Rational Irrationality

The real reason that capitalism is so crash-prone.

by John Cassidy – The New Yorker

On June 10, 2000, Queen Elizabeth II opened the high-tech Millennium Bridge, which traverses the River Thames from the Tate Modern to St. Paul’s Cathedral. Thousands of people lined up to walk across the new structure, which consisted of a narrow aluminum footbridge surrounded by steel balustrades projecting out at obtuse angles. Within minutes of the official opening, the footway started to tilt and sway alarmingly, forcing some of the pedestrians to cling to the side rails. Some reported feeling seasick. The authorities shut the bridge, claiming that too many people were using it. The next day, the bridge reopened with strict limits on the number of pedestrians, but it began to shake again. Two days after it had opened, with the source of the wobble still a mystery, the bridge was closed for an indefinite period.

Some commentators suspected the bridge’s foundations, others an unusual air pattern. The real problem was that the designers of the bridge, who included the architect Sir Norman Foster and the engineering firm Ove Arup, had not taken into account how the footway would react to all the pedestrians walking on it. When a person walks, lifting and dropping each foot in turn, he or she produces a slight sideways force. If hundreds of people are walking in a confined space, and some happen to walk in step, they can generate enough lateral momentum to move a footbridge—just a little. Once the footway starts swaying, however subtly, more and more pedestrians adjust their gait to get comfortable, stepping to and fro in synch. As a positive-feedback loop develops between the bridge’s swing and the pedestrians’ stride, the sideways forces can increase dramatically and the bridge can lurch violently. The investigating engineers termed this process “synchronous lateral excitation,” and came up with a mathematical formula to describe it.

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