3 - Experiments and observations

Summary

A number of experiments and observations have been undertaken to test for SOC in the ‘real world’. Ultimately, these observations motivate the research based on analytical and numerical tools, although the latter provide the clearest evidence for SOC whereas experimental evidence is comparatively ambiguous. What evidence suffices to call a system self-organised critical? One might be inclined to say scale invariance without tuning, but as discussed in Sec. 9.4, the class of such systems might be too large and comprise phenomena that traditionally are regarded as distinct from criticality, such as, for example, diffusion.

In most cases, systems suspected to be self-organised critical display a form of scaling and a form of avalanching, suggesting a separation of time scales. Because of the early link to 1/f noise (Sec. 1.3.2), some early publications regard this as sufficient evidence for SOC. At the other end of the spectrum are systems that closely resemble those that are studied numerically and whose scaling behaviour is not too far from that observed in numerical studies. Yet it remains debatable whether any numerical model is a faithful representation of any experiment or at least incorporates the relevant interactions.

At first sight, solid experimental evidence for scaling or even universality is sparse among the many publications that suggest links to SOC. This result is even more sobering as evidence for SOC is heavily biased – there are very few publications (e.g. Jaeger, Liu, and Nagel, 1989; Kirchner and Weil, 1998) on failed attempts to identify SOC where it was suspected.