Analysis of the PPN Two-Body Problem Using Non-Osculating Orbital Elements

Abstract: The parameterised post-Newtonian (PPN) formalism is a weak-field slow-motion approximation for both GR and some of its generalisations. It permits various parameterisations of the motion, among which are the Lagrange-type and Gauss-type orbital equations. Often, these equations are developed under the Lagrange constraint, which makes the evolving orbital elements parameterise instantaneous conics tangent to the orbit. Arbitrary mathematically, this choice of a constraint is convenient under perturbations dependent only on positions. Under perturbations dependent also on velocities (like relativistic corrections) the Lagrange constraint unnecessarily complicates solutions that can be simplified by introducing a freedom in the orbit parameterisation, which is analogous to the gauge freedom in electrodynamics and gauge theories. Geometrically, this freedom is the freedom of nonosculation, i.e. of the degree to which the instantaneous conics are permitted to be non-tangent to the actual orbit. Under the same perturbation, all solutions with different degree of nonosculation look mathematically different, though describe the same physical orbit. While non-intuitive, the modeling of an orbit with a sequence of nontangent instantaneous conics can at times simplify calculations. The appropriately generalised ("gauge-generalised") Lagrange-type equations, and their applications, appeared in the literature hitherto. We in this paper derive the gauge-generalised Gauss-type equations and apply them to the PPN two-body problem. Fixing the gauge freedom in three different ways (i.e. modeling an orbit with non-osculating elements of three different types) we find three parameterisations of the PPN two-body dynamics. These parameterisations render orbits with either a fixed non-osculating semimajor axis, or a fixed non-osculating eccentricity, or a fixed non-osculating argument of periastron.