Abstract
We compute string scattering amplitudes in an orbifold of
Minkowski space by a boost, and show how certain divergences in the
four point function are associated with graviton exchange near the
singularity. These divergences reflect large tree-level backreaction
of the gravitational field. Near the singularity, all excitations
behave like massless fields on a 1+1 dimensional cylinder. For
excitations that are chiral near the singularity, we show that
divergences are avoided and that the backreaction is milder. We
discuss the implications of this for some cosmological
spacetimes. Finally, in order to gain some intuition about what
happens when backreaction is taken into account, we study an open
string rolling tachyon background as a toy model that shares some
features with 
1,1/
.
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