The synchronisation of two pendulum clocks hanging from the same wall may be caused by the exchange of sound pulses, a paper in Scientific Reports this week suggests. A combination of theoretical models and experimental results help to provide an explanation for this strange phenomenon, first observed in the 17th century by the inventor of the pendulum clock, Christiaan Huygens.
Huygens observed the pendulums of two clocks hanging from a wall or from a board standing on two chairs would eventually swing together in unison. Explanations for both systems have been proposed, although the first case (two clocks hanging from a wall) has been harder to model. Now, Henrique Oliveira and Luis Melo hypothesise that travelling sound pulses might pass from clock to clock, perturbing the swing of the pendulums and causing them to synchronise. They test this theory by observing the pendulums of two pendulum clocks attached to an aluminium rail fixed to a wall. Their results suggest that a change in the speed of the pendulum swings coincides with a sound-pulse producing cycle.