Why Pendulum Clocks Mysteriously Sync Up

Pendulum clocks swinging in exactly the opposite direction from each other.l

The 350-year-old mystery of why pendulum clocks hanging from the same wall synchronize over time may finally be solved, scientists say.
In 1665, Dutch physicist Christiaan Huygens, inventor of the pendulum clock, was lying in bed with a minor illness and watching two of his clocks hanging on a wall, said Henrique Oliveira, a mathematician at the University of Lisbon and co-author of a new study detailing the findings. Huygens noticed something odd: No matter how the pendulums on these clocks began, within about a half-hour, they ended up swinging in exactly the opposite direction from each other.
The cause of this effect — what Huygens called an "odd kind of sympathy" — remained a mystery for centuries. But recently, scientists analyzing two pendulum clocks hanging from the same beam found that the clocks could influence each other through small forces exerted on the supporting beam. However, "nobody tested properly the idea of clocks hanging on the same wall," Oliveira told Live Science.