They Split an Atom's Wave in Two, Let One Half Fall, and Watched Einstein's Oldest Idea Survive

Galileo's experiment is the one everyone remembers: drop two objects of different mass and they hit the ground together. An international team has now run a version of it in which the falling object is a single atom that is in two places at once, and the answer still came out the way Einstein said it would.

The result, published Wednesday in Science Advances, comes from a collaboration led by Ben-Gurion University of the Negev with the University of Ulm and the University of Oxford, and including the University of Southampton, the German Aerospace Center and Texas A&M. The lead author is Ron Folman of Ben-Gurion. The co-authors include Vlatko Vedral of Oxford, the Nobel laureate Roger Penrose, and Or Dobkowski, a doctoral student in Folman's group.

The apparatus is what the group calls a Quantum Galileo Interferometer. Rubidium atoms are cooled to within a hair of absolute zero and manipulated by an atom chip, a surface carrying wires that generate precisely shaped magnetic fields. The chip splits each atom's quantum wave into two branches. One branch is held in place, supported against gravity. The other is released and allowed to fall freely. After a controlled interval the two branches are brought back together and made to interfere.

What comes out of that interference is a phase, the quantum equivalent of asking how far out of step the two halves have drifted. Gravity acts differently on a supported path and a free-falling one, and general relativity makes a specific prediction about the difference that should show up in the recombined atom. The measured phase matched it. Einstein's equivalence principle, the statement that gravitation and acceleration are locally indistinguishable and that all bodies fall the same way regardless of composition, held in a regime where the falling body is a delocalized quantum superposition rather than a classical mass.

That is the point of doing it at all. General relativity describes gravity as geometry and assumes objects with definite trajectories. Quantum mechanics permits an object to have no definite trajectory. The two frameworks have never been made to fit together, and every experiment that puts a genuinely quantum object into a gravitational field is a test of whether the seam holds. "This unique paper combines a hard experiment with far-reaching theoretical interpretation," Folman said, describing the work as a step toward unifying gravity and quantum theory.

No violation was found, which is the usual and useful outcome: Einstein's framework survives another regime it was never built for, and the boundary of where it might finally break moves further out. The interferometer is a platform as much as a result, and the group's aim is to push toward superpositions held apart longer and farther, where any deviation would grow large enough to see.

The paper is indexed under DOI 10.1126/sciadv.aec8045.