Confused about quantum gravity theory?

[carl]

confused about quantum gravity theory?

I know that quantum gravity is used to describe the effects of the gravitational interaction under differentg circumstandes, But can anyone please explain,under what circumstances is the theory appropriate, also some examples of the phenomena that are explained or predicted by this theory. Thankyou.

 

[tom.stoer]

First we should specify what we mean by Quantum Gravity.

We can use this phrase rather cloudy and say it applies whenever both quantum and gravitational effects are to be considered. In that case all we can say is that QG needs to be considered when GR predicts singularites or something like that. There are some generic features every theory of QG should incorporate, but no specific predictions are possible.

We can say that string theory IS already THE theory of QG. I don't think that's true but somehow I have to accept what the majority is telling me ... In that case there are some predictions from s.t., e.g. concerning black hole microstates.

Then there is Loop Quantum Gravity, a theory that claims to be a theory of QG only - w/o any GUT attitude or something like that. This theory is rather well developed and has indeed something to say about QG phenomena (quantized length, area and volume, black hole entropy, resolution of singularities like the big bang that is replaced by a big bounce, anomalous dispersion relation of el.-mag. waves in the vacuum with c != const.)

And then there are other QG theories like Horava's recent approach, causal dynamical triangulation (Loll et al.), causal sets (Sorkin) and others; none of them is as well developed as string theory and Loop Quantum Gravity, but as long as NO QG theory makes predictions that can definitely be falsified by experiment it's hard to say which road we should follow.

Do have some specific theory in mind?
Are you talking about specific regimes, processes, phenomena etc.?

 

[Haelfix]

Quantum gravity is the attempt to quantize a classical theory based on the Einstein-Hilbert action. The two most popular approaches is path integral quantization or canonical quantization.

Both approaches will yield some predictions under the guise of a particular approximation (called semiclassical gravity) whereby you will get the equations of motions + smallish quantum corrections. Unfortunately both approaches are sick, in that they lose predictivity as you get very close to the Planck scale (when gravity and curvature becomes strong). This manifests itself in a number of guises (pathologies of the hamiltonian constraint, nonrenormalizability and so forth). Otoh, there are concrete predictions for a number of phenomena, the most famous is the prediction of Hawking radiation and particular thermodynamics for black holes.

String theory is not a theory of pure quantum gravity, its action contains the EH term (amongst infinitely other terms), but there is no way to consider gravity by itself and in fact it would be inconsistent to do so under that guise.

LQG in its original form follows the canonical quantization program more closely and is about as far as anyone has taken it, of course its much more than that nowdays.

CDT is a lattice approach to quantization and is numerical in nature.

 

[tom.stoer]

One should add that LQG is a special form of QG, where one starts with new variables. Instead of the metric (as in old-fashiooned QG) one uses the so-called tetrad formalism plus a new connection one-form. Via these new variables (Ashtekar-variables) it is possible to avoid many problems of the old-fashioned QG approach.

In classical general relativity both sets of variables (metric, Ashtekar-variables) produce identical results.

 

[ExactlySolved]

The difficulty of constructing a consistent theory of quantum gravity has led to one of the boldest predictions in the history of physics: the existence of extra spatial dimensions besides the usual 3.

This prediction come about because we know that quantum particles (point-like objects) cannot describe gravitons (this technical result is called the Witten-Weinberg theorem). Therefore gravitons must not be zero dimensional objects, they are either 1-dimensional strings or higher dimensional branes.

Then we can do a variety of calculations which all show the same result: that the only way to combine quantum mechanics, special relativity, and the notion of strings and branes is if the dimension of spacetime is exactly 26.

This prediction does not involve any speculation, the only input is the standard model and general relativity, the rest is necessitated by the mathematics.

In contrast, there are many calculations in string theory which strongly suggest the existence of supersymmetry (every particle having a superheavy symmetric partner), but I would still count this prediction as speculative (although 99% likely). By the way, supersymmetry effectively reduces the dimension of spacetime to be
10, which is the more oft quoted prediction of the number of extra dimensions.

Then there is Loop Quantum Gravity, a theory that claims to be a theory of QG only - w/o any GUT attitude or something like that. This theory is rather well developed

Are there any review papers on LQG of a pedagogical nature that have been published in a peer reviewed journal e.g. mod phys rev ?

My advice to students is to ignore non-string theories of quantum gravity. Even though there is a lot of chatter about these on internet forums such as this one, there is virtually zero discussion of LQG in physics research departments or peer reviewed journals. If you are a student, then any time spent studying LQG is time spent away from learning physics, and if and when you learn enough physics to see that LQG is nothing, you'll probably regret the time you spent on it.

 

[tom.stoer]

Regarding extra dimensions: if you study loop quantum gravity you will find out that it uses a rather basic toolbox:
standard canonical quantization +
new variables for the gravitational field (which leads to a structure similar to SU(2) gauge theory) +
tools to implement the diffeomorphism constraint (which is not know from usualk gauge theory)
LQG does not predict or require extra spatial dimensions. Nevertheless the derivation is rather sound and well-defined. Therefore the conclusion could simply be: they do not exist!

