Einstein's theory of general relativity just passed its most rigorous test yet

Weak Equivalence Principle

The weak equivalence principle is a critical component of the theory of general relativity. Simply put, the principle suggests that all objects should fall freely under gravity at the same rate, independent of their mass and composition, and when no other forces are acting on them.

The experimental equipment aboard the MICROSCOPE satellite consisted of a chamber in which two cylinders made of different materials (titanium and platinum alloys) were placed. The cylinders were suspended in free fall in Earth's gravitational field.

The experimental instrument used electrostatic forces to keep the two test masses in the same position relative to one another. Any deviations in acceleration- a metric known as the Eötvös ratio – would be seen as a change in the electrostatic forces, which would have signaled that the two cylinders were falling at slightly different rates and that the equivalence principle was violated.

Still, this signally did not occur. Instead, the results revealed that acceleration in pairs of objects in free fall differed by no more than one part in 10¹⁵, or 0.000000000000001, meaning they found no violations in principle larger than that.

Can it be proved wrong in the future?

This is the strictest test on the weak equivalence principle to date and is unlikely to be exceeded soon. It means that scientists can continue relying on general relativity more confidently than ever.

The theory of general relativity, published by Albert Einstein in 1916, describes how gravity works and relates to time and space. But because it does not account for the observations of quantum phenomena, researchers look for deviations from the theory at increasing levels of precision and in various situations. If they find a violation, it could reconcile the impasse regarding quantum theory and general relativity.

The team predicts that enhancing the experimental setup would allow measuring the weak equivalence principle potential violations at the level of one part in 1017. However, it won't be feasible for some time yet. For now, the MICROSCOPE experiment will remain the best test of the weak equivalence principle.

"For at least one decade or maybe two, we won't see any improvement with a space satellite experiment," Manuel Rodrigues, a MICROSCOPE team member and a scientist at ONERA, a French research institute specializing in aerospace, said in the same statement.

The team's work has been published in Physical Review Letters and a special Classical and Quantum Gravity issue.