The Strangest, Most Spectacular Bridge Collapse (And How We Got It Wrong)

Bridge building has been bedeviling humans for a long time, probably since the 1st century. That may explain why, even when they can't carry lots of people or things, bridges are particularly good at carrying lots of meaning: breaking, burning, going too far, going nowhere; the bridges between cultures, across generations, the ones we’ll cross when we come to them. To this day, however, the meanings of Gertie's collapse and that unforgettable footage—"among the most dramatic and widely known images in science and engineering," wrote one engineer—remain murky.

For physics teachers, the footage of Gertie has proved irresistible as a lesson in wave motion—and, specifically, a textbook example of the power of forced resonance. The image of the undulating bridge left its mark on scores of students (including me) as a demonstration of what one canonical version of the film calls “resonance vibrations." Since then, scores of books and articles, from Encyclopedia Britannica to a Harvard course website, have reported that the Tacoma Narrows was destroyed by resonance.

But it turns out it wasn't. And yet, while science has known that for years, lots of people (including me) apparently didn't get the memo.