On glass

Chad Orzel (Uncertain Principles) points to a New York Times article on the nature of glass and the glass transition. Understanding the structure of glass and how the glass transition occurs is one of the large remaining problems in condensed matter and solid state physics.

Glass, of course, is a solid, insofar as it holds it shape without a container; yet, it exhibits the properties of a liquid in that it lacks any long range molecular structure. To quote the article:

When a liquid solidifies into a glass, [the] organized stacking [of a crystal] is nowhere to be found. Instead, the molecules just move slower and slower and slower, until they are effectively not moving at all, trapped in a strange state between liquid and solid.

Further complicating the issue is the fact that the transition temperature and final structure of a glass is dependent on the rate at which the liquid is cooled. The complexity of the problem has led one physicist working on the problem to joke, "There are more theories of the glass transition than there are theorists who propose them."

In any case, the article provides a great summary of the questions involved in glass and glass transitions and the current theories proposed to address them. Definitely worth a read.