
You are getting sleeeeeeepy ... - ColinWright
http://blogs.discovermagazine.com/cosmicvariance/2012/10/01/you-are-getting-sleeeeeeepy/
======
lutusp
I have a story seemingly remotely associated with this, but actually very much
the same.

Years ago, at highway construction sites, sawhorses would be erected that were
equipped with flashing lights for safety after dark. In those days the
flashing lights were gas-discharge lamps connected to a simple charging
circuit consisting of a capacitor connected across the lamp and charged by way
of a resistor -- a very simple arrangement, but one that would reliably flash
the light all night long.

It turns out that the specific moment the lamp broke down and flashed could be
affected by nearby lights of the same kind. As a result, if a large number of
sawhorses were erected in a dark location, eventually all the lights would get
into synchronization. I can remember on a number of occasions on long trips
through dark countrysides, cresting a hill and seeing a construction site in
the distance, with all the lights flashing as one.

As I would approach closer, as my headlights shone more brightly on the
construction lights, they would go out of sync.

This is a purely historical note, because modern construction sawhorses use
LEDs instead of high-voltage gas-discharge lamps -- cheaper and more reliable.
The LED's never become synchronized, of course.

~~~
jonmrodriguez
How would your headlight desynchronize the construction lights just by shining
on them, if it was powered by a separate source? Did the construction lights
have light sensors?

~~~
lutusp
> Did the construction lights have light sensors?

The construction lights _were_ light sensors. The reason they got into
synchronization is because they were triggered to break down at a specific
time by the flashes of light from the other lamps.

In a system like this, the voltage on all the lamps is increasing because a
charging current is applied to a capacitor that'a connected across the lamp.
But the specific time of breakdown, the exact voltage at which the lamp breaks
down, depends on the ambient light level. And a nearby flash of light is
sufficient to trigger the lamp to break down in sync.

This effect washes out in daylight, or in a place with normal background
light. Or if a light source interferes with the delicate balancing act
responsible for the synchronization -- like the headlights on a passing car.

~~~
raldi
Can you explain what you mean by "break down" in this context?

~~~
gryphonic
Plasma breakdown. The voltage across the gas eventually accelerates electrons
fast enough that they can ionize atoms when they collide, and an avalanche
occurs creating a plasma and light. Same effect as a neon lamp

------
alan_cx
There is a place in the UK where this happens with spooky regularity with
people walking. There is a covered walk way between Waterloo and Waterloo East
railway stations. The mob of people enter it unsynchronised and by the time
they leave, they are usually all synchronised. I assume this happens in many
other places too, I just haven't been every where else to see!!!!

I always assumed this was because people can hear each other and subliminally
fall in to syc, whether this is the same or not, Im already internally
debating!!!

~~~
lutusp
> There is a place in the UK where this happens with spooky regularity with
> people walking.

Sometimes in the military, when crossing a bridge, troops are ordered not to
walk in step, because of the stress it places on the bridge.

<http://en.wikipedia.org/wiki/Angers_Bridge>

A quote: "As usual in crossing that bridge, the soldiers had been ordered to
break step and to space themselves farther apart than normal."

More dramatic:

<http://en.wikipedia.org/wiki/Broughton_Suspension_Bridge>

A quote: "On 12 April 1831, the 60th Rifle Corps carried out an exercise on
Kersal Moor under the command of Lieutenant P. S. Fitzgerald, the son of John
Fitzgerald. As a detachment of 74 men returned to barracks in Salford by way
of the bridge[6] the soldiers, who were marching four abreast, felt it begin
to vibrate in time with their footsteps. Finding the vibration a pleasant
sensation some of them started to whistle a marching tune, and they began to
"humour it by the manner in which they stepped", causing the bridge to vibrate
even more.[6] The head of the column had almost reached the Pendleton side
when they heard "a sound resembling an irregular discharge of firearms".[6]
Immediately, one of the iron columns supporting the suspension chains on the
Broughton side of the river fell towards the bridge, carrying with it a large
stone from the pier to which it had been bolted. The corner of the bridge, no
longer supported, then fell 16 or 18 feet into the river, throwing about 40 of
the soldiers into the water or against the chains. As the water was only about
two feet (60 cm) deep at that point none of the men were killed, but 20 were
injured, including six who suffered severe injuries including broken arms and
legs, severe bruising, and contusions to the head."

~~~
huhtenberg
> _troops are ordered not to walk in step, because of the stress it places on
> the bridge_

