  Since its introduction, liquid viscosity and relaxation time $\tau$ have been
considered to be an intrinsic property of the system that is essentially local
in nature and therefore independent of system size. We perform dielectric
relaxation experiments in glycerol, and find that this is the case at high
temperature only. At low temperature, $\tau$ increases with system size and
becomes non-local. We discuss the origin of this effect in a picture based on
liquid elasticity length, the length over which local relaxation events in a
liquid interact via induced elastic waves, and find good agreement between
experiment and theory.
