(ORDO NEWS) — “What we perceive as the force of gravity is actually a curvature of space-time. Any body simply moves along a trajectory in the space time of the Earth, regardless of whether it is made of dense platinum, lighter titanium, or any other material”
Experiment is the most accurate confirmation of the key postulate of the general theory of relativity. Gravity makes no difference. An experiment in orbit confirmed, with an accuracy hundreds of times greater than previous attempts, that under the influence of gravity everything falls the same way.
This result is the most rigorous test of the equivalence principle, a key postulate of Einstein’s general theory of relativity. This principle works to about one part in a thousand trillion, the researchers report Sept. 14 in the journal Physical Review Letters.
The idea that gravity affects all things in the same way may not seem surprising. But the slightest hint to the contrary can help explain how the general theory of relativity, which underlies the theory of gravity, is consistent 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 particles of matter and energy. Combining them into a unified theory of everything has been a pipe dream of scientists since the time of Einstein.
“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 that it will eventually have to be changed because in its current form it cannot account for quantum effects.”
To look for possible changes, the MICROSCOPE experiment tracked nested metal cylinders a 300-gram titanium outer cylinder and a 402-gram platinum inner cylinder as they orbited the Earth in near-perfect free fall.
Any difference in the effect of gravity on the respective cylinders caused them to move relative to each other. Small electrical forces applied to push the cylinders back into place would reveal a potential violation of the equivalence principle.
From April 2016 to October 2018, the cylinders were inside the satellite, which protected them from the riot of solar winds, the meager pressure of sunlight, and the residual atmosphere at an orbital altitude of just over 700 kilometers.
By conducting the experiment in orbit, the researchers were able to compare the free fall of two different materials over long periods of time without the interference of vibrations or nearby objects that could exert a gravitational pull, says Manuel Rodriguez, a member of the MICROSCOPE team and a physicist at the French aerospace laboratory ONERA in Palaiseau.
“One of the lessons learned from MICROSCOPE is… that space is the best way to achieve a significant improvement in accuracy for this kind of testing.”
In its two and a half years of operation, MICROSCOPE found no evidence of cracks in the equivalence principle, according to a new study. This conclusion is based on a previous interim report of an experiment in which the same thing was found, but with less accuracy.
Some physicists suspect that the limits of the equivalence principle may never be found in experiments, and that Einstein will forever prove himself right.
Even a 100-fold increase in the accuracy of the follow-up MICROSCOPE 2 mission, tentatively scheduled for the 2030s, is unlikely to reveal a violation of the equivalence principle, says Clifford Will, a physicist at the University of Florida at Gainesville who is not involved in the experiment.
“It’s really still the same basic idea that Einstein taught,” he says. What we perceive as the force of gravity is actually the curvature of spacetime. “Any body just moves along a trajectory in the space time of the Earth”, whether it is made of dense platinum,
But even if physicists never prove Einstein wrong, Hossenfelder says, experiments like MICROSCOPE are still important.
“These tests are not just about the principle of equivalence,” she says. “They are implicitly looking for all other kinds of deviations, new forces and so on” that are not part of general relativity. “So it’s really a multi-purpose high-precision measurement.”
Now that the mission is complete, the MICROSCOPE satellite will slowly spiral out of orbit. “It’s hard to say where it will fall in 25 years,” says Rodriguez. Together with a reference set of platinum cylinders on board, “that’s several million euros in platinum”.
Where this precious platinum metal will land, no one knows, but the force of gravity that will pull it down will push the titanium just as hard, at least to within a part in a thousand trillion.
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