Why is LQG a dead end but not m-theory?

[eigenguy]

hi,

I went to a lecture last january where it was mentioned that physicists don't work much on LQG and why it's such a dead end but m- theory isn't. Could someone explain this?

Also, can someone give me references to explain why the problems of LQG are so serious and so why the above is true but m-theorys serious problems don't make it a dead end? I think it is safe to assume the above would not be thought for no good reason, I just couldn't understand the lecture.

Also, is it better to start for m-theory with green schwartz witten or polchinski and what mininmum knowledge do I need? Also, is there an introduction or paper on line? Thanks.

 

[quartodeciman]

I have a suspicion. It is subject to scoff, but here it is anyway.

M-theory and its supersymmetric neighbors use tons of advanced mathematics, which means employment and career advancement opportunities galore for the mathematical adept. In other words, it has developed a flourishing life of its own. Maybe Loop Quantum Gravity uses only a normal range of mathematical techniques.

Pat Schwartz's math preparation for string theory --->
http://superstringtheory.com/math/index.html

 

[lethe]

Originally posted by eigenguy

Also, is it better to start for m-theory with green schwartz witten or polchinski and what mininmum knowledge do I need? Also, is there an introduction or paper on line? Thanks.

the importance of d-branes and m-theory were discovered after greene schwartz and witten was written, so you won t learn any m-theory from there. polchinski is newer.

but i think it is easier to learn basic string theory from GSW.

also, there is a new book by zwiebach that is supposed to hit the shelves in january, string theory for undergrads, which will teach you quite a bit of string theory with a minimum of prerequisites. but this book is not available to the public yet.

 

[marcus]

Originally posted by eigenguy
hi,

I went to a lecture last january where it was mentioned that physicists don't work much on LQG and why it's such a dead end but m- theory isn't. Could someone explain this?

Also, can someone give me references to explain why the problems of LQG are so serious and so why the above is true but m-theorys serious problems don't make it a dead end? I think it is safe to assume the above would not be thought for no good reason, I just couldn't understand the lecture. ..

It's a groundless claim, so it would be difficult to find a recent (2003) reputable paper explaining why one is a dead end and the other isnt. However there is a polarized perspective! So you can find papers by Relativists (GR theory people) who have one viewpoint and papers by Particle Theorists who have a different one. Neither kind---if he is responsible and speaking to peers---will say that the other guy's line is a dead end. But they will see things from radically different perspectives.

There is a survey "Loops versus Strings" written by a string theorist Enrique Alvarez for an audience of particle theorists as invited survey talk to a recent conference "What comes after the Standard Model?". If you want a reputable String-side view try him: search arxiv with Alvarez or the title Loops versus Strings.

For an opposing viewpoint, read Smolin's recent survey "How far are we from the quantum theory of gravity". It is respectful of string theory and makes a careful (objective it seems to me) comparison, one from the loop-side however.

Interestingly (to me at least) the most serious recent critiques of stringy research have been coming not from loop-people (who mostly mind their own business) but from people such as Tom Banks and Peter Woit whose research interests have nothing to do with LQG.

What members of the string community say on the outside, to members of the general public, is a separate chapter and requires a separate discussion

 

[eigenguy]

Originally posted by marcus
It's a groundless claim, so it would be difficult to find a recent (2003) reputable paper explaining why one is a dead end and the other isnt. However there is a polarized perspective! So you can find papers by Relativists (GR theory people) who have one viewpoint and papers by Particle Theorists who have a different one. Neither kind---if he is responsible and speaking to peers---will say that the other guy's line is a dead end. But they will see things from radically different perspectives.

You sound confident but the idea that relatively speaking LQG generates little interest among physics people is not a "claim" it is a fact and if you don't know this I'm pretty sure that you won't be able to tell me about the problems with LQG. But if you can explain to me the problems I would appreciate it because I don't want to spend eons trying to figure it out when the experts don't think it is a worthwhile theory. Also there must be some serious problems since physicists aren't interested in it very much. Also, what do you mean by polarized perspective? do you mean your perspective?

But thankyou very much for the references I appreciate it.

 

[Hurkyl]

Also, what do you mean by polarized perspective? do you mean your perspective?

The impression I've gotten at this point is:

People who study LQG think that ST is fundamentally misguided because it ignores the most important question about merging GR with quantum mechanics, "What does quantized GR look like?"


