Galileo, legend has it, dropped balls from the Tower of Pisa to prove that heavy and light objects fall equally. This law formed the foundation of Einstein's theory of gravity. Today, scientists have turned an entire space station into such a tower. Only instead of stone balls, they use two clouds of rubidium atoms: one type of atom slightly lighter, the other slightly heavier.
In zero gravity, the atoms aren't just dropped: they're tossed upward, and they drift slowly, like fluffs, before falling. Laser beams, like a perfect ruler, measure their motion to within thousandths of a hair's breadth. The goal is to catch the slightest difference in how gravity pulls each type.
No difference was found. Both clouds feel the curvature of spacetime exactly the same. The error is no more than three ten-millionths. This space experiment is a thousand times more precise than Earth-based measurements with atoms. In the future, it will enable building orbital instruments for catching gravitational waves and searching for dark matter.
Galileo probably never actually dropped balls from the tower. But modern physicists had to toss atoms in space: on Earth, free fall would last less than a second, but in zero gravity it's a few precious moments.
In the movie 'Interstellar,' gravity allows communication across dimensions. Experiments like this are a real step toward understanding how the fabric of the universe curves.