This energy gives a 'kick' that over days alters the swing of the pendulums

It is a mystery that has puzzled scientist for nearly 350 years, but now researchers claim to have solved what causes pendulum clocks to synchronise their swings.

A team of physicists and mathematicians say sound from the ticking clocks transfers energy between them and eventually causes the pendulums to move in time.

In effect the clocks 'communicate' with each other and over several days or even hours they begin to nudge each other until their swing matches.

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Researchers have found that minute amounts of energy transmitted between two pendulum clocks connected to the same beam can cause them over time to become synchronised. This is because the sound produced by the mechanisms inside each clock, above, gives the other a 'kick' that causes them to exchange energy

However, the researchers found the synchronisation would only occur if the clocks were both connected to a beam capable of transferring the energy between them.

The phenomenon was first spotted by Dutch scientist Christiaan Huygens, who in 1665 after inventing the pendulum clock noticed two of them attached to the same wall swung in time with each other no matter if the clocks had been started at different times.

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Huygens at the time said he believed the clocks somehow 'spoke' to each other either through the air or though the material they were connected to.

Since then many scientists have attempted to understand what lay behind the effect but most have struggled to measure what has been going on.

Now Dr Henrique Oliveira, a mathematician and Dr Luis Melo, a physicist, both at the University of Lisbon, have used computer modelling and high precision optical lasers to answer the question.

Writing in the journal Scientific Reports, the pair said: 'We assume that the coupling is obtained through the exchange of sound pulses between the clock propagated through the rail.

'When one clock receives the kick, the impact propagates in the wall slightly perturbing the second clock.

'In this model each clock transmits once per cycle a sound pulse that is translated in a pendulum speed change.'

The researchers set up two pendulum clocks attached to an aluminium beam before setting them ticking.

No matter when the clocks were started, they would eventually start to move into synchronisation over the course of around 18 hours.

This graph shows show the clocks, one shown by the red light and other other by the black line, became synchronised after a few hours. The phase difference between the clocks is shown by the green line and shows how after about three days they were completely in phase with each other

Even if the pendulums moved in opposite directions, the period – the time it takes to swing an entire cycle to the left and right - would become the same.

Their models predicted that each clock would sends out tiny vibrations as the pendulum swings and causes the gears inside the clock to move.

However, when they changed the material the clocks were attached to – to MDF or fiberglass – the pendulums would fail to couple properly.

Cuckoo clocks, like those above, use a pendulum system. Clock makers have often noticed the strange effect that causes the swinging pendulums to synchronise over time and it appear to be due to sound passing through the wall they are connected to causing the pendulums to exchange energy with each other

Dutch scientist Christiaan Huygens, above, was the first person to spot the phenomenon in two pendulum clocks hanging on his wall in 1665 when he noticed they both began to synchronise over time no matter how out of sync they were when they were started. The new research has finally solved the mystery of why

This is because the more flexible material absorbed much of the energy from the clocks and prevented it from being transmitted.

This suggests the vibration is transmitted as sound pulses through the material the clocks are attached to rather than through the air.

This causes a small change with each swing of the pendulum in the neighbouring clock until they eventually reach equilibrium.

In the clocks used by Huygens, which were monsters using weights weighing 50-60lbs, a stiff wooden beam connected them.

Dr Melo told the Smithsonian Magazine that with patience a similar experiment could be carried out by anyone at home, but warned that while watching the clocks was mesmerizing, 'one gets very anxious after a while'.