Unifying Quantum Mechanics and General Relativity: A Debate on Strings vs LQG

[marlon]

Strings or LQG and why?

Hi,
I am currently studying LQG and i would like to have an idea on how opinions are devided between String Theory and LQG.

I would like to use the outcome of this poll as a reference to the introductory text i am writing on LQG. View the Loop Quantum Gravity-thread if you are interested.

So here's the question. What theory do you think is the best approach in order to unify QM and general relativity ( to describe the quantum gravity).


Thanks a lot in advance

regards
marlon

 

[marcus]

...
So here's the question. What theory do you think is the best approach in order to unify QM and general relativity ( to describe the quantum gravity).
...

String is more of an approach towards unifying the treatment of all fields and forces. By contrast, LQG has the limited aim of applying QM to general relativity and seems more apt to achieve that narrowly defined goal. But that is what your poll is explicitly asking about!

So although I answered Loop, Marlon, I don't think it is really a fair contest as the poll is stated. That's because quantizing GR is not String's main purpose, interest, or value. If, in the future, some version of String theory should emerge that makes experimentally testable predictions, it would be valuable based on other considerations than what you mention here.

 

[PRodQuanta]

Yes, marcus takes the words right out of my mouth. String theory is a TOE, where lqg is a theory of quantum gravity. Now, a good question might be... I think I'll start another poll.

Paden Roder

 

[humanino]

I feel LQG might demonstrate that there is no need for strings. Yet, string theories are so beautiful and have produce so many mathematical results, that this attempt might be considered better. I totally agree with Marcus too, and answered strings for other reasons

 

[sol2]

Finally, string theory started out as a generalization of quantum field theory where instead of point particles, string-like objects propagate in a fixed spacetime background. Although string theory had its origins in the study of quark confinement and not of quantum gravity, it was soon discovered that the string spectrum contains the graviton, and that "condensation" of certain vibration modes of strings is equivalent to a modification of the original background.

So LQG does not have this feature to describe point particles, where a one dimensional string includes gravity.

According to Wikipedia:

1.loop quantum gravity makes too many assumptions
2. according to the logic of the renormalization group, the Einstein-Hilbert action is just an effective description at long distances
3. loop quantum gravity is not a predictive theory
4. loop quantum gravity has not offered any non-trivial self-consistency checks
5. loop quantum gravity is isolated from particle physics
6. loop quantum gravity does not guarantee that smooth space as we know it will emerge as the correct approximation of the theory at long distances
7. loop quantum gravity violates the rules of special relativity
8. the discrete area spectrum is not a consequence, but an assumption of loop quantum gravity
9. the discrete area spectrum is not testable
10. loop quantum gravity provides us with no tools to calculate the S-matrix
11. loop quantum gravity does not really solve any UV problems
12. loop quantum gravity is not able to calculate the black hole entropy, unlike string theory
13. loop quantum gravity has no tools to answer other important questions of quantum gravity
14. the criticisms of loop quantum gravity regarding other fields of physics are completely misguided
15. loop quantum gravity calls for "background independence" are misguided
16. loop quantum gravity is not science

The numbered points are connected to deeper explanations.

Criticisms of string theory can follow in someone else's post. With the group in favor of LQG, they should be able to gather their heads and come up with lots of things

 

[humanino]

You are really unfair sol2. It is absolutely necessary that some of us check deeply that we really need new hypothesis such as : supersymmetry, extra-dimensions, extended objects.

Especially your first point is wrong. Your all post is not a scientific argument. The simple argument I quote above justify that great scientists as Rovelli and Smolin among other spend time on LQG. Please read Rovelli's paper.

 

[marlon]

sol2, I think you are going to have to learn a little bit about LQG. This is not hwo you do science.

Did you know that when Einstein published his General Relativity many scientists of whom we will never remenber the name were against it. 100 scientist published a paper in order to convince everybody else that Einstein was wrong.

Einstein replyed : Why a hundred scientists? If I was wrong, 1 would be enough !

regards
marlon

 

[sol2]

Of course I have to rebuttel you too

I present the source, and you call that unfair? I didn't write it I only repeated the claims. Now it is up to the people that signed their name to the LQG perspective to present there reasons why they like it. I expect these people to be deep thinkers

I have been more then fair in my research, and do not fault Smolin and the road he has choosen. It is a honourable imention, and my posts reflect that honour, when I ever refer to Smolin.

For heavens sake I even named my site in honour

People have to step up and say why.

