  A microscopically motivated theory of glassy dynamics based on an underlying
random first order transition is developed to explain the magnitude of free
energy barriers for glassy relaxation. A variety of empirical correlations
embodied in the concept of liquid "fragility" are shown to be quantitatively
explained by such a model. The near universality of a Lindemann ratio
characterizing the maximal amplitude of thermal vibrations within an amorphous
minimum explains the variation of fragility with a liquid's configurational
heat capacity density. Furthermore the numerical prefactor of this correlation
is well approximated by the microscopic calculation. The size of heterogeneous
reconfiguring regions in a viscous liquid is inferred and the correlation of
nonexponentiality of relaxation with fragility is qualitatively explained. Thus
the wide variety of kinetic behavior in liquids of quite disparate chemical
nature reflects quantitative rather than qualitative differences in their
energy landscapes.
