  This elementary introduction to string field theory highlights the features
and the limitations of this approach to quantum gravity as it is currently
understood. String field theory is a formulation of string theory as a field
theory in space-time with an infinite number of massive fields. Although
existing constructions of string field theory require expanding around a fixed
choice of space-time background, the theory is in principle
background-independent, in the sense that different backgrounds can be realized
as different field configurations in the theory. String field theory is the
only string formalism developed so far which, in principle, has the potential
to systematically address questions involving multiple asymptotically distinct
string backgrounds. Thus, although it is not yet well defined as a quantum
theory, string field theory may eventually be helpful for understanding
questions related to cosmology in string theory.
