  Although several theories relate the steep slowdown of glass formers to
increasing spatial correlations of some sort, standard static correlation
functions show no evidence for this. We present results that reveal for the
first time a qualitative thermodynamic difference between the high temperature
and deeply supercooled equilibrium glass-forming liquid: the influence of
boundary conditions propagates into the bulk over larger and larger
lengthscales upon cooling, and, as this static correlation length grows, the
influence decays nonexponentially. Increasingly long-range susceptibility to
boundary conditions is expected within the random firt-order theory (RFOT) of
the glass transition, but a quantitative account of our numerical results
requires a generalization of RFOT where the surface tension between states
fluctuates.
