Drop-testing general relativity, Galileo’s way

About 425 years ago, legend has it, Galileo Galilei climbed the Leaning Tower of Pisa. Before a throng of scholars and students, the savant dropped pairs of balls of different weights and materials—say, wood and lead—to show that regardless of their weight or composition, all objects accelerate at the same rate under gravity’s pull. About a year from now, a satellite will blast into orbit to perform the legendary test more precisely than Galileo could have imagined.

Rather than dropping things to the ground, the Drag-Compensated Micro-Satellite for the Observation of the Equivalence Principle (MicroSCOPE) will contain two free-floating weights of different materials and will monitor whether one feels a stronger tug from Earth’s gravity than the other. If so, the result would sink general relativity, Albert Einstein’s theory of gravity. After more than 15 years of development, “the instrument is done, definitely done,” says Pierre Touboul, a physicist at the French aerospace laboratory, ONERA, in Chatillon. “Now we cross our fingers.”

Funded primarily by the French National Center for Space Studies, MicroSCOPE will test a key assumption of general relativity called the equivalence principle, which marries two conceptions of mass. Inertial mass determines how much an object resists moving when pushed by a force—as when you shove a car. Gravitational mass determines how strongly gravity pulls on the object. According to the equivalence principle, the two masses are one and the same, regardless of how heavy a thing is or what it’s made of. That explains Galileo’s experiment: If the two types of mass are identical, then for all objects the pull of gravity varies in strict proportion to the resistance to motion, ensuring that all things fall at the same rate. MicroSCOPE aims to test whether the two masses are the same with a precision 100 times better than any previous experiment, and other efforts could go even further. Via Feature: Drop-testing general relativity, Galileo’s way

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