|Since 12 August, 2008.|
Strings can be open or closed. As they move through spacetime they sweep out an imaginary surface called a worldsheet.
These strings have certain vibrational modes which can be characterized by various quantum numbers such as mass, spin, etc. The basic idea is that each mode carries a set of quantum numbers that correspond to a distinct type of fundamental particle. This is the ultimate unification: all the fundamental particles we know can be described by one object, a string! [A very loose analogy can be made with say, a violin string. The vibrational modes are like the harmonics or notes of the violin string, and each type of particle corresponds to one of these notes.]
As an example let's consider a closed string mode which looks like:
This mode is characteristic of a spin-2 massless graviton (the particle that mediates the force of gravity). This is one of the most attractive features of string theory. It naturally and inevitably includes gravity as one of the fundamental interactions.
Strings interact by splitting and joining. For example the anihilation of two closed strings into a single closed string occurs with a interaction that looks like:
Notice that the worldsheet of the interaction is a smooth surface. This essentially accounts for another nice property of string theory. It is not plagued by infinities in the way that point particle quantum field theories are. The analogous Feynman diagram in a point particle field theory is:
Notice that the interaction point occurs at a topological singularity in the diagram (where the 3 world-lines intersect). This leads to a break down of the point particle theory at high energies.
If we glue two of the basic closed string interactions together, we
get a process by which two closed strings interact by joining into an intermediate
closed string which splits apart into two closed strings again: