  Strain correlation functions in two-dimensional isotropic elastic bodies are
shown both theoretically (using the general structure of isotropic tensor
fields) and numerically (using a glass-forming model system) to depend on the
coordinates of the field variable (position vector r in real space or
wavevector q in reciprocal space) and thus on the direction of the field vector
and the orientation of the coordinate system. Since the fluctuations of the
longitudinal and transverse components of the strain field in reciprocal space
are known in the long-wavelength limit from the equipartition theorem, all
components of the correlation function tensor field are imposed and no
additional physical assumptions are needed. An observed dependence on the field
vector direction thus cannot be used as an indication for anisotropy or for a
plastic rearrangement. This dependence is different for the associated strain
response field containing also information on the localized stress perturbation

Statistical Mechanics