Comment on “Effective Refractive Index Tensor for Weak-Field Gravity” by P. Boonserm, C. Cattoen, T. Faber, M. Visser and S. Weinfurtner

P. Boonserm et. al have tried to describe some anisotropic optical effects in the gravitation field in frame of the general relativity and by introducing the so-called “effective refractive index tensor” similarly to anisotropic crystals. We suppose that this approach is inconsequent because of the following: if one develops this claim, it would be necessary to attribute the gravitation field to the properties of matter, including its electric properties, the refractive index is not a tensorial quantity and the analogy to the situation in anisotropic crystals does not lead to artificial “effective refractive index tensor” but to naturally introduced tensor of optical-frequency dielectric impermeability of matter.