Testing the Equivalence Principle with laser ranging to the Moon

Abstract

Equality of the rate of acceleration of Earth and Moon by the Sun is confirmed to 5×10 −13 using laser ranging data. This experiment not only matched those in the laboratory in showing composition-independence of gravity's acceleration, but it also has become the tightest constraint on general relativity's 1/c 2 order, non-linear structure which couples the Sun's accelerating gravity to the Earth's internal gravitational binding energy. But the ability of LLR data to test theory at even higher levels of precision is restrained by presence of systematic range residual signals from unmodeled physics. This suggests the need for a new priority and approach — redesigning the data-fitting hypotheses for mapping out the unmolded signal, whatever its source, rather than simply trying to fit for a pre-defined relativity signal. Initial work in this direction is described. The goal is model improvement and the ability to measure gravity theory signals closer to the inherent noise limits of the data, being in this case a factor 4 or 5 smaller than the present ‘realistic uncertainties’ allowed for incomplete modeling.