A fundamental principle of general relativity has just passed its toughest test yet

A core principle of Einstein’s general theory of relativity has just passed its toughest test yet.

Using a specially developed satellite, an international team of scientists measured the accelerations of pairs of free-falling objects in orbit. Results based on five months of data showed that the accelerations did not differ by more than one part in 10^fifteenthereby precluding violations of the weak equivalence principle to this extent.

The weak equivalence principle is relatively easy to observe and states that all objects, regardless of their mass or composition, accelerate identically in the same gravitational field if they are not acted upon by other influences.

It was perhaps most famously demonstrated in 1971 when astronaut Dave Scott dropped a hammer and feather simultaneously from the same height while standing the moon. With no air resistance, the spring slows the two objects down fell to the lunar surface at the same speed.

The new experiment, called MICROSCOPE, led by the late physicist Pierre Touboul, was slightly more rigorous than Scott’s demonstration. It was a satellite that orbited the earth from April 25, 2016 until deactivation on October 18, 2018.

During this time, the team conducted several free-fall mass experiments, providing a total of five months of data. Two-thirds of this data concerned pairs of test masses of different compositions, alloys of titanium and platinum. The remaining third concerned a pair of reference masses of the same platinum composition.

The test equipment used electrostatic forces to keep the two test masses in the same position relative to each other. If there was a difference in acceleration – a metric known as the Eötvös ratio – the equipment would register changes in the electrostatic forces holding the masses in place.

First results published in 2017 were promising and found no violation of the weak equivalence principle up to an Eötvös parameter of −1 ± 9 x 10⁻¹⁵. However, the satellite was still operational and producing data, meaning the work was not yet complete. The full data set cements these early findings and limits the Eötvös parameter to 1.1 x 10⁻¹⁵.

This is the weakest limit of the weak equivalence principle to date, and is unlikely to be exceeded anytime soon. This means scientists can continue to rely more confidently than ever on general relativity and impose new constraints on the interface between general relativity and quantum mechanics, two regimes governed by different rules.

“We have new and much better constraints on any future theory, because these theories must not violate the equivalence principle at this level,” explains astronomer Gilles Métris of the Côte d’Azur Observatory in France.

This is a spectacular result considering that equipment designed to operate in the microgravity environment of Earth orbit could not be tested prior to launch. Now that the MICROSCOPE project has been successfully completed, the team can use the results to design an even more rigorous test.

These tests will help explore the limits of general relativity, a framework that describes gravity in physical spacetime. However, at the atomic and subatomic level, general relativity breaks down and quantum mechanics takes over. Scientists have been trying to resolve the differences between the two for quite some time. Figuring out where exactly general relativity breaks down could be one way to do it.

We now know that this breakdown is not to one part in 10^fifteen for weak equivalence. Specific improvements that can be made to the next iteration of the satellite could probe it down to the part in 10 level¹⁷. However, this will still take some time.

“For at least a decade or maybe two, we don’t see any improvement with a space satellite experiment,” says physics engineer Manuel Rodrigues of France’s national center for aerospace research (ONERA).

But we suspect these results will be enough to keep us going for now.

The team’s incredible work was published in Physical Verification Letters and a special edition of Classical and Quantum Gravity.