Tuesday, July 29, 2008

Broken Glass

You have some unlearning to do, this time about glass:

It is well known that panes of stained glass in old European churches are thicker at the bottom because glass is a slow-moving liquid that flows downward over centuries.

Well known, but wrong. Medieval stained glass makers were simply unable to make perfectly flat panes, and the windows were just as unevenly thick when new.
The old saw is not totally without a basis in fact, however -- there is such a thing as a glass transition temperature (the temperature at which it goes from liquid to glass), and it turns out to be an unusual phase transition because glass, even after it hardens, refuses to fit nicely in either the solid or liquid category:
In liquids, molecules jiggle around along random, jumbled paths. When cooled, a liquid either freezes, as water does into ice, or it does not freeze and forms a glass instead.

In freezing to a conventional solid, a liquid undergoes a so-called phase transition; the molecules line up next to and on top of one another in a simple, neat crystal pattern. When a liquid solidifies into a glass, this organized stacking 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.
It's remarkable what you can find by taking a careful look at the most pedestrian of things.

(H/T 3 Quarks Daily)

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