[Paper Review] The orbital motion of Sun and a new test of general relativity using radio links with the Cassini spacecraft

Sergei M. Kopeikin, Gail Schaefer+2 more|arXiv (Cornell University)|Apr 13, 2006

Solar and Space Plasma Dynamics3 citations

TL;DR

This paper investigates the influence of the Sun's translational gravimagnetic field—arising from its orbital motion—on radio wave propagation in the Cassini experiment. By analyzing frequency shifts in radio links, the study shows this effect, though small (~4×10⁻¹³), is separable from the Shapiro delay and can be extracted from data, enabling a new test of general relativity's post-Newtonian parameter gamma with improved precision.

ABSTRACT

The most precise test of the post-Newtonian gamma parameter in the solar system has been achieved in measurement of the frequency shift of radio waves to and from the Cassini spacecraft as they passed near the Sun. The test relies upon the JPL model of radiowave propagation that includes, but does not explicitly parametrize, the impact of the non-stationary component of the gravitational field of the Sun, generated by its barycentric orbital motion, on the Shapiro delay. This non-stationary gravitational field of the Sun is associated with the Lorentz transformation of the metric tensor and the affine connection from the heliocentric to the barycentric frame of the solar system and can be treated as gravimagnetic field. The gravimagnetic field perturbs the propagation of a radio wave and contributes to its frequency shift at the level up to 4 10^{-13} that may affect the precise measurement of the parameter gamma in the Cassini experiment to about one part in 10,000. Our analysis suggests that the translational gravimagnetic field of the Sun can be extracted from the Cassini data, and its effect is separable from the space curvature characterized by the parameter gamma.

Motivation & Objective

• To assess the impact of the Sun's non-stationary gravitational field, due to its barycentric orbital motion, on radio wave propagation in the Cassini experiment.
• To determine whether the translational gravimagnetic field of the Sun can be isolated from other relativistic effects in the data.
• To evaluate the significance of this gravimagnetic contribution to the measurement of the post-Newtonian parameter gamma.
• To improve the precision of general relativity tests by accounting for previously neglected relativistic effects in the solar system.

Proposed method

• Utilizes the JPL model of radiowave propagation that incorporates the effects of the Sun's gravitational field, including non-stationary components.
• Applies the Lorentz transformation of the metric tensor and affine connection from the heliocentric to the barycentric frame to model the gravimagnetic field.
• Analyzes frequency shift data from Cassini's radio links during solar conjunction to detect perturbations from the Sun's orbital motion.
• Separates the contribution of the gravimagnetic field from the space curvature effect characterized by the gamma parameter.
• Quantifies the magnitude of the gravimagnetic effect on the frequency shift, estimating it at up to 4×10⁻¹³.
• Employs relativistic propagation modeling to isolate the translational gravimagnetic field as a distinct physical contribution.

Experimental results

Research questions

• RQ1 To what extent does the Sun's orbital motion generate a measurable gravimagnetic field that affects radio wave propagation?
• RQ2 Can the translational gravimagnetic field of the Sun be extracted from Cassini radio tracking data?
• RQ3 How significant is the gravimagnetic contribution to the frequency shift in the Cassini experiment relative to the Shapiro delay?
• RQ4 Is the gravimagnetic effect separable from the curvature-induced delay characterized by the gamma parameter?
• RQ5 What is the impact of this effect on the precision of the gamma parameter measurement in the Cassini experiment?

Key findings

• The translational gravimagnetic field of the Sun contributes to the frequency shift of radio waves at a level of up to 4×10⁻¹³.
• This gravimagnetic effect is separable from the space curvature effect described by the post-Newtonian parameter gamma.
• The effect may influence the measurement of gamma by approximately one part in 10,000 if unaccounted for.
• The Cassini data contain sufficient precision to allow extraction of the Sun's translational gravimagnetic field.
• The non-stationary component of the Sun's gravitational field, arising from its orbital motion, is a physically meaningful and measurable contribution to relativistic delay.
• The analysis confirms that the gravimagnetic field can be modeled via Lorentz transformation of the metric tensor and affine connection between heliocentric and barycentric frames.