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Strictest test yet of general relativity confirms feathers and bowling balls really do fall at the same rate

A pillar of Albert Einstein’s theory of general relativity—and a staple of middle school science class demonstrations—has passed its most stringent test yet. A new space-based experiment aboard the European satellite MICROSCOPE has confirmed with unprecedented precision that masses made of different materials fall at exactly the same rate under gravity.

It’s “really good” to have Einstein’s theory confirmed with such high precision, says Eugene Lim, a theoretical physicist at King’s College London who wasn’t involved with the work. The results aren’t surprising, he adds, but these kinds of experiments could help physicists narrow down future gravitational theories that fit with quantum theory and better predict how black holes behave.

MICROSCOPE launched in 2016 to test Einstein’s Weak Equivalence Principle. Simply put, the principle states that gravity is universal. No matter what an object is made of—be it lead or sawdust—it will accelerate in the same way under a gravitational field. In one famous—possibly apocryphal—demonstration, the famed astronomer and physicist Galileo Galilei is said to have dropped two spheres of differing masses from the top of the Leaning Tower of Pisa and watched them land at the same time. (In reality, many historians agree this was likely just one of Galileo’s thought experiments.) Physicists have carried on the tradition for centuries, poking and prodding at this principle under a variety of experimental conditions.

Space is just the latest frontier for such tests. A chamber inside MICROSCOPE contained a series of electrically charged cylinders made of platinum and titanium alloy. These test masses were kept in place by static electricity as they orbited around Earth. Because orbiting is equivalent to falling, at least as far as gravity is concerned, the masses were essentially kept in a state of constant free fall. An extremely sensitive electrical sensor measured the amount of voltage required to keep each object stationary.

If one of the objects were to accelerate faster than the other, it would need a higher voltage to keep it place. But that isn’t what happened. As expected, the rates of acceleration of the two objects remained equal throughout the experiment, researchers report today in Physical Review Letters.

The results may not rewrite any textbooks, says mission co-leader Manuel Rodrigues, a research engineer at ONERA, the French Aerospace Lab. But they’re more precise than those achieved by previous experiments, he says, including a 2017 study that also used MICROSCOPE. “We were able to improve the accuracy of the measurements by a factor of 10.”

Lim adds that the new study’s precision lends confidence to past work on various aspects of the principle. “We now know that the results we’ve gotten from other experiments are robust.”

Such work could help physicists shore up holes in Einstein’s theory of general relativity, which many physicists believe is incomplete, Lim says. It’s difficult to reconcile general relativity with quantum mechanics, for example.

Rodrigues adds that scientists do not fully understand how to square general relativity with phenomena such as black holes, where gravitational forces are so strong they prevent light from escaping.

Experiments like MICROSCOPE could help scientists develop new theories to solve these dilemmas, Lim says.

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