Self-organized criticality describes systems that organize themselves to their critical point and remain there. At this point there is no simple causality, which means a dramatic effect does not require a dramatic cause. In fact, there is no way to predict what the effect of dropping the next grain of sand on a pile of sand will be.
Self-organized criticality describes a lot more than just sand piles. Many complex systems, including the weather, economy, and fault lines display SOC behavior. A butterfly flapping its wings in Africa may or may not cause a hurricane in Florida. A few trades by one person could or could not crash the market. A small change in the strain or stress along a fault line might not cause an earthquake, but it might. There is no way to tell what a small change will do.
Most naturally occuring systems that display self-organized criticality prove difficult to study in a laboratory due to their size and the fact that they cannot be easily manipulated. As a result, most researchers studying self-organized criticality use computer models. Our project, however, deals with a physical system. We have replaced the sand pile with a pile of small beads, about 3 mm in diameter. Beads are more uniform and less affected by moisture and static electricity than sand. Working with beads enables us to eliminate extra variables present in sand piles.
Our bead pile fits the requirements of self-organized criticality. It organizes itself to its critical angle of repose and remains there. Avalanches are size independent and cannot be predicted. We are continuing to test the theory and make new discoveries.