The people who study ST think that it is literally impossible to quantize 4-D General Relativity, and thus LQG is doomed from the start. Instead, they look to expand the standard model in novel ways hoping to find an alternate theory of gravity.

 

[selfAdjoint]

I've been looking into some LQG papers about trying to get to current physics from LQG. And I've also seen papers from the stringy side trying to do that too. And just today I read an intro paper on doing that with Connes' noncommutative geometry. That's three independent ways of "going deeper" than the standard model, and trying to get the standard model as a limiting case. They all fail. None of them is any closer than the others.

String partisans like to say that LQG fails to have a low energy limit of GR. But they don't mention that for twenty years now they have claimed to have a low energy limit of the Standard model and have never truly achieved it. Their models look a bit like the SM if you squint and cross your eyes, but not up close.

Similarly noncommutative geometry can produce things that remind you of the standard model, but aren't.

The LQG case is interesting because they are trying to fit quantum physics from the ground up into their background free webs. And they get so far and run into very subtle math problems. They have had breakthroughs in the past that weren't as widely broadcast as the stringy duality breakthrough, but they need another one now.

For Strings, there is reason to think that producing things that look like the standard model isn't a big achievement. The Coleman-Mandula theorem says that in Minkowski spacetime, with point particles, any theory that goes beyond the standard model would look like it it. So the low energy limits of stringy physics, where the strings, etc. look like points would fall under that theorem, and anything consistent they might produce in the lower energy range would have to look a lot like the standard model. Even with that help, they can't close the gap.

 

[eigenguy]

Originally posted by Hurkyl
The impression I've gotten at this point is:

People who study LQG think that ST is fundamentally misguided because it ignores the most important question about merging GR with quantum mechanics, "What does quantized GR look like?"

The people who study ST think that it is literally impossible to quantize 4-D General Relativity, and thus LQG is doomed from the start. Instead, they look to expand the standard model in novel ways hoping to find an alternate theory of gravity.

Originally posted by selfAdjoint
I've been looking into some LQG papers about trying to get to current physics from LQG. And I've also seen papers from the stringy side trying to do that too. And just today I read an intro paper on doing that with Connes' noncommutative geometry. That's three independent ways of "going deeper" than the standard model, and trying to get the standard model as a limiting case. They all fail. None of them is any closer than the others.

String partisans like to say that LQG fails to have a low energy limit of GR. But they don't mention that for twenty years now they have claimed to have a low energy limit of the Standard model and have never truly achieved it. Their models look a bit like the SM if you squint and cross your eyes, but not up close.

Similarly noncommutative geometry can produce things that remind you of the standard model, but aren't.

The LQG case is interesting because they are trying to fit quantum physics from the ground up into their background free webs. And they get so far and run into very subtle math problems. They have had breakthroughs in the past that weren't as widely broadcast as the stringy duality breakthrough, but they need another one now.

For Strings, there is reason to think that producing things that look like the standard model isn't a big achievement. The Coleman-Mandula theorem says that in Minkowski spacetime, with point particles, any theory that goes beyond the standard model would look like it it. So the low energy limits of stringy physics, where the strings, etc. look like points would fall under that theorem, and anything consistent they might produce in the lower energy range would have to look a lot like the standard model. Even with that help, they can't close the gap.

Very interesting! But the point of my question is still being missed. Given the evenhanded way you guys talk about the LQG versus string theory/m-theory you would think that the interest in them would be equal but really the opposite is the case. So either string theory people are just irrational and prejudiced against LQG or there are good reasons why the problems with LQG are more serious than in string theory. I think the former possibility is implausible since physics is an exact science and not politics and the people who work on this stuff are very very bright and just want to find the truth any way they can before they die. At least that's how I feel. So what I want to know is why LQG is inferior to string theory, which virtually must be the case given that hardly anybody works on LQG. What other reason could there be since string theorists probably understand LQG too, or at least as well as anyone here. Do you guys not think the lack of interest in LQG says something about LQG worth considering?

For example why do " people who study ST think that it is literally impossible to quantize 4-D General Relativity, and thus LQG is doomed from the start."?

 

[marcus]

Originally posted by eigenguy

But thankyou very much for the references I appreciate it.

You are very welcome! The claim that "LQG a dead end but not m-theory" does not have a logical basis. It belongs to the realm of propaganda. So the question you asked, namely "why is it true" does not make sense.

