Physicists Observe Einstein's Gravity Effect On Quantum Object In First-Of-Its-Kind Experiment

BEERSHEBA, Israel — An international team of physicists, including Nobel Prize-winning physicist Sir Roger Penrose, has directly observed a long-predicted effect of gravity on a falling quantum object for the first time, offering new experimental evidence that Einstein's theory of gravity remains consistent with the strange behavior of matter at the quantum scale.

The study, led by researchers at Ben-Gurion University of the Negev, the University of Ulm and the University of Oxford, was published Sept. 2 in the journal Science Advances. The research probes one of the most persistent unresolved questions in modern physics: how quantum mechanics, which explains the behavior of atoms and other tiny particles, fits together with Einstein's theory of general relativity, which explains how gravity shapes the motion of objects and the structure of the universe.

At the center of the experiment is Einstein's equivalence principle, a foundational element of his theory of gravity, which holds that for an observer in free fall, gravity should locally disappear. A person falling freely inside an elevator, for example, would experience apparent weightlessness. While that principle has withstood extraordinarily precise tests using ordinary matter, physicists had not previously found a way to test it directly using quantum objects, which can behave as waves and effectively travel along multiple paths simultaneously.

To test the principle at the quantum level, researchers built a new apparatus they call the Quantum Galileo Interferometer. The device allowed the team to split the quantum wave associated with an individual atom into two separate paths, holding one path stationary while allowing the other to fall freely under gravity, before reuniting the two paths to measure how the falling wave had changed relative to the stationary one.

The experiment was carried out at Ben-Gurion University using clouds of rubidium atoms cooled to just above absolute zero and manipulated near the surface of a specially designed atom chip. Researchers, including Ph.D. student Or Dobkowski, first used microwave pulses to place the ultracold atoms into a quantum superposition, effectively allowing each atom to exist along two different paths simultaneously. Tiny electrical wires embedded in the chip then generated precisely controlled magnetic fields, with one portion of the atomic wave responding to that field in a way that produced an upward force exactly counteracting the downward pull of gravity, effectively holding it stationary relative to the laboratory and Earth.

The second portion of the wave was pushed upward using a precisely controlled magnetic pulse, then switched into a state largely unaffected by the magnetic field, allowing it to fall freely under gravity along a ballistic trajectory, similar to the arc of a ball thrown into the air. At the end of the fall, researchers used another carefully controlled magnetic pulse to bring the two separated portions of the wave back together. When reunited, the two waves interfered with one another, allowing researchers to measure the tiny difference in quantum phase that had accumulated between the falling portion and the stationary portion during the experiment.

That measured phase matched precisely what physicists had predicted would occur if Einstein's equivalence principle applies to a quantum wave. While previous experiments have used quantum particles to measure the effects of gravity in other ways, researchers say this marks the first direct measurement of the specific quantum phase predicted for a freely falling object.

Ron Folman, a professor at Ben-Gurion University of the Negev and the study's lead author, described the significance of combining a difficult physical experiment with a question central to the deepest unresolved issues in physics.

"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: How can gravity, described by Einstein's theory of relativity, and quantum theory be unified into one understanding of the universe?" Folman said. "These two pillars of modern physics have so far eluded all attempts at a unified theoretical framework, but this complex experiment gives more hints as to how such a unification may be achieved."

Study co-author Vlatko Vedral, a professor in the Department of Physics at the University of Oxford, emphasized that the result adds to a long track record of quantum mechanics holding up under increasingly demanding experimental tests.

"We have no consistent theory telling us why quantum physics should fail," Vedral said. "This experiment pushes quantum mechanics into one of its most intriguing frontiers, gravity, and shows that, once again, its predictions hold."

Researchers were careful to clarify the limits of what the experiment demonstrates. The result does not unify quantum mechanics and gravity into a single theoretical framework, nor does it prove that gravity itself behaves as a quantum phenomenon. Instead, it shows that Einstein's equivalence principle remains consistent with quantum mechanics within the specific conditions the experiment was able to test.

The findings also do not overturn a separate theoretical argument previously advanced by study co-author Sir Roger Penrose of the University of Oxford, suggesting that quantum mechanics could potentially break down for sufficiently massive objects held in quantum superposition for long enough periods of time. The current experiment did not involve objects massive enough, or superposition states held for long enough durations, to directly test that particular hypothesis. Researchers say they hope the techniques developed for this experiment will serve as a stepping stone toward future experiments involving significantly heavier objects, including nanodiamonds, that could eventually probe Penrose's proposed limits on quantum superposition. Such a follow-up experiment is reportedly already underway within the same research group at Ben-Gurion University of the Negev.

The broader international research effort behind the study included scientists from Ben-Gurion University of the Negev, the University of Oxford, the University of Southampton, the German Aerospace Center, the Institute of Quantum Technologies in Ulm, Universität Ulm, and Texas A&M University.

The research adds to a growing body of experimental work probing the boundary between quantum mechanics and gravity, an area physicists have long regarded as one of the most significant unresolved frontiers in the field, given the profound theoretical difficulty of reconciling Einstein's smooth, continuous description of spacetime with the fundamentally probabilistic, wave-like behavior that governs particles at the quantum scale. While Wednesday's findings stop well short of resolving that broader challenge, researchers involved in the study said the experiment represents a meaningful incremental step toward better understanding how, and under what conditions, the two foundational pillars of modern physics ultimately connect.