15th Capra Meeting on Radiation Reaction in General Relativity

Self-force as probe of internal structure

Eric Poisson

University of Guelph
[SLIDES]

The self-force acting on a (scalar or electric) charge held in place outside a massive body contains information about the body's composition, and can therefore be used as a probe of internal structure. We explore this theme by computing the (scalar or electromagnetic) self-force when the body is a spherical ball of perfect fluid in hydrostatic equilibrium, under the assumption that its rest-mass density and pressure are related by a polytropic equation of state. The body is strongly self-gravitating, and all computations are performed in the context of exact general relativity. The dependence on internal structure is best revealed by expanding the self-force in powers of $r_0^{-1}$, with $r_0$ denoting the radial position of the charge outside the body. To the leading order, the self-force scales as $r_0^{-3}$ and depends only on the square of the charge and the body's mass; the leading self-force is universal. The dependence on internal structure is seen at the next order, $r_0^{-5}$, through a multiplicative structure factor that depends on the equation of state. We compute this structure factor for relativistic polytropes, and show that for a fixed mass, it increases linearly with the body's radius in the case of the scalar self-force, and quadratically with the body's radius in the case of the electromagnetic self-force. In both cases we find that for a fixed mass and radius, the self-force is smaller if the body is more centrally dense, and larger if the mass density is more uniformly distributed.