Which way is the best one for studying quantum gravity: LQG or Strings?

[jvdornas]

I am looking for a master program in QG. I would like to study LQG at first, but I am not exactly confident that this is the best choice.

Please, correct me if I am wrong, but as far as I know the biggest problems in LQG and Strings are:

LQG: is background independent, but its classical limit has to be found yet.
Strings: GR is its classical limit, but the theory is not background independent.

I agree that background independence is very important, as someone can learn studying GR. But if a theory of QG wants to prevail, it has to have GR as its classical limit too. So, considering also the fact that both theories postulates and are built based on a 1 dimensional object (strings and loops), couldn't one suppose that maybe both theories are complementary?

Knowing this, which way is the best one: LQG or Strings?

 

[bapowell]

It's important to point out that LQG is not a unified theory. It currently does not incorporate matter. Also, as far as I know the two theories are not complimentary and don't go beyond superficial similarities like the one you mentioned. I recall reading somewhere that Lee Smolin (LQG guy) worked briefly on M-theory and tried to find some common ground between the theories. He failed.

As for which to study, tough to say. Job prospects are pretty poor for both fields right now...

 

[Physics Monkey]

In my opinion it is useful to consider a number of factors. Here are a few I think are important.

As far as technical content goes, you should be aware that not everyone agrees with the dichotomy you set up between LQG and strings. For example, I would say that LQG is background independent but without a classical limit at the moment. On the other hand, string theory has a known classical limit which is background independent, thus string theory must also be background independent in that limit. Also, holographic duality provides another background independent formulation of string theory. People may disagree with me about this, but that's partially my point. LQG and strings people seem to talk past each other constantly. What I said above makes it sound (to me) like strings is better technically, but LQG is not without virtue. For example, LQG has a nice kinematic Hilbert space even if the Hamiltonian constraint can't be implemented simply.

But there is a lot more to think about than merely technical content. In terms of mathematical technology, LQG seems to favor a more formal algebraic approach to the problem. You may need to learn some sophisticated mathematics including operator theory for "big" vector spaces. Perhaps this aspect of the theory excites you. String theory also has a lot of math, but it is of a more familiar type to many people, namely quantum field theory and lots of classical geometry. On the other hand, if you don't like supersymmetry, I would be wary of string theory as susy is ubiquitous there (though I'm not saying you can't find non-susy problems by any means).

String theory is a much bigger community, and in my opinion the pace is faster and large collaborations are more common. I think LQG has a smaller more tight knit community.

Neither field is connected to experiment as far as quantum gravity goes. On the string theory side, you may be able to make some limited experimental contact through holographic duality, but I wouldn't count on it.

Hope this helps.

 

[jvdornas]

In my opinion it is useful to consider a number of factors...

(...) As for which to study, tough to say. Job prospects are pretty poor for both fields right now...

Thank you for your response.
I think this is really a difficult question (which way...). From a point of view from who is far distant from the full technical details, it seems to me that maybe a good researcher in QG should try to learn from both theories.
Besides that, I think that my choice will be made based more in practical facts. For example, I am in Brazil and here we don't have a researcher specialized in LQG, so if I would study that theory I would have to do my Phd in a field near but not exactly in LQG (somebody told me to study QFT in curved space-time). On the other hand, we do have Strings researchers, so from this point of view is easier for me to study Strings than LQG.
I think that the best way to answer this question, at least for now, is talking with the researchers we have available here and see which way they are following in their respective subjects and see if they are available as an adviser for the next term.

best,

Joao Dornas

 

[atyy]

How about condensed matter? Some of them are doing LQG related stuff. http://arxiv.org/abs/0907.2994

 

[bapowell]

Thank you for your response.
I think this is really a difficult question (which way...). From a point of view from who is far distant from the full technical details, it seems to me that maybe a good researcher in QG should try to learn from both theories.

I think this is a very healthy way to look at it. And you are right that it's also very important to be practical about it. It's of course very easy to get too involved in your own field to spend much time looking into the competing theory, but I believe it can be done if you're truly interested.

 

[marcus]

... I am in Brazil and here we don't have a researcher specialized in LQG,...

I'm curious to know which university in Brazil.

Just as general information, the researcher in South America with the best connections to the LQG community worldwide is Rodolfo Gambini, in Montevideo Uruguay.

Gambini's longtime collaborator is Jorge Pullin at Louisiana State U. (LSU)
Pullin is building up the LQG sector of the department at LSU and recently announced 3 new tenure-track faculty openings. Pullin runs the International LQG Seminar (ILQGS) that connects the different QG centers in a conference call several times a month.

