            From its humble beginning as a theory that predicted the existence of gravitons, String Theory has evolved into what some physicists hope to be the Theory of Everything. To date, it is expected to supply computable answers to some of the biggest questions on the minds of physicists today: the underlying symmetries of nature, the quantum behavior of black holes, the existence and breaking of supersymmetry, and the quantum treatment of singularities.

            The fundamental premise of string theory is that all fundamental particles are made up of Planck length strings, which can be open or closed. As these strings move through spacetime, they are supposed to sweep out a worldsheet, a surface which is used to describe the interactions between strings. The key property of these strings is that they vibrate: different vibrational modes can be used to describe the mass, spin and charge of particles, which is why so many people love the theory. As long as you can make the string move in enough directions, every property of every particle can be adequately described. The problem is that in order to make the string move in enough directions, there must be enough dimensions for the string to move in, several beyond our own three (plus time).

            Various kinds of boundary conditions in strings add yet another aspect of versatility to String Theory: strings can be periodic (closed) or open; of the open strings, a Neumann boundary condition describes a string whose endpoint is free to move, but can not transform momentum. A Dirichlet boundary condition describes a string which has one end attached to a manifold brane, on which it can move. The manifold is called a D-brane or Dp-brane (where ‘p’ is the integer number of spatial dimensions of the manifold). When p=0, all of the spatial coordinates are fixed at a single point in space- the D0-brane is called a D-particle. Superstrings exist in 10 dimensional spacetime: its Dirichlet boundary condition would describe a string attached to a D-9 brane: its end could move through 9 spatial dimensions. The case where p=-1 describes a state in which all space and time coordinates are fixed- an instanton or D-instanton.