About 425 years ago, Galileo Galilei supposedly dropped pairs of balls of different sizes and materials from the Leaning Tower of Pisa to show that all objects accelerate at the same rate under gravity's pull. Now, physicists are performing various versions of this classic experiment (which Galileo probably never performed) to test a basic premise of Einstein's theory of gravity, or general relativity, called the equivalence principle. In April 2016, a French satellite will blast off to see whether two free-floating cylinders of different materials orbit Earth at exactly the same altitude. Another team of physicists aims to reproduce Galileo's freefall experiment with two different types of atoms. Such work compares an object's inertial mass, which determines how much it resists moving when subjected to a force, and its gravitational mass, which determines how strongly gravity pulls on the thing. That equivalence explains Galileo's experiment. According to general relativity, it must hold exactly. If it doesn't, general relativity cannot be the ultimate theory of gravity.