  This paper first reviews the shoving model for the non-Arrhenius viscosity of
viscous liquids. According to this model the main contribution to the
activation energy of a flow event is the energy needed for molecules to shove
aside the surrounding, an energy which is proportional to the instantaneous
shear modulus of the liquid. Data are presented supporting the model. It is
shown that the fractional Debye-Stokes-Einstein relation, that quantitatively
expresses the frequently observed decoupling of, e.g., conductivity from
viscous flow, may be understood within the model. The paper goes on to review
several related explanations for the non-Arrhenius viscosity. Most of these are
also "elastic models," i.e., they express the viscosity activation energy in
terms of short-time elastic properties of the liquid. Finally, two new
arguments for elastic models are given, a general solid-state defect argument
and an Occam's razor type argument.
