  We propose that there exists a generic class of glass forming systems that
have competing states (of crystalline order or not) which are locally close in
energy to the ground state (which is typically unique). Upon cooling, such
systems exhibit patches (or clusters) of these competing states which become
locally stable in the sense of having a relatively high local shear modulus. It
is in between these clusters where ageing, relaxation and plasticity under
strain can take place. We demonstrate explicitly that relaxation events that
lead to ageing occur where the local shear modulus is low (even negative), and
result in an increase in the size of local patches of relative order. We
examine the ageing events closely from two points of view. On the one hand we
show that they are very localized in real space, taking place outside the
patches of relative order, and from the other point of view we show that they
represent transitions from one local minimum in the potential surface to
another. This picture offers a direct relation between structure and dynamics,
ascribing the slowing down in glass forming systems to the reduction in
relative volume of the amorphous material which is liquid-like. While we agree
with the well known Adam-Gibbs proposition that the slowing down is due to an
entropic squeeze (a dramatic decrease in the number of available
configurations), we do not agree with the Adam-Gibbs (or the Volger-Fulcher)
formulae that predict an infinite relaxation time at a finite temperature.
Rather, we propose that generically there should be no singular crisis at any
finite temperature: the relaxation time and the associated correlation length
(average cluster size) increase at most super-exponentially when the
temperature is lowered.
