Engineers called it “an absolute statement of our capabilities at the beginning of the 21st century.” At the opening ceremony, thousands of enthusiastic spectators swarmed all over the bridge.
Then something unexpected happened. The bridge began to sway slightly from side to side.
The wobble became more pronounced. Pedestrians didn’t know what to do. They began to walk with wide strides to counteract the bridge’s motion. Like entrained pendulums, they stepped to the left and then to the right.
The wobble increased, and the people scrambled off the bridge as soon as they could and the bridge was closed immediately. This opening day event earned the bridge the nickname “the Wobbly Bridge.”
Why did the bridge fail, after the brightest design and engineering minds in the world had done their utmost to make it perfect?
When they felt the first tiny sway, the pedestrians on the bridge adjusted their behavior to compensate. This unintentionally brought them into resonance. They began to walk together, exacerbating the movement of the bridge.
In an example of what scientists call emergent systems, there was no plan or leader guiding the Millennium Bridge wobble. It took place as an emergent response to resonance.
The bridge remained closed for two years while the problem was investigated and eventually solved by installing dampers to cushion the bridge’s movement. The bridge was once again opened to pedestrian traffic in February 2002. But the event remains an example of how resonance can trigger unexpected consequences in complex systems.