However the string research establishment is well entrenched and the sheer volume of stringy research is impressive. If that is what you want to talk about, numbers of pre-prints in the archive, you are very welcome to discuss it! Here are some numbers. I did a search at arxiv.org, by year from 1998 to present, using keywords "string", or "brane", or "M-theory" appearing in the abstract:

1998-----1260 papers
1999-----1373 papers
2000-----1459 papers
2001-----1400 papers
2002-----1211 papers
"past year"-----715 papers

The "past year" list is for November 2002 thru October 2003, a substitute to make up for not having the full year for 2003. Personally I do not consider numbers of papers in theoretical physics signifies much----the research fields go thru changes of fashion---especially when the counts are not weighted by a citations gauge of how influential or uninfluential the papers are.
I wouldn't expect anybody else to attribute much importance to the numbers either. But you may be curious to compare these with the much smaller LQG numbers for the same time period.

I did a search on keywords "loop quantum gravity" or "loop quantum cosmology" or "spin foams" appearing in the abstract and found:

1998-----27
1999-----44
2000-----46
2001-----48
2002-----64
"past year"-----68

Since these are just the los alamos preprint archive robot's raw response, I would not take them too seriously. Sorry about the meaninglessness but its the only source of numbers readily at hand. Ignore any apparent trends. At most they show a sort of order of magnitude imbalance in the research effort----at least 10 times more stringy research is being supported. Maybe 50 times! There are one would guess at least 10 times more research positions---just as a guesstimate, it may be much more. Probably a good many more than 10 times as many people. I don't want to suggest that this is necessarily an advantage to the progress of the theory! Or indicative of some intrinsic merit---it's just the condition things are in.

Instead of looking at the raw numbers of papers---which indicate rather more about fashion and the control of funding institutions---you might learn more that's real about the merits and progress of the research itself by reading Smolin's 90 page side-by-side progress comparison: "How far are we from the quantum theory of gravity?"

http://arxiv.org/hep-th/0303185

Or the counterpart survey "Loops versus Strings" by the string theorist Enrique Alvarez

 

[marcus]

Originally posted by eigenguy
given that hardly anybody works on LQG. What other reason could there be since string theorists probably understand LQG too, or at least as well as anyone here. Do you guys not think the lack of interest in LQG says something about LQG worth considering?

For example why do " people who study ST think that it is literally impossible to quantize 4-D General Relativity, and thus LQG is doomed from the start."?

Do you want just to argue with someone or do you want to read articles and learn something. People in particle physics had some mistaken ideas early on about the impossibility of quantizing GR.
(who knows eventually it may prove impossible, but the dire predictions have not panned out)

Rovelli who is a science historian as well as a GR expert, analyses the reasons in his new book "Quantum Gravity". It is interesting to see how so many HEP people got on the present track. You are asking why so many had the (now questionable) notion that it was "doomed from the start" and Rovelli devotes part of an interesting history chapter to this! So if you want to know you can read up on it.

But in the present this is not so interesting, I think, because I see people switching interest to Loop Gravity and a growing amount of energy in the work done in this field. When I see new authors of papers and look at their PAST papers (which the LANL archive makes easy) then I sometimes see that the new Loop author has earlier been writing String papers.

Rather than dwell on past mistakes I would suggest trying to understand recent trends. Like this months Strings meets Loops symposium at Berlin---a first of its kind---I think it presages a more balanced attendance at future conferences and eventually a more balanced distribution of funding and research positions. This presupposes continued rapid progress in LQG (which has made significant gains this year)

 

[selfAdjoint]

Just to say, Marcus, your arxiv search probably missed most of the Ashtekar, et al. papers, as well as those of Rovelli and associates since they both tend to avoid the term Loop Quantum Gravity and its abbreviation LQG.

I've posted before that it's a mistake to judge a field by the number of people working in it. Until the late 1930s the annual congresses of people working in quantum theory only had a dozen or so people attending. As late as the 1950s it was a small enough community world wide that everybody knew each other (except for the USSR, of course).

Over on s.p.r. it's been pointed out that "all particle physicists" is a tiny subset of "all physicists".

 

[eigenguy]

Originally posted by marcus
Rather than dwell on past mistakes I would suggest trying to understand recent trends.

I'm not talking about past mistakes. I'm talking about the unresolved problems of LQG. Since part of "recent trends" is going to include new ways to address old problems we should understnd what those problems are. It's getting hard not to assume that your nonrespones to this question are either just irrational prejudice or lack of understanding or unwillingness to even consider in an honest way what the challenges of LQG are. Of courese I can try to understand the papers but I don't think I know enough to really understand them. That's why I'm asking you to just outline them for me to give me persective before I read papers. I 'm not being argumentative.

