Laboratory tests of a high-precision laser interferometry readout for the GG experiment in space

On 25 April the Microscope satellite was successfully launched to test the weak equivalence principle, which is the founding pillar of General Relativity, to to 1 part in 10
¹⁵
(100-fold improvement). A possible violation will potentially be a major discovery and call for urgent checking. The GG (“Galileo Galilei”) satellite experiment can do that with one hundred times better precision, to 10
^-17
. To do that GG is required to measure the relative displacements of two concentric test cylinders with a precision of about half a picometer at the frequency of 1 Hz. A laser interferometry gauge has been designed for this purpose and is under development at INRIM with a required noise level of at 1 Hz. We report recent experimental results which demonstrate that the requirement can be met with a free running laser (no frequency lock needed) and consequent reduced complexity.