Various String Theories

Different string theories may employ different forms of strings for their basic building blocks. Four of the five mainstream theories use closed strings as the basic building blocks. One uses open strings. Three of the five string theories are superstring theories, that is, they combine string theory with supersymmetry. The other two combine bosonic string theory with superstrings, resulting in heterotic string theory 2.

Closed strings have periodic boundary conditions, the string comes back on itself. Open strings can have Neumann and Dirichlet boundary conditions. Neumann boundary conditions allove the endpoint to move but no momentum to flow out. Dirichlet boundary conditions allow the endpoint to move only on a manifold, called a D-brane 3.

Closed string becoming open using a D-brane

With dualities, one theory can be transformed to look just like another. This gives support to M-theory. T-duality obscures the ability to distinguish between large and small distance scales. It relates IIA to IIB and E8xE8 to SO(32). S-duality obscures the ability to distinguish between large and small coupling constants . Weak coupling can be understood by expanding in a series, so if two theories are related by S-duality the strong theory can be understood through transformation into the weak theory and subsequently expanding the series. It relates I to SO(32) and IIB to itself 5.