For example what is the hamiltonian constraint and why can't it be solved? Is this related to why they can't prove that LQG is realistic since it can't get GR? Why can't they get GR? I don't see how you can be confident without being familiar with all of this.

 

[wolram]

november issue focus mag,

Witten showed that all five superstring theories are just rough descriptions of a single,overarching
idea, which he dubbed M theory, some theorists argued that the M represented mother,
mysterious, or even magic,but its connection with superstrings is clearest if it stands for membrane,
the five superstring theories then emerge as merely the multi-dimensional edges of 11 dimensional
membranes, all but four of whose dimensions are curled up to small for us to see.
-----------------------------------------------------------------
clip.
even M theory may ultimatly prove to lack the power requierd to answer all these questions.
edward witten advanced study Princeton.
----------------------------------------------------------------
this is what joe public is being told

 

[eigenguy]

Originally posted by wolram
even M theory may ultimatly prove to lack the power requierd to answer all these questions.

Yes, but why are physicists so much more confident in M theory than LQG?

 

[marcus]

Originally posted by selfAdjoint

I've posted before that it's a mistake to judge a field by the number of people working in it.

I heartily agree. For a theoretical field it seems like half a dozen or a dozen good ones has been enough in the past, and fashion does not have so much to do with validity as it does with other circumstances. I don't think those numbers mean anything---but it seemed to me that one of the other posters was asking about numbers and wanted to discuss them, so for the sake of concreteness there they are.

 

[wolram]

posted by eigenguy.

Yes, but why are physicists so much more confident in M theory than LQG?
-----------------------------------------------------------------
plebs view.

1 reward
2 endless possibilities
3 years of future work
4 they are not

 

[eigenguy]

Originally posted by selfAdjoint
it's a mistake to judge a field by the number of people working in it.

No, it's a mistake to judge a field ONLY by the number of people working in it. For people like us who are not experts it makes sense to ask why most experts feel this or that way.

Originally posted by marcus
I don't think those numbers mean anything

Why stand on personal opinion when you can find out for sure? That's all I'm trying to do.

Again I ask what is the hamiltonian constraint and why can't it be solved? Is this related to why they can't prove that LQG is realistic since it can't get GR? Why can't they get GR? I don't see how you can be confident without being familiar with all of this.

 

[marcus]

Originally posted by selfAdjoint
Just to say, Marcus, your arxiv search probably missed most of the Ashtekar, et al. papers, as well as those of Rovelli and associates since they both tend to avoid the term Loop Quantum Gravity and its abbreviation LQG.

I've posted before that it's a mistake to judge a field by the number of people working in it. Until the late 1930s the annual congresses of people working in quantum theory only had a dozen or so people attending. As late as the 1950s it was a small enough community world wide that everybody knew each other (except for the USSR, of course).

Over on s.p.r. it's been pointed out that "all particle physicists" is a tiny subset of "all physicists".

Just to re-iterate, we are both skeptical that the number of people who happen to be working in a field directly reflects the merit of that line of research. But frivolously or not, this morning I set out to verify that my search did NOT underrepresent LQG and found, to my amusement, that the first thing I looked for "Quantum Gravity and the Big Bang" a survey of loop quantum cosmology by Bojowald given this summer at a cosmology and fundamental physics conference was in fact not on the robot's search results list! I wanted to be able to reassure you that my search was effective and instead immediately found anecdotal evidence that it was not. (but I still think it gives a roughly accurate idea of the disproportion---something like a factor of 10 to 50)

You are right that several senior loop people don't use the term LQG---they simply say quantum gravity, or quantum geometry, or quantum GR, and omit the word loop. But I was searching in the abstract text and one would imagine that even if the title does not say "loop" that it will occur in the abstract somewhere. So I still think that all or nearly all of ashtekar and rovelli papers were caught.

indeed some stuff that shouldn't have been caught got included by mistake but not enough to distort the proportion---this occurred in the search of abstract text on["string" OR "brane" OR "M-theory"]as well but didnt seem significant there either.

