More than a century later, scientists are still proving Albert Einstein right.

The famed physicist’s revolutionary general theory of relativity debuted in 1915, positing that gravity can be understood as objects falling along the curvature of spacetime. One well-tested product of this is frame-dragging, in which a heavy, rotating object — like a black hole or the Earth itself — drags spacetime and anything in its orbit around with it. (Some researchers have compared the effect to a spoon spinning in honey, moving the honey and anything in the honey as it turns.)

Now researchers have managed to measure this phenomenon with more precision than ever before, confirming Einstein’s greatest theory once again in a study published Wednesday in Nature.

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“We improved by a factor more than 10 the measurement of frame-dragging — and in physics that’s a lot. This measurement helped us to put validity limits on alternative theories of gravity,” says Ignazio Ciufolini, lead author on the paper and physicist at the Sapienza University of Rome.

The study depended on data from the Laser Relativity Satellite 2 (LARES-2), a mission led by Ciufolini. Launched into orbit by the Italian Space Agency in 2022, LARES-2 is a follow-up to the earlier work of NASA’s two Laser Geodynamics Satellite (LAGEOS) missions. The LARES-2 and LAGEOS satellites are both mirror-covered orbs resembling galactic disco balls—scientists bounce laser beams off the mirrors to precisely determine the orbs’ exact orbital positions.

The satellites orbit thousands of kilometers above the Earth, well above altitudes where wisps of atmosphere can perturb their orbits. This, Ciufolini says, means that if our planet were a perfect sphere, frame-dragging would be the sole source for changes in their orbits. Alas, the gravitational pull of the Sun and the moon—which we know as tides—makes our world a little lopsided, complicating the orbital motions of satellites. But by combining the measurements of LARES-2 and LAGEOS, he and his team canceled out those effects to pin down frame-dragging with an uncertainty of one part in a thousand.

That’s an impressive feat on its own, says Daniel Holz, an astrophysicist at the University of Chicago who was not involved in the study. But it’s even more remarkable when compared to previous missions such as NASA’s $750-million Gravity Probe B, a spacecraft launched in 2004 that used onboard gyroscopes to measure frame-dragging with far less precision.

“This thing is 100 times better, and cost a lot less, because they’re treating the entire orbit of the satellite as a gyroscope—which is a very nice, elegant way to do it,” Holz says.

Discerning exactly how the lunar and solar tides influenced the orbit of LARES-2 was crucial for the best-yet frame-dragging measurement. While most tidal effects were easily canceled out with the combination of data from the two satellites, one lunisolar tide called K1 brought uncertainty into the equation.

The scientists had to track the K1 tide’s impacts on the satellites for three years to understand its effects. On the bright side, the team’s work placed new limits on K1’s strength—a finding that could help scientists studying earthquakes and our planet’s oceans.

By more precisely measuring this consequence of general relativity, this study also serves to constrain alternate theories that call Einstein’s conclusions into question. Because the team measured frame-dragging within the solar system, however, they were working in relatively weak gravitational fields, says Paul Lasky, an astrophysics professor at Monash University who was not involved in the study. Experiments done in stronger gravitational fields can provide more certainty about the validity of such alternate theories, he says.

“The work presented here is a more pristine measurement, albeit one that does not probe regimes of stronger gravity where any deviation from general relativity would be more likely to show up,” Lasky concludes.

For now, Holz says, the research adds “another feather in Einstein’s cap,” once again showing the ongoing success of general relativity.

“The result does not change relativity, and some theories that creative theorists were excited about that would maybe break relativity are ruled out. But that’s how progress happens,” he says. “Now we go onto the next one.”

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