Response to comment on “The glassy state of matter: Its definition and ultimate fate”

Abstract The comment by Popov raises two questions related to (a) whether the glassy state can more appropriately be described as metastable or unstable and (b) whether the supercooled liquid state exists below the glass transition temperature, Tg. We argue that the glassy state is thermodynamically unstable, as it continuously and spontaneously relaxes toward the metastable supercooled liquid state at any nonzero temperature. Moreover, the supercooled liquid state—the target for the glass relaxation process—exists at any temperature below the liquidus temperature of a system. There is no problem with having a missing target for the glass relaxation process, since this metastable supercooled liquid state can always be defined.