COMMENTARY

atural structures are often hierarchical, for example, with a sponge-like topology. Self-similar topologies can be mathematically well described by fractal geometry1. Let us consider objects of a specifi ed family, for example, cracks and fragments2 or dislocation cells3, having a size larger than a given value; self-similar structures imply an inverse proportionality between the number of objects and their size raised to a positive real number D, the so-called fractal exponent (Table 1, fractal law). Interestingly, similar laws can be found in nature with respect to time, not only to space, that is, they describe the distribution of the number of events (earthquakes, for example) having a duration larger than a given value