Editing Power law (section)

===Universality===
The equivalence of power laws with a particular scaling exponent can have a deeper origin in the dynamical processes that generate the power-law relation. In physics, for example, [[phase transition]]s in thermodynamic systems are associated with the emergence of power-law distributions of certain quantities, whose exponents are referred to as the [[critical exponent]]s of the system. Diverse systems with the same critical exponents—that is, which display identical scaling behaviour as they approach [[critical point (thermodynamics)|criticality]]—can be shown, via [[renormalization group]] theory, to share the same fundamental dynamics. For instance, the behavior of water and CO<sub>2</sub> at their boiling points fall in the same universality class because they have identical critical exponents.{{citation needed|date=July 2015}}{{clarify|date=July 2015}} In fact, almost all material phase transitions are described by a small set of universality classes. Similar observations have been made, though not as comprehensively, for various [[self-organized criticality|self-organized critical]] systems, where the critical point of the system is an [[attractor]].  Formally, this sharing of dynamics is referred to as [[universality (dynamical systems)|universality]], and systems with precisely the same critical exponents are said to belong to the same [[renormalization group#Relevant and irrelevant operators and universality classes|universality class]].
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COMMENT: Rather than spin-glasses I'd like a concrete reference of two distinct systems that share the same universality class.  I'd also like a reference about the how-common-is-universality issue. -->
<!--RESPONSE: Discussion of water and CO2 satisfies first request. Can someone else satisfy the second part? -->
<!--COMMENT: Note that scale-invariance is not necessarily observed for power-law-''tailed'' equations.  For example, the [[Lévy distribution]] does not display the above property.-->
<!--RESPONSE: Any function that asymptotically follows a power law relation is scale invariant, by the definition given in this article. -->