The Kovacs effect is a classic memory phenomenon in glassy systems. Subject a system to a sudden protocol change, and its subsequent relaxation depends on its history — not just its current state. The effect demonstrates that macroscopic variables (like temperature or volume) don't fully describe the system. Hidden internal degrees of freedom carry memory of past conditions, influencing future relaxation.
Garzó, Brito, and Soto (arXiv:2602.20716) search for the Kovacs effect in a vibrated thin granular layer and find something unexpected: memory exists, but only during the fast initial relaxation. Once the system enters the slower relaxation toward equilibrium, memory vanishes. The system has two regimes with qualitatively different physics.
The fast regime is kinetic — the granular temperature splits between horizontal and vertical components, and the coupling between these components carries memory of the initial preparation. The vertical and horizontal degrees of freedom haven't equilibrated, and their imbalance remembers how the system was prepared. This is where the Kovacs-like anomaly appears.
The slow regime is hydrodynamic — the temperature components have equilibrated into a single granular temperature, and the system's state is fully described by this macroscopic variable. No hidden degrees of freedom, no memory. The system relaxes to equilibrium on a path determined entirely by its current state.
The standard Kovacs effect never appears because the memory is consumed during the fast transient. By the time the system reaches the slow regime where the classic Kovacs protocol operates, the memory-carrying degrees of freedom have already relaxed.
The general observation: memory in a physical system requires unrelaxed internal degrees of freedom. When those degrees of freedom relax faster than the macroscopic variables, the window for observing memory effects is confined to the transient. The system forgets its history on the same timescale it equilibrates internally. Fast internal relaxation kills slow macroscopic memory.