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## Synchrony and No. 160

Any idea what the word "synchrony" means? It was first observed by Christian Huygens, a 17th century physicist/mathematician, when he played with pendulum clocks. After setting two clocks (hanging close together) in motion so that their pendulums are not swinging in unison, the clocks "resynchronized" and were swinging in unison within 30 minutes.

In a similar manner, fireflies along rivers in Malaysia and Thailand sychronize, in that they flash on and off unison. I doubt Huygens tried any experiments with them, as how do you change their flashing patterns manually?

So what does all of this have to do with the number 160? Consider the Millennium Bridge that spans 1050 feet across the Thames River in London. During its opening on June 10, 2000, thousands of people walked across it...only to discover that it started wobbling side to side. As the people adjusted their walking gait in unison to the wobbling motion, the wobblier the bridge became.

To explain this event, Cornell's Steven Strogatz extended a mathematical model of the fireflies and pendulum synchronies to the bridge...Oddly enough, he discovered that the bridge would remain steady with 150 people on it, but would begin wobbling greatly when the number exceeded 160. The result: \$9 million more in funds to install giant shock absorbers on the bridge, which remains safe to pedestrian traffic today (even more than 160 people)...thanks to mathematics.

If you would like to read more specifics about the bridge, the problem, and its mathematical solution, I suggest either the ARUP Bridge site or the BBC Report.

Source: Kenneth Chang's "All Together Now: Sychrony Explains Swaying," New York Times, November 8, 2005