Redshift and frequency comparison in Schwarzschild spacetime

TL;DR

This paper derives exact formulas for gravitational redshift between Earth-based and various orbiting observers in Schwarzschild spacetime, analyzing effects of different orbits, relativistic precession, and multipole moments.

Contribution

It provides new exact expressions for redshift in Schwarzschild spacetime for diverse observer trajectories, including elliptical orbits and effects of quadrupole moments.

Findings

Redshift depends on orbit type and relativistic effects.

Post-Newtonian expansion separates Doppler and gravitational contributions.

Quadrupole moments modify the Schwarzschild redshift results.

Abstract

We derive exact expressions for the relativistic redshift between an Earth-bound observer, that is meant to model a standard clock on the Earth's surface, and various (geodesic) observers in the Schwarzschild spacetime. We assume that the observers exchange radial light signals to compare the frequencies of standard clocks, which they transport along their respective worldlines. We calculate the redshift between an Earth-bound clock and static observers, observers in radial free fall, on circular geodesics, and on arbitrary bound quasi-elliptical orbits. For the latter case, we consider as examples an almost circular orbit, the Schwarzschild analog of Galileo satellites 5 orbits with a moderate eccentricity, and a highly elliptical orbit as special examples. Furthermore, we also use orbits close to a Schwarzschild black hole to highlight the influence of the relativistic perigee precession on the redshift signal. Calculating a post-Newtonian expansion of our results, the total redshift is decomposed into its special relativistic Doppler parts and the gravitational part due to the theory of General Relativity. To investigate the impact of higher order relativistic multipole moments on the gravitational redshift, we consider static observers in a general Weyl spacetime. We give a general expression for the mutual redshift of their standard clocks and consider in particular the effect of the relativistic quadrupole as a modification of the Schwarzschild result.