Gravity does not discriminate. An experiment in orbit has confirmed, with a hundred times greater precision than previous efforts, that everything falls in the same way under the influence of gravity.
The finding is the strictest test yet of the equivalence principle, a key tenet of Einstein’s theory of general relativity. The principle holds to about one part in a billion trillionresearchers report on September 14 in Physical Review Letters.
The idea that gravity affects all things equally may not seem surprising. But the slightest hint to the contrary could help explain how general relativity, the fundamental theory of gravity, fits in with the standard model of particle physics, the theoretical framework that describes all fundamental particles of matter. General relativity is a classical theory that sees the universe as smooth and continuous, while the standard model is a quantum theory involving granular particles of matter and energy. Combining them into a single theory of everything has been an unfulfilled dream of scientists dating back to Einstein (Serial number: 01/12/22).
“The equivalence principle is the most important cornerstone of Einstein’s general theory of relativity,” says Sabine Hossenfelder, a physicist at the Frankfurt Institute for Advanced Study in Germany who was not involved in the study. “We know [it] eventually it has to be altered because in its current form it cannot take quantum effects into account.”
To help search for possible disturbances, the MICROSCOPE experiment tracked the movement of nested metal cylinders (a 300-gram titanium outer cylinder and a 402-gram platinum inner cylinder) as they orbited Earth in near-perfect free fall. Any difference in the effect of gravity on the respective cylinders would cause them to move relative to each other. Small electrical forces applied to realign the cylinders would have revealed a possible violation of the equivalence principle.
From April 2016 to October 2018, the cylinders were shielded inside a satellite that protected them from the onslaught of solar winds, the miniscule pressure exerted by sunlight and residual atmosphere at an orbital altitude of just over 700 kilometers. height.
By conducting the experiment in orbit, the researchers were able to compare the free fall of two different materials over long periods without the confounding effects of vibrations or nearby objects that might exert gravitational forces, says Manuel Rodrigues, a member of the MICROSCOPE team and physicist at the French aerospace laboratory ONERA in Palaiseau. “One of the lessons learned by MICROSCOPE is … that space is the best way to get a significant improvement in accuracy for this type of test.”
During its two-and-a-half-year mission, MICROSCOPE found no signs of cracks in the equivalence principle, the new study reports. The finding is based on an earlier interim report of the experiment. who found the samebut with less precision (Serial number: 4/12/17).
Some physicists suspect that the limits of the equivalence principle may never show up in experiments, and that Einstein will always be proven right.
Even 100 times greater precision from a follow-up MICROSCOPE 2 mission, tentatively planned for the 2030s, is unlikely to reveal a breakdown of the equivalence principle, says Clifford Will, a physicist at the University of Florida in Gainesville who is unaffiliated. to the experiment. “It’s really still this basic idea that Einstein taught,” he says. What we see as the force of gravity is actually the curvature of space-time. “Any body just moves along Earth’s path in space-time,” whether it’s made of dense platinum, lighter titanium, or any other material.
But even if physicists never prove Einstein wrong, Hossenfelder says, experiments like MICROSCOPE are still important. “These tests are not just about the equivalence principle,” she says. “Implicitly looking for all other kinds of deviations, new forces, etc.”, which are not part of general relativity. “So it really is a multi-purpose, high-precision measurement.”
Now that the mission is complete, the MICROSCOPE satellite will slowly move out of orbit. “It’s hard to bet where it will fall in 25 years,” says Rodrigues. Along with a reference set of platinum cylinders on board, “it is [a] couple of million euros [in] platinum.” Where that precious platinum metal will land is anyone’s guess, but the gravity pulling it down will pull on the titanium just as strongly, at least one part in a thousand trillion.