ON THE DIMENSIONALITY OF SPACETIME

Some superstring theories have more than one effective low-energy limi t corresponding to classical spacetimes with different dimensionalitie s. We argue that all bur the (3 + 1)-dimensional one might correspond to 'dead worlds', devoid of observers, in which case all such ensemble theories would actually predict that we should find ourselves inhabit ing a (3 + 1)-dimensional spacetime. With more or less than one time d imension, the partial differential equations of nature would lack the hyperbolicity property that enables observers to make predictions. In a space with more than three dimensions, there can be no traditional a toms and perhaps no stable structures. A space with less than three di mensions allows no gravitational force and may be too simple and barre n to contain observers.