# Synchrony of mechanical objects. How is this possible?

The Science section of today’s NY Times has a brief article about “synchrony”. This is a phenomenon whereby people, animal and things, given half a chance will begin to act in unison.

There was this sentance in the article:

How can mechanical objects display this phenomenon?

Vibrations transmitted through the wall or a table that they are touching:

http://www.apa.org/monitor/mar05/lockstep.html

I would imagine this normally happens when the objects involved are part of a system and the actions of each will affect the other, often in ways that are not immediately apparent.

For example, the pendulum example is probably explainable by air currents. While the pendulums are not synchronized, turbulence is created, which will probably slow down one pendulum more than the other. Once they begin to come into sync, turbulence would, I imagine, decrease, and thus encourage them to stay in sync.

If they are hanging from the same supportive bar, or from bars that are in close enough contact that vibrations from one would travel to the other, that’s another possible source of interrelation.

don’t think the example given is evidence they do, it’s just a single and incomplete observation. I observe the same behavor between the turn signal and and those of the car in front of me. They start out of sync but within not too many cycles are out of sync. Did the clocks observed by Huygens stay in sync for good or were they out of sync soon after?

Mechanical objects could self syncronize if there is a feedback loop between them but I don’t think the clocks would do this.

One possibility for the clock pendulums is that they were each swinging with a very slightly different period and that what was actually observed was just the fact of them coming into phase (i.e. if they had been observed for longer, that they would no longer appear to be swinging in unison, then later, they would again and so on).

If that were the case, then the clocks would go out of phase again. From what I remember about the Huygens experiment, once synchronized the clocks stay that way.

In that case, there must be some physical feedback link between the two; this could be quite subtle - air currents or imperceptible vibrations through the framework upon which the pendulums were mounted.

Either that, or they are both falling int sync with something else that is not observed.

Huygens’ clocks are a frequently cited example of coupled harmonic oscillators, the study of which is important in physics (and differential equations classes). The feeling is that vibrations through the walls were coupling the clocks together – an altogether believable notion. I think we can rule out the possibility that Huygens just happened to catch the clock pendulums when they happened to be in phase. He was a pretthy good observer. Coupled harmonic oscillators will do just that.

Huygens was a Class A Scientist. He thoroughly studied it.

Indeed, pendulums that are close enough that even small vibrations can travel between them will sync up.

For a thorough, readable, discussion about such phenomena, read Sync by Steven Strogatz.

Awww.

(disappointed that the consensus has apparently ruled against his air turbulence idea, though he did mention vibrations too.)

I certainly wouldn’t rule it out absolutely.

Has nobody tried the experiment in a vacuum?

Pendulum? We don’t need no stinkin’ pendulum.

I don’t know for a fact if anyone has. The mechanical-coupling idea is pretty well supported by theory and experiment (coupled oscillators is a common undergrad experiment), and UI’d certainly expect mechanical coupling through the support to be present and much larger than coupling forces through air movement. On the other hand, it doesn’t take much to induce synchrony, and it would be an interesting experiment to see if you could, in the absence of other mechanical coupling, show that you could couple them via air vibrations alone. Be a lotta work,though.

Indeed; rewinding the thread a bit, the pendulums are quite likely to drift into/through synchony anyway; all that the mechanical link has to do is apply moderate restraint to the system drifting back out again.

Beside the already mentioned forces, couldn’t mass attraction help the effect along if the pendulums are parallel to each other? It’s a week force compared to many other forces, but it still links the two pendulums.

Gravitational attraction between objects the size of your typical pendulum, on distances the scale of room size, are so tiny that air currents are enormous by comparison. I suspect any such effects would get drowned out by random air movements ans brownian motion. On the other hand, it’s a regular effect, so maybe it could have a cumulative effect. I seriously doubt it, though.
Interesting Board Name for this question, Harmonious Discord.

Also does the Earth’s magnetic flux tend to put the pendulums in sync over time? The magnetic field could pull a bit more for a given time. The pendulum in a rising stroke would lose a small bit of momentum. The pendulum in a down stroke would gain a little momentum. The action could put the pendulums in sync, and help keep them in sync. Once they are in sync they would both lose or gain momentum the same. I believe this could be a viable mechanism for this to occur.

This is indeed the interlinking of two systems, and is a part of chaos theory known as mode locking. It accounts for all sorts of syncronous events, from the phenomenon of the Moon always presenting the same face to the Earth, to unwittingly syncronising pendulums and electronic oscillators. With pendulums, the coupling between the two systems is by mechanical vibration, and with electronic oscillators it’s either directly coupled signals over a common conductor, or electromagnetic coupling. It makes designing two analogue oscillators of slightly different frequencies almost impossible, as their respective frequencies will tend to pull in towards each other.

I used to see a high-voltage example of mode locking when I had a job designing gas cooker igniters one time. They were set to spark at just under once per second (to comply with EMC regs), and when a dozen were wired up to the mains and turned on, they would all start by sparking at their own sweet rhythm, but slowly they would start to influence each other, and after a few hours they would all be sparking in unison.

I’ve a feeling the same effect can be seen on a pheremonal-feedback level when several post-pubescent and pre-menopausal women share the same dwelling for any long period of time. Their menstrual cycles will often come into sync, sometimes after a difficult readjustment for some of the women. IIRC in these instances, some women tend to have rhythms that dominate the others.

Gravitational coupling of the pendula would be miniscule, but it would be there, and if other couplings were well-controlled, it would eventually suffice. In fact, the experiment could probably be done with current technology: It’s basically a variation of Cavendish’s experiment.

Pumping from the Earth’s magnetic field, however, would be negligible, and I think completely undetectable. The Earth’s magnetic field does vary, but only on timescales hugely longer than that of any laboratory-sized pendulum. Add to that, that you’d have to control for the gravitational coupling between the pendula, which would likely prove impossible (there is no known way to shield against gravitational influences).

You could certainly have magnetic interactions between the pendula themselves, however, especially if they were made of iron or other ferromagnetic material. But if you didn’t want magnetic coupling, a wise choice of material could eliminate this.

Fridgemagnet, menstrual synchronization is a somewhat controvertial subject. I’ve seen studies which purport to be able to induce it, but I’ve also seen studies which purport to show that it’s never above the expected chance level of frequencies beating against each other. I’m not sure which are more reliable.