The MICROSCOPE mission presents the most precise test of the weak equivalence principle of general relativity

In new studies published in Physical Verification Letters and a special edition of Classical and Quantum Gravity On September 14, a team of researchers presents the most precise test yet of the weak equivalence principle, a key component of general relativity. The report describes the final results of the MICROSCOPE mission, which tested the principle by measuring the accelerations of free-falling objects in an Earth-orbiting satellite. The team found that the accelerations of object pairs differed by no more than about one part in 10^fifteen Exclusion of violations of the weak equivalence principle or deviations from the current understanding of general relativity at this level.

“We have new and much better constraints for any future theory, because these theories must not violate the equivalence principle at this level,” says Gilles Métris, scientist at the Côte d’Azur Observatory and member of the MICROSCOPE team.

The General Theory of Relativity, published by Albert Einstein in 1915, describes how gravity works and relates to time and space. But because it doesn’t take into account the observations of quantum phenomena, researchers are looking for deviations from the theory with increasing accuracy and in different situations. Such violations would suggest new interactions or forces that could unify relativity with quantum physics. Testing the weak equivalence principle (WEP) is one way to look for possible extensions of general relativity.

According to WEP, objects in a gravitational field fall in the same way when no other forces are acting on them, even if they have different masses or compositions. To test the principle, the MICROSCOPE team designed their experiment to measure the Eötvös ratio – which relates the accelerations of two free-falling objects – with extremely high precision. When the acceleration of one object differs from that of the other by more than about one part in 10^fifteenthe experiment would measure these and detect this violation of the WEP.

To measure the Eötvös ratio, researchers monitored the accelerations of platinum and titanium alloy test masses as they orbited the Earth in the MICROSCOPE satellite. The experimental instrument used electrostatics

forces to keep pairs of test masses in the same position relative to each other, and looked for possible differences in these forces that would indicate differences in the objects’ accelerations.

A major challenge of the experiment was finding ways to test the instrument on Earth to ensure it would work as designed in space. “The difficulty is that the instrument we launched cannot be operated on the ground,” says Manuel Rodrigues, scientist at the French aerospace laboratory ONERA and member of the MICROSCOPE team. “So it’s kind of a blind test.”

The MICROSCOPE mission tested the weak equivalence principle by measuring the accelerations of free-falling objects in an Earth-orbiting satellite. The researchers found that the accelerations of object pairs differed by no more than about one part in 10^fifteen, thereby ruling out violations of the weak equivalence principle or deviations from the current understanding of general relativity at this level. The final results will be published in Physical Verification Letters and a special edition of Classical and Quantum Gravity on September 14th. Source: (c) CNES 2015

When the instrument was ready, the team launched it in 2016. They published preliminary results in 2017, but they continued to analyze the data, accounting for perturbations and systematic uncertainties after the mission ended in 2018. They finally found no violation of the WEP, which puts the principle’s strictest restrictions yet.

The team’s work paves the way for even more precise testing of the WEP with satellite experiments. Their analysis includes ways to improve the experimental setup, such as reducing crackling in the satellite coating that affected acceleration measurements, and replacing wires in the setup with contactless devices. A satellite experiment implementing these upgrades should be able to measure potential violations of the WEP at the part in 10 level¹⁷, the researchers say. But the MICROSCOPE results will likely remain the most accurate constraint for WEP for a while.

“We haven’t seen improvement with a space satellite experiment for at least a decade or maybe two,” says Rodrigues.