My plan this morning was to look for information on the energy values of the particles we assign in those colliders( this should give some clues as to why LQG cannot work here in regards to quantum gravity as a explanation).

I am making a brief appearance here from being up all night. You must know I hold high esteem all those theoreticist, for what ever approach they come from.

Humanino you must explain why you like strings, from being more then simple and beautiful. I a explaining myself constant onthis board when it comes to strings. A geometrical discription cannot arise from empty space, so you need a back to begin.

 

[marlon]

First of all the claims you are referring to are not your own, so did you really think about LQG?

It is in fact a model that is in development stage so nobody will say that it is cristal clear...just as string aren't

As an answer to these claims, just check out the many references made by marcus in the Loop Quantum Gravity thread. I especially recommend the book of Carlo Rovelli on this matter.

The reason I prefer LQG over strings is the fact that we adopt the notions of General relativity and rewrite the QFT in such a way that it fits the background independence and the gauge-invariance...
String Theory starts the other way around. There, the general relativity is not considered the basis of the theory. as a matter of fact in string theory one gives up background independence, then try to quantize gravity in some way and finally try to restore the backgorund-independence. ofcourse I do realize that the main attempt of string theory is not to quatize gravity but to unify the fundamental interactions...


regards
marlon

 

[humanino]

You are probably more competent that I am to talk about strings. I have some books, I did not read them entirely yet. You ask, I have to try : the main reasons why I claim strings are elegant :

1) It is very natural from the assumption of extended objects, and when one lists the axioms leading to QFT, only point particles looks non-fundamental
2) Once the free theory is settled, there is no need to introduce interactions
3) There is only one diagram at each order given by a simple quantity : the genus
4) It has one, or even no free-parameter
5) I work in QCD. The Maldacena conjecture takes us very near to describe strongly coupled objects with strings, alas one supersymmetry is required

OK, would it only be those, this already justify why I love strings. However, the previous argument I quoted still holds : given the fact that there is not (yet) any experimental test, we must explore theories without new physical hypothesis.

Besides, what has strings done : who said "predicted gravity" ? See, I am not competent.

Also, LQG has reproduced Hawking result without the assumption of extreme black-holes, contrary to strings (AFAIK)

Again, I am not competent to judge. I think it is important to keep up for all physicists. After all, the only reason why strings and/or LQG might have some relevance, can be due to the fact that there are models for Connes' geometry. But here, I am totally incompetent.

 

[sol2]

You are probably more competent that I am to talk about strings. I have some books, I did not read them entirely yet. You ask, I have to try : the main reasons why I claim strings are elegant :

1) It is very natural from the assumption of extended objects, and when one lists the axioms leading to QFT, only point particles looks non-fundamental
2) Once the free theory is settled, there is no need to introduce interactions
3) There is only one diagram at each order given by a simple quantity : the genus
4) It has one, or even no free-parameter
5) I work in QCD. The Maldacena conjecture takes us very near to describe strongly coupled objects with strings, alas one supersymmetry is required

OK, would it only be those, this already justify why I love strings. However, the previous argument I quoted still holds : given the fact that there is not (yet) any experimental test, we must explore theories without new physical hypothesis.

Besides, what has strings done : who said "predicted gravity" ? See, I am not competent.

Also, LQG has reproduced Hawking result without the assumption of extreme black-holes, contrary to strings (AFAIK)

Again, I am not competent to judge. I think it is important to keep up for all physicists. After all, the only reason why strings and/or LQG might have some relevance, can be due to the fact that there are models for Connes' geometry. But here, I am totally incompetent.

Your points reveal competency to me, as much as any who are endeavoring to understand this issue.

Thank you for responding

 

[sol2]

First of all the claims you are referring to are not your own, so did you really think about LQG?

Oh yes, Marlon I have, and others perspective as well. Smolin was very kind to us in showing us those three roads and his distilliation process he went through. Because I give sound reasons for LQG rejection should not matter what my opinion is? I recognize the source of who wrote it, so I knew that from that perspective it would have been a legitmate one, and any disgust with me was a sore point about facing reality then?

It is in fact a model that is in development stage so nobody will say that it is cristal clear...just as string aren't

I have been monitoring and have spent a few year piece mealing, as strings has been piece mealing You had to look for the obvious differences, and who do you thnk brought Glast to this board?

As an answer to these claims, just check out the many references made by marcus in the Loop Quantum Gravity thread. I especially recommend the book of Carlo Rovelli on this matter.

