Twin pendulums mirror each other's movements.

US physicists have solved a 350-year-old riddle of why the pendulums of two clocks become synchronized. The clocks were the first example of spontaneous synchronization, a phenomenon found throughout nature from cells to the Solar System.

In 1665, the Dutch physicist Christiaan Huygens, who patented the pendulum clock, was working on a way to calculate longitude on board ships. His approach was to put two clocks in a single housing, so that if one broke down, the other could provide a back-up.

Ill in bed, with nothing better to do than watch his clocks, Huygens noticed that no matter where they started, within about half an hour the two pendulums swung towards and away from each other in unison. His observations - made 20 years before Newton devised the laws of mechanics - have gone unexplained until now.

Physicist Michael Schatz and his colleagues at the Georgia Institute of Technology reconstructed Huygens' two-in-one clock box from his lab notes and diagrams.

They found that the clocks' behaviour resulted from the 40 kg of lead ballast in the apparatus, intended to hold the clocks fast at sea.

The puzzled Huygens thought that the synchrony was due to "imperceptible movements" of the clocks' support. He was right: as each pendulum swings, it transfers energy to the housing, some of which will reach the other pendulum.

But with too little ballast, the interaction would be too great, and one or both clocks would stop, Schatz's team found. More ballast weakens the link between the clocks, allowing differences in their ticking speeds to prevent synchronization.

Modern marvel: A perfect replica of Huygens' clock .

"The two clocks need to interact, but only by just the right amount to lock together," says Schatz. Huygens got lucky in his twin-clock apparatus, with the weight of his ballast and the quality of his clocks, Schatz says: "The circumstances all converged."

"It's wonderful that this problem has finally been analysed," says mathematician Steven Strogatz of Cornell University, Ithaca, New York. "We had to wait for chaos theory to happen," says Strogatz - newtonian physics is good at explaining smooth movements, but not jerky clock mechanisms.

Modern chaos theory shows that even tiny changes in the weight of ballast can have large effects on whether the pendulums will stop or swing.

Synchronicity surrounds us

As well as solving an ancient conundrum, Schatz's findings might help us to explain the spontaneous synchronization that pervades nature. Fireflies, for example, come to flash in unison as the evening wears on. Epilepsy results from nerve cells firing in synchrony when they shouldn't. The particles in superconducting materials, and the orbits of moons around planets, show similar behaviour.

The Georgia team has made a mathematical model of Huygens' clock phenomenon that could help engineers understand how and when synchronization will set in, and what form it will take. The knowledge might help to build more powerful lasers, for example, says Schatz.