More than a hundred years after its introduction, Albert Einstein’s theories continue to be validated by scientists.
Einstein’s groundbreaking general theory of relativity, presented in 1915, suggested that gravity could be seen as objects moving along the warp of spacetime. One of the well-documented effects of this theory is frame-dragging, where a massive, spinning object—such as a black hole or Earth—twists spacetime and anything orbiting it. Some scientists liken this to a spoon stirring honey, causing the honey and anything in it to move.
Researchers have now achieved unprecedented precision in measuring this effect, reaffirming Einstein’s landmark theory in a study published on 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 — and this measurement helped us to put validity limits on alternative theories of gravity,” says Ignazio Ciufolini, lead author of the paper and physics professor at the Sapienza University of Rome.
The research utilized data from the Laser Relativity Satellite 2 (LARES-2), a mission under Ciufolini’s leadership. The Italian Space Agency launched LARES-2 into orbit in 2022, building on the work of NASA’s two Laser Geodynamics Satellite (LAGEOS) missions. The LARES-2 and LAGEOS satellites both feature mirror-covered surfaces, resembling cosmic disco balls, which allow scientists to bounce laser beams off them to accurately track their orbital positions.
Orbiting thousands of kilometers above Earth, these satellites travel well beyond the atmospheric effects that could disturb their paths. According to Ciufolini, if Earth were a perfect sphere, frame-dragging would solely influence their orbits. However, the gravitational forces exerted by the Sun and the Moon, known as tides, create asymmetrical forces, complicating satellite trajectories. By merging data from LARES-2 and LAGEOS, Ciufolini and his colleagues eliminated these disturbances, narrowing down frame-dragging to a precision of one part in a thousand.
This achievement stands out on its own, remarks Daniel Holz, an astrophysicist at the University of Chicago who was not part of the study. It is even more impressive when compared to past missions, like NASA’s $750-million Gravity Probe B from 2004, which used gyroscopes to measure frame-dragging with less accuracy.
“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.
Understanding how lunar and solar tides affected LARES-2’s orbit was essential for achieving the most precise frame-dragging measurement to date. While most tidal impacts were neutralized by combining data from both satellites, the K1 lunisolar tide introduced some uncertainty.
Researchers had to monitor the effects of the K1 tide on the satellites for three years. However, the team’s efforts to gauge K1’s influence could aid scientists in studying earthquakes and ocean dynamics.
By more accurately measuring this aspect of general relativity, the study also helps limit alternative theories challenging Einstein’s conclusions. As the team worked within the solar system’s relatively weak gravitational fields, stronger fields could provide more certainty about these alternate theories, explains Paul Lasky, an astrophysics professor at Monash University.
“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 the moment, Holz notes, the findings add “another feather in Einstein’s cap,” demonstrating 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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