What neutrons do tell us about the nature of (spin) glasses?

A common feature of all glassy systems is the decisive role of dynamics in the evolution of macroscopic properties, while the short and eventually partial long-range order shows little difference between the different phases. Our recent neutron scattering results on a Cu-Mn spin glass dramatically illustrate this point, and provide unambiguous evidence against recent suggestions that behaviour in Cu-Mn is driven by spin density waves rather than glassy dynamics. Recent findings in Au-Fe provide the first circumstantial evidence for the theoretically predicted extremely long correlation lengths at the spin-glass transition. Recent experimental advances opened up the way to investigate dynamics in glasses and liquids on extended length scales, where only spin correlations could be studied by now.