An Illness, Two Clocks, and a Mystery
In 1665, Dutch scientist Christiaan Huygens lay sick in bed, watching two pendulum clocks he had built swing from a beam above him. Though started at different times and angles, the clocks mysteriously began to synchronize their swings.
He noticed that they eventually moved in perfect opposition, like mirror images. Huygens recorded the phenomenon, theorizing that some kind of subtle motion in the air or beam caused the effect. Without the tools to explore further, he left the puzzle unsolved.
Pendulums and the Passing of Time
Pendulum clocks operate by using a swinging arm to regulate the movement of gears powered by descending weights. Each tick is accompanied by small mechanical nudges, and Huygens’ clocks were fitted with stabilization weights of 50 to 60 pounds.
That heft may have helped transmit force between the clocks through the beam. When Huygens saw the clocks fall into rhythm, he coined the term “an odd sympathy.” He didn’t know that the secret might lie in the sound of their ticks.
Soundwaves in the Beam
In a 21st-century laboratory in Lisbon, physicists Henrique Oliveira and Luís Melo recreated Huygens’ setup. They attached two modern pendulum clocks to a rigid aluminum beam and monitored them using high-precision optical sensors.
As in Huygens’ bedroom centuries earlier, the clocks began to synchronize. Their data suggested that sound energy—transmitted through the connecting beam—nudged the clocks into sync. Clocks only coupled when mounted close together on a solid sound-conducting material like aluminum and ticking at similar frequencies.
Still Not the Final Word
While Oliveira and Melo’s work supports the sound transmission theory, others remain cautious. Researcher Jonatan Peña Ramirez noted that synchronization can still occur even if the ticking motion is smoothed out, removing the tiny sound pulses.
This raises questions about whether sound is the only factor. Some physicists suggest that other mechanical interactions or structural vibrations might still be at play. Nearly 360 years after Huygens first noticed the phenomenon, the question of why clocks sync continues to tick on.
In 1665, Christiaan Huygens was bedridden when he noticed something strange.
Two pendulum clocks hanging from the same beam began to swing in perfect rhythm, even though they were started at different times. He couldn’t explain it—but the clocks seemed to “talk” to each other. pic.twitter.com/Ue2HlYOAQ3
— Detective Tiger's Stories (@TigerDetective) June 16, 2025