3.4 Refractive Index Modeling Given that velocity-of-light effects in a spacetime-altered region, as viewed from an external frame, are governed by Eq. (7), it is seen that the effect of spacetime alteration on light propagation can be expressed in terms of an optical refractive index n, defined by 00 , e L g c v n n g − = = (8) where n is an effective refractive index of the (spacetime- altered) vacuum. This widely-known result has resulted in the development of refractive-index models for GR [14-17] that have found application in problems such as gravitational lensing [18]. The estimated electric or magnetic field strengths required to generate a given refractive index change are given by standard GR theory (the Levi-Civita Effect) and can be found in In engineering terms, the velocity of light c is given by the expression 0 1 c µ where and are the magnetic permeability and dielectric permittivity of undistorted vacuum space ( µ 0 = 4 π x 10 -7 H/m and ε 0 = 8.854 x Fm. The generation of an effective refractive index 00 1 n g g = by technological means can from an engineering viewpoint be interpreted as manipulation of the vacuum parameters and. In GR theory such variations in µ 0 , ε 0 , and hence the velocity of light, c, can be treated in terms of a “TH εµ” formalism used in comparative studies of gravitational theories As discussed in Section 4.4 below, a number of striking effects can be anticipated in certain engineered spacetime re- gions. 3.5 Effective Mass in Spacetime-Altered Regions In a spacetime-altered region E = mc 2 still holds in terms of local (proper coordinate) measurements, but now energy E and the velocity of light c take on altered values as observed from an exterior (undistorted) spacetime region. Reference to the definitions for E and c in Table 1 permits one to define an effective mass as seen from the exterior undistorted region as therefore taking on the value 00 m m g g → − providing a sixth entry for Table 1. Depending on the values of g 00 and g 11 the effective mass maybe seen from the viewpoint of an observer in an undistorted spacetime region to have either increased or decreased.