Originally posted by (Q)
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No problem. Thed has a good handle on string theory.
Catches buck thrown at me. :) I'm not that sharp with string theory, it's a little more complex than I like. See the first link below for the hoary maths.
String theory comes out of a thing called Kaluza Klein theory (http://www-th.phys.rug.nl/~schaar/htmlreport/node12.html) and Yang Mills thoeries (http://www.claymath.org/prizeproblems/yangmills.htm).
Right, are we sitting comfortable. First off a dimension is only a thing needed to uniquely measure some event in spacetime. There is nothing sci-fi'ish about it, you can't travel to 'other dimensions' as they are in integral part of our Universe. In normal life we percieve 3 dimensions length, depth, breadth. Of course, you also need to specify <i>when</i> an event happened so you have to include time, t. In other words, the Universe is 4 dimensionsal.
The big change between Newton and Einstein is that in Newtonian physics time is the same for all people and independant of space. In Einsteinian physics time and space are interelated as 4 dimensional spacetime. Though Relativity comes out of problems with Electromagnetism in Newtonian physics Relativity does not include a description of Electromagnetism in General Relativity.
What Kaluza and Klein done was assume a 5th dimension. That is, a requirement for a 5th data point to uniquely measure things. In this 5 dimensional description of things you can describe the 4 dimensional space time and electromagnetism. The extra dimension 'wraps up' on a small scale (planck lengths) which is why we can't see it. What's more this is all done in a geometric description favoured by Relativists.
One of the big things favoured by Physics is that the Universe should be fundamentally simple. Maybe a bad assumption but 'simple' theories are favoured over complex theories. So, what happens next.
Back in the early 1900's Physics started finding 'fundamental; particles like the neutron, electron and proton. As detectors got better more and more fundamental particles where found. Things like the muon and kaon. Within a shortish time hundreds of 'fundamental' particles where found. Invoking the rule of simplicity it was obvious they where not fundamental at all. This led to the development of <A href="http://www.particleadventure.org">The Standard Model</a>. This is a description of particles using mathematical set theory. The set generators are used to produce tuples of particles in families. The set generators are called 'quarks', you need 6 of them. But you also have quantum chromodynamics that describes the interaction of the strong force, that needs 3 types of particles called gluons that come in 3 colours. You also have quantum electrodynamics that descibe weak nuclear forces, that requires a further 3 particles. Neither of these are predicted in the standard model. Alongside this lot you have electrons and neutrinos as fundamental particles.
Sounds a bit complex doesn't it. Too many particles and too many theories required. Let's invoke the rule of simplicity. Maybe the quarks, gluons, vector bosons and electrons are actually caused by something more simple.
I don't know much about Yang-Mills to be honest and don't know much it's historical imporance, perhaps James R does?
But anyway, what Brian Green and others done was assumed that all these particles where different examples of the vibrational modes of simple 1 dimensional strings. That is all there is to string theory. It describes particles as open or closed strings that vibrate. To do this though you have to play with complex maths and describe the Universe in several dimensions, just like Kaluza Klein done. That is where the theory was resurrected.
Great idea, on the face of it. The next trick was to then incorporate this string theory into General Relativity just like Kaluza Klein. To do that you need even more dimensions, about 11 at last count. The problem was that these 'simple' 1 dimensional strings threw wierd infinities in to the solutions, a sign something was wrong. So, they went a stage further and used 2 dimensional membranes, Brane Theory.
After that the theory looses me. You get into wierd, stange maths describing particles with highly advanced geometrical terms and using things like Kaluza Klein to combine it with Relativity.
None of this should be confused with another string theory involving Cosmological Strings created in the Big Bang. Same names, different ideas.