  The noncrystalline glassy state of matter plays a role in virtually all
fields of materials science and offers complementary properties to those of the
crystalline counterpart. The caveat of the glassy state is that it is out of
equilibrium and therefore exhibits physical aging, i.e., material properties
change over time. For half a century, the physical aging of glasses has been
known to be described well by the material-time concept, although the existence
of a material time has never been directly validated. We do this here by
successfully predicting the aging of the molecular glass
4-vinyl-1,3-dioxolan-2-one from its linear relaxation behavior. This
establishes the defining property of the material time. Via the
fluctuation-dissipation theorem, our results imply that physical aging can be
predicted from thermal-equilibrium fluctuation data, which is confirmed by
computer simulations of a binary liquid mixture.
