In 2034, two unusual satellites will launch into space — heavy spheres with not a single instrument onboard. Their surfaces are covered in corner reflectors that always bounce light straight back. No engines, no electronics. They'll act as a giant tuning fork for gravitational waves — ripples in spacetime first described by Albert Einstein in the last century.

The idea is simple: if the frequency of an incoming wave matches the sphere's orbital motion, it will begin to sway more and more, just as an opera singer's voice makes a crystal glass tremble. Laser stations on Earth fire short light pulses and catch the reflection; that's how they detect even millimeter shifts. Day after day, the effect builds up, letting us hear extremely faint signals.

This 'tuning fork' will open up a whole new realm — frequencies from ten-millionths to ten-thousandths of a hertz. That's where the sounds of merging supermassive black holes at galactic centers are hiding. Perhaps we'll catch the vibrations of dark matter: that mysterious invisible substance we know about thanks to the work of Vera Rubin. Or the whisper of inflation (the sudden expansion of the universe after the big bang), proposed by Alan Guth. We might also capture echoes of phase transitions, abrupt changes in the early universe, and traces of topological defects, giant folds in spacetime. And along the way, test alternative modified gravity theories that might describe the world more accurately than Einstein's equations.