What is a spin glass? A glimpse via mesoscopic noise

It is an open question whether the slow dynamics of spin glasses is more accurately described by a model based on rather compact droplets which can flip thermally, or by a picture of more diffuse sets of flipping spins with properties described by hierarchical dynamics. Techniques have been developed for analyzing spontaneous fluctuations in mesoscopic samples which can directly address this question. In a well-known spin glass, $\mathrm{Cu}\mathrm{Mn}$, the experimental results are better fit by the hierarchical picture. Previously unexplored properties of the space of metastable configurations are directly measurable by these mesoscopic methods.