고체 같지 않은 고체: 700년 동안 흐른 유리창의 비밀

Walk into a Gothic cathedral and the stained-glass windows tell a strange story: their lower edges are visibly thicker than the upper ones. For centuries this asymmetry has been offered as proof that glass is a very slow-moving liquid, gradually sagging under its own weight. The story is appealing because glass, lacking the crystalline lattice of metals or ice, is technically an amorphous solid — a frozen liquid at the molecular level. Yet in 1998 the Brazilian materials scientist Edgar Zanotto subjected the popular claim to a numerical test. Using the measured viscosity of soda-lime silicate glass, he calculated that a flow of merely one nanometer at room temperature would require a span of time vastly exceeding the 13.8-billion-year age of the universe. The drooping panes of medieval Europe were never cosmic creep but the signature of pre-industrial craft, in which glaziers spun molten discs of uneven thickness and routinely oriented the heavier rim downward. What the windows record is not the patience of physics but the hand of the workman.