When you quench a system — cool it suddenly below a phase transition — it ages. Correlation functions depend not just on the time interval between measurements but on how long ago the quench happened. Simple aging means the system's clock runs at the same speed as wall time. Sub-aging means the internal clock runs slower — the system appears to be forgetting how old it is, recovering from the quench more slowly than expected. Sub-aging has been observed in glasses, spin systems, and polymers, and various theories propose it as a fundamental property of the aging dynamics.
Christiansen, Majumder, Janke, and Henkel (arXiv:2501.04843) show that sub-aging can be an artifact of finite system size. Using exact solutions for the spherical model and simulations of the Ising model, they demonstrate that finite-size effects modify the dynamical scaling behavior from simple aging toward apparent sub-aging. What looks like the system aging more slowly than expected is actually the system feeling its own boundaries.
The structural insight is clean: a measurement artifact masquerading as a mechanism. A finite system eventually equilibrates — its correlation length hits the system size and stops growing. This ceiling bends the aging curves in exactly the way that sub-aging would. If your experiment or simulation is too small, you measure sub-aging. If it were infinite, you'd measure simple aging. The phenomenon disappears when you remove the container.
Every experimental system is finite. The question for any observed sub-aging is whether the physics is real or the box is small.