In Mexico the person most active in QG is Alejandro Corichi at Morelia.
He will chair the LQG session at the next big GR conference, GR19, which will be held in Mexico City.
http://www.gr19.com/
He is hosting an international QG school at Morelia for two weeks right before the big GR19 meeting.
http://www.physics.ucsb.edu/~pasi/
He goes to Penn State a lot to collaborate with members of Ashtekar's group. He also has several PhD students at Morelia---plenty of QG activity there around Corichi, as evidenced by the papers.

To get advice, from a Latin American perspective, about LQG and about Masters study, I would recommend to immediately write email to these three people:

Rodolfo Gambini
Jorge Pullin
Alejandro Corichi

They will know the best people and the best prospective programs in either Sao Paolo or Rio. To find the email address, if you wish to write, one simple way is to look at their publications on arxiv. On each paper's PDF file, at the bottom of the first page, it usually gives the email address.

http://arxiv.org/find/grp_physics/1/au:+Gambini_R/0/1/0/all/0/1
http://arxiv.org/find/grp_physics/1/au:+Pullin_J/0/1/0/all/0/1
http://arxiv.org/find/grp_physics/1/au:+Corichi_A/0/1/0/all/0/1

Pullin is currently teaching a one-semester undergraduate course in LQG at LSU. The homework problems and information about the course are posted at his LSU faculty website. He is distributing NOTES for the students to study from, instead of using a textbook. Since you are an advanced undergrad student already thinking about the Masters, these course notes might be accessible to you. You could ask Jorge Pullin to give you a copy of the Introduction to LQG lecture notes he is preparing for his students.

I have a link to Pullin's website in this other thread, in case you want to check it out.
https://www.physicsforums.com/showthread.php?p=2595126#post2595126
Look down at the bottom of that post. If you have any trouble with the links, or getting email addresses, please write me PM and let me know.

 

[francesca]

LQG: is background independent, but its classical limit has to be found yet.

Calculating the classical limit of the full theory is an hard job, but there are a lot of promising indications (see the results in the asymptotic of the graviton propagator). The last one is, for example, in the cosmological framework by deriving the Friedmann dynamics from the spinfoam vertex amplitude.

Strings: GR is its classical limit, but the theory is not background independent.

This is not true, "GR is its classical limit" is definitely an overstatement. As far as I know, they have a limit but it is not exactly GR.

I am in Brazil and here we don't have a researcher specialized in LQG, so if I would study that theory I would have to do my Phd in a field near but not exactly in LQG (somebody told me to study QFT in curved space-time).

I suggest you to write to Alejandro Perez for two reasons: he is a LQG guy who can write papers also about strings, and he use to collaborate with people in Brazil, so he can advise someone to you. But, if you want to do this kind of job, I think you should consider the possibility to study abroad.

Knowing this, which way is the best one: LQG or Strings?

The best of course is LQG but this is not the point: how much do you have fun studying this or that? As someone already said, you are not going to get a job studying this, so my only advice is to do something that makes you happy: happy time is never wasted.

For further readings:
http://arxiv.org/pdf/hep-th/0310077"

 

[jvdornas]

I'm curious to know which university in Brazil. (...) If you have any trouble with the links, or getting email addresses, please write me PM and let me know.

Thank you Marcus, for your response. I did my undegraduate course at Unicamp and UFMG (the former in Campinas, Sao Paulo state, the other in Belo Horizonte, Minas Gerais state). I have some indications of researchers at Unesp (Sao Paulo) and CBPF (Rio de Janeiro). I will try to talk with them soon.

I contacted Gambini last month and he told me he has a lot of graduate students and a lot of projects, so he will be very busy for the next terms. He advised me to contact people in Morelia, I will do that.

Anyway, I think its better for me to do my master in Brazil, at least for now. Maybe I could try a Phd abroad. Indeed, Brazil government only give scholarships for courses abroad if you are going to get a Phd, the masters you have to do here.

You replied an older post of mine indicating a path I should follow to self study LQG. I have all noted and I hope I will have time to do that.

best,

joao

 

[jvdornas]

(...) how much do you have fun studying this or that? As someone already said, you are not going to get a job studying this, so my only advice is to do something that makes you happy: happy time is never wasted.

For further readings:
http://arxiv.org/pdf/hep-th/0310077"

Francesca,

Thank you for correcting me.

Funny paper this one from Rovelli. Sure it makes one like more LQG than Strings. I agree that, whether the theory you choose is the correct one or not, the most important is that you enjoy the process. And as more math it has more I like it.

I already have a tendency to like more LQG than Strings, and I will try to follow this path.
I will try to talk with some researchers in Sao Paulo and Rio, and see if the are available (if they have time for more students) and see what are exactly their interests.

best,

joao