| Summary: | This thesis examines the interaction of both bosonic- and superstrings with
various backgrounds with a view to understanding the interplay between
tachyon condensation and world-sheet conformal invariance, and to understanding
the d-branes that overlap with closed string modes. We briefly
review the development of both background independent string field theory
and cubic string field theory, as these provide insight into the problem of
tachyon condensation. We then develop the boundary state and show that in
backgrounds of interest to tachyon condensation the conformal invariance of
the string world-sheet is broken, which suggests a generalized boundary state
obtained by integrating over the conformal group of the disk. We find that
this prescription reproduces particle emission amplitudes calculated from the
string sigma model for both on- and off-shell boundary interactions. The
boundary state appears as a coherent superposition of closed string states,
and using this a method for calculating amplitudes beyond tree level is developed.
The interaction of closed strings with other backgrounds is also
discussed. An extension of the boundary state to encode fields other than
a gauge or tachyon field is described. A modification of the boundary state
which encodes the time dependence of tachyon condensation is reviewed, and
an examination of spherically symmetric tachyon condensation in the 1/D
expansion is presented.
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