On the nanometer scale, a seemingly smooth crystalline surface is not only bumpy, it's also in motion. Tiny mesas and depressions appear and disappear; escarpments range over the surface like waves on a beach. These thermal fluctuations are visible, thanks to advances in imaging techniques, which exploit electrons to divine the nanoscale motions (see figure 1). But although experiments can capture the spatialstructure of surface fluctuations with atomic resolution, they lack the temporal resolution to follow the hops of individual atoms. Instead, observations yield a set of parameters that characterize how the surface changes on longer, millisecond timescales.
Before you build things in the nanoworld, you'd better make sure you know how atomic fluctuations affect your structure's size and shape.