T-duality has long tantalized us in giving a window into stringy geometry.
In recent years, we have developed metastring theory, a string theory in
which the target space is not interpreted a priori as space-time but rather
as a sort of phase space. By generalizing in this way, we obtain a theory in
which space-time, and space-time locality, can be regarded as emergent.
T-duality is then realized as a sort of Fourier transform, selecting a
space-time based on how the theory is probed. We find that metastring theory
is essentially unique; there is a purely quantum notion of space-time that
we call modular space-time, and all of the usual string compactifications
arise from it by various `classical’ limits.
