Distance-as-time in physical aging

Understand

Although it has been known for half a century that the physical aging of glasses in experiments is described well by a linear thermal-history convolution integral over the so-called material time, the microscopic definition and interpretation of the material time remains a mystery.

• We propose that the material-time increase over a given time interval reflects the distance traveled by the system's particles.
• Different possible distance measures are discussed, starting from the standard mean-square displacement and its inherent-state version that excludes the vibrational contribution.
• The viewpoint adopted, which is inspired by and closely related to pioneering works of Cugliandolo and Kurchan of the 1990s, implies a "geometric reversibility" and a "unique-triangle property" characterizing the system's path in configuration space during aging.