Galloping Gertie:

As engineering failures go, the 1940 collapse of the Tacoma Narrows Bridge ranks among the most visually arresting. When it was unveiled in June 1940, the bridge was the thrid largest suspension bridge in the world. It didn't stay open long, because the bridge began to sway in high winds, so all traffic was banned from it. The bridge became known as Galloping Gertie, and on November 7, 1940, Gertie collapsed in a 42 mile per hour wind. The bridge was actually resonating, like a tuning fork, caused by the wind flowing over it. Here it is collapsing.

The 'tuning fork' theory has been known since ancient times: roman armies were trained to break their step going across bridges for fear of having the same effect as the wind on Galloping Gertie. One can see the effect of many people moving together on Norman Foster's Millenium Bridge, below.

What has this to do with economics? Find out below.

Markets as Wind Tunnels

Ross M. Miller suggests the following analogy: Because of the collapse of Gertie, the engineers building bridges now have to run models of the bridge through wind tunnels, to simulate the effects of high winds on bridges of this type.

The wind tunnel is a controlled, simulated environment in which one can subject a bridge to strains it would encounter in the real world, only with a smaller version of th bridge. The wind tunnel works because the tendency for the bridge to resonate is approximately the same across many scales---that is, it is a scaling phenomenon.

As long as the scale model captures the essential aerodynamic features of the real bridge, we can apply what we learn from the wind tunnel to a real world sized bridge in it's natural physical setting, because the same physical laws apply to both. We need only include the most important features of the full size bridge in the reduced size model.