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Aging in nonequilibrium systems means that the measured response or correlation depends on how long the system has been evolving since preparation, not only on the interval between measurements. A two-time correlation $C(t_w+t,t_w)$ depends separately on waiting time $t_w$ when time-translation invariance has not been established.

One probing protocol quenches a disordered magnet, waits different times and then applies a small field. Comparing the resulting responses distinguishes preparation history from a single equilibrium relaxation time. Analytical spin-glass models exhibit such nonequilibrium correlation and response behavior; simple scaling with $t/t_w$ is a model-dependent form, not the definition of aging.^[ref]

J. Kurchan coauthored a 1993 analytical study of relaxation in a long-range spin glass with Leticia Fernanda Cugliandolo. They solved the long-time behaviour of correlation and response functions in a spherical model with interactions involving several spins.^[ref]

The solution exhibits aging in nonequilibrium systems: the response after a disturbance depends on how long the system has already been evolving, rather than only on elapsed time since the disturbance. It supplies an explicit model in which slow history-dependent dynamics can be studied analytically. The initials are those printed in the primary-paper byline.