  Understanding glass formation is a challenge because the existence of a true
glass state, distinct from liquid and solid, remains elusive: Glasses are
liquids that have become too viscous to flow. An old idea, as yet unproven
experimentally, is that the dynamics becomes sluggish as the glass transition
approaches because increasingly larger regions of the material have to move
simultaneously to allow flow. We introduce new multipoint dynamical
susceptibilities to estimate quantitatively the size of these regions and
provide direct experimental evidence that the glass formation of molecular
liquids and colloidal suspensions is accompanied by growing dynamic correlation
length scales.
