A team of scientists at the University of Lisbon claim that they have solved the 350-year-old mystery of why pendulum clocks sync with each other over time. The team stated that energy from sound waves causes the pendulum clocks to sync.

Christiaan Huygens (1629-1695), a Dutch scientist, and inventor of the pendulum clock noticed a strange phenomenon that even if two pendulum clocks are hanging from the same beam, their swings at one point synchronize in opposite directions, despite the point that they started out at different times and positions.

Now, the University of Lisbon scientists in Portugal think that they have solved this centuries-old conundrum. The team that presented their findings this week in the journal Scientific Reports stated that the answer to this long held mystery lied in sound waves.

Co-author Luís V. Melo, said, “We tried different beam materials and conditions and could only get coupling when the beam was made of a very good conductor, the clocks were close, and the frequencies were close enough”.

Results of the study showed that a transfer of energy occurs from the sounds of the ticking clocks, causing the two pendulums to swing in synchrony.

Researchers said that the coupling is likely obtained through the exchange of sound pulses between the clocks propagated through the rail. They further said when one clock receives the kick, the impact propagates in the wall slightly perturbing the second clock.

Another researcher who appeared unconvinced about the secret of Huygens’ told the Smithsonian that even if two clocks have mechanisms that do not apply impulses, synchronization nevertheless occurs.

Co-author Dr. Henrique M. Oliveira told LiveScience that the idea came to the two over coffee, because nobody had ever tested properly the idea of clock hanging on the same wall.