String theory provides a deeper understanding of the four dimensional phenomena that we can directly measure. In particular, gauge symmetry and parity violation follow naturally from string theory.
Consider our four dimensional world as a D3-brane embedded in 10 dimensional spacetime. All the fundamental particles such as electrons, quarks, muons and so on are the end-points of open superstrings.
The quantum states which arise from open strings beginning and ending on the 3-brane can effectively be described by gauge fields confined to the 3-brane coupling to charges which are the end points of open strings.
For consistency, a superstring has to have both bosonic and fermionic excitations which are coupled in a supersymmetric manner.
There are five consistent superstring theories
It has been shown that these five superstring theories are all just different ways of looking at the same theory.
Both heterotic and IIB closed superstrings are chiral in the sense that string excitations propagate only say clockwise and not counter-clockwise. This geometrical property ultimately translates into parity violation in our four-dimensional world.
Last updated: 06 March, 2000