It's been 350 years since Dutch physicist Christiaan Huygens, the illustrious inventor of the pendulum clock, noticed that no matter how his oscillating masterpieces started, within 30 minutes they would always end up swinging the opposite direction to each other if mounted on the same beam.

The observation, made in 1665 while Huygens was sick in bed with nothing else to stare at, gave rise to the phrase, "an odd kind of sympathy", and teams of scientists and mathematicians have struggled to explain it ever since. But a pair of researchers from Portugal say they've solved the mystery, attributing the spooky coordination to the tiny forces created by sound pulses.

Back in the 17th century, Huygens proposed that the syncing - or 'anti-syncing' - action was achieved through the sharing of air currents between the two pendulums. Experiments soon disproved this hypothesis, and Huygens instead proposed that the sympathetic motion of pendulums was down to the almost imperceptible movement in the beam from which they were both suspended.

Since then, scientists have been experimenting with two pendulum clocks hanging from the same beam, and while the conclusion was that forces exerted on this moveable beam were causing the syncing action, no one could agree on how this actually works. But what about if you took the supporting beam out of the equation altogether?

"Nobody tested properly the idea of clocks hanging on the same wall," Henrique Oliveira, a mathematician at the University of Lisbon and the co-author of a new study, told Charles Q. Choi at Live Science.  

Oliveira and his colleague Luís V. Melo developed a mathematical model to work with a physical experiment for which they attached an aluminium optical rail to a wall, and fixed two pendulum clocks to to it, 230 millimetres apart. According to Rina Marie Doctor at Tech Times, they said this is the smallest possible distance that the clocks can be from one another without clashing. 

"The anchor-pendulum clocks used were mass-driven and, according to the scientists, one mass travel can provide energy that can last for up to five days," Doctor reports. "The clocks won't die down instantly, as it would take approximately one day for it to relax under its final frequency after winding. The period is adjusted by lengthening the actual pendulum through manipulation of the screws at the bottom of the clocks."

Using these perfectly strung pendulums, Oliveira and Melo calculated that the speed of their swings corresponded to the cycles of the sound pulses they produced, which travelled through the wall from clock to clock. "We could … verify that the energy transfer is through a sound pulse," Melo told the AFP. 

"The two clocks interact, giving two 'kicks,' one in one direction and another one in the opposite direction," Oliveira told David Freeman at The Huffington Post. "Only when the clocks are at phase opposition the effects of the perturbation cancel," he said, which causes the pendulums to swing in opposite directions.

The results were published in the journal Scientific Reports.

The team says this isn't just an explanation for a centuries-old mystery - understanding how "very small interactions can add up and in the end synchronise very large systems" can have huge implications in everything from economics and electronics to the biology of how cells sync up to produce a heartbeat, Oliveira said

And Huygens would be proud to know his invention - which was considered the world's most accurate timekeeper for over 270 years - is finally back in the spotlight.