As far as we can tell, gravity doesn’t care who you are. Using a level of precision one hundred times greater than earlier efforts, an experiment conducted in space has verified that all objects fall in the same direction when subjected to the force of gravity. Einstein’s equivalence principle is central to general relativity, and this discovery puts it to its toughest test to date. The researchers report that the theory holds up to within one in a thousand trillion.

To some, the idea that everything is equally subject to gravity may come as no surprise. Any evidence to the contrary, however, could shed insight on the compatibility between general relativity and the standard model of particle physics. The world is continuous and seamless in general relativity, whereas quantum particles and forces are seen as discrete and granular in the standard model. Since Einstein’s day, scientists have hoped for a unifying theory that could explain everything.

Sabine Hossenfelder, a physicist at the Frankfurt Institute for Advanced Studies in Germany who was not involved in the work, says, “The equivalence principle is the most crucial cornerstone of Einstein’s theory of general relativity.” Since “it” in its current form “cannot account for quantum effects,” we know that it will need to be revised at some point.

The MICROSCOPE experiment monitored the orbits of two metal cylinders, one weighing 300 grams of titanium and the other 402 grams of platinum, as they fell nearly freely from space and circled the Earth. The cylinders would shift with regard to one another if the gravitational pull on each was different. Applying slight electrical forces to realign the cylinders would have shown that the equivalence principle was perhaps being broken. The cylinders were enclosed in a spacecraft from April 2016 until October 2018 to keep them safe from the solar winds, the tiny pressure that sunlight exerts, and the remaining atmosphere at an orbital altitude of about 700 kilometers.

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According to Manuel Rodrigues, a physicist with the French aerospace lab ONERA in Palaiseau and a member of the MICROSCOPE team, performing the experiment in orbit allowed the researchers to compare the free fall of two different materials for extended periods without the confounding effects of vibrations or of objects nearby that could exert gravitational forces. According to MICROSCOPE’s findings, “the best method to gain an important improvement in the accuracy for this kind of test is to do it in space.”

The equivalence principle was not compromised during MICROSCOPE’s two and a half year mission, according to the latest study. This confirms a less precise intermediate report from the trial that came to a similar conclusion (SN: 12/4/17). The possibility that experimental evidence of equivalence principal restrictions would never be found has led some physicists to conclude that Einstein will always be vindicated.

According to physicist Clifford Will of the University of Florida in Gainesville who is not connected with the experiment, even a follow-up MICROSCOPE 2 mission with 100 times more precision, provisionally scheduled for the 2030s, is unlikely to find a breakdown of the equivalence principle. It’s still “just this basic principle” that Einstein taught, he argues. Gravity appears like a force, but it is really just the spacetime curvature. Whether it’s constructed of platinum, titanium, or something else entirely, “every body merely goes along the path in Earth’s spacetime.”

However, Hossenfelder claims that investigations like MICROSCOPE are still significant even if physicists never prove Einstein wrong. She explains that “the equivalence principle is only one part of these tests.” According to the author, “they implicitly look for all other kinds of aberrations, new forces, and so on” that aren’t a part of general relativity. Therefore, “it is a high-precision measurement that may be used for a variety of purposes.”

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The MICROSCOPE satellite will gradually fall out of orbit now that its primary purpose has been fulfilled. Rodrigues says, “It’s hard to bet where in 25 years it will tumble down.” “it’s [a] few of millions of euros [in] platinum,” including a reference set of platinum cylinders, on board. No one knows exactly where the expensive platinum metal will fall, but the gravity that draws it down will exert an equal and opposite force on the titanium, at least to within one part in a thousand trillion.

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