Editing Scale invariance (section)

==Universality==
A phenomenon known as [[universality (dynamical systems)|universality]] is seen in a large variety of physical systems. It expresses the idea that different microscopic physics can give rise to the same scaling behaviour at a phase transition. A canonical example of universality involves the following two systems:
* The [[Ising model]] phase transition, described above.
* The [[liquid]]-[[vapour]] transition in classical fluids.

Even though the microscopic physics of these two systems is completely different, their critical exponents turn out to be the same. Moreover, one can calculate these exponents using the same statistical field theory. The key observation is that at a phase transition or [[critical point (thermodynamics)|critical point]], fluctuations occur at all length scales, and thus one should look for a scale-invariant statistical field theory to describe the phenomena. In a sense, universality is the observation that there are relatively few such scale-invariant theories.

The set of different microscopic theories described by the same scale-invariant theory is known as a [[universality class]]. Other examples of systems which belong to a universality class are:
* [[Avalanche]]s in piles of sand. The likelihood of an avalanche is in power-law proportion to the size of the avalanche, and avalanches are seen to occur at all size scales.
* The frequency of [[network outage]]s on the [[Internet]], as a function of size and duration.
* The frequency of citations of journal articles, considered in the network of all citations amongst all papers, as a function of the number of citations in a given paper.{{Citation needed|date=February 2017}}
* The formation and propagation of cracks and tears in materials ranging from steel to rock to paper. The variations of the direction of the tear, or the roughness of a fractured surface, are in power-law proportion to the size scale.
* The [[electrical breakdown]] of [[dielectric]]s, which resemble cracks and tears.
* The [[percolation]] of fluids through disordered media, such as [[petroleum]] through fractured rock beds, or water through filter paper, such as in [[chromatography]]. Power-law scaling connects the rate of flow to the distribution of fractures.
* The [[diffusion]] of [[molecule]]s in [[Solution (chemistry)|solution]], and the phenomenon of [[diffusion-limited aggregation]].
* The distribution of rocks of different sizes in an aggregate mixture that is being shaken (with gravity acting on the rocks).

The key observation is that, for all of these different systems, the behaviour resembles a [[phase transition]], and that the language of statistical mechanics and scale-invariant [[statistical field theory]] may be applied to describe them.