order to decide (for example after the PhD), where I want to work. I am not completely sure that working in LQG you are going to end in the same situation.

I think you are exaggerating the situation somewhat. If you do a pure string PhD you won't find switching fields all that easy either. It really depends where you're looking at. If by physics you mean particle physics, especially in the context of high energy experiments then yes.

But physics is vast. If you want to get into topological orders your LQG toolbox might be a better starting place. If you want to go into algebraic/axiomatic quantum field theory it's exactly the right one. The former is a hype subject the latter dramatically out of fashion.

The study of black holes in LQG did produce some very interesting results for classical GR, etc...