Quantum Gravity Test: Falling Atoms Confirm Einstein's Rule

Physicists have directly measured how gravity changes a quantum object in free fall for the first time, and the result matches a prediction rooted in Albert Einstein's equivalence principle. The experiment, led by Ben-Gurion University of the Negev with collaborators at the University of Ulm and the University of Oxford, was published September 2 in the peer-reviewed journal Science Advances. Nobel laureate Sir Roger Penrose is among the study's coauthors. It is a rare direct measurement in quantum gravity research, the field that asks how the two most successful theories in physics fit together.

The equivalence principle says that someone in free fall feels weightlessness because gravity's effects effectively disappear for a freely falling observer. It underpins Einstein's general theory of relativity. The idea has passed extremely precise tests with ordinary matter, but measuring it with objects that behave as waves and can follow more than one path at once posed a different kind of experimental challenge. For more than a century, quantum mechanics and general relativity have each passed test after test, yet physicists still lack a single framework covering both, which is the central open problem of quantum gravity research.

How the experiment worked

The team built a device called the Quantum Galileo Interferometer to separate and reunite atomic waves. At Ben-Gurion, the experimental group, including PhD student Or Dobkowski, worked with clouds of rubidium atoms cooled to just above absolute zero. At those temperatures the atoms became a Bose-Einstein condensate, a state of matter in which thousands of atoms act as one unified particle. The cloud held about 20,000 rubidium atoms and was manipulated just beneath a specially patterned wire chip, roughly 113 micrometres below its surface.

Microwave pulses put the atoms into a quantum superposition, a state in which each atom effectively follows two paths at once. Tiny electrical wires on the chip generated precise magnetic fields. For the wave the researchers wanted to keep still, they applied an upward magnetic force that balanced gravity's downward pull, holding that part stationary relative to the laboratory. The other part received a magnetic push upward, then was switched into a state almost unaffected by the magnetic field, leaving it to fall freely, like a ball tossed into the air. The two trajectories stayed about 7.5 micrometres apart, with the longest flight lasting only two milliseconds. A final magnetic pulse brought the two parts back together. Their waves interfered, and the interference pattern revealed the difference in quantum phase accumulated along the two paths.

That phase difference matched the prediction obtained by applying Einstein's equivalence principle to a quantum object, according to the study. Earlier experiments used quantum particles to measure gravity, but the researchers describe this as the first direct measurement of the predicted quantum phase of a freely falling object.

Why a falling wave matters

Oxford has had a busy year in quantum physics: earlier, researchers at the university demonstrated quadsqueezing, a fourth-order quantum interaction previously thought practically unreachable, in the journal Nature Physics.

People have measured the acceleration of falling objects since Galileo. The 1975 Colella-Overhauser-Werner experiment showed that gravity affects the interference patterns of neutrons, and later tests confirmed that different atomic species fall at the same rate. What no experiment had done was track how the phase of a quantum wave itself changes while falling. "While people have measured the acceleration of free-falling objects ever since Galileo, no one has ever measured the quantum features of a free-falling quantum object, and specifically, how its phase changes due to free-fall," lead author Ron Folman told The Press Service of Israel.

The result does not unite quantum mechanics and gravity into one theory, and it does not show that gravity itself is quantum. What it does show is that the equivalence principle stays consistent with quantum mechanics under the conditions tested. "We have no consistent theory telling us why quantum physics should fail," coauthor Vlatko Vedral of Oxford's Department of Physics said in a statement. "This experiment pushes quantum mechanics into one of its most intriguing frontiers, gravity, and shows that, once again, its predictions hold."

"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? 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," Folman said in a statement.

What comes next

One proposal still awaits a heavier test. Penrose has suggested that quantum mechanics could break down when sufficiently massive objects stay in quantum superpositions for long enough. The rubidium experiment reached neither the masses nor the timescales needed to test that idea, as reported by SciTechDaily. The team hopes to extend the technique to much heavier objects, including nanodiamonds, pushing the same method toward the mass scales where quantum gravity debates get concrete. An experiment pursuing that goal is already underway in the same group at Ben-Gurion University.

The work could also lead to more sensitive quantum sensors for measuring gravity and acceleration, with potential uses in surveying, geological mapping, and navigation that does not rely on GPS, according to reporting on the study. The full findings appear in Science Advances, with additional details reported by SciTechDaily.