Introduction

Abstract

Understanding cooperative phenomena far from equilibrium poses one of the most challenging research problems of present-day many-body physics. At the same time, the practical handling of many of these materials has been pushed to great sophistication, and a lot of practical knowledge about them exists since prehistoric times. Glasses are one example of such systems. In many cases, they are made by rapidly cooling (“quenching”) a molten liquid to below some characteristic temperature-threshold. If this cooling happens rapidly enough, normal crystallization no longer takes place and the material remains in some non-equilibrium state. These non-equilibrium states may at first and even second sight look very stationary – everyone has probably seen in archaeological museums intact specimens of Roman glass or even older tools from the Paleolithic or old-stone-age – after all, obsidian or fire-stone is a quenched volcanic melt. But since the material is not at equilibrium, at least in principle it is possible (and it does happen very often in practice) that over time the properties of the material change – in other words, the material ages.¹ Finally, one may wonder what happens from a thermodynamic point of view to a glass-forming material quenched to below its characteristic threshold for glass-formation. Is the hardening of the quenched glass-former merely a kinetic effect such that a glass-transition can be arbitrarily said to occur when motion has slowed down over so many orders of magnitude that any ongoing motion simply escapes the attention or patience of the experimentalist? Or does the system pass through a true thermodynamic phase-transition and enters a new physical regime with a qualitatively different behaviour?