Again, if you look you will see my name along Marcus's at almost every turn. I understood his position very well and did not need this thread to tell me what he thought

In fact if you find the post of the soccer ball by John Baez and the Monte Carlo demonstration, maybe come back to be about your perspective. I mean that in a nice way

The reason I prefer LQG over strings is the fact that we adopt the notions of General relativity and rewrite the QFT in such a way that it fits the background independence and the gauge-invariance...

Then I have to ask how did the model of LQG ever think it could have some geometrical association that begins from nothing? Did Loops always exist? That's a fundamental question about it's logic.

String Theory starts the other way around. There, the general relativity is not considered the basis of the theory.

As I said above, the search for geometical consistancy has been my bend one could say, and there is no perspective here that has shown itself. But I have some ideas here that I am working. Also you must understand, that the one dimensional strng already includes gravity, and you must also undertsand that this is a discription of a point particle. How shall you, with LQG show me this feature in quantum gravity?

as a matter of fact in string theory one gives up background independence, then try to quantize gravity in some way and finally try to restore the backgorund-independence.

In joing QM with GR, strings recognize the background dependancy as necesaary feature in how the geometry must emerge? There is a version of string theory that B.I., although I woud have to dig deeper for this. Maybe someone could bring it forward here, above in sci physics strings?

of course I do realize that the main attempt of string theory is not to quatize gravity but to unify the fundamental interactions...

to unify all of them of course and where shall we do this. We have set targets for ourselves and how shall the standard model arise. J Pierre has a graph for your consideration, which I shall bring later. It includes gravity.


Regards,

Originally Posted by sol2
http://assets.cambridge.org/0521837.../0521837332.jpg

Quantum gravity is perhaps the most important open problem in fundamental physics. It is the problem of merging quantum mechanics and general relativity, the two great conceptual revolutions in the physics of the twentieth century. The loop and spinfoam approach, presented in this book, is one of the leading research programs in the field. The first part of the book discusses the reformulation of the basis of classical and quantum Hamiltonian physics required by general relativity. The second part covers the basic technical research directions. Appendices include a detailed history of the subject of quantum gravity, hard-to-find mathematical material, and a discussion of some philosophical issues raised by the subject. This fascinating text is ideal for graduate students entering the field, as well as researchers already working in quantum gravity. It will also appeal to philosophers and other scholars interested in the nature of space and time.

http://titles.cambridge.org/catalog...isbn=0521837332

It will be a must buy for myself as well.

But I wonder about the issue of quantum geometry. How will this be formulated into the LQG perspective, as it has in strings?

The Elegant Universe, by Brian Greene, pg 231 and Pg 232

"But now, almost a century after Einstein's tour-de-force, string theory gives us a quantum-mechanical discription of gravity that, by necessity, modifies general relativity when distances involved become as short as the Planck length. Since Reinmannian geometry is the mathetical core of general relativity, this means that it too must be modified in order to reflect faithfully the new short distance physics of string theory. Whereas general relativity asserts that the curved properties of the universe are described by Reinmannian geometry, string theory asserts this is true only if we examine the fabric of the universe on large enough scales. On scales as small as Planck length a new kind of geometry must emerge, one that aligns with the new physics of string theory. This new geometry is called, quantum geometry."

I am seeing similarities arising not only from this perspective but from the current link Marcus supplied on the cosmological association (LQC ).

https://www.physicsforums.com/showthread.php?t=30419

 

[sol2]

meteor, PRodQuanta, setAI, yanniru, YZer

So we have a couple here, who have not really address the issue, but have voted?

 

[meteor]

String theory is a TOE, where lqg is a theory of quantum gravity

http://arxiv.org/abs/gr-qc/0409045

Yes, but LQG can incorporate the other interactions. In the paper above, is proposed an unification of LQG with Yang-Mills theory. QCD and electroweak theory, the 2 pillars of the Standard model, are both Yang-Mills theories. If this path becomes fruitful then LQG will also become a TOE

 

[Curious6]

A really good argument for string theory is that the five original theories have all been connected to form M-Theory by means of duality: isn't that a lot of coincidence?

 

[selfAdjoint]

A really good argument for string theory is that the five original theories have all been connected to form M-Theory by means of duality: isn't that a lot of coincidence?

If string physics stood by itself it would be. But it is an instantiation of a lot of interconnected mathematics, so the fact there is a non-perturbative zone that unifies the five original perturbative theories is not quite so surprising.

 

[sol2]

Self Adjoint,

You seem a bit reserved to voting here?

