Gravity does not distinguish. An in-orbit experiment confirmed, with a hundred times greater accuracy than previous efforts, that everything similarly falls under the influence of gravity.
This discovery is the most rigorous test yet of the equivalence principle, a fundamental principle of Einstein’s theory of general relativity. The principle applies to about one thousandth of a trillionresearchers report September 14 in physical review messages.
The idea that gravity affects all things equally may not seem surprising. But the slightest hint could otherwise help explain how general relativity, the foundational theory of gravity, is intertwined with the Standard Model of particle physics, the theoretical framework that describes all the 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 that includes grainy bits of matter and energy. Combined into one theory of everything An unfulfilled dream for scientists back to Einstein (SN: 1/12/22).
“The equivalence principle is the most important cornerstone of Einstein’s general theory of relativity,” says Sabine Hosenfelder, a physicist at the Frankfurt Institute for Advanced Studies in Germany who was not involved in the study. “We know [it] Ultimately it must be changed because it cannot in its current form take into account quantitative effects.”
To help search for potential modifications, the MICROSCOPE experiment tracked the motion of the two nested metal cylinders — an outer cylinder of 300 grams of titanium and an inner cylinder of 402 grams of platinum — as they orbited the Earth in near-perfect free fall. Any difference in the effect of gravity on the cylinders in question will cause them to move relative to each other. The small electrical forces applied to bring the cylinders back into alignment would have revealed a possible violation of the equivalence principle.
From April 2016 to October 2018, the cylinders were shielded inside a satellite to protect them from the solar wind, the tiny pressure exerted by sunlight and the atmosphere remaining at an orbital altitude of just over 700 km.
By conducting the experiment in orbit, researchers can compare the free fall of different materials for long periods without the confounding effects of vibrations or nearby objects that can exert gravitational forces, says Manuel Rodriguez, a MICROSCOPE team member and physicist with the French. Aerospace Laboratory ONERA in Palaiseau. “One of the lessons that MICROSCOPE has learned is…that space is the best way to get a significant improvement in the accuracy of this type of test.”
During its two-and-a-half-year mission, MICROSCOPE found no sign of cracks in the equivalence principle, the new study reports. The result is based on a previous interim report from the experiment that found the samebut with less precision (SN: 12/4/17).
Some physicists suspect that the limits of the equivalence principle may never appear in experiments, and that Einstein will always prove him right.
Even a hundred times greater accuracy than the MICROSCOPE 2 follow-up mission, initially planned for the 2030s, says Clifford Weil, a physicist at the University of Florida in Gainesville, who is not affiliated with the experiment, is unlikely to reveal a breakdown of the equivalence principle. “It’s still the basic idea that Einstein taught,” he says. What we see as the force of gravity is actually the curvature of spacetime. “Any object that simply moves along the path in Earth’s spacetime,” whether it is made of dense platinum, light titanium, or any other material.
Even if physicists never prove Einstein wrong, Hosenfelder says, experiments like MICROSCOPE still matter. “These tests are not just about the equivalence principle,” she says. “They are implicitly looking for all kinds of other deviations, new forces and so on,” which is not part of general relativity. “It is truly a multi-purpose, high-accuracy measurement.”
Now that the mission is complete, the MICROSCOPE satellite will slowly be de-orbited. “It’s hard to bet where it will fall in 25 years,” Rodriguez says. Along with a reference set of platinum cylinders on board, “It [a] a few million euros [in] platinum; Where this precious platinum metal will land is anyone’s guess, but the gravity pulling it down will attract titanium with the same force, to at least one part in a thousand trillion.