Scientists Test How Objects Fall in Space

In the most precise experiment to date, researchers confirm a principle of Einstein’s Theory of General Relativity

A gold satellite orbits Earth
A rendering of the MICROSCOPE satellite in space.  CNES - Juillet 2012 / Illust. D. Ducros

As the story goes, Italian astronomer Galileo Galilei ascended the Leaning Tower of Pisa in the 16th century to drop two spheres of different masses at the same time—proving that they would land on the ground simultaneously.

While the famed demonstration might not have taken place (many historians believe it was just a thought experiment), it illustrates one of Albert Einstein’s big ideas: the weak equivalence principle. Essentially, the idea states that two falling objects should accelerate at the same rate, regardless of their mass or what material they’re made from.

On Earth, of course, air resistance and other factors might change how an object falls. So, scientists conducted an experiment testing the weak equivalence principle aboard a satellite in space.

The new research, published last week in the journal Physical Review Letters, provides the most precise confirmation of Einstein’s theory, ever, reports Science’s Jacklin Kwan.

In 2016, the researchers sent masses made of titanium and platinum alloys to space aboard the French satellite MICROSCOPE, per Space.com’s Robert Lea. As the satellite orbited Earth, the objects inside were in a state of constant free fall, but static electricity worked to hold them still. An electrical sensor measured the amount of voltage needed to keep each mass in place, which revealed the masses’ accelerations, writes Science.

By the weak equivalence principle, the masses should have experienced the same amount of acceleration. And as far as the sensor could tell, they did—any differences in their accelerations would have had to have been smaller than one part in 10^15, per Space.com.

“The most thrilling part during the project was to develop an instrument and a mission that nobody has done before at such a level of accuracy,” Manuel Rodrigues, co-leader of the mission and a research engineer at the French aerospace lab ONERA, tells CNET’s Monisha Ravisetti. These results improve upon 2017 observations from MICROSCOPE by a factor of ten, he says to Science.

The weak equivalence principle is part of Einstein’s theory of general relativity, which describes gravity as a warping of the fabric of spacetime. Eugene Lim, a theoretical physicist at King’s College London in England who did not contribute to the research, tells Science that precise experiments like the one in the new paper could help physicists narrow down future gravitational theories. Many physicists believe general relativity is incomplete, he says, in part because it’s incompatible with the current theory of quantum mechanics, per Science.

The new knowledge from MICROSCOPE could help tighten up the theory of general relativity, experts say. “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 in France and a member of the MICROSCOPE team, said in a press release.

Future improvements in the experiment, which might be years away, could be 100 times as sensitive as MICROSCOPE and could potentially detect violations of the weak equivalence principle too subtle for this recent experiment to have observed, per Space.com.