We have seen in the first part of Statistical mechanics of phase transitions that when a phase transition occurs the free energy of the system is such that the response functions exhibit singularities, often in the form of divergences.
To make a concrete example (but of course all our statements are completely general) if we consider a magnetic system we can suppose to write its free energy density as:
is the "regular" part of the free energy (which does not significantly change near a critical point), while
is the "singular" one, which contains the non-analytic behaviour of the system near a critical point (i.e.
Widom's static scaling hypothesis consists in assuming that the singular part of the free energy is a generalized homogeneous function, i.e.:
in the appendix Homogeneous functions
we discuss the main properties of such functions.
Note that assuming that one thermodynamic potential is a generalized homogeneous function implies that all the other thermodynamic potentials are so.
The exponents and are not specified by the scaling hypothesis; however, we are shortly going to show that all the critical exponents of a system can be expressed in terms of and ; this also implies that if two critical exponents are known, we can write and in terms of them (since in general we will have a set of two independent equations in the two variables and ) and therefore determine all the critical exponents of the system. In other words, we just need to know two critical exponents to obtain all the others.
As shown in the appendix Homogeneous functions, an important property of generalized homogeneous functions is that with a proper choice of we can remove the dependence on one of their arguments; for example, if in our case we choose then:
is sometimes called gap exponent