# Tic toc: Why pendulums swing in harmony

Almost 350 years ago, Dutch inventor and scientist Christiaan Huygens observed that two pendulum clocks hanging from a wall would synchronise their swing over time.

What causes the phenomenon has led to much scientific head-scratching over the centuries, but no consensus to date.

On Thursday, a study in the Nature journal Scientific Reports proposed a solution: the pendulums transfer energy to one another through .

A pair of Portuguese scientists hypothesised that these pulses might move from clock to clock, perturbing the swing of the pendulums and eventually causing them to synchronise.

They developed a complex mathematical model before conducting experiments with a pair of clocks attached to a rail fixed to a wall.

The and simulation matched, they found.

"We could... verify that the energy transfer is through a sound pulse," co-author Luis Melo from Lisbon University's physics department told AFP by email.

This not only solves "an old, fundamental problem," it also boosts understanding of other types of oscillators, he said.

Huygens is credited with making the first clock in 1656—the most accurate timepiece built until then, with an error margin of less than one minute a day. He later improved this to under 10 seconds.

In 1665, the scientist reported observing a strange phenomenon—two clocks hanging from the same structure would start swinging in unison, though in opposite directions.

The speed of a pendulum—a weight suspended from a string, rod or wire—is determined mainly by its length.

Attempts to explain the synchronisation have ranged from the impacts of tiny air currents to minute movements in the base on which the clocks are mounted.

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Jul 23, 2015
Do pendulums swing in harmony in vacuum?

Jul 23, 2015
Do pendulums swing in harmony in vacuum?

If they are not attached to any common object (so that waves can travel through any kind of medium): no

Unless you count that photons emitted when the pendulums move towards the respectively other clok are slightly blue shifted (Doppler effect) and those from the pendulum moving away will be redshifted. Than in itself would constitute a difference in energy transferred, which would synchronize the clocks eventually just like with sound waves.
But that would take a LOT longer than synchronization via sound.

Jul 23, 2015
in other words, items within close proximity(tbd) with the same period communicate over the ambient noise(level tbd) in sync, therefore moving toward stability, i.e. in synch

Jul 23, 2015
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Jul 23, 2015
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Jul 23, 2015

I'm guessing we are NOT talking sound pulses through air, but rather vibrational pulses through their common fixture.

Keep in mind this sentence
"In 1665, the scientist reported observing a strange phenomenon—two clocks hanging from the same structure would start swinging in unison, though in opposite directions."

If it was through air, then this synchronisation would also occur even if the clocks were attached to different structures, but within "hearing" distance of one another.

I thought this was already known????
What is new about any of this??

Jul 23, 2015
Do pendulums swing in harmony in vacuum?
In this case, with the same experimental setup, the answer is yes, because the sound was transmitted through the aluminum rail that both clocks were attached to. They know this because they substituted different materials for the rail (wood, iron and fiberglass) to check.

Additionally, if there was more than 0.01 seconds delay between the clocks to start with, then they would _not_ synchronize. They also would not synchronize if the distance between the clocks was greater than 8.7 inches (22 cm). I found that info here: http://www.iflsci...periment

Good question, I wondered the same thing. AA's answer was what I thought too, before checking.

Jul 23, 2015
@AA
If they are not attached to any common object (so that waves can travel through any kind of medium): no
I would say that it depends of what you define as pendulums. Here are some interresting examples: https://commons.w...tion.gif , http://www.nature...469.html

Jul 23, 2015
Perhaps they aren't syncing to each other, but are all syncing to a universal influence.

Jul 24, 2015
It's the spin of the earth that causes this.

Jul 24, 2015
I would say that it depends of what you define as pendulums.

Good examples. We could probably extend the definition to anything that is in a restricted/periodic motion (whether by mechanical limits or by being bound in an orbit to a larger body as in teh case of the moons) and where there is an exchange of forces dependent on relative position/motion (i.e. not a constant/quantized interchange).

Jul 24, 2015
@AA
Because there are a lot of people that have funny ideas on PO (e.g. the earlier comment of jeffreyjoemiller) it would be wise to be more specific. In the case of this article the pendulums are physical bodies in periodic motion exchanging kinetic energy mediated by sound waves (vibrations are sound waves and they do not travel only through the air). In the case of the moons of Jupiter and Pluto they are physical bodies in periodic motion exchanging kinetic energy mediated by gravitational interactions. In all cases the physics of resonance is well understood so it is a good idea to read the scientific paper to understand what this article is all about: http://www.nature...rep11548

Jul 27, 2015
It should be taken into account that the wall is a part of some room (closed space). Consequently, some standing wave is forming which synchronizes all the clocks in both a phase or anti-phase, depending on the distance between them. Moreover, that "is fundamentally different from the problem of two clocks hanging from a moveable base" (http://www.nature...p11548).
The real "discovery" is an introduction of the mysterious "sound pulses". Because it is wellknown that a sound is some wave with the frequency 0.02÷16 kHz! May be the author could tell us how these "sound pulses" are sounding or to compose by them a great melody?