Back Reflection of Scalar and Vector Waves in Gravitational Fields

The back reflection of mass-zero scalar or vector waves passing through a gravitational field is calculated. In the Newtonian (equivalence-principle) approximation to Einstein's or Brans and Dicke's gravitational theory, no back reflection occurs. The non-Lorentz part of the spatial metric components ($g_{\mathrm{xx}}, g_{\mathrm{yy}}, g_{\mathrm{zz}}$) produce the back reflection of the waves. For optimum wavelengths, the reflection coefficient of a wave passing a mass $M$ at distance $d$ is of order $\frac{\mathrm{GM}}{c^{2}d}$.