Almost a hundred years ago, physicists theorized what free fall should do to a quantum wave. If wrong, quantum mechanics and Einstein’s theory of gravity contradict each other. Now, a team led by Ron Folman at Ben-Gurion University of the Negev, with collaborators including Nobel laureate Roger Penrose, has built a new interferometer.

They gave a single atom two possible paths: one in free fall, one held still. Both end at the same place and time, allowing measurement of what fall does to the atom's wave-like phase. "Every particle, doesn’t matter if it’s a car or a spaceship or an atom, is a wave," Folman says.

The wave nature shows only when atoms are cooled to nearly absolute zero—possible only since the late 1990s. A phase cannot be measured alone; it requires splitting a single particle into two trajectories and bringing them together. Free fall adds complexity: "Drop a stone and after one second it is 5 meters below you."

The team put an atom into a superposition where one path is thrown upward and gravity returns it. "You have to shoot one with a cannon, into a ballistic motion like artillery," Folman says. The challenge was designing a cannon that affects only one wave packet.