A core principle of Einstein's general theory of relativity has just passed its most stringent test yet.
Using a specially designed satellite, an international team of scientists measured the accelerations of pairs of free-falling objects in Earth's orbit. Results based on five months' worth of data indicated the accelerations didn't differ by more than one part in 1015, ruling out any violations to the weak equivalence principle down to that scale.
The weak equivalence principle is relatively simple to observe, stating all objects accelerate identically in the same gravitational field when no other influences act upon them, regardless of their mass or composition.
It was perhaps most famously demonstrated to dramatic effect in 1971 when astronaut Dave Scott dropped a hammer and a feather simultaneously from the same height while standing on the Moon. Without air resistance to slow the feather, the two objects dropped to the Moon's surface at the same speed.
The new experiment, called MICROSCOPE and headed by the late physicist Pierre Touboul, was somewhat more rigorous than Scott's demonstration. It involved a satellite circling over Earth in orbit from April 25, 2016 until deactivation on October 18, 2018.
During this time, the team ran multiple experiments using masses suspended in free-fall, providing a total of five months of data. Two-thirds of this data involved pairs of test masses of different compositions, alloys of titanium and platinum. The remaining third involved a reference pair of masses of the same platinum composition.
The experimental equipment used electrostatic forces to keep the two test masses in the same position relative to one another. If there was any difference in the acceleration – a metric known as the Eötvös ratio – the equipment would register changes in the electrostatic forces holding the masses in place.
Early results released in 2017 were promising, finding no violation of the weak equivalence principle down to an Eötvös parameter of −1±9 x 10−15. However, the satellite was still operational, and producing data, which meant the work was not complete. The full dataset cements those early findings, constraining the Eötvös parameter to 1.1 x 10−15.
This is the tightest bound on the weak equivalence principle to date, and unlikely to be exceeded soon. It means that scientists can continue to rely on general relativity with more confidence than ever, and place new constraints on the intersection between general relativity and quantum mechanics, two regimes that operate under different rules.
"We have new and much better constraints for any future theory, because these theories must not violate the equivalence principle at this level," explains astronomer Gilles Métris of Côte d'Azur Observatory in France.
This is a spectacular result, given that the equipment, designed to work in the microgravity environment of Earth orbit, could not be tested before launching. Now that the MICROSCOPE project has been successfully completed, the team can use the results to design an even more stringent test.
These tests will help probe the limitations of general relativity, a framework that describes gravitation in physical space-time. On atomic and subatomic scales, however, 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 precisely where general relativity breaks down could be one way to do it.
We know now that that breakdown does not occur down to one part in 1015 for weak equivalence. Specific improvements that can be made to the next iteration of the satellite could probe it down to the level of one part in 1017. That is going to take some time to accomplish, however.
"For at least one decade or maybe two, we don't see any improvement with a space satellite experiment," says physics engineer Manuel Rodrigues of the French national aerospace research centre (ONERA).
But we suspect these results will be quite enough to be getting on with for the time being.