Undergrad Understanding the Derivation of the Ginzburg Criterion for the Ising Model

[thatboi]

For the Ising Model, the Ginzburg Criterion is, for $m_{0}$ the order parameter and $\delta m$ the fluctuations: $$\langle\delta m\left(x\right)\delta m\left(x^{\prime}\right)\rangle << m_{0}^{2}$$. I want to understand how to derive the left hand side of the inequality from $\langle M^{2} \rangle - \langle M \rangle ^{2}$ where $M = m_{0} + \delta_{m}$. Just from plugging in, I'm not sure how most of the terms cancel out, or what the fate of a term like $\langle \delta m\rangle \langle \delta m \rangle$ is.

 

[thatboi]

Ok, I think the best way of understanding that $\langle \delta m \rangle = 0$ is by calculating the probability associated with $\delta m$ i.e. calculating its Boltzmann weight and noticing that it is a Gaussian random variable with 0 mean. The other more physical way of arguing it is that we know $m_{0}$ is the value of the order parameter that minimizes the Helmholtz Free Energy so we expect $\langle M \rangle = m_{0}$.