I have always known you to be open to the possibilties, but always well grounded in that respective view. Would this thread present something of a
dilemma, becuase you might have not voted on either, as there could be other possibilities?

 

[PRodQuanta]

sol2

You are right in saying that ST describes particles, thus is coherent with our standard model. Well, LQG doesn't exactly define the particles, but more of the strings themselves.

String theory is formulated with the need for supersymmetry.

String theory is not a background independant theory. I know, I know, neither is LQG. But it is more so than ST.

I don't know, maybe both theories are a part of one greater theory.

My $.02

Paden Roder

 

[sol2]

String theory is formulated with the need for supersymmetry.

Oh you cannot revote. You made your bed, so you have to sleep in it. The only hope is that you wake from your dreams in peace

There is no other way in which to look at the cosmo in my book. The idea is to look where we might have found this trail that leads us to where this supesymmetry could exist. You had to go to the early universe and this is not inconsistent with the idea of how to tap that energy(oops I let it out)

Every feature around us leads us to some consideration about how it all began. If not at this singuarity, then how? It had to ask us to consider, if existence always was, and if so how we might have interpreted it.

Splitting the atom takes us to the the very realization of supersymmetry, and how such events, take us to the doorway, but have never allowed us to tap it. So in the mean time, there is this vast world out there developing mathematcially. If you cannot geometrically construct the foundations of this geometry from that event, then you do not hold much hope of getting anywhere but understanding the basis of photon intersection at the glast level?

Quantum mechanics recognizes the string, and the oscillatory nature this energy defines for us. That's my point of view anyway. Pure speculation

 

[humanino]

String theory is formulated with the need for supersymmetry.

Unless of course, there is no fermions in your world

 

[humanino]

String theory is not a background independant theory. I know, I know, neither is LQG.

Why do you say that ? I read several times that this is a feature of LQG ! I am puzzled

 

[marlon]

Why do you say that ? I read several times that this is a feature of LQG ! I am puzzled

Background-independency is one of the basis-postulates of LQG. This theory is constructed always keeping this in mind. This is the reason why we don't work with just one metric but with wilson loops and spin networks and spin foams...

So, your remark is justified, humanino, i agree with you
regards
marlon

 

[sol2]

FRom the STone Tablets--Item 15

I will exorcise the demons from you yet



loop quantum gravity calls for "background independence" are misguided
[/URL]

The first constraint for a correct physical theory is that it allows the (nearly) smooth space(time) - or the background - which we know to be necessary for all known physical phenomena in this Universe. If a theory does not admit such a smooth space, it can be called "background independent", but it is definitely a useless theory and a physically incorrect theory. It is a totally different question whether a theory treats all possible shapes of spacetime on completely equal footing. However, it is not a priori clear on physical grounds whether it must be so (it can be just an aesthetic feature of a particular formulation of a theory, not the theory itself), and moreover, for a theory that does not predict many well-behaved backgrounds the question is meaningless altogether. Physics of string theory certainly does respect the basic rules of general relativity exactly - general covariance is seen as the decoupling of unphysical modes of the graviton. This exact decoupling can be proved in string theory quite easily. It can also be seen in perturbative string theory that a condensation of gravitons is equivalent to a change of the background; therefore physics is independent of the background we start with, even if it is hard to see for the loop quantum gravity advocates.

The World According to Quantum Gravity Be gone demons

And now to my soft side... why doesn't anyone want to believe?

 

[PRodQuanta]

Firstoff, the post that sol2 just posted is what I read about LQG's background independancy being iffy.

2nd off, you confuse me sol2. I know you're a smart guy, but sometimes I don't know what you're talking about. like this post:

sol2

Originally Posted by ProdQuanta
String theory is formulated with the need for supersymmetry.

Oh you cannot revote. You made your bed, so you have to sleep in it. The only hope is that you wake from your dreams in peace

There is no other way in which to look at the cosmo in my book. The idea is to look where we might have found this trail that leads us to where this supesymmetry could exist. You had to go to the early universe and this is not inconsistent with the idea of how to tap that energy(oops I let it out)

Every feature around us leads us to some consideration about how it all began. If not at this singuarity, then how? It had to ask us to consider, if existence always was, and if so how we might have interpreted it.

Splitting the atom takes us to the the very realization of supersymmetry, and how such events, take us to the doorway, but have never allowed us to tap it. So in the mean time, there is this vast world out there developing mathematcially. If you cannot geometrically construct the foundations of this geometry from that event, then you do not hold much hope of getting anywhere but understanding the basis of photon intersection at the glast level?

