Study presents the most precise test of general relativity’s weak equivalence principle

New and much better constraints for any future theory.


The Weak Equivalence Principle is a critical component of the theory of general relativity. The principle suggests that objects in a gravitational field fall similarly when no other forces are acting on them, even if they have different masses or compositions.

In a new study, a team of scientists tested the principle by measuring accelerations of free-falling objects in a satellite orbiting Earth. They found that the accelerations of pairs of objects differed by about one part in 1015 ruling out any violations of the Weak Equivalence Principle or deviations from the current understanding of general relativity at that level. 

The report describes the final results of the MICROSCOPE mission.

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

In their experiment, scientists measured the Eötvös ratio to extremely high precision, which relates to the accelerations of two free-falling objects. If there is a difference in the accelerations of the objects by more than one part in 1015, the experiment would measure it and detect this violation of the WEP.

To measure the Eötvös ratio, the scientists monitored the accelerations of platinum and titanium alloy test masses as they orbited Earth in the MICROSCOPE satellite. The experimental instrument used electrostatic forces to keep pairs of test masses in the same position relative to each other. It looked for potential differences in these forces, indicating differences in the objects’ accelerations.

Manuel Rodrigues, a scientist at the French aerospace lab ONERA and member of the MICROSCOPE team, said“a major challenge of the experiment was finding ways to test the instrument on Earth to make sure it would work as designed in space. The difficulty is that the instrument we launch cannot operate on the ground. So it’s a kind of blind test.”

The team’s work lays the path for satellite research to test the WEP even more precisely. Their research includes recommendations for enhancing the experimental setting, such as minimizing satellite coating crackles that hampered acceleration measurements and swapping out wires for contactless devices.

A satellite experiment that implements these upgrades should be able to measure potential violations of the WEP at the level of one part in 1017, the researchers say. But the MICROSCOPE results will likely remain the most precise constraints on the WEP for a while.

Journal Reference:

  1. Pierre Touboul et al. MICROSCOPE Mission: Final Results of the Test of the Equivalence Principle. Phys. Rev. Lett. 129. DOI: 10.1103/PhysRevLett.129.121102
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