 

[eigenguy]

Originally posted by marcus
Just to re-iterate, we are both skeptical that the number of people who happen to be working in a field directly reflects the merit of that line of research. But frivolously or not, this morning I set out to verify that my search did NOT underrepresent LQG and found, to my amusement, that the first thing I looked for "Quantum Gravity and the Big Bang" a survey of loop quantum cosmology by Bojowald given this summer at a cosmology and fundamental physics conference was in fact not on the robot's search results list! I wanted to be able to reassure you that my search was effective and instead immediately found anecdotal evidence that it was not. (but I still think it gives a roughly accurate idea of the disproportion---something like a factor of 10 to 50)

You are right that several senior loop people don't use the term LQG---they simply say quantum gravity, or quantum geometry, or quantum GR, and omit the word loop. But I was searching in the abstract text and one would imagine that even if the title does not say "loop" that it will occur in the abstract somewhere. So I still think that all or nearly all of ashtekar and rovelli papers were caught.

indeed some stuff that shouldn't have been caught got included by mistake but not enough to distort the proportion---this occurred in the search of abstract text on["string" OR "brane" OR "M-theory"]as well but didnt seem significant there either.

Even if there are 1000 times more string theory papers than LQG ones, it still doesn't explain to me why this is the case. What is it about LQG that makes it so less interesting to researchers in this field? I just don't get it. Is there a "fatal" flaw and if so what is it and if there isn't what is it about LQG that is less attractive than string theory? Is it really just that the math is less interesting or it's a smaller theory ? Isn't there some physics thing about LQG that people don't like compared to string theory. I feel discouraged to giving up looking for an answer here.

 

[eigenguy]

I'm getting worried that I'm offending people here. I really don't mean to do that and I am very sorry if it comes across that way. I am just having a hard time understanding the papers or finding where it says what's wrong with LQG which explains why this is more bothersome to people than what's wrong with string theory.

 

[selfAdjoint]

LQG is done in a different style than string physics. String physics is carried on in the treaditional way of particle physics, a mathematical trick from here, another from there, all put together cleverly. Particle physicists are just exhasperated by mathematical physicists who want to procede carefully and rigorously.

LQG is mostly done by mathematical physicists. Certainly that term applies to Baez, and I think it does to Ashtekar too. The breakthroughs have been in aspects of functional analysis that they can apply. They worry about dense subsets of Hilbert space. All of this is booooring to particle physicsts. Where's the physics?

As far as content is concerned - remember content? before we got tied up in head counting? - both M-theory and Ashtekar style quantum geometry are moving ahead slowly. Motl just had a good paper on M-theory on the arxiv, and the Ashtekar-Thiemann-Sahlmann papers on the generalization of the Stone-von Neumann theorem, which started all this posting, are the answer to the problems Thiemann encountered in his attempt to build quantum mechanics up within interacting quantum geometry. More progress in both areas can be expected. Neither field is dead, in spite of claims to the contrary.

 

[eigenguy]

Originally posted by selfAdjoint
LQG is done in a different style than string physics. String physics is carried on in the treaditional way of particle physics, a mathematical trick from here, another from there, all put together cleverly. Particle physicists are just exhasperated by mathematical physicists who want to procede carefully and rigorously.

LQG is mostly done by mathematical physicists. Certainly that term applies to Baez, and I think it does to Ashtekar too. The breakthroughs have been in aspects of functional analysis that they can apply. They worry about dense subsets of Hilbert space. All of this is booooring to particle physicsts. Where's the physics?

As far as content is concerned - remember content? before we got tied up in head counting? - both M-theory and Ashtekar style quantum geometry are moving ahead slowly. Motl just had a good paper on M-theory on the arxiv, and the Ashtekar-Thiemann-Sahlmann papers on the generalization of the Stone-von Neumann theorem, which started all this posting, are the answer to the problems Thiemann encountered in his attempt to build quantum mechanics up within interacting quantum geometry. More progress in both areas can be expected. Neither field is dead, in spite of claims to the contrary.

So it is not about the problems with these theories it is about the style of them? If so than I guess reading papers to figure out the logic of all of this is a waste of time. Also I realize that LQG is not dead. I said most researchers around this think it is a dead end which is different than kaput. If they said LQG is dead I probably wouldn't bother with this because it is hard enough to understand this to begin with. I just think that since neither LQG or m-theory is known to be correct there must be problems with them and I just wanted to know what they are. I guess maybe I should just forget about this and learn about what interests me which is m-theory. But I know you are trying to help. Thankyou.