Quantum mechanics recognizes the string, and the oscillatory nature this energy defines for us. That's my point of view anyway. Pure speculation

Plus, here are several critisisms by LQGist on ST. String critisisms
Eat your heart out.

I find both theories fascinating.

Paden Roder

 

[sol2]

Thank you ProdQuanta for telling me that you do not understand. So which part, and if it is all of it, let's start with one question at a time.

I appreciate you taking the time to respond in a most approrpriate manner.

 

[PRodQuanta]

Not a problem. The whole making my bed part threw me off guard.

I understood what you were talking about for the symmetry.

Paden Roder

 

[sol2]

The bed thingy.

"marcus, marlon, meteor, PRodQuanta, setAI, yanniru, YZer" is the list for pro LQG, bed, sleeping all related to choices we make once we sign our names to a view. If we change our perceptions and titter, you can't change the list.


Whew thanks Paden...as I am under some most personal constraints here in this forum intellectually...it rubs off as not being relevant when through question it becomes apparent. I could be quickly lashed for 2 points. Ouch!

So hopefully you understand my predicament. Your right about the symmetry issue.

Plus, here are several critisisms by LQGist on ST. String critisisms
Eat your heart out.

I still have the stone tablets Paden.

 

[setAI]

I've yet to see an argument against LQG's background independence that holds up even to lay scrutiny- it seems to me a case of "we don't have BI so you can't either" envy-

LQG's background independence is self-evident as far as I'm concerned- it's inherent in the very structure of the framework-

I too think that LQG and Strings are part of the same theory- but that LQG provides a more fundamental glimpse at this theory- and the basic conjecture of connectionist relationships out of a matrix/graph/lattice generating spacetime and matter fields [and strings] is a solid model that resembles the nature of the Causality that we observe-

to me strings seem more like emergent phenomenology of some deeper relationships/dynamics [LQG]- to make a poor computer science analogy- strings/branes are like Turing Machines- while LQG describes the Lambda Calculus which defines what computable functions are-

 

[sol2]

I've yet to see an argument against LQG's background independence that holds up even to lay scrutiny- it seems to me a case of "we don't have BI so you can't either" envy-

LQG's background independence is self-evident as far as I'm concerned- it's inherent in the very structure of the framework-

I too think that LQG and Strings are part of the same theory- but that LQG provides a more fundamental glimpse at this theory- and the basic conjecture of connectionist relationships out of a matrix/graph/lattice generating spacetime and matter fields [and strings] is a solid model that resembles the nature of the Causality that we observe-

to me strings seem more like emergent phenomenology of some deeper relationships/dynamics [LQG]- to make a poor computer science analogy- strings/branes are like Turing Machines- while LQG describes the Lambda Calculus which defines what computable functions are-

Both use computer modelling. To understand strings function, it still has to follow the math. All you need is a good programmer. He has to be younger then sixty(Turing age of reason?)

Look at Andrey Kravtsov simulations, and I have yet to assign Calabi Yau model, as one of those mathematical functions, but it has been done(the equation has to work bothways). The Monte Carlo feature is a good one to from the QG perspective, for LQG

 

[PRodQuanta]

I understand your predicament sol2.

I wonder how the viewpoint of the public will sway once Rovelli's book comes out on LQG. You think it will be a hit? Much like Green's book on strings (Elegant Universe)?

Only time will tell, I guess.

I am still sleeping in my bed, however. But sometime I will have to wake up, and surrender my (our) dreams to reality. Whatever it may be.

Paden Roder

 

[humanino]

I really want to add to those statements that

it is definitely a useless theory and a physically incorrect theory.

seems too strong to me. The guys in LQG are not totally stupid. Once again sol2 you are unfair. This is not what I call a scientific attitude. If you took time to read recent papers by Ravelly, he made very strong claims about soon to be issued articles. My position is : we do not know yet, and whichever will eventually be picked, the other attempts were worth.

I also doubt that Wikipedia can furnish reliable interpretations for yet to be decided and speculative fields. One can refer to Wikipedia for elementary scientific information, but even there some statements are inaccurate and should be taken with care.

 

[pat777]

I really want to add to those statements that

seems too strong to me. The guys in LQG are not totally stupid. Once again sol2 you are unfair. This is not what I call a scientific attitude. If you took time to read recent papers by Ravelly, he made very strong claims about soon to be issued articles. My position is : we do not know yet, and whichever will eventually be picked, the other attempts were worth.

