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 consistency 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, because 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 independent 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 independent 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