can have various kinds of boundary conditions
, which limit their behaviour. When the strings are 'open', that is not closed loops, they can have two kinds of boundary conditions, Neumann and Dirichlet
. With the Neuman boundary condition, one end point is fixed into some manifold
, and the other is free to move about, however no momentum
flows out of this system. With the Dirichlet boundary condition, the ends are fixed into the manifold, and can only move within this manifold.
The manifold is known as a D-brane, or more generally as a Dp-brane where p is an integer ranging from -1 to the number of spatial dimensions in the space-time you're looking at! So for the 10 dimensional superstring theories, you can have up to D9-branes. The -1 brane refers to a kind of null state, where the space and time coordinates are fixed. D0-branes are point like, D1-branes are string like, D2-branes are like bounded membranes, D3-branes are like soap bubbles, and past there, I don't know how to visualise. Presumably they have the same kinds of geometry of higher dimensional solids such as the hypercube and hypersphere.