Regarding review papers: I am surprised that you talk about quantum gravity and string theory w/o having studied loop quantum gravity papers. There are a lot detailed research articles and review papers regarding loop quantum gravity. Most of them have been published in http://arxiv.org/archive/gr-qc and they have passed reviews, of course. A good starting point is http://relativity.livingreviews.org/Articles/lrr-1998-1/ where you will find about 200 references. Another very good source are two books of Rovelli and Thiemann; I would recommend Rovelli's as a starting point which has been published in Camebridge University Press and which is (or was) availabe as online-draft.

Regarding your advice to students: I am wondering how you can advice people not to study a certain subject when (at the same time !) you have to admit that you do not know one single review paper.

Sorry for being harsh, but that's your blind spot - not LQG's.

Thomas

 

[Finbar]

The difficulty of constructing a consistent theory of quantum gravity has led to one of the boldest predictions in the history of physics: the existence of extra spatial dimensions besides the usual 3.

This prediction come about because we know that quantum particles (point-like objects) cannot describe gravitons (this technical result is called the Witten-Weinberg theorem). Therefore gravitons must not be zero dimensional objects, they are either 1-dimensional strings or higher dimensional branes.

Then we can do a variety of calculations which all show the same result: that the only way to combine quantum mechanics, special relativity, and the notion of strings and branes is if the dimension of spacetime is exactly 26.

This prediction does not involve any speculation, the only input is the standard model and general relativity, the rest is necessitated by the mathematics.

In contrast, there are many calculations in string theory which strongly suggest the existence of supersymmetry (every particle having a superheavy symmetric partner), but I would still count this prediction as speculative (although 99% likely). By the way, supersymmetry effectively reduces the dimension of spacetime to be
10, which is the more oft quoted prediction of the number of extra dimensions.



Are there any review papers on LQG of a pedagogical nature that have been published in a peer reviewed journal e.g. mod phys rev ?

My advice to students is to ignore non-string theories of quantum gravity. Even though there is a lot of chatter about these on internet forums such as this one, there is virtually zero discussion of LQG in physics research departments or peer reviewed journals. If you are a student, then any time spent studying LQG is time spent away from learning physics, and if and when you learn enough physics to see that LQG is nothing, you'll probably regret the time you spent on it.

This seems to me to be a dangerous point of view there is certainly no proof that shows LQG is not the correct theory of gravity. We should deal with facts alone not opinions. The WW theorem is a no go theorem for some theories but not LQG. Even if ST is a more promising theory we as scientists must explore all the options. We must be humble we can't just work on a theory because most people believe it is true.

 

[tom.stoer]

Thanks!

 

[ExactlySolved]

Regarding review papers: I am surprised that you talk about quantum gravity and string theory w/o having studied loop quantum gravity papers. There are a lot detailed research articles and review papers regarding loop quantum gravity. Most of them have been published in http://arxiv.org/archive/gr-qc and they have passed reviews, of course. A good starting point is http://relativity.livingreviews.org/Articles/lrr-1998-1/ where you will find about 200 references. Another very good source are two books of Rovelli and Thiemann; I would recommend Rovelli's as a starting point which has been published in Camebridge University Press and which is (or was) availabe as online-draft.

I asked for peer reviewed review papers. It may surprise you that books from Camebridge University Press can be arbitrarily speculative, they do not go through a pure reviewed process. I'm sure you know this also true for the papers on the ArXiV that are not cross-published in peer reviewed journals. Reviews of Modern Physics is a journal that goes back to before the 1930s, and within one or two years of a ground breaking discovery one can usually find various review papers in this journal, e.g. there are many such peer-reviewed reviews of ADS/CFT (although this example is 10 years old, at least it is well known).

For those who have not been through the experience of being peer reviewed, I can understand why this seems like a minor point, but believe me its not. I don't have a problem with the majority of papers being ArXiV pre-prints for the sake of rapid communications, but every so often someone needs to bundle this up into a review and get it published in a rigorous journal.

LQG does not predict or require extra spatial dimensions. Nevertheless the derivation is rather sound and well-defined. Therefore the conclusion could simply be: they do not exist!

I know that LQG does not predict extra spatial dimensions, but it also violates Lorentz-Invariance according to the standard meaning of the term in physics. The following dichotomy is availible:

String Theory: (quantum gravity) + (unified forces) - (extra dimensions)

LQG: (Quantum Gravity) -(Lorentz Invariance) - (QFT)

According to these equations and my own values I choose String Theory >> LQG.

Regarding your advice to students: I am wondering how you can advice people not to study a certain subject when (at the same time !) you have to admit that you do not know one single review paper.

Sorry for being harsh, but that's your blind spot - not LQG's.

The accusation of a blind spot is ironic when your examples appear to be blind to the words "peer review."

I decided to look up the answer myself, and even I was shocked by the answer. The following link takes you to the search page for the entire system of APS journals (Phys rev letters, Phys rev A,B,C,D,E, Mod Phys Rev):

http://prola.aps.org/search/query

Searching for LQG or Loop Quantum Gravity returns zero matches!

