The Surprising Secret of Synchronization

The second law of thermodynamics tells us that everything in the universe tends towards disorder and in complex systems chaos is the norm. So you naturally expect the universe to be messy. And yet we can observe occasions of spontaneous order. The synchronization of metronomes, the perfectly timed orbits of moons, the simultaneous flashes of fireflies, and even the regular beating of your heart.

What puts these things in order in spite of nature's tendency for disorder? On June 10th, 2000, the Millennium Bridge, a new foot bridge across the river tempames in London, was opened to much excitement. But as crowds filled the bridge, it began to wobble back and forth. Police started restricting access to the bridge, but that only resulted in long lines to get on.

The wobble was unaffected. 2 days later, the bridge, which had cost £18 million, was fully closed and it wouldn't reopen for another 2 years. So, what went wrong? Well, it's long been known that armies should break step when crossing bridges.

This dates back to an accident in 1831 when 74 men from the 60th Rifle Corps were marching across the Brotten Suspension Bridge in Northern England. It collapsed under their synchronized footsteps. 60 men fell into the river, 20 of whom suffered injuries like broken bones or concussions. Luckily, no one was killed.

But after this, the British army ordered all troops to break step when crossing bridges. Now, look at the people walking across the Millennium Bridge. Most of them are walking in step with each other, but they are not part of an army. They're random members of the public.

So why are they walking together? And why couldn't a modern bridge designed for heavy pedestrian traffic handle this? Well, to understand it, we have to go back 350 years. In 1656, famous Dutch physicist Christian Huygens created the first working pendulum clock.

The goal was to help sailors figure out where they were on the globe. Latitude can be judged by measuring the position of the sun or stars. But for longitude, you also need to know the time at some fixed location, say your home port. But clocks at the time were routinely out by around 15 minutes a day, so they were effectively useless.

Huygens's pendulum clocks, by contrast, were accurate to around 10 to 15 seconds per day. Huygens's plan was to attach ...