Universal clues in noisy skews

Crackling noise is a familiar phenomenon that arises when a system responds to slowly changing external conditions with the emission of a series of sudden avalanche-like pulses, and displays many universal characteristics over a wide range of time and length scales and in a diverse range of contexts ¹ . So the crackle of a candy wrapper as it is crumpled, the crackle of a tree as it is felled and the crackle of the Earth as it quakes under the movement of tectonic plates, all follow similar laws. Remarkably, most of the essential elements of this behaviour are successfully predicted by simple, cartoon-like models of the systems in which it occurs. Yet despite their success, the basic nature of these models, which either neglect or make crude approximations of the complex microscopic details of the systems they describe, means our understanding of the subtler aspects of crackling noise is incomplete. On page 46 of this issue ² , Zapperi and colleagues identify a microscopic origin for the previously enigmatic asymmetric shape of the noise pulses emitted by a magnet when its magnetization is reversed by an external field. Not only does this resolve a leading deviation between theory and experiment, it could have significant implications for other crackling-noise systems.