 

[tom.stoer]

Please have a look at the references in http://relativity.livingreviews.org/Articles/lrr-1998-1/

You will find
- Phys. Rev. and Phys. Rev. Lett.
- Class. Quantum Grav.
- Nucl. Phys.
- Int. J. Theor. Phys.
to mention a few

The living reviews are reviewed as well :-)

Btw.: you should explain why you require Lorentz invariance to be a fundamental symmetry of nature; diffeomorphism invariance is much stronger.

 

[Finbar]

String Theory: (quantum gravity) + (unified forces) - (extra dimensions)

LQG: (Quantum Gravity) -(Lorentz Invariance) - (QFT)

According to these equations and my own values I choose String Theory >> LQG.

Nice to see some good hard science on these forums!

I decided to look up the answer myself, and even I was shocked by the answer. The following link takes you to the search page for the entire system of APS journals (Phys rev letters, Phys rev A,B,C,D,E, Mod Phys Rev):

http://prola.aps.org/search/query

Searching for LQG or Loop Quantum Gravity returns zero matches!

Erm...i did the search i found plenty of matches!? I think tom is right; your blind spot for LQG is quite severe.

 

[Fra]

My advice to students is to ignore non-string theories of quantum gravity. Even though there is a lot of chatter about these on internet forums such as this one, there is virtually zero discussion of LQG in physics research departments or peer reviewed journals. If you are a student, then any time spent studying LQG is time spent away from learning physics, and if and when you learn enough physics to see that LQG is nothing, you'll probably regret the time you spent on it.

Unless this was irony I think this doesn't sound like good advice at all to me. You sound exactly like my former supervisor who is a string professor. I'm sure he meant well, but all he accomplished was to loose a student.

I would expect that any serious student, should be capable of finding their own focus.

I am not advocating LQG, but I am not impressed by string theory either. I think we need something new, although I think there are parts of string theory that is interesting, I think the research program as a whole is not appealing me and maybe we need new ideas. And to in such situtaion suggest that new people coming into this, perhaps with new, odd and interesting ideas, are more or less wasting their time but not doing string theory is just completely whacked out! because it discourages new ideas. When has string theory wasted enough resources in this way?

That's my opinion.

/Fredrik

 

[Fra]

> Even though there is a lot of chatter about these on internet forums such as this one, there is virtually zero discussion of LQG in physics research departments

This was correct to where I came from. But for a very simlpe reason. The head of the departments was people with this attitude. The preserved their own ideas, and supressed any competing ideas.

To argue against equal competitors and compete for funding is I think normal, but to try to give such biased advice to innocent students makes my pulse go up.

/Fredrik

 

[tom.stoer]

Thanks again

 

[Finbar]

I am not advocating LQG, but I am not impressed by string theory either. I think we need something new, although I think there are parts of string theory that is interesting, I think the research program as a whole is not appealing me and maybe we need new ideas. And to in such situtaion suggest that new people coming into this, perhaps with new, odd and interesting ideas, are more or less wasting their time but not doing string theory is just completely whacked out! because it discourages new ideas. When has string theory wasted enough resources in this way?

That's my opinion.

/Fredrik

I'm in strong agreement with your here. I don't think i know enough about strings or LQG. But quantum gravity is probably the most open question in fundamental physics and i also i feel the most interesting. I think to solve the problem of QG will need new ideas in both maths and our preconceptions of spacetime and matter. I think string theory has benn good for physics becauses its opened door ways by considering extra dimensions, added symmetry and a greater number of degrees of freedom. But really it suffers from no experimental data. String theory seems to be an approach based on extending mathematical concepts. LQG on the otherhand is a lot more modest and instead of introducing more degrees of freedom it wants to ask more questions about the theories we already have. One could probably argue that we do now have a reasonable understanding of QM and GR in situations where only one applies. But even with QFT in curved space problems do arise which don't seem to be fully resolved.

I think its important to sepearte politics in the physics community from actual science. In a perfect world there would be no politics and scientists would only seek the truth and not have to worry about grants. There are some interesting new ideas eg CDT, causal sets. Even if they turn out to be wrong its healthy for science to have competing ideason the table.

 

[tom.stoer]

After we agree that we don't agree with ExactlySolved, we should try to move forward.

Thanks to Finbar for making a clear statement reagarding the differences between s.t. and LQG. S.t. is on the GUT-track, whereas LQG tries to harmonize existing theories w/o adding too much additional stuff (Occam's razor). Both approaches are important!

Nevertheless there is one new element (just one statement copied from another thread): "we try to develop a new theory w/o one single experimental hint. Even worse, QG tells us that these new hints may be simply not there, except in regimes and processes that are not tractable experimentally."

We must focus on two aspects:
- rigorous maths and consistency
- physical predictions disprovable by experiments

Regarding the second topic: is there a list of such predictions? I know the non-linear vacuum dispersion relations, but nothing else.