Crackling noise

Crackling noise arises when a system responds to changing external conditio ns through discrete, impulsive events spanning a broad range of sizes. A wi de variety of physical systems exhibiting crackling noise have been studied , from earthquakes on faults to paper crumpling. Because these systems exhi bit regular behaviour over a huge range of sizes, their behaviour is likely to be independent of microscopic and macroscopic details, and progress can be made by the use of simple models. The fact that these models and real s ystems can share the same behaviour on many scales is called universality. We illustrate these ideas by using results for our model of crackling noise in magnets, explaining the use of the renormalization group and scaling co llapses, and we highlight some continuing challenges in this still-evolving field.