Editing Critical phenomena (section)

{{Short description|Physics associated with critical points}}
In [[physics]], '''critical phenomena''' is the collective name associated with the
physics of [[critical point (thermodynamics)|critical point]]s. Most of them stem from the divergence of the 
[[Correlation function (statistical mechanics)|correlation length]], but also the dynamics slows down. Critical phenomena include [[scaling (geometry)|scaling]] relations among different quantities, [[power-law]] divergences of some quantities (such as the [[magnetic susceptibility]] in the [[ferromagnetism|ferromagnetic phase transition]]) described by [[critical exponent]]s, [[universality (dynamical systems)|universality]], [[fractal]] behaviour, and [[ergodicity]] breaking. Critical phenomena take place in [[second order phase transition]]s, although not exclusively.

The critical behavior is usually different from the [[Mean field theory|mean-field approximation]] which is valid away from the [[phase transition]], since the latter neglects correlations, which become increasingly important as the system approaches the critical point where the correlation length diverges. Many properties of the critical behavior of a system can be derived in the framework of the [[renormalization group]].

In order to explain the physical origin of these phenomena, we shall use the [[Ising model]] as a pedagogical example.