  Physical aging is the generic term for irreversible processes in glassy
materials resulting from molecular rearrangements. We present multi-speckle
dynamic light-scattering data on an aging sample of the molecular glass former
1-phenyl-1-propanol following temperature jumps close to the glass transition,
starting from and ending in thermal equilibrium. It is demonstrated that the
material time of the Tool-Narayanaswamy aging formalism can be determined from
the time-autocorrelation function of the scattered-light intensity
fluctuations. These fluctuations are shown to be stationary and reversible when
regarded as a function of the material time. The glass-forming colloidal
synthetic clay Laponite, as well as a chemically aging curing epoxy, are shown
also to have material-time-reversible scattered-light intensity fluctuations.
Our findings, besides showing direct measurements of the material time,
identify a fundamental property of aging in quite different contexts, which
presents a challenge to the current theories of aging.
