10 - Summary and discussion

Summary

In his review of SOC, Jensen (1998) asked four central questions paraphrased here.

1. Can SOC be defined as a distinct phenomenon?

2. Are there systems that display SOC?

3. What has SOC taught us?

4. Does SOC have any predictive power?

As discussed in the following, the answers are positive throughout, but slightly different from what was expected ten years ago, when the general consensus was that the failure of SOC experiments and computer models to display the expected featureswasmerely amatter of improving the setup or increasing the system size. Firstly, this is not true: larger and purer systems have, in many cases, not improved the behaviour. Secondly, truly universal behaviour is not expected to be prone to tiny impurities or to display such dramatic finite size corrections. If the conclusion is that this iswhat generally happens in systems studied in SOC over the last twenty years, critical phenomena may not be the most suitable framework to describe them.

Can SOC be defined as a distinct phenomenon?

In the preceding chapters, SOC was regarded as the observation that some systems with spatial degrees of freedom evolve, by a form of self-organisation, to a critical point, where they display intermittent behaviour (avalanching) and (finite size) scaling as known from ordinary phase transitions (Bak et al., 1987, also Ch. 1). This definition makes it clearly distinct from other phenomena, although generic scale invariance has been observed elsewhere.