1) Why do the extra dimensions of string theory need to be curled up rather than extended? Why not 10 or 11 extended spatial dimensions?


2) I like to get both sides of an issue and right now I can't seem to find any good literature on any anti-string-theory POVs. Surely someone here is adamently opposed to it. Are there ANY non-string based theories right now that united GR and QM the way string theory buffs claim their theory does? Is the holographic universe theory an equal contender?

Thank you in advance for your honest responses.

The biggest criticism of string theory that anyone can make at the moment is that there isn't any way to test it. The math behind string theory works, so you can't really argue about that. The problem is that it's currently impossible for us to actually see if it's correct or not. Experimental verification is probably the most important part of science; it's what separates science from philosophy. We can design experiments to test string theory, but we can't actually perform any of them with our current technology. It seems likely that we won't be able to test string theory for quite a long time. Some physicists think that because of this it doesn't even qualify as science.

1) Why do the extra dimensions of string theory need to be curled up rather than extended? Why not 10 or 11 extended spatial dimensions?

in short, they have to be curled to agree with what we see. our universe is 4 dimensional. string theory has 10, so we have to do something with the other dimensions. compactification of these extra dimensions also gives us a lot of freedom to control the spectrum of the theory. this is, i suppose, both a strength and a weakness.

as far as i know, we do not know a mechanism for why the compactification occured, for why we don't actually see all 10. good thing we don't though; atoms and solar systems would not be stable in such a universe.

2) I like to get both sides of an issue and right now I can't seem to find any good literature on any anti-string-theory POVs. Surely someone here is adamently opposed to it. Are there ANY non-string based theories right now that united GR and QM the way string theory buffs claim their theory does? Is the holographic universe theory an equal contender?

Thank you in advance for your honest responses.
oh, you want to read some of the polemics by the anti string theorists?

ok....

read this (http://arxiv.org/abs/hep-th/0310077), this (http://www.arxiv.org/abs/physics/0102051) and this (http://www.americanscientist.org/template/AssetDetail/assetid/18638)

although those articles are pretty mean.

that first article is written by Rovelli, who is one of the main architects of a theory called Loop Quantum Gravity. you asked for other possibilities for quantizing gravity, well here is one. LQG is a theory of quantum gravity, but not a theory of everything (the way string theory is). in other words, it says nothing about stuff other than gravity. LQG does not require extra dimensions or supersymmetry the way string theory does. instead, it requires that spacetime is discrete at the Planck scale.

Hum,
we do not know a mechanism for why the compactification occurred,

i have an idea that perhaps ALL the dimensions were curled up once,(big-bang), and that during a symmetry-breaking (early on) of the BB, the 4 macro dimensions that we see today expanded...
The exact mechanisms is still vague at the moment, but it must have occurred at the Planck time, 10^-43 of a second, (when gravity would be created)...i have a feeling that the creation of the Higgs field generated the energy for the expansion of the 4 dimensions we see today (er, this is the BB, right)...

So at the wall of the original cosmos bubble, Super-symmetry was broken, making the bubble grow. And i imagine that inside(or near to) this wall Higgs particles were releasing their energy as they decayed. So this baby bubble was gradually filled with energy. As bubbles of the "true vacuum" (with a nonzero Higgs field) percolate and grow, baryogenesis occurred at or near the bubble walls. and viola! One expanding universe....

Yes, I've heard of this Loop quantum gravity. I will definitely get into it thank you. Thank you also for the links.

Slight tangent. For my own curiousity's sake, could you expound a little, lethe, on why planets and stars would be unstable in a higher-dimensional space.

Both String theory and LQG seem to necessitate space being discrete at the planck scale, yet both achieve this in completely different ways. Whats inportant is that they HAVE to get rid of the chaoticness of the spatial fabric below the planck scale in order to link GR with QM. I'd like to review all the so-far conjectured ways that people have circumvented this potential quality of space.

Slight tangent. For my own curiousity's sake, could you expound a little, lethe, on why planets and stars would be unstable in a higher-dimensional space.
well, i don't know a good "why" to explain why keplerian orbits are unstable in higher dimensions.... if your force law is an inverse square law, then your orbits are conic sections. if the force law is inverse some higher power, then you no longer have conic sections, but you have some kind of orbits. only, they are not stable. it just comes out of comparing exponents, i guess.

Both String theory and LQG seem to necessitate space being discrete at the planck scale
well, the final word is not in yet, but it doesn't seem to me that string theory requires spacetime to be discrete. only LQG does that.
yet both achieve this in completely different ways. Whats inportant is that they HAVE to get rid of the chaoticness of the spatial fabric below the planck scale in order to link GR with QM. I'd like to review all the so-far conjectured ways that people have circumvented this potential quality of space.