Relaxational Dynamics, Avalanches, and Disorder in Real Sandpiles

Abstract

Nothing, it seems, could be simpler than a sandpile—this was certainly the general view of physicists until recently. Sandpiles were therefore used as illustrations for complicated physical systems¹ and concepts, with the implicit assumption that their innate simplicity would illuminate the difficulties of the system or concept being explained. The most recent example of this arose about 5 years ago, when Bak, Tang, and Wiesenfeld (BTW) proposed their theory of self-organised criticality (SOC), using “a simple pile of sand” for purposes of illustration (Bak et al., 1987). This has proved to be a rather perilous paradigm in retrospect, because it is now almost universally accepted that real sandpiles are in fact much more complicated than that model; in fact, the avalanche of interest in these systems in recent years has not resulted in anything approaching even a crude understanding of their microscopic behaviour. Whatever the relevance of theories of self-organised criticality to real physical systems may turn out to be, there is no doubt at all that their choice of the “wrong” paradigm has led to a positive outcome; there has been an explosion of interest in the physics of real sandpiles, as physicists have responded in increasing numbers to the challenges of describing the many hidden complexities of what can no longer be regarded as a “simple” pile of sand.