Unification with low string scale

I argue that in open-string theory with hierarchically small (or large) extra dimensions, gauge groups can unify naturally with logarithmically-running coupling constants at the high Kaluza-Klein (or string-winding) scale. This opens up the possibility of rescuing the standard logarithmic unification at $MU/sim 1015-18$ GeV even if the fundamental-string scale is much lower, at intermediate or possibly even electroweak scales. The argument, however, also shows why a low type-I string scale may not suffice to obliterate the ultraviolet problems associated with the gauge hierarchy. (author)