 

[marcus]

Last month a poster here described his difficulty completing a PhD in LQG, which caused him to switch fields. This is a 4 September post by Javier in an ElJose thread about "quantization of Minkowskian metric". The point is, whether or not quantum gravity interests you, concrete CONDITIONS may cause you to study string theory----funding, availability of thesis advisors, job prospects, desire to have conferences to go to, a postdoc position, colleagues, tenure.

Javier's story is quite mild and par for gradschool, but it was told here at PF just last month and illustrates how other considerations besides interest affect choice of field.

---------------------------
...Certainly Carlo is one of the important players in the development of the loop formulation of qg. I had started my PhD work with a friend of his, Lee Smolin, but he left after a couple of years for a new institute in Waterloo, Ontario, so that ended that. Then I went over to the "dark side": the string/supergravity camp.
Consequently, I am familiar with Smolin's development of loop formulation of qg (which he worked on with Roveli). But after some time, I realized that this formulation was taking a back seat and something borne out of it was taking the center stage: spin networks. Some people still use the term "loop qg" to decribe the non-perturbative qg program, but that term doesn't properly title the program...it played a role, though.
There are currently two attempts at quantization of pure gr. One is the "canonical" quantization program, which uses spin networks as a basis for a space on which operators (like the area and volume operators) act. This network then represents a (quantum) state of space. There are a number of constraints in the theory to take into account, and all but one was. The remaining thorny issue is how these networks should advance in time; that is, how is the Hamiltonian constraint applied?
The second method is the "covariant" quantization, which is the analog of Feynman's sum over histories in point particle quantum mechanics, represented by "spin foams". These are like "world-foams", the time advancement of a spatial graph. But how exactly the spin networks (which are graphs) in the canonical formulation relate, if at all, to the spin foams is unknown.
Check out Abhay Ashtekar's review articles on the arXiv for some of the story. John Baez has written popular stuff on spin foams.
------------------------------

Recently I've noticed more new people working in quantum gravity in Europe and Latin America than in USA. There seem to be more postdoc positions and more financial support for LQG abroad---Germany, France, India, Argentina,...
So people who were in this country a couple of years ago seem to be percolating out.

The String "party" seems to be so influential both in the agencies that decide funding and in the popular media that whether we in US like it or not we seem to be putting all our eggs in a string basket. I hope this imbalance is addressed before more damage is done.

 

[eigenguy]

Originally posted by marcus
Last month a poster here described his difficulty completing a PhD in LQG, which caused him to switch fields. This is a 4 September post by Javier in an ElJose thread about "quantization of Minkowskian metric". The point is, whether or not quantum gravity interests you, concrete CONDITIONS may cause you to study string theory----funding, availability of thesis advisors, job prospects, desire to have conferences to go to, a postdoc position, colleagues, tenure.

The String "party" seems to be so influential both in the agencies that decide funding and in the popular media that whether we in US like it or not we seem to be putting all our eggs in a string basket. I hope this imbalance is addressed before more damage is done.

I want to know if string researchers can back up their attitudes with good physics reasons and what they are. If they can't then this whole thing is nutty. I just find it hard to believe that string theorists are just a bunch of quantum gravity biggots. When I went to that lecture in january, the reasons sounded like they had to do with physics and not with other stuff. But I didn't really understand so I can't say for sure. This was my impression.

Originally posted by marcus
I realized that this formulation was taking a back seat and something borne out of it was taking the center stage: spin networks. Some people still use the term "loop qg" to decribe the non-perturbative qg program, but that term doesn't properly title the program...it played a role, though.

So "spin networks" is more correct than "LQG". Good to know. I will try to use it to embarrass someone at my next quantum gravity party.

 

[meteor]

Superstring theory relies in a particle called graviton to explain gravity, however the graviton is impossible to find
An hypothesis about why there's more research in superstring theory than in LQG: is it possible that in the universities the major part of people choose studies related with quantum theory and QFT rather than studies related with relativity? If so, it is more easy for them to choose string theory like research object
And the last thing: It is not an advantage for LQG to be founded upon a mathematically consistent theory, general relativity, over string theory that is founded over a mathematically inconsistent theory, Quantum field theory?

 

[marcus]

Originally posted by meteor
Superstring theory relies in a particle called graviton to explain gravity, however the graviton is impossible to find
...
And the last thing: It is not an advantage for LQG to be founded upon a mathematically consistent theory, general relativity, over string theory that is founded over a mathematically inconsistent theory...

for sure

who would you rather go out on a date with? choose the
mathematically consist one every time! many more possibilities

 

[Mentat]

eigenguy,
Welcome to the PFs.
A question: how many people were working along with Einstein on Relativity?

