Self-organised criticality in the dynamics of granular systems (The great Sesame Street crash)

2004

YEAR BOOK

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Lumen Christi College, Derry

Breand�n Hill & Rois�n McCloskey

Self-organised criticality in the dynamics of granular systems (The great Sesame Street crash)

Sesame seeds fed continually onto a pile produce avalanches which exhibit the same statistics as earthquakes, wars and stock market crashes. Self-organised criticality may be a general theory explaining many features of complex systems.

Despite a fundamental drift to increasing disorder, we observe order at every scale in the universe � from super clusters of galaxies to the hexagonal symmetry of snowflakes. This order has culminated, at least on earth, with the human brain. Recent advances in our understanding of the ability of multi-component systems to apparently self-organise into ordered structures in which characteristic length and time scales disappear, so called Self-Organised Criticality (SOC), may explain the origin of spatial and temporal fractals which are common in nature. They may also represent a first step in explaining the fundamental transition between a chemical soup governed by the laws of physics, to complex self-reproducing molecules whose further development was governed by the laws of natural selection.

In this project, we examined the physics of sand piles as the stereotypical example of systems which produce structure from disorder without tuning. We showed that, while ideal computer sand pile models give rise to ideal critical behaviour, reasonable adaptations of these models which more closely represent reality exhibit enhanced numbers of large events due to the effect of finite system size. We reviewed experimental work on real sand and seed piles and note that the results are littered with examples of exactly this large event enhancement. We designed and constructed an apparatus in which a seed-pile is continually fed, sometimes for days, with a steady, tuneable supply of individual grains. We examined the distributions of events on these piles using a photographic technique which, we believe for the first time, allows the examination of small events on the surface of the pile. We showed that these events do indeed exhibit power law scaling indicative of SOC, but that their development is punctuated by large catastrophic collapses due to a fundamental change in causative mechanism. A qualitative model is developed for the occurrence of these events.

Breand�n Hill & Rois�n McCloskey entered their project in the Senior Group Section in the Chemical, Physical and Mathematical Sciences Category at the EsatBT Young Scientist & Technology Exhibition in January 2004. They won one of the top prizes The Best Group Award. They also won a Special Award sponsored by the Institute of Physics in Ireland. Their teacher was Ms Martina Gillen.

This article was sponsored by The Institute of Physics