What weighs more: a kilogram of feathers or a kilogram of lead? This old joke is actually no joke to physicists – since the time of Galileo, they have been trying to demonstrate that objects in a gravitational field experience the same acceleration. This is the universality of free fall, and it has now proven to be correct to less than one part per 1,000 trillion.

As reported in papers in Physical Review Letters and a special issue of Classical and Quantum Gravity, researchers used a special satellite to precisely test the so-called Weak Equivalence Principle, a cornerstone of Einstein’s general relativity. In a gravitational field, with no other forces acting upon them, two objects made of different materials and with different masses are expected to be subjected to the same acceleration.

To test this, scientists built the MICROSCOPE satellite. In it, they placed two concentric cylindrical test masses: one titanium and one platinum-rhodium alloy. These masses were kept motionless and monitored by electrostatic accelerometers. A violation of the equivalence principle will see the two masses having different accelerations.

Galileo is said to have demonstrated that by dropping cannon balls of different masses from the Leaning Tower of Pisa. Apollo 15 commander David Scott did it with a feather and a hammer on the Moon. This experiment brought that understanding to a precision of one part in 10¹⁵.

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

The latest data is 10 times better than the initial results published in 2017 and 100 times better than what can be done on Earth. The team suggests that there are ways to push the precision even further with a new satellite, maybe making it 100 times more precise.

“For at least one decade or maybe two, we don’t see any improvement with a space satellite experiment,” added Manuel Rodrigues, a scientist at the French aerospace lab ONERA and member of the MICROSCOPE team.

Testing the Weak Equivalence Principle is not just a way for scientists to prove that Einstein was wrong. General Relativity is our best theory of gravity so far, but it doesn’t work well with quantum mechanics. Investigating gravity at the highest precision might give us insight into how to reconcile the two.