I also doubt that Wikipedia can furnish reliable interpretations for yet to be decided and speculative fields. One can refer to Wikipedia for elementary scientific information, but even there some statements are inaccurate and should be taken with care.

What claims did Ravelly make? Can you link to one of his papers? I agree with you on the part with Wikipedia not being reliable. Wikipedia was created for general information, not specific/complex information. It's unreliable in a situation like this.

 

[selfAdjoint]

Wikipedia is updated by users, so you can't depend on it as an authoirity; it's just some other guy's opinion. The author humanino referred to is Carlo Rovelli; he has an online book and many papers on LQG. See the early posts on Marcus's Rovelli's progrm thread for links.

 

[pat777]

Wikipedia is updated by users, so you can't depend on it as an authoirity; it's just some other guy's opinion. The author humanino referred to is Carlo Rovelli; he has an online book and many papers on LQG. See the early posts on Marcus's Rovelli's progrm thread for links.

If LQG is correct, then I hope it's possible to turn it into a TOE. I like the paper meteor linked to.

 

[jeff]

Did you know that when Einstein published his General Relativity many scientists of whom we will never remenber the name were against it. 100 scientist published a paper in order to convince everybody else that Einstein was wrong.

Einstein replyed : Why a hundred scientists? If I was wrong, 1 would be enough !

By this remark einstein meant that the criticism of his work by german scientists had nothing to do with science and was simply an act of anti-semitism, general relativity being derided at that period in germany's history as "jewish" physics.

 

[marcus]

By this remark einstein meant that the criticism of his work by german scientists had nothing to do with science and was simply an act of anti-semitism, general relativity being derided at that period in germany's history as "jewish" physics.

I agree with jeff's inference as to what he meant. His comment also
points up their servile intellectual conformity----political correctness by the standards of the day.

 

[marcus]

http://arxiv.org/abs/gr-qc/0409045
Yes, but LQG can incorporate the other interactions. In the paper above, is proposed an unification of LQG with Yang-Mills theory. QCD and electroweak theory, the 2 pillars of the Standard model, are both Yang-Mills theories. If this path becomes fruitful then LQG will also become a TOE

If LQG is correct, then I hope it's possible to turn it into a TOE. I like the paper meteor linked to.

thanks meteor, pat777, I had not registered the existence of this new paper by Gambini, Pullin, and somebody else. I'll put the title and abstract here to make it clear what meteor his talking about and go have a look. It appears at first sight to be only a tentative beginning at trying to include other interactions---one that does not yet work in an important case.

Unified model of loop quantum gravity and matter
R. Gambini, S. Jay Olson, J. Pullin
4 pages, dedicated to Michael P. Ryan on the occasion of his sixtieth birthday
http://arxiv.org/abs/gr-qc/0409045

"We reconsider the unified model of gravitation and Yang--Mills interactions proposed by Chakraborty and Peldan, in the light of recent formal developments in loop quantum gravity. In particular, we show that one can promote the Hamiltonian constraint of the unified model to a well defined anomaly-free quantum operator using the techniques introduced by Thiemann, at least for the Euclidean theory. The Lorentzian version of the model can be consistently constructed, but at the moment appears to yield a correct weak field theory only under restrictive assumptions, and its quantization appears problematic."

 

[pat777]

Marcus, Is Thiemann's unification of LQG and ST a TOE? I'm just a layperson who's interested in TOE.

 

[selfAdjoint]

thanks meteor, pat777, I had not registered the existence of this new paper by Gambini, Pullin, and somebody else. I'll put the title and abstract here to make it clear what meteor his talking about and go have a look. It appears at first sight to be only a tentative beginning at trying to include other interactions---one that does not yet work in an important case.

Unified model of loop quantum gravity and matter
R. Gambini, S. Jay Olson, J. Pullin
4 pages, dedicated to Michael P. Ryan on the occasion of his sixtieth birthday
http://arxiv.org/abs/gr-qc/0409045

"We reconsider the unified model of gravitation and Yang--Mills interactions proposed by Chakraborty and Peldan, in the light of recent formal developments in loop quantum gravity. In particular, we show that one can promote the Hamiltonian constraint of the unified model to a well defined anomaly-free quantum operator using the techniques introduced by Thiemann, at least for the Euclidean theory. The Lorentzian version of the model can be consistently constructed, but at the moment appears to yield a correct weak field theory only under restrictive assumptions, and its quantization appears problematic."

Note the last sentence. And Thiemann's methods are the same ones he applied to quantize the closed string, which have been slammed by string physicists, and in that case seem not to work. Not to say this isn't good work, but to continually cry TOE over it is as silly as the stuff Peter Woit mocks in string physics.

