  The zero modes of closed strings on a torus --the torus coordinates plus dual
coordinates conjugate to winding number-- parameterize a doubled torus. In
closed string field theory, the string field depends on all zero-modes and so
can be expanded to give an infinite set of fields on the doubled torus. We use
the string field theory to construct a theory of massless fields on the doubled
torus. Key to the consistency is a constraint on fields and gauge parameters
that arises from the L_0 - \bar L_0=0 condition in closed string theory. The
symmetry of this double field theory includes usual and 'dual diffeomorphisms',
together with a T-duality acting on fields that have explicit dependence on the
torus coordinates and the dual coordinates. We find that, along with gravity, a
Kalb-Ramond field and a dilaton must be added to support both usual and dual
diffeomorphisms. We construct a fully consistent and gauge invariant action on
the doubled torus to cubic order in the fields. We discuss the challenges
involved in the construction of the full nonlinear theory. We emphasize that
the doubled geometry is physical and the dual dimensions should not be viewed
as an auxiliary structure or a gauge artifact.

High Energy Physics - Theory