Figure 4: Time dependence of the susceptibility ${\chi }_4(t)$ that quantifies spontaneous fluctuations of the relaxation function in a molecular dynamics simulation of a supercooled liquid. For each temperature, ${\chi }_4(t)$ has a maximum, which shifts to larger times and has a larger value when $T$ is decreased, revealing the increasing length scale of dynamic heterogeneity in supercooled liquids that approach the glass transition.