  We propose that the finite-frequency susceptibility of matter near a class of
zero-temperature phase transition exhibits distinctive excitonic structure
similar to meson resonances. The specific case of a Landau level undergoing a
transition to antiferromagnetism is considered as a prototype. A physical
analogy is drawn between the behavior calculated for such systems by
epsilon-expansion techniques and the behavior of 1-dimensional spin chains
calculated by exact diagonalization. It is proposed that the correct low-energy
description of such critical points is a relativistic gauge theory in
spin-fractionalized coordinates, and conversely that the application of gauge
theories to solids amounts to perturbation theory about critical points.
