Understanding the call of the monopole

Mysterious magnetic noise of elusive particles is finally understood Noise is often thought of as merely a hindrance to detecting a signal, but it frequently contains vital clues to otherwise hidden behavior. Recently, noise has been used to hunt for elusive particles called emergent magnetic monopoles, which are predicted to exist within crystals called spin ices (1). Monopoles would feature a magnetic charge that resembles the north pole of a magnet moving independently of the south pole. In 2019, the first noise measurements on monopole dynamics revealed distinctive spontaneous noise in the magnetization of spin ices (2). However, the noise differed from the simplest expectations for magnetization coming from moving magnetic particles. On page 1218 of this issue, Hallén et al. (3) propose that natural constraints in a spin ice restrict monopole motions to a lattice (within the spin ice crystal) that bears a fractional number of dimensions (known as a fractal). Understanding the nature of monopoles in spin ice might have important practical uses, in much the same way that understanding the nature of electrons facilitated modern electronics.