
T Duality
Sometimes
Theory A with large
compact dimensions is equivalent to Theory
B with small compact dimensions.
This
tells us about relationships among the five superstring theories:
Type IIA string theory on
a circle of radius R
is equivalent to Type IIB
string theory on a circle of radius L_{st}^{2}/R
Heterotic E_{8} X E_{8}
string theory on a circle of radius R
is equivalent to Heterotic SO(32)
string theory on a circle of radius L_{st}^{2}/R
In
each case, the duality transformation interchanges the momentum excitations
and the windingmode excitations.

