Large-volume string compactifications, revisited Original Research Article

We reconsider the issue of large-volume compactifications of the heterotic string in light of the recent discoveries about strongly coupled string theories. Our conclusion remains firmly negative with respect to classical compactifications of the ten-dimensional field theory, albeit for a new reason: When the internal sixfold becomes large in heterotic units, the theory acquires an additional threshold at energies much less then the naive Kaluza-Klein scale. It is this additional threshold that imposes the ultimate limit on the compactification scale MKK > 4 × 107 GeV for any compactification; for most compactifications, the actual limit is much higher. (Generically, MKK ≳ αGUTMPlank in either SO(32) or E8 × E′8 heterotic string.)