Not really, not because of the "stress." The walking rhythm may match the
natural oscillation of the bridge thus resulting in a resonance effect. At
least that's what they taught us in high school.

~~~
ColinWright
I've always been told this, and it's always seemed plausible, but it was
declared busted by Mythbusters.

Having said that, as quoted elsewhere, the Millennium Bridge across the Thames
in London did exhibit resonant swaying as pedestrians crossed it, and extra
dampers needed to be added.

 _Added in edit ..._

Auto-hoist, petard-wise. It seems some people have assumed that I meant that
this never happens, and cannot ever happen, simply because it was the subject
of a Mythbusters episode in which they declared it busted, even though I then
added that something similar has in fact happened in real life.

I thought the implication was clear, but obviously not. So let me say more
explicitly ...

It is clear that people walking on a bridge can cause resonance, it has been
seen in real life.

[http://en.wikipedia.org/wiki/Millennium_Bridge_%28London%29#...](http://en.wikipedia.org/wiki/Millennium_Bridge_%28London%29#Resonance)

<http://www.youtube.com/watch?v=eiaM_LZUsqM>

Further, that was a non-extreme case, so it is totally plausible that more
vigorous marching, more definite synchronisation, and closer matching to the
structure's resonant frequency is not only possibly going to cause problems,
but _likely_ to cause problems.

It is, however, _unlikely_ that marching will match the resonant frequency.
Structures these days don't resonate at 2 Hz. Football stadiums are designed
to resonate at above 12 Hz. Further, structures often have explicit energy
dissipation system built in to them. Taipei 101, for example, has a tuned mass
damper to change the resonant frequencies and help absorb, then dissipate
energy (yes, I'm speaking loosely here):

<http://en.wikipedia.org/wiki/Taipei_101#Structural_design>

So yes, I knew all that, and yes, a single Mythbusters episode is not proof of
anything, but it does provide a collection of data from which to start forming
your own hypotheses, designing your own experiments, and understanding that
things aren't necessarily as simple as you thought.

~~~
huhtenberg
> _declared busted by Mythbusters_

Oh, goodie, case closed then. Mythbusters have this tendency to "bust" stories
that revolve around rare and unique circumstances by running just few
experiments. _Obviously_ that's a flawed approach.

~~~
DrStalker
And then there was the time they tested shock absorbers on a car's bumper
exploding in a fire and sending the bumper flying away. They couldn't
replicate it.

They spoke to a woman with scars on her legs from a bumper that hit her after
shock absorbers on a burning car exploded and sent the bumper flying away.

So they called it confirmed, since it could happen, they just couldn't get
everything right in the lab.

I know it's entertainment first and science second, but a complete reversal of
policy just because there is someone you're not willing to look at and call a
liar? Weak.

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tptacek
Network protocols with periodic updates will do the same thing; it used to be
a problem, so much so that designers would add noise to timing in
implementations. Here's a great paper from Sally Floyd and Van Jacobsen on it:

<http://ee.lbl.gov/papers/sync_94.pdf>

~~~
makmanalp
Here is an example for multicast dns (used in bonjour etc):

[http://files.multicastdns.org/draft-cheshire-dnsext-
multicas...](http://files.multicastdns.org/draft-cheshire-dnsext-
multicastdns.txt) (page 9)

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plg
After looking at various papers on the topic I found some equations of motion,
did some algebra and wrote some python code. Here is the result

video of simulation results animated:
<http://www.youtube.com/watch?v=U1QqYBOrYjA>

python code: git://github.com/paulgribble/metronomes.git

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morganpyne
This looks like a scaled up version of this experiment that has been on
youtube for about 5 years: <https://www.youtube.com/watch?v=W1TMZASCR-I> (only
5 metronomes in that one, but I remember being fascinated when I first saw the
effect)

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tgb
The linked article claims that metronomes start 'pairing up' with their
nearest neighbors saying that "strongest forces on a metronome will initially
be from its nearest neighbors." But if the forces are spread by the string-
suspended table swaying back and forth, I wouldn't expect adjacent metronomes
to be significantly more connected than further away ones. Instead, I wonder
if we just happen to be better at noticing metronome synchronicity when the
metronomes are adjacent. Any pairs that formed several steps away would escape
our notice.

~~~
derekp7
Probably the initial synchronization happens due to vibration, which would be
felt by the closest neighbor. Then the macro synchronization happens due to
the table swaying.

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raldi
Order from chaos! And without any external source of energy.

I wonder what other physical phenomena emerge this way. Magnetic fields, I
bet.

~~~
ChuckMcM
Uh, no. The article points out the metronomes are on a suspended table. Each
metronome adds energy to the table in phase with its cycle, as more metronomes
get into phase, their summed energy edition begins to push back on other
metronomes bringing them all into alignment. By the end of the video you can
see the table is oscillating exactly out of phase with the metronomes as
expected by that point.

~~~
raldi
What difference does that make? The metronomes and table are a closed system,
so the fact that energy is transferred between them is moot.

I think lutusp's explanation makes a lot more sense: that when the metronomes
sync up, they're at a lower energy level. They spend energy to decrease
entropy.

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drjenkins
Steven Strogatz has done a lot of work on things like this in the past -
including this experiment.

<http://www.ted.com/talks/steven_strogatz_on_sync.html>

In the simple case of just two metronomes on a single moving plane, the
tendency for sync is governed by a non linear differential equation. The math
gets rather complicated: <http://salt.uaa.alaska.edu/dept/metro.pdf>

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OldSchool
Wow, we can certainly conclude that conformity is the lowest acute energy
state of this system in motion. An apt metaphor for mass social behavior; it
eventually sounds frighteningly like soldiers marching.

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user24
I have a feeling a similar phenomenon was responsible for the Millennium
Bridge being closed:

[http://en.wikipedia.org/wiki/Millennium_Bridge_%28London%29#...](http://en.wikipedia.org/wiki/Millennium_Bridge_%28London%29#Resonance)

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Jun8
Cool! This was first observed by Huygens on two pendulum clocks.

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thornofmight
I'm surprised nobody picked up on the seemingly obvious metaphor these
metronomes are for people and behavior.

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yafujifide
Does this violate the second law of thermodynamics?

~~~
mikeash
Of course not. Nothing does, that's why it's a law!