 

[marcus]

... Not to say this isn't good work, but to continually cry TOE over it is as silly as [the string hype that Woit mocks.]

that's a pretty harsh accusation of silliness because some of the string hype is pretty dreadful

but what I said, selfAdjoint, was
"It appears at first sight to be only a tentative beginning at trying to include other interactions---one that does not yet work in an important case."

I do not believe anyone here is "continually crying TOE"----I usually make a point of contrasting the narrow objective of quantizing gravity with the broader more inclusive aims of stringy thought. And I thought Meteor was reserving judgement when he qualified his notice of the Gambini et al paper by saying "if the approach is fruitful"

So who are you suggesting is indulging in silly hype (if that is the suggestion). Thomas Thiemann? Robert Helling? Gambini and Pullin?
Or is it actually me, or meteor?

That is not meant rhetorically, your point just needs clarification. My personal suspicion is that Hermann Nicolai has a reason for encouraging people like Thiemann and Helling to build bridges between Loop and String,

even if the bridges do not work perfectly and can be "slammed" as you say, and bearing in mind that even individually Loop and String are not perfect either.

and even if the aim of Loop theorizing is not to explain all fields and interactions (not the same as String's)

I am not sure why Hermann Nicolai encourages this kind of crossover research---maybe it is partly an instinctive effort to break down the sense of division into "camps"---or a hunch that it will shed some unexpected light on one or both research lines.

 

[pat777]

I too think that LQG and Strings are part of the same theory- but that LQG provides a more fundamental glimpse at this theory- and the basic conjecture of connectionist relationships out of a matrix/graph/lattice generating spacetime and matter fields [and strings] is a solid model that resembles the nature of the Causality that we observe-

Although I think LQG underlies string theory, I believe M-theory underlies LQG and ST if ST and LQG are both true. Loops are probably made from zero branes.

 

[selfAdjoint]

Although Thomas Larsson's criticism of LQG is only one man's opinion (see https://www.physicsforums.com/showthread.php?p=334373#post334373 for the latest iteration), it seems to be well founded. There is a deep aspect of generating a physical hamiltonian based theory that LQG hasn't addressed and seems to have missed entirely. It has to do with an anomaly that is NECESSARY for unitarity. Larsson says LQG as currentlly constructed is anomaly-free, and hence non-unitary, a physical no-no.

It was Thiemann's LQG quantization of the closed string that brought these methods to the attention of the non-LQG theorist community (not only string theorists!), and the criticism that resulted seems now to have been justified. This does not mean that LQG is sunk; it is currently in a period of changing it's inner methods (chunkymorphisms and all) and may yet arise justified. But we have to wait for that event.

 

[marcus]

... anomaly-free, and hence non-unitary, a physical no-no.
...

but is non-unitary evolution a no-no or is it, instead, a logical requirement for successfully quantizing GR (a physical "yes-yes" in other words)?

Rovelli addressed the non-unitary issue one year ago in
hep-th/0310077. We have come full circle back to October 2003, it seems

---exerpt from page 18 of "Dialog"---
Simp - ... Is loop gravity unitary?
Sal – No, as far as I understand.
Simp – This is devastating.
Sal – Why?
Simp – Because unitarity is needed for consistency.
Sal – Why?
Simp – Because without unitarity probability is not conserved.
Sal – Conserved in what?
Simp – In time.
Sal – Which time?
Simp – What do you mean “which time?”. Time.
Sal – There isn’t a unique notion of time in GR.
Simp – There is no coordinate t?
Sal – There is, but any observable is invariant under change of t, therefore everything is constant in this t just by gauge invariance.
Simp – I am confused.
Sal – I know, it is always confusing. . . Nonperturbative GR is quite different from physics on Minkowski . . .
Simp – Do we really need to get in the conceptual complications of GR?
Sal – Well, if we are discussing the theory that is supposed to merge GR and QM . . .
---end exerpt---

BTW, just a side comment. This spring Gambini and Pullin posted a paper which quantified a necessary rate of non-unitariness using any real clock and produced an argument to show that perfect unitariness is a no-no, namely it is non-physical. they used what I thought was a clever thought experiment to bound how good a realistic quantum clock could be.
and proceeded to resolve the black hole information paradox (which is based on the questionable assumption of perfect unitariness). I thought Gambini et al was neat---I will get the link

 

