Written and Illustrated by Ed Witten. (Slides 11 to 20 of 48) 11.. So a single string describes all at once a huge assortment of different particles. Since one learns to correctly 'quantise' the string and 'compute' the 'spectrum' one finds that one of these particles has just the right properties to be the 'graviton', or quantum of gravitational waves. ---- This is the beginning of the discovery that ....
12.. In contrast to convential quantum field theory, which makes gravity impossible, string theory requires gravity. Other modes of the string look like the photon, electron, muon, quarks, neutrinos, etc. That is, with very simple assumptions, 'everything' fits neatly into a string.
13.. In a way that is uncannily like what we see in nature. For a simple explanation of why string theory turns out to have such nice features ... lets go back to standard quantum theory.
14.. According to Feynman, one calculates the details of physical processes in quantum (field) theory via 'Feynman Diagrams'. (Slide 14 below)
15.. The lines in a feynman diagram represent free propagation of particles. (Slide 15 above) But the vertices. (Slide 15 above) are interaction events where particles branch and rejoin.
16.. There are lots of great things about Quantum Mechanics but its limits. (Slide 16 below)
can be seen here: 1.. There are too many theories, differing by what kind of vertices one allows. 2.. Infinities (which eventually make gravity impossible) come when X = Y = Z = W. (Slide 16 above)
17.. In String theory one replaces the propagating point particle. (Slide 17 below) by a propagating string. Slide 17 below)
18.. So ... (Slide 18 below) The 'vertices' have disappeared, so a) very few theories b) Finite - no trouble with inverse square law. [possibly jump to 22 .. 19, 20, 21 appear to have been added later... greg]
19.. The statement that there are 'very few theories' means, to be precise, that once one understands the motion of a free string. (Slide 19 below) one automatically understands
the interactions, since the pictures look everywhere locally the same. (Slide 19 below)
20.. This contrasts with field theory, where after describing the free motion. (Slide 20 below) one must still describe the interactions or vertices. (Slide 20 below)
continues in next post .......