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Galileo's 400-year-old theory of free-falling objects passes space test

A key tenet of Albert Einstein's general theory of relativity has passed yet another test with flying colors—and for the first time in space. A French satellite experiment has shown that objects with different masses fall at exactly the same rate under gravity, just as relativity dictates. The result is the most precise confirmation yet of the equivalence principle, first tested more than 400 years ago by Galileo Galilei. "The mission appears to have performed fantastically," says Clifford Will, a theoretical physicist at the University of Florida in Gainesville.

Physicists scrutinize the equivalence principle because any violation could point to new forces of nature that might resolve a long-standing impasse between general relativity and quantum theory. The satellite, called MICROSCOPE, found no discrepancy in the acceleration of two small test masses to about one part in 100 trillion (10¹⁴).That's more than 10 times better than the most sensitive ground-based experiments, which look for disparities in the response of weights to Earth's spin.

[...] A proposed Italian satellite, aptly named Galileo Galilei, would test equivalence to a precision of one part in 10^17, partly by spinning rapidly and isolating any signal from more slowly varying systematic effects. Researchers at Stanford University in Palo Alto, California, have proposed a satellite that aims to reach one part in 10^18 using noise-reducing cryogenics. Still other researchers hope to use Bose-Einstein condensates—clouds of cold atoms that behave as a single quantum wave [DOI: 10.1126/science.357.6355.986] [DX]—to reach tight limits.

Equivalence principle.

Determination of the Equivalence Principle violation signal for the MICROSCOPE space mission: optimization of the signal processing

Relevance of the weak equivalence principle and experiments to test it: lessons from the past and improvements expected in space

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