Radiation drag in Brans-Dicke theory of gravity

The orbital motion of a test particle near a non-rotating star with a high and uniform luminosity is explored in the context of the Brans-Dicke gravity. Just as in the Einstein gravity, from the equation of motion for the test particle, we found that a “suspension surface” exists, where the test particle comes to rest. Unlike in the Einstein gravity, the radial position of the suspension surface depends on the BD parameter ω as well as the luminosity of the star, but is independent of the initial position and velocity of the particle. As the BD parameter ω gets larger, the position of the “suspension surface” in the BD gravity approaches that in the Einstein gravity as it should. However, when the value of the parameter ω is small, i.e., ω ~ 1, the suspension’s position in the BD gravity deviates significantly from that in the Einstein gravity. Among others, however, the most remarkable lesson we learned in the present study is that the true and detailed nature of an alternative theory of gravity is not available in the complete absence of a test particle with motion.