  The slowing down of molecular dynamics when approaching the glass transition
generally proceeds much stronger than expected for thermally activated motions.
This strange phenomenon can be formally ascribed to a temperature-dependent
activation energy E(T). In the present work, via measurements of the
third-order nonlinear dielectric susceptibility, we deduce the increase of the
number of correlated molecules Ncorr when approaching the glass transition and
find a surprisingly simple correlation of E(T) and Ncorr(T). This provides
strong evidence that the non-canonical temperature development of glassy
dynamics is caused by a temperature-dependent energy barrier arising from the
cooperative motion of ever larger numbers of molecules at low temperatures.
