Einstein’s Gravity Enters the Quantum World: Scientists Measure a Hidden Phase for the First Time

Scientists have measured a long-predicted quantum effect of gravity for the first time, showing that Einstein’s equivalence principle can also describe the behavior of a quantum object under the conditions tested.

The international team, led by researchers at Ben-Gurion University of the Negev, the University of Ulm and the University of Oxford, published the study in Science Advances on September 2, 2026, with the findings reported by the University of Oxford on September 8.

The experiment did not simply drop an atom and watch it fall. Researchers used ultracold rubidium-87 atoms and turned their quantum wave into two paths. One part was held stationary against gravity while the other was allowed to fall freely.

When the two paths were brought back together, they produced an interference signal containing a tiny difference in quantum phase. That measured phase matched the value predicted by theory, giving scientists a new experimental test at the boundary between quantum mechanics and Einstein’s gravity.

The strange experiment that made gravity visible in a quantum wave

At everyday scales, gravity seems straightforward. Drop a ball, and it falls toward Earth.

Quantum objects behave very differently. An atom can be described by a wave, and that wave can be placed into a superposition, allowing two different paths to be considered at the same time.

The researchers designed a new instrument called the Quantum Galileo Interferometer (QGI) specifically to take advantage of this behavior.

They cooled a cloud of rubidium atoms to extremely low temperatures and prepared the atoms in a quantum superposition. The experiment then separated each atom’s wave into two parts.

One wave packet was held in place using a carefully controlled magnetic force that balanced Earth’s gravity. The other was launched upward and then allowed to move freely under gravity.

This created the unusual situation the researchers needed: one quantum wave was stationary relative to Earth while its counterpart was freely falling.

The falling atom picked up a quantum phase

The key measurement was not simply the distance the atom traveled.

As the freely falling wave evolved, it accumulated a quantum phase relative to the stationary wave. When the researchers reunited the two paths, the waves interfered with each other.

That interference made the otherwise tiny phase difference measurable.

The experiment produced about 13 oscillations, corresponding to roughly 80 radians of phase accumulation. The measured result agreed closely with the theoretical prediction for the phase of a freely falling quantum wave.

The researchers also compared their measurements with numerical simulations and analytical calculations, finding good agreement.

The result matters because this particular phase had been predicted nearly a century ago but, according to the researchers, had never previously been directly measured.

What Einstein’s equivalence principle has to do with it

The experiment tests a central idea behind Einstein’s description of gravity: the equivalence principle.

In simple terms, the principle says that a freely falling observer locally experiences gravity as if it has disappeared. A familiar example is an astronaut in free fall, who feels weightless even though Earth’s gravity is still acting on the spacecraft.

Scientists have tested this principle with ordinary objects to extraordinary precision.

The difficult question has been whether the same idea can be applied to something behaving quantum mechanically.

The new experiment provides evidence that, at the low masses and energies tested, the principle remains consistent with quantum mechanics.

Professor Ron Folman of Ben-Gurion University of the Negev, the study’s lead author, said the experiment addresses one of the biggest unresolved 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 the experiment could provide clues toward understanding how the two major theories might coexist.

This does not mean scientists have solved quantum gravity

The result is significant, but it is easy to overstate what it proves.

The researchers have not created a unified theory of quantum mechanics and gravity. They also have not shown that gravity itself is definitively a quantum force.

Instead, the experiment demonstrates that the predicted quantum phase of a freely falling object agrees with the consequence expected when Einstein’s equivalence principle is applied to a quantum wave packet.

Professor Vlatko Vedral of the University of Oxford, a co-author of the study, emphasized the importance of testing quantum mechanics in this unusual setting.

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

That distinction is important. The experiment tests the interface between quantum theory and gravity, rather than providing the final theory physicists have been searching for.

Why the experiment could lead to an even bigger test

The new interferometer may become useful for experiments involving objects much larger than individual atoms.

One particularly interesting direction involves nanodiamonds. Researchers want to investigate whether quantum behavior could eventually break down when an object becomes sufficiently massive or remains in a quantum superposition for a long enough time.

That question is connected to an idea proposed by Roger Penrose, the Nobel Prize-winning physicist who is also a co-author of the study.

Penrose has proposed that gravity could play a role in causing quantum superpositions to collapse. The current experiment does not test that proposal because its atoms are far too small and the quantum superpositions do not last long enough.

However, the researchers say the new technique could eventually allow experiments to move into that more challenging territory.

A new way to probe one of physics’ biggest mysteries

For decades, quantum mechanics and general relativity have worked remarkably well in their respective domains, yet physicists have struggled to combine them into a single framework.

The new experiment does not bridge that gap completely. What it does is provide a new experimental tool for examining the boundary between the two theories.

By making a tiny quantum phase caused by free fall measurable, the researchers have turned an idea that existed largely in equations into something that can be tested in the laboratory.

The next challenge will be to push the experiment further, using larger quantum objects and more demanding conditions. If future experiments find a deviation from the predictions tested here, that could point toward new physics.

For now, however, the result delivers a simpler message: an atom can behave quantum mechanically while falling under gravity, and the measured phase of that quantum free fall agrees with Einstein’s equivalence principle.