On the first test of the Weak Equivalence Principle in low Earth orbit

TL;DR

The Microscope experiment in low Earth orbit tested the Weak Equivalence Principle with unprecedented precision, revealing thermal noise effects and data handling challenges that impact the interpretation of its results.

Contribution

This work critically assesses the Microscope experiment's results, focusing on thermal noise, acceleration spikes, and data processing methods, providing insights for future tests.

Findings

Tested WEP to 1e-15 accuracy in orbit

Identified thermal noise and stray potentials affecting measurements

Highlighted data processing issues with artificial gap filling

Abstract

The Weak Equivalence Principle is the founding pillar of General Relativity and as such it should be verified as precisely as possible. The Microscope experiment tested it in low Earth orbit, finding that Pt and Ti test masses fall toward Earth with the same acceleration to about 1e-15, an improvement of about two orders of magnitude over ground tests. Space missions, even if small, are expensive and hard to replicate; yet, the essence of physics is repeatability. This work is an assessment of the Microscope results based on the laws of physics and knowledge from previous experiments, focusing on the limiting thermal noise and the treatment of acceleration outliers. Thermal noise reveals anomalies that we explain by stray sub-microVolt potentials caused by patch charges, giving rise to an unstable zero. The measurements were affected by numerous acceleration spikes occurring at the synodic frequencies relative to the Earth (the signal frequency) and the Sun, which we interpret as evidence of a thermal origin. In Microscope authors' analysis, the spikes were removed and the resulting gaps replaced with artificial data (up to 35, 40 per cent of the sessions data), which retain memory of the gaps and may simulate or cancel an effect (signal or systematic). An alternative approach based exclusively on real measured data would avoid any ambiguity. The lessons of Microscope are crucial to any futures improved mission.