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.
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.
Now, scientists analysing two pendulum clocks hanging from the same beam found that the clocks could influence each other through small forces exerted on the supporting beam.
"Nobody tested properly the idea of clocks hanging on the same wall," co-author Henrique Oliveira, a mathematician at the University of Lisbon was quoted as telling Live Science.
Oliveira analszed how two pendulums might interact through an immobile wall. Earlier studies investigated how they might interact through a movable beam.
"As pendulums move back and forth, sound pulses could travel through the wall from clock to clock," Oliveira said.
These pulses can interfere with the swings of the pendulums, eventually causing them to synchronise.
The investigators tested their idea with experiments involving two pendulum clocks attached to an aluminium rail fixed to a wall.
Their results showed that changes in the speed of the pendulum swings coincided with cycles of those sound pulses.
Oliveira and Melo detailed their findings online in the journal Scientific Reports.
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