Flying Circus of Physics

Flying Circus of Physics

Newsletter for Nov. 2013 - January 2014

This newsletter is about self-synchronizing systems. Well, I don’t mean situations like synchronized swimmers who must practice long hours to perform a synchronized swim/dance routine. Rather I mean situations where unsynchronized things bring themselves into synchrony in a surprising way, one that makes me laugh. (I have never laughed while watching synchronized swimmers.)

Metronomes

Metronomes are inverted pendulums that are designed to oscillate at a set rate, to keep time in music. Here is a system of five metronomes that are started at different times and out of step. They are mounted on a flat board. When the board is on the table, the metronomes remain out of step. Nothing to laugh about there. But when the board is supported by two cans such that it can oscillate left and right, the metronomes soon fall into step. You can hear the difference: at first there is an audio clutter of clicking, but soon there is an orderly sequence of clicking, as if a single metronome is in action.

http://www.youtube.com/watch?v=W1TMZASCR-I

When they were on the stationary and rigid table, the two metronomes could not affect each other. But when they are on the mobile board on top the cans, the swinging motion of each metronome causes the board to oscillate left and right, which then affects the swinging of the other metronome. The metronome thus communicate their motions.

That worked for five metronomes. How about 34?

http://www.youtube.com/watch?v=kqFc4wriBvE

Millenium Bridge in London

The Millennium Bridge is a footbridge across the Thames in London, connecting the Tate Modern art gallery with the vicinity of St. Paul’s Cathedral. This sleek, modern bridge was built to celebrate the new millennium, hence its name. However, when the first surge of people began to walk over the bridge, the bridge began to oscillate so vigorously that many people had difficulty keeping their balance. Here is a video of the opening day.

http://www.youtube.com/watch?v=eAXVa__XWZ8 

The bridge was closed while it was fitted with dampers to decrease or eliminate the oscillations. Later, researchers explored why the oscillations occurred on that opening day. The ultimate cause was the pedestrians themselves, which is surprising because they did not walk across locked in step like a marching band. Rather, their walking was nonsynchronous (independent). Studies revealed, however, then when the density of walkers exceeds a certain critical value, even the random impacts of the footsteps sets up small oscillations of the bridge at its natural frequencies (the frequencies at which it would oscillate if it were somehow pulled to one side and released and then allowed to oscillate on its own). If the density of walkers is above a certain critical value, then the oscillations grow somewhat, causing some of the people to fall into step with them in order to better maintain their balance. In turn, that falling-into-step increases the extent of the oscillations, which causes even more people to fall into step. And so on.

    You might experience similar synchronous oscillations in a stadium stand or a concert hall balcony if spectators stomp or jump in unison. That is always my signal to leave because I don’t want to be part of the experiment to see if the engineers who designed the structure allowed for significant oscillations in the construction plans.

Starlings in flight

The sight of a flock of starlings can be awesome, if not frightening. A flock, which might consist of hundreds to tens of thousands of birds, can undergo a fluid-like, synchronized motion. Here is an example:

http://www.youtube.com/watch?v=XH-groCeKbE&NR=1&feature=fvwp flocks of starlings near Oxford, England

The stunning feature of starling flocks is that in spite of the flock’s three-dimensional twists and turns, none of the birds collide. How can tens of thousands of closely-spaced flying birds avoid collisions? How do they manage to fly and land without any leader? How is the “decision” for the flock to, say, turn to the left communicated across the flock, or is the flight simply a result of chance decisions, with many random decisions being made and changed? How do the birds respond when the density of the flock suddenly changes?

These questions have fascinated scientists in many disciplines, including physics, because the starling flocks are an example of collective behavior without a leader. The questions still have not been fully answered, but recent studies of starling flocks have led researchers to fairly successful models. In particular, the models suggest that any given bird within a flock monitors the six or seven closest birds, regardless of how close they really are. That is, the bird’s flight is affected by those closest birds regardless of the density of birds.

Presumably, the bird maintains its flight orientation relative to those six or seven closest birds. If they begin to, say, veer to the right, then the given bird does also. Then information about this veering is fairly rapidly transmitted across the flock so that it (or at least part of it) veers to the right. In the video you can see that the flight change is not always transmitted to the full flock, with the flock tending to split into branches. But then corrections are made to bring the branches together.

The simulations also suggest that this flight behavior works to the advantage of the birds if they are attacked by a falcon. If each bird were affected by the density, then the flock would tend to splinter at the point of attack, leaving individual birds as easy prey for the falcon. However, if each bird tends to monitor only the six or seven closest birds, the flock tends to veer away from the falcon at the point of attack, without leaving any stragglers as prey for the falcon.

Fireflies

The fireflies back home in Texas were charming but completely independent. That is, each firefly randomly flashed with no coordination with the other fireflies in my backyard. However, certain species of fireflies in Asia and the United States Smokey Mountains are far more coordinated because they will flash in unison. Here is a description from an early scientific report:

“Imagine a tenth of a mile of river front with an unbroken line of … trees with fireflies on every leaf flashing in unison, the insects on the trees at the ends of the line acting in perfect unison with those between.”

The river front must resemble a lane with flashing billboards

Here are two videos. In the first, audio is missing after a few seconds but you can still see the flashing of the fireflies.

http://www.youtube.com/watch?v=OBO_PKstYzc

http://www.youtube.com/watch?v=sC2OkCEdqU4

Synchronized firefly flashing has been the subject of much research as investigators attempt to model it with electrical oscillators that will recognize one another and then shift their oscillations to be locked in step. However, finding the neurological correlation between a firefly and an electrical circuit has been challenging.

Cheers,

Jearl Walker

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