Space test of the Equivalence Principle: first results of the MICROSCOPE mission

TL;DR

The MICROSCOPE mission tested the Weak Equivalence Principle in space with unprecedented precision, finding no violation and setting new limits on possible deviations from Einstein's theory of gravity.

Contribution

First space-based test of the WEP achieving a sensitivity of 10^{-15}, improving ground tests by two orders of magnitude.

Findings

No detected violation of WEP within measurement uncertainties.

Set a new upper limit on WEP violation at approximately 1.3×10^{-14}.

Demonstrated the feasibility of high-precision space experiments for fundamental physics.

Abstract

The Weak Equivalence Principle (WEP), stating that two bodies of different compositions and/or mass fall at the same rate in a gravitational field (universality of free fall), is at the very foundation of General Relativity. The MICROSCOPE mission aims to test its validity to a precision of $10^{-15}$, two orders of magnitude better than current on-ground tests, by using two masses of different compositions (titanium and platinum alloys) on a quasi-circular trajectory around the Earth. This is realised by measuring the accelerations inferred from the forces required to maintain the two masses exactly in the same orbit. Any significant difference between the measured accelerations, occurring at a defined frequency, would correspond to the detection of a violation of the WEP, or to the discovery of a tiny new type of force added to gravity. MICROSCOPE's first results show no hint for such a difference, expressed in terms of E\"otv\"os parameter $\delta(Ti,Pt)=[-1\pm{}9{\rm (stat)}\pm{}9{\rm (syst)}] \times{}10^{-15}$ (both 1$\sigma$ uncertainties) for a titanium and platinum pair of materials. This result was obtained on a session with 120 orbital revolutions representing 7\% of the current available data acquired during the whole mission. The quadratic combination of 1$\sigma$ uncertainties leads to a current limit on $\delta$ of about $1.3\times{}10^{-14}$.