In 1665, Dutch scientist Christiaan Huygens noticed something strange. Two swinging pendulum clocks were found to eventually synchronize when hung together, and it wasn’t until 2002 that the mystery was solved – the swings were transferring energy to the support, causing the oscillations of the pendulums to “couple,” or swing at the same frequency.

But now, researchers at the University of Lisbon in Portugal have taken this effect even further. Hanging two pendulums from a rigid support that couldn’t move, they found that the synchronization effect still occurred – albeit to a much, much lesser degree – and they reason that it is due to sound pulses passing from clock to clock.

When Huygens observed the effect, his pendulums synchronized in half an hour no matter their swing rates – or frequency. For this latest research, published in Nature Scientific Reports, the two separate pendulums took 12 hours to synchronize in opposition – and only if their swing rates were out of sync by no more than 0.01 seconds.

“We saw, looking at Huygens’ original letter to his father, he had looked at the clocks being synchronized when hanging from the beam of a house,” Luís Melo from the University of Lisbon, who co-authored the paper with Henrique Oliveira, told IFLScience. “We decided to query it with a non-movable stand.”

In their tests, two brass pendulums were attached to an aluminum rail on a wall, and each separate pendulum had a ticking clock that it was attached to. The researchers theorize that the sound pulse produced by the tick of each swing imparted some energy onto the other pendulum through the rail, “kind of like pushing someone on a swing,” said Melo. Very, very gradually this would cause them to synchronize.

Melo said the research took months to complete, as each synchronization took half a day and all other factors had to be ruled out, although it was “a lot of fun.”

^{Above is a video of the two pendulums synchronized in opposition. Henrique Oliveira/Luís Melo.}

It should be noted that the sound pulses are not traveling through the air, but instead through the aluminium rail to the other pendulum. This could be surmised by using other materials for the rail – wood, iron and fiberglass – and observing that synchronization did not occur. “Since the stand is not moving itself, the way to propagate mechanical energy is through sound,” said Melo.

So small is the force of the sound pulse, though, that 0.01 seconds was found to be a strict limit for how delayed they could be to each other. Any more, and they simply would not synchronize. In addition, the two clocks needed to be no further than 8.7 inches (22 centimeters) from each other.

It might not seem that important, but research such as this is useful for working out how so-called coupling oscillations occur in physics. And it helps to further solve the mystery of what Huygens observed all those years ago.