The point is that the amount of people working on a theory is irrelevant to the merit of the theory itself.

As to why people constantly say that strings are far ahead of loops, it's probably because of the utter elegance and potential power of M-Theory...LQG is a great theory (from what I know of it - I'm more into strings myself), but it doesn't have the potential impact of string theory.

 

[Mentat]

Originally posted by meteor
Superstring theory relies in a particle called graviton to explain gravity, however the graviton is impossible to find
An hypothesis about why there's more research in superstring theory than in LQG: is it possible that in the universities the major part of people choose studies related with quantum theory and QFT rather than studies related with relativity? If so, it is more easy for them to choose string theory like research object
And the last thing: It is not an advantage for LQG to be founded upon a mathematically consistent theory, general relativity, over string theory that is founded over a mathematically inconsistent theory, Quantum field theory?

meteor, I'm pretty positive you're wrong about this. IIRC, one of the great things about superstring theory was that it's equations yeilded the graviton.

 

[selfAdjoint]

Mentat, I think that Meteor's point is that although the graviton is well defined within string theory, there is no physical evidence of it. There is at least some physical evidence for GR, enough to make a consistent picture for the astrophysicists and cosmologists. Note that dark matter is not a problem for GR (which is not a theory of matter) and dark energy has a solution within GR, the cosmological constant. As far as observation and experiment goes, GR has been conditionally validated at the length scales it concernes.

 

[lumidek]

Dead ends

Hi!

Strictly speaking, we never know for sure that a theory is a dead end - until it gives precise and strict enough predictions that are ruled out by the experiment. This has almost happened with loop quantum gravity many times, but not quite because every time this theory is ruled out, its defenders construct a not-quite-working comment that it might be fixed sometime in future. Most high energy physicists today are convinced that loop quantum gravity is *probably* a dead end, and therefore they don't work on it. Of course, they can be wrong. However, I don't think that they are wrong.

Someone has said that string theory smells like God, but loop quantum gravity smells like Man. What do we mean? String theory is full of nice surprises. We often do a calculation, and the mathematics is so complex and contains so many possibilities to give meaningless answers, that normally you would expect that a meaningless, anomalous, divergent result can be the only outcome.

Nevertheless, in string theory all the nonsensical things always cancel out, and a unique answer follows. String theory is a structure that is able to teach us; we are not inserting the answers as input. There is one string theory only; no variations exist. The answers are derived from nontrivial calculations. We are often forced to think about subtle details to avoid fast conclusions that would look like contradictions.

String theory seems to contain all good ideas in physics. It is a very vast field, and it has the necessary capacity to describe everything in the real world: quarks and leptons with the correct charges, interacting with the correct forces, and most importantly these forces include gravity. We did not create string theory in order to describe gravity: gravity is an inevitable consequence of string theory that appeared surprisingly.

Strings in a certain vibrational pattern interact exactly in the same way as if they were curving the space around. This is true at long distances - long distances is where we know that gravity must follow these Einstein's laws (which are approximated well by Newton's laws for weak gravitational fields) - we know it from observations. At very short distances, string theory predicts much richer physical phenomena than ordinary general relativity, and these rich effects don't contradict observations.

String theory seems to be a totally unique structure: all of its different versions have been shown to be connected, especially in the second (duality) superstring revolution in the middle 1990s. We are permanently learning new things about string theory: we are learning that our idea about the number of dimensions of space was probably wrong. We are learning that our geometrical intuition is wrong at very short distances. We are learning many facts about the previous theories of reality - quantum field theories; string theory illuminates so many miraculous and unexpected interrelations between quantum field theories (as well as gravitational theories based on string theory) that even people who are not interested in fundamental physics and quantum gravity simply must observe the developments in string theory; these developments seem to be essential for many other subfields of physics (and mathematics).

String theory is responsible for solving many problems in pure mathematics - problems that were unsolvable previously. Mirror symmetry is a good example. It is string theory that links two superficially very different six-dimensional shapes (physics on them is equivalent!), and allows us to answer difficult questions about one of them using simple properties of the other. And there are hundreds of such miraculous mathematical properties.

Loop quantum gravity is a different type of theory. Its goal is not to explain the origin of quarks; leptons; electromagnetic force; weak force; strong force. Its only goal is to claim that Einstein's gravity can be relatively easily reconciled with quantum mechanics. Most leaders of theoretical physics believe that this opinion has been proved wrong beyond any doubts. General relativity treated as quantum theory has a lot of technical as well as conceptual problems - and the divergences in Feynman's diagrams are the most obvious problem.