[marcus]

this was a series of 4 papers that appeared in February through August of this year, the authors were Rodolfo Gambini, Rafael Porto, Jorge Pullin

A relational solution to the problem of time in quantum mechanics and quantum gravity induces a fundamental mechanism for quantum decoherence
http://arxiv.org/gr-qc/0402118

No black hole information puzzle in a relational universe
http://arxiv.org/hep-th/0405183

Realistic clocks, universal decoherence and the black hole information paradox
http://arxiv.org/hep-th/0406260

Fundamental decoherence from relational time in discrete quantum gravity: Galilean covariance
http://arxiv.org/gr-qc/0408050

they present an argument to show that any theory which implies strictly unitary time evolution must be unphysical (must not be using a real physical clock to define time). maybe I should quote some of what they say in the abstracts, have to go out at the moment but will get back to this in a few hours

 

[pat777]

I think a good argument for string theory is that it predicts(or should I say postdict)that we can only observe 3 spatial dimensions. Strings and anti-strings are likely to collide in 3 spatial dimensions or less. Thus, it expands the 3 dimensions. Strings and anti-strings are very unlikely to collide in 4 spatial dimensions.

 

[selfAdjoint]

this was a series of 4 papers that appeared in February through August of this year, the authors were Rodolfo Gambini, Rafael Porto, Jorge Pullin

A relational solution to the problem of time in quantum mechanics and quantum gravity induces a fundamental mechanism for quantum decoherence
http://arxiv.org/gr-qc/0402118

No black hole information puzzle in a relational universe
http://arxiv.org/hep-th/0405183

Realistic clocks, universal decoherence and the black hole information paradox
http://arxiv.org/hep-th/0406260

Fundamental decoherence from relational time in discrete quantum gravity: Galilean covariance
http://arxiv.org/gr-qc/0408050

they present an argument to show that any theory which implies strictly unitary time evolution must be unphysical (must not be using a real physical clock to define time). maybe I should quote some of what they say in the abstracts, have to go out at the moment but will get back to this in a few hours

Correct me if I'm wrong, but it appears these authors are proposing (yet) another way to quantize gravity. They "promote all the variables to quantum operators" before establishing one of them as their time operator. Does this help LQG? Many people have tried to produce a quantum time operator, and maybe this one will work. We'll see. It is brave of them to simply accept the self destruction of pure states, even if they can show the decay to be very slow.

 

[marcus]

Correct me if I'm wrong, but it appears these authors are proposing (yet) another way to quantize gravity.

Hi selfAdjoint. Gambini and Pullin are major LQG figures, have done loop quantum gravity for quite some time. Pullin, as I think you know, does Matters of Gravity the APS newsletter on gravity and he has occasionally dropped in here at PF under the name Jorge. I think non-unitary knows them.

I am glad your interest was stimulated in the Gambini Pullin work. Here is another recent paper of theirs, to give a more complete perspective.

R. Gambini, S. Jay Olson, J. Pullin
Unified model of loop quantum gravity and matter
http://arxiv.org/abs/gr-qc/0409045
( 4 pages, dedicated to Michael P. Ryan on the occasion of his sixtieth birthday)

Gambini's approach diverges from that of Ashtekar and others but it is hard to say how far the divergence goes. Do Gambini et al constitute a separate species? In my opinion not. They give talks at the same conferences as the others, and cite a common core of Loop Gravity references. Maybe the taxonomical term is "variety".

 

[marcus]

... Rodolfo Gambini, Rafael Porto, Jorge Pullin
...
No black hole information puzzle in a relational universe
http://arxiv.org/hep-th/0405183

Realistic clocks, universal decoherence and the black hole information paradox
http://arxiv.org/hep-th/0406260
...
they present an argument to show that any theory which implies strictly unitary time evolution must be unphysical (must not be using a real physical clock to define time)...

selfAdjoint, you correct me if I am wrong but I think the arguments in these two papers are not meant to apply narrowly to one quantum theory or another.
They made them generally to apply to any quantum theory of gravity and not just gravity (it seems to me) but to any quantum theory at all. To the extent that the theory pretends that time evolution is perfectly unitary, the theory is unphysical.

this is because time must have an operational meaning.

 

[RingoKid]

for what it's worth I'm going for strings cos I like the simple idea that they can tie everything together...

...so what if it means a fixed background there are always hidden dimensions to explore

 

[pat777]

http://arxiv.org/PS_cache/hep-th/pdf/9208/9208027.pdf
Ed Witten wrote a paper about getting a background independent string theory. Yes, I know it's an old paper.