T-duality

Summary

T-duality is an equivalence between string theories on different backgrounds. Let one direction of spacetime be curled into a circle of radius $R$. Then closed strings give the same physics as on a circle of radius $\alpha'/R$, where $\alpha'$ is the Regge slope. The duality exchanges the momentum of a string around the circle with the number of times the string winds around it. It shows that string theory has a minimal length scale of order $\sqrt{\alpha'}$. In superstring theory it relates the type IIA and type IIB theories.