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Posts Tagged ‘Nature’

I came across a report on the new year eve of an old problem concerning the fantastic pattern formation and columnar jointing in nature  resolved. This particular problem  has been close to my heart as I elaborate in the paragraph below hence it was fun to read about it.

A Childhood Story: When I was in grade 2, my pappa got me a set of Childcraft Worldbook. It had some very interesting pictures. I was too young to understand the text completely but I used to love looking at the pictures. There was a very nice picture and a short accompanying article on the Giant’s Causeway in Volume 6 – Our World, on page 31 . However, I enjoyed reading this particular article as it had a story of the kind that kids are attracted to. It said that a lot of people before the turn of the century thought it was made by a giant, Fionn Mac Cumhail, to travel from Ireland to Scotland, and that this explained why the mostly regular hexagonal blocks that made up the Giant’s Causeway were so huge. I was fascinated by the story, but there was a short note at the end – It is believed that these columns were made by volcanic activity. I think fascination is fundamental to most science and art, when talking of science it has to be coupled with its seemingly opposite characteristic – skepticism. I think the line on volcanic activity making the structure did just that. Over the years whenever I saw a picture of the Causeway, I used to wonder how it (specifically the shapes) might have formed. And a PhD student has answered that question now!

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The Giant’s Causeway: On the coast of northern Ireland, there is an area of about 40,000 interlocking basalt columns extending into the sea. Most of the columns are hexagonal and fit very neatly, the neatness of this structure inspired a number of legends of an “intelligent designer”.

giantscauseway_1[A view of the Giant’s Causeway on the northern Irish Coast : Image Source]

Click to Enlarge

giants_causeway[A view of the Basalt Columns on the Causeway: Image Source -BBC]

A structure similar somewhat to the Giant’s Causeway is the Devil’s Postpile in California.

What is known is that in the Paleogene period, there was intense volcanic activity in the region (Antrim), and that it is what formed the structures. However, what was not known was how was it that such specific forms, shapes were formed.

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Research: PhD student Lucas Goehring and his adviser Dr Stephen Morris of the department of Physics  of the University of Toronto have been working on the problem above. They found out that the size of the columns that varies from one site to another varies as according to the speed of cooling of the lava from an eruption/flow. Using a combination (as required for such a task) of solid mathematical theory provided by Harvard Prof L Mahadevan, good experimentation and field work at the actual sites the researchers solved the problem of what determines the size of the columns.The field work involved making measurements in-situ i.e on the columns to determine at what rate had the lava cooled to form them.

The key to understanding and confirming their ideas was reproducing the phenomenon in the lab, that they did using simple materials. The idea was to use water and corn- starch, which cracks when dried and forms very similar columns as the ones talked about. Controlling the drying rate, a relationship between the size of the columns and the cooling rate was worked out.

starch_expt[Image Source – Reference 1]

The above is the experimental setup for the one just described above. According to Goehring : “The columns are formed as a sharp front of cooling moves into the lava flow, assisted by the boiling of groundwater, As the front advances, it leaves behind a crack network which evolves into an almost hexagonal arrangement. This network carves out the columns.”. They found out that the slower the cooling the much larger the columns.

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For more detailed information on the research carried out in this regard, I’d direct you to pages from Goehring’s website.

>> Experiments

>> Field Work

>> Models

>> Extensions

He in the above pages gives a quite accessible picture of their findings and their work. Perfect, especially for somebody like me who is not trained in the field in which this research has been carried out, but are very interested in how it was done.

The findings of this work appeared in the December issue of the proceedings of the national academy of sciences.

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Quick Links:

1. Order and Disorder in Columnar Joints

2. Workshop on Emergent Pattern Formation

3. U-Toronto Press Release

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Onionesque Reality Home >>

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This post is following the previous post in which i mentioned about the rules governing the motion of a bird swarm.

The video below is a breathtaking, sublime, amazing recording of thousands of starlings in a flock before roosting. This is from the UK country-side. While it is another awesome demonstration of how the “imagination” of nature can be like, it also gives a perfect example of how the entire swarm is organized and how it moves. Modern researchers are trying to imitate such emergent behavior to use in fields like robotics, data mining, internet mathematics, optimization etc etc.

How do birds exactly do this?

Here is an excerpt from a talk that i had given at a technical event in October ’07.

In a swarm if we say that there are ‘N’ number of agents, then we can say that these autonomous agents are in a way co-operating to achieve a global objective. This global objective can be better foraging, constructing shelter, serving as a defence mechanism among others. This apparent collective intelligence emerges from very simple individual agents. The actions of these agents are governed by local rules and through the interactions of the N agents the swarm achieves a global objective. A kind of “self organization” emerges in these systems. We see that there is no central controller in such cases. Swarm intelligence gives a basis which makes it possible to explore collective (or distributed) problem solving without centralized control or without the provision of a global model. [1]

The individual (but autonomous) agent does not follow directives from a central authority or work according to some global plan. As a common example, a bird in a flock, only adjusts its movements to coordinate with the movements of its flock mates or more precisely the members that are its neighbors. It simply tries to stay close to its neighbors, but avoid collisions with them. Each bird does not take commands from any leader bird since there is no lead bird. Any bird can fly anywhere in the swarm, either in the middle or the front or the back of the swarm. Swarm behavior gives the birds some distinct advantages like protection from predators, and searching for food (effectively in a swarm each bird is exploiting the eyes of every other bird). Scientists are trying to find out how these birds, fish etc move in flocks, schools in a way that appears orchestrated. A school of fish and a flock of birds move as if all the “steps” were pre planned. For one moment they are moving towards the left and in another they are darting towards the right. Among these researchers in 1987, Reynolds created a boidor bird-oid (bird like) model. This is a distributed behavioral model, to simulate the motion of a flock of birds on a computer [2]. Each boid is implemented as an agent which moves according to its own understanding of the dynamic environment. A boid observes the following rules. First, is that a boid must move away from boids that are too close, so as to reduce the chance of collisions. Second, boid must fly in the general direction that the flock is moving. Third, a boid should minimize exposure to the flock’s exterior by moving toward the perceived center of the flock. Flake later [3] added a Fourth rule, a boid should move laterally away from any boid that blocks its view. This boid model seems reasonable if we consider it from another point of view, that of it acting according to attraction and repulsion between neighbors in a flock. The repulsion relationship results in the avoidance of collisions and attraction makes the flock keep shape, i.e., copying movements of neighbors can be seen as a kind of attraction.

This is what i was talking about in the previous post. A certain set of rules is followed that would give a certain shape to the swarm, if the set is altered so will be the shape and maybe functionality as well!

Sounds nice, now how can these simple rules be modeled on a computer? They can be done using NetLogo. Here is a sample model . To get a good idea about the intricacies adjust the population, the turn angles and the vision. For playing around with the model and making your own you are instructed to go through the writeup to this model on the above page.

References:

[1] E. Bonabeau, M. Dorigo, and G. Theraulaz, Swarm Intelligence: From Natural to Artifcial Systems. NY: Oxford Univ. Press, 1999.

[2] C. Reynolds, ‘Flocks, herds, and schools: A distributed behavioral model,” Comp. Graph, vol. 21, no. 4, pp. 25{34, 1987.

[3] G. Flake, The Computational Beauty of Nature. Cambridge, MA: MIT Press, 1999.

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