Physicists observe Einstein's equivalence principle at work in a falling quantum atom

An international team of physicists has directly measured, for the first time, the tiny quantum phase that a falling atom accumulates under gravity, and found it matches exactly what Albert Einstein's century-old equivalence principle predicts. The result, published September 2 in the journal Science Advances, does not unify gravity with quantum mechanics or prove that gravity itself is quantum. But it closes a small, previously untested gap between two theories that have never been reconciled, showing that one of the pillars of general relativity still holds when the falling object is not a person or a planet but a single ultracold atom in superposition.

The experiment was carried out at Ben-Gurion University of the Negev in Israel, by a collaboration that also included the University of Oxford, the University of Ulm, the University of Southampton, the German Aerospace Center's Institute of Quantum Technologies in Ulm, and Texas A&M University. The lead author is physicist Ron Folman of Ben-Gurion University; the theoretical framework was developed with University of Oxford physicist Sir Roger Penrose, who shared the 2020 Nobel Prize in Physics for work on black holes.

What the numbers show

Researchers in Folman's Atom Chip Laboratory cooled a cloud of rubidium atoms to just above absolute zero and held them near the surface of a purpose-built chip lined with electrical wires. Using microwave pulses, the team, including graduate student Or Dobkowski, put each atom into a quantum superposition, effectively sending its matter wave along two paths simultaneously. Magnetic fields held one branch of the wave in place while the other was released to fall freely under gravity for a fraction of a second, before a further magnetic pulse brought the two branches back together. Where they recombined, the atoms formed an interference pattern, and the shift in that pattern revealed the quantum phase the falling branch had picked up during its brief drop.

That measured phase matched the value calculated by applying Einstein's equivalence principle, the idea that gravity is locally indistinguishable from acceleration, to the mathematics of a quantum wave rather than a classical object. The apparatus, which the team calls the Quantum Galileo Interferometer, is described in the paper "Observation of the quantum phase of free fall and the consistency with the equivalence principle," available in full on the National Institutes of Health's PubMed Central archive.

A century-old idea meets the quantum world

Einstein's equivalence principle underpins general relativity and has been checked to extraordinary precision with falling apples, orbiting satellites and free-falling test masses in space. What had not been directly measured before, according to the researchers, was the specific quantum phase that the principle predicts should build up in a falling matter wave, as opposed to the classical trajectory of a falling object. Earlier atom-interferometry experiments had tested related aspects of free fall, but none had isolated and confirmed this particular quantum-mechanical signature.

Physicists have long known that general relativity and quantum mechanics, the two most successful theories in physics, resist being combined into a single framework. Gravity bends space and time smoothly and continuously; quantum mechanics deals in discrete jumps and probabilities. Experiments like this one probe the edges of that divide by testing whether the predictions of one theory survive when applied, mathematically, to the domain of the other.

"This is a unique paper, in the sense that it combines a hard experiment with a far-reaching theoretical interpretation, about one of the most fundamental questions in physics" — Ron Folman, Ben-Gurion University of the Negev

Who is watching this work

The result is mainly of interest to physicists working at the boundary of gravitational and quantum theory, a small but closely watched field that includes groups building ever-larger matter-wave interferometers and space agencies developing ultra-precise atomic sensors for navigation and geodesy. Oxford physicist Vlatko Vedral, a co-author, framed the finding as reassurance rather than revelation. "We have no consistent theory telling us why quantum physics should fail," he said. "This experiment pushes quantum mechanics into one of its most intriguing frontiers, gravity," and shows its predictions hold.

The finding was covered by outlets including the American Association for the Advancement of Science's EurekAlert news service, ScienceDaily, and Space.com, each summarizing the same set of results independently confirmed by the research team's peer-reviewed paper.

Caveats and what comes next

The authors are careful to note what the experiment does not do. It does not merge gravity and quantum theory into a single description, and it does not demonstrate that gravity is itself quantum in nature, a question that remains open. Nor does it rule out a competing idea championed by Penrose, who has argued for decades that quantum mechanics itself might break down once an object is heavy enough to be placed in superposition, with gravity acting as the trigger for that collapse. Rubidium atoms are far too light to test that hypothesis.

• Study published September 2, 2026, in Science Advances
• Experiment conducted at Ben-Gurion University's Atom Chip Laboratory in Beersheba, Israel
• Collaboration spans five institutions across Israel, the United Kingdom, Germany and the United States
• Researchers plan to repeat the test with heavier particles, including nanodiamonds

To press further into that question, Folman's group is already working on a follow-up generation of experiments at Ben-Gurion University that will use heavier particles, including nanodiamonds, in place of single atoms. If quantum superposition begins to break down as the mass of the falling object increases, in the way Penrose has proposed, later versions of the Quantum Galileo Interferometer could be the instrument that catches it happening. For now, the physicists say, Einstein's ideas have passed a small but genuinely new quantum test.