Lunar Laser Ranging Tests of the Equivalence Principle

TL;DR

Lunar Laser Ranging experiments provide highly precise measurements of the Earth-Moon distance, enabling stringent tests of the Equivalence Principle and contributing to our understanding of relativistic gravity.

Contribution

This paper reviews and analyzes LLR data and techniques, demonstrating the robustness of current tests of the Equivalence Principle and discussing future improvements.

Findings

Current LLR results constrain violations of the EP to within 1.3 x 10^{-13}

Multiple data analysis approaches confirm the robustness of the tests

Future data and model improvements will enhance relativistic gravity research

Abstract

The Lunar Laser Ranging (LLR) experiment provides precise observations of the lunar orbit that contribute to a wide range of science investigations. In particular, time series of highly accurate measurements of the distance between the Earth and Moon provide unique information that determine whether, in accordance with the Equivalence Principle (EP), both of these celestial bodies are falling towards the Sun at the same rate, despite their different masses, compositions, and gravitational self-energies. Analyses of precise laser ranges to the Moon continue to provide increasingly stringent limits on any violation of the EP. Current LLR solutions give (-0.8 +/- 1.3) x 10^{-13} for any possible inequality in the ratios of the gravitational and inertial masses for the Earth and Moon, (m_G/m_I)_E - (m_G/m_I)_M. Such an accurate result allows other tests of gravitational theories. Focusing on the tests of the EP, we discuss the existing data and data analysis techniques. The robustness of the LLR solutions is demonstrated with several different approaches to solutions. Additional high accuracy ranges and improvements in the LLR data analysis model will further advance the research of relativistic gravity in the solar system, and will continue to provide highly accurate tests of the Equivalence Principle.