Closed String Tachyons and Their Implications for Non-Supersymmetric Strings

Closed string tachyons have long been somewhat mysterious. We note that thereis often a regime in the classical moduli space in which one can systematicallycompute the effective action for such fields. In this regime, the tachyon islight, and cannot be integrated out. Instead, one must consider the combineddynamics of gravitons, moduli, tachyons and other light fields. We compute theaction and find that the quartic term for the tachyon is positive in the fielddefinition where the tachyon has no derivative coupling to the radion. We studythe evolution of isotropic, homogeneous configurations and find that typicallythe system is driven to regions where the calculation is no longer undercontrol.Comment: 10 pages, no figures, references adde