The loop quantum gravity people argue that these calculations that clearly show the inconsistency of quantized general relativity should not be taken seriously. We should not trust our perturbative calculations, they say, and they believe that if we just reparameterize the variables in a relatively obscure way, the problem will be solved.

There is a lot of evidence that it is not true, and there is no evidence that it is true. Loop quantum gravity essentially plays with a sort of randomly chosen simple discrete system that is, in some formal sense, equivalent to Einsteinian gravity at very short distances. Nevertheless we know that if we discretize a system, we are not guaranteed that the physics will follow the original laws at the long distance scale. Just the opposite: there is a principle that everything, every term, every correction, every possible interaction whose existence is not forbidden by a symmetry, WILL occur.

This principle has been tested on hundreds of discrete systems, and it has always been true. For example, if we put quantum chromodynamics on lattice, the long-distance physics will NOT follow the naive equations that we discretized. There will be a lot of new terms generated. If we apply this to loop quantum gravity, it turns out that more or less smooth space will NOT exist at all. There is no evidence whatsoever that smooth space can emerge from loop quantum gravity. Most likely, the spin networks want to be crumpled to a very chaotic web whose radius is comparable to the Planck length.

Imagine. It is a theory based on all possible - and very unlikely - prejudices that people had about the real world decades ago, and it is very likely that it does not allow the smooth space to exist. This simply means that most likely, it contradicts *all* observations that have ever been made. Is there something in the real physics Universe that can exist without the space?

Loop quantum gravity contains no successful self-consistency checks - unlike thousands of mathematical checks that just work out in string theory. Every attempt to calculate something in loop quantum gravity always leads to a result that seems wrong. The black hole entropy was calculated with a wrong coefficient by a factor of ln(2)/sqrt(3).pi. Ridiculous numbers. Attempts have been made to remove this discrepancy, and they have led to other predictions - for example about the quasinormal modes of the black holes - and these predictions have been shown incorrect again. Nothing seems to work, and loop quantum gravity seems to be a perfect example of the disasters and inconsistencies that one encounters if she wants to describe quantum gravity without string theory.

String theory really seems to be the only game in town.

Loop quantum gravity also contains a lot of prejudices that have been inserted and that are believed to be incorrect. It assumes that the dimension of space is always four; it assumes that the metric is a good variable at all distances scales; it assumes that the form of Einstein's equations essentially does not depend on the scale on which we study physics; it assumes that non-gravitational physics can be safely ignored when we study gravitational physics; it assumes that our intuitive idea about the independence of things separated in space (for example inside and outside black holes) are correct; it assumes that there is qualitative difference between momenta and other charges, between geometry and other fields. Moreover, it seems clear that loop quantum gravity predicts that Einstein's special relativity must be heavily wrong.

We have accumulated overwhelming indirect evidence that most of these assumptions - and probably all of them - are just plain wrong. Loop quantum gravity is one of trillions of possible wrong theories derived from about 30 incorrect prejudices that dominated physics in the 1920s, and it predicts no result whatsoever that seems to agree with another fact that we know about Nature.

All "successes" of loop quantum gravity seem to be artificial parodies on true successes of string theory. When Strominger and Vafa calculated the correct black hole entropy from string theory in 1995, loop quantum gravity people wanted to do the same. It did not quite work. Because today we are expecting the experiments (not only accelerators) to confirm many possible cosmic scenarios that emerged from string theory and that are very exciting and reasonable. Loop quantum gravity people wanted to be connected with experiments, too, and therefore they propose totally undefendable experiments to check their prediction that special relativity is wrong (which we know is probably not true). It's just their inability to construct a theory that agrees with the well-tested principles of Nature, not a serious scientific proposal.

If someone says that the reasons not to study loop quantum gravity - and focus on string theory - are not rational, he or she is simply wrong (or a liar, in the worse case). There might be some young people who really don't understand string theory well and who work on it only because of some random pressures, but be sure that all senior and famous physicists know what they're doing - and most of the active ones are doing string theory and they know very well why they don't study loop quantum gravity!

I know it is difficult to understand the different situation of string theory and loop quantum gravity without actually going into the technical details, but I also know that loop quantum gravity simply can't compare with strings once you *do* study the details.

Best wishes
Lubos