It is well known pendulum clocks hanging on the same wall synchronize themselves over time. Now researchers at the University of Lisbon have revealed the reason behind the phenomenon: sound waves.

During the study, the researchers analyzed the motion of two pendulum clocks hanging on the same beam. The researchers found the swinging clocks influenced each other with the help of the small force that was being exerted on the supporting beam.

Researchers Luis V. Melo and Henrique M. Oliveira placed the two pendulum clocks 230 millimeters apart so the two pendulums do not bump into each other on a wall with a standard optical rail. The researchers observed the to and fro of the clocks and found sound pulses transmitted from one clock to the other.

The researchers said the sound pulses transmitted from each clock had an impact. In addition, the researchers explained the waves traveled along to the aluminum bar, synchronizing one pendulum with the other. The team thus confirmed that the energy transfer that takes place between the pendulums is through a sound pulse.

Christiaan Huygens was the first person to observe the synchronization of wall clocks in 1665 when he was lying in bed with a minor illness. Huygens observed no matter where the two wall clocks were placed or when they started, the devices always swayed in sync. Huygens called it an "odd kind of sympathy."