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Picture two choirs in soundproof rooms on a wobbly stage. They sing, but can't hear each other — the air doesn't carry the tune. Yet if the stage trembles, its vibrations link their songs. That's the idea behind a new test for quantum gravity. The 'choirs' are clouds of atoms chilled into a single quantum wave, a Bose-Einstein condensate colder than deep space. Their 'song' is internal sound waves. In a vacuum, these waves are silent. But the stage, spacetime itself, has a faint shudder from hypothetical particles called gravitons, akin to microscopic gravitational waves. When the clouds are extremely close, this shudder entangles their sound, creating a ghostly harmony.

The nearer the clouds, the stronger the link. Success would prove gravity is quantum, bringing it into the standard model and revealing spacetime's grainy nature.