  Characterizing the glass state remains elusive since its distinction from a
liquid state is not obvious. Glasses are liquids whose viscosity has increased
so much that they cannot flow. Accordingly there have been many attempts to
define a static length-scale associated with the dramatic slowing down of
supercooled liquid with decreasing temperature. Here we present a simple method
to extract the desired length-scale which is highly accessible both for
experiments and for numerical simulations. The fundamental idea is that low
lying vibrational frequencies come in two types, those related to elastic
response and those determined by plastic instabilities. The minimal observed
frequency is determined by one or the other, crossing at a typical length-scale
which is growing with the approach of the glass transition. This length-scale
characterizes the correlated disorder in the system, where on longer
length-scales the details of the disorder become irrelevant, dominated by the
Debye model of elastic modes.
