Three Roads To Quantum Gravity
Three Roads To Quantum GravityA book by LeeSmolin.
A forward-looking book about current theories and research leading towards a theory of QuantumGravity, which is essentially an updated term for a GrandUnifiedTheory, focusing on StringTheory, LoopQuantumGravity? and a third road which I mistakenly recalled as being based on the Standard Model of Elementary Particles, but which discussion below covers.
The Standard Model has nothing to do with quantum gravity. As for loop quantum gravity, it's got a bad reputation among theorists. LQG is an artificial construction (one among an infinite number of equally artificial/arbitrary constructions) and completely useless besides; it doesn't even unify gravity with electromagnetism! There is only one serious candidate for quantum gravity and that's the theory that used to be superstrings. So I'm at a loss as to what the other two "roads" to quantum gravity are supposed to be.
As for terminology, let's be clear:
To string theorists, "theory of everything" is a propaganda term which means the same thing as "quantum gravity" and refers to the theory that used to be superstrings uniquely.
To LQG theorists (the miniscule number there are of them), LQG is not a Grand Unified Theory. It can't be since LQG doesn't deal with the electroweak or colour forces. LQG is a theory of quantum gravity only.
Grand Unified Theory refers to extensions of the Standard Model like the MSSM (Minimal Supersymmetric Standard Model) which predict Higgs bosons, inflatons and the like. GUTs unify the electroweak and colour forces under one formalism.
It was my mistake regarding the "third road" in the book, and as I indicated above, I wasn't sure if I had the third road right. I pulled out the book to see if they were summarized easily, and they were not, but in one of the Amazon reviews they seem to imply the third road is from "fundamental principles" which doesn't mean much to me, but it did stir the recollection that I think the third road as described in the book is actually an attempt to bridge the best and most promising work from both StringTheory and LoopQuantumGravity?. As for what exactly qualifies as a GrandUnifiedTheory, well, I said above that QuantumGravity was a kind of updated form of the earlier notion of a GrandUnifiedTheory. I didn't say they were the same or that LoopQuantumGravity? in particular was being addressed in this book or elsewhere as a GrandUnifiedTheory, only that it was one of the three roads addressed in this book which might lead to a theory of QuantumGravity. You seem to have an animosity towards LoopQuantumGravity? that I have no interest in seeing indulged here, though if you wish to discuss some of the problems you understand with respect to LoopQuantumGravity? please feel free to do so here or on that page, as I'd be interested to understand.
One source of information I came across last October may be of interest here, as it was a step forward for Loop Quantum Gravity that showed that it "does not suffer from certain mathematical 'infinities'," and can be read about here http://www.aip.org/enews/physnews/2001/split/562-1.html and the actual paper is available here http://xxx.lanl.gov/abs/gr-qc/0104057.
Also, in the interest of full disclosure, though LeeSmolin remains open-minded on the issue, he seems to expect that ultimately LoopQuantumGravity? will be the path to QuantumGravity, though he speculates that at a different scale the correct understanding of LoopQuantumGravity? may be very closely approximated by some version of StringTheory. In particular, the reason Smolin cites as being disinclined towards believing in StringTheory as the final version of QuantumGravity is that like NewtonianPhysics?, StringTheory is not BackgroundIndependent?, and Smolin feels in that famous debate between IsaacNewton and GottfriedWilhelmLeibniz that all of 20th century physics has proven Leibniz right (and he seems to doubt that Newton really believed it himself, but rather that since he could not conceive of a workable system any other way, he concluded of logical necessity that the universe must be absolute, though that's just my reconstruction of Smolin's opinion).
Okay, first of all neither LQG nor string theory are updated versions of GUTs. LQG because it does not achieve, or even set out to do, what even the most mundane GUT achieves. And string theory because it wasn't created as a GUT and shares no properties of GUTs. If the same researchers were involved in the creation of GUTs and string theory, it's only because the theoretical physics community is so small. So saying that quantum gravity is "an updated version of GUT" is wrong by any sense of the term quantum gravity.
Furthermore, the notion of taking "the best things from LQG and string theory" is nonsense. As I understand it, there isn't any "best of" LQG. LQG hardly has any researchers or papers to its name. Secondly, they are inimical to each other. There are no common concepts between them nor "fundamental principles" more fundamental than string theory. Thirdly, there is nothing "beyond" string theory. There is no fact of our universe that is beyond string theory. There is no way to bridge string theory with "something else" the way you can between QM and GR, because there is nothing else. And fourthly, LQG concerns itself only with gravity; it doesn't unify anything and is thus a limited throwback theory. For these reasons, saying that string theory might "approximate" LQG is laughably nonsensical. It would be like seeking to unify General Relativity with Newtonian mechanics.
If you're interested in this subject, I suggest reading the 'loop quantum gravity' threads in sci.physics.research (just use Google). Lubos Motl deals with your, and many other, objections to string theory. In particular, the notion that string theory is background dependent is nonsense. There is nothing static, there is no fixed background, in string theory. Everything changes according to the rules of string theory. It must be recalled here that the theory that used to be superstrings has a dynamical number of dimensions. This is one reason why most string theorists think Lee Smolin just doesn't know string theory.
All the people working on loop quantum gravity are friends of mine, and I just don't have that many friends who do this stuff.
The number of people working actively on both loop quantum gravity and string theory could be counted on the fingers of one hand after a serious accident with a cleaver.
Some people even argue that the equations describing any theory of physics can themselves can be considered a background structure, because they hold regardless of the state of the system. To get rid of *this* background structure, we would need to do physics without any fundamental equations. Believe it or not, folks like Lee Smolin and Holger Nielsen have tried this! -- JohnBaez?
The background is a coherent state of strings themselves - as you can see easily because its infinitesimal change has the same effect as inserting a string vibrating in the appropriate (graviton) vibration pattern.
[IOW, string theory is background independent.]
John Baez made a good presentation of LQG. For me it is a system of ideas as large as say D-branes on orbifolds; it also fits the number of papers etc. I do not see a reason why should one consider LQG to be more relevant than D-branes on orbifolds, only because its proponents claim that it is a competition of the whole building of M-theory. Pride is not enough. When Fidel Castro says that Cuba will defeat the whole imperialistic United States of America, it does not mean that Cuba becomes much more important than Florida. ;-) -- LubosMotl?
I took a fourth road, and that has made all the difference. http://WikiWorld.com/wiki/index.php/InformationPhysics -- JimScarver
Actually, you might be quite interested in Smolin's book then, as there's a great deal of discussion of the signifigance of the flow of information in physics, and a particular new kind of math who's name escapes me right now dealing with systems where limited amounts of information is available. Its not exactly liek EdFredkin's ideas about physics, but Smolin does give some treatment there (though it may have been more explicit in his earlier book, TheLifeOfTheCosmos)
Or you could read something both challenging and mainstream, like DavidDeutsche?'s paper on the information structure of the multiverse. In any case, what does information physics have to do with quantum gravity?
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