Radiation Pressure on a Test Particle in General Relativity

Abstract

RADIATION pressure is believed to be the dominant force on gas in or near quasi-stellar objects (QSOs) under some conditions^1,2, and I have studied the acceleration of a particle to relativistic velocities by radiation pressure in an inverse-square (1/r²) radiation field with the gravitational force neglected². But although arguments have been given^3,4 that the line emission regions of QSOs are not close to Schwarzschild singularities, it is nevertheless possible that a small portion of the redshifts of QSOs may be gravitational, and it is also likely that highly ionized or stripped material lies close to the central source, where the effects of general relativity are larger. In Newtonian theory, both the radiation pressure and the gravitational force vary as r⁻², so that a particle of constant cross-section σ released at rest is either swallowed up or driven out, independent of its initial location. Here, I show that according to general relativity the result is different. For stationary particles the force of radiation pressure varies more rapidly than that of gravity with changing distance r to the central source. This makes stable equilibria possible. Collective effects, the effects of time variations in the strength of the radiation field and the changing opacity of the gas may well dominate the general relativistic effect discussed here. Thus, I am not proposing that the emission or absorption clouds of QSOs are necessarily stabilized in this way, but rather pointing out the, presence of this stabilizing effect, in contrast with a previous suggestion in the literature⁵. Although Fowler and Hoyle⁵ do not explicitly state that stabilization could not occur in the relativistic case, they imply this by favourably comparing the stabilization in their model with that of a point source in the Newtonian approximation.