  Supercooled liquids exhibit spatial heterogeneity in the dynamics of their
fluctuating atomic arrangements. The length and time scales of the
heterogeneous dynamics are central to the glass transition and influence
nucleation and growth of crystals from the liquid. We report direct
experimental visualization of the spatially heterogeneous dynamics as a
function of temperature in the supercooled liquid state of a Pt-based metallic
glass, using electron correlation microscopy with sub-nanometer resolution. An
experimental four point space-time intensity correlation function demonstrates
a growing dynamic correlation length, $\xi$, upon cooling of the liquid toward
the glass transition temperature. $\xi$ as a function of the relaxation time
$\tau$ data are in the good agreement with the Adam-Gibbs, inhomogeneous mode
coupling theory and random first order transition theory of the glass
transition. The same experiments demonstrate the existence of a nanometer
thickness near surface layer with order of magnitude shorter relaxation time
than inside the bulk.
