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.

## SupersymmetryFor consistency, a superstring has to have both bosonic and fermionic excitations which are coupled in a supersymmetric manner. There are five consistent superstring theories - Open string (O).
- Heterotic closed string (Het)
- Non-chiral closed string (IIA)
- Chiral closed string (IIB)
- M Theory
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