Physicists led by Ron Folman at Ben-Gurion University, working with international collaborators including Nobel laureate Roger Penrose, have built a new type of interferometer to test a near century-old theoretical prediction about how free fall affects a quantum wave. The experiment matters because if the long-standing theoretical prediction turns out to be wrong, it would reveal a direct contradiction between quantum mechanics and Einstein's theory of gravity, two pillars of modern physics that have never been reconciled. Testing the idea had previously been impossible, since no one had managed to construct an interferometer capable of making the required measurement.

The device places a single atom into a superposition of two paths, one where it falls freely and another where it is held motionless, with both paths converging at the same point and moment so the "phase" of the atom's wave-like nature can be compared, similar in principle to the classic double-slit experiment. Achieving this required cooling atoms to near absolute zero, a capability only available since the late 1990s, and devising a way to send part of the atom's wave packet on a ballistic, cannon-like trajectory upward before gravity returned it, without destroying the delicate quantum state through measurement or disturbance.