Scientists made gravity act on an atom in a quantum superposition — the surprising result could bring Einstein’s theory and quantum physics closer together

For more than a century, physics has been built around two extraordinarily successful theories. Quantum mechanics explains the strange behaviour of atoms and other microscopic objects, while Albert Einstein’s theory of gravity describes everything from falling bodies to the structure of the Universe.

The problem is that the two theories have never been fully reconciled.

Now, an international team of physicists has carried out an experiment that brings the two worlds together in an unusually direct way. Researchers placed ultracold atoms into a quantum superposition, effectively allowing parts of an atom’s wave to follow different paths. One part was held almost motionless while another was allowed to fall under Earth’s gravity.


When the two paths were brought back together, the scientists detected a tiny change in the atom’s quantum phase — an effect that matched a prediction based on Einstein’s equivalence principle.

The study, involving researchers from Ben-Gurion University of the Negev, the University of Oxford, the University of Ulm and other institutions, was published in Science Advances.

An experiment at the boundary of quantum physics and gravity

The result is significant because gravity has long occupied an awkward position in modern physics.

Quantum mechanics works extraordinarily well for particles and atoms, but Einstein’s general theory of relativity describes gravity on much larger scales. Despite decades of theoretical work, physicists still lack a complete framework that combines quantum mechanics with gravity.

The new experiment does not solve that problem. Instead, it tests whether one of the central ideas behind Einstein’s description of gravity continues to hold when the object being studied behaves according to quantum mechanics.

That principle is known as the equivalence principle.

In simple terms, Einstein’s principle says that an observer in free fall should locally experience the effects of gravity as if gravity had disappeared. A person inside a freely falling elevator, for example, would feel weightless.

The principle has been tested with extraordinary precision using ordinary objects. Testing it with quantum objects is much more complicated because atoms can behave like waves and can exist in superpositions of different states.

Scientists put an atom on two different paths

To make the experiment possible, the researchers developed an instrument called the Quantum Galileo Interferometer.

The device uses ultracold rubidium atoms cooled to temperatures just above absolute zero. The atoms were manipulated close to a specially designed chip containing tiny electrical wires capable of generating precisely controlled magnetic fields.

The researchers first used microwave pulses to place the atoms into a quantum superposition.

That meant the atomic wave could effectively be divided into two components.

One component was subjected to a carefully controlled magnetic force that counteracted Earth’s gravitational pull. It therefore remained essentially stationary relative to the laboratory.

The other component was given an upward push and then placed into a state in which it was largely unaffected by the magnetic field. Gravity could then take over, sending that part of the quantum wave into free fall.

In other words, the experiment created a situation in which the two components experienced very different motion: one was held in place while the other moved under gravity.

The crucial moment came when the waves met again

The researchers then reunited the two components.

This produced a phenomenon known as quantum interference.

Instead of simply asking where the falling atoms ended up, the scientists examined the difference in the quantum phase accumulated along the two paths.

That difference was incredibly small, but it carried information about what happened while one part of the wave was falling and the other remained stationary.

The measured phase agreed with the value predicted when Einstein’s equivalence principle is applied to a quantum object.

According to the researchers, this represents the first direct measurement of this predicted quantum phase associated with a freely falling object.

The experiment therefore provides a rare experimental connection between Einstein’s theory of gravity and quantum mechanics.

Why this matters for Einstein’s theory

The finding is not simply another measurement showing that objects fall under gravity.

Scientists have already used quantum particles in experiments involving gravity.

What makes this experiment different is the way the researchers exploited quantum superposition and interference to measure a predicted phase associated with free fall.

The result shows that the equivalence principle continues to be compatible with quantum mechanics under the conditions tested.

That is important because the behaviour of quantum objects can appear fundamentally different from the behaviour of everyday objects. An atom can display wave-like properties, exist in a superposition and produce interference patterns — all while remaining subject to gravity.

The new measurement shows that Einstein’s principle survives that transition into the quantum regime.

It does not mean scientists have solved quantum gravity

Despite the striking implications, the experiment should not be interpreted as a discovery of a complete theory of quantum gravity.

The researchers have not shown that gravity itself is quantum.

Nor have they demonstrated that general relativity and quantum mechanics can now be combined into a single theory.

Instead, the experiment establishes that Einstein’s equivalence principle continues to agree with quantum mechanical behaviour within the range that was tested.

That distinction is important. The long-standing conflict between quantum mechanics and general relativity remains one of the biggest unresolved problems in theoretical physics.

Roger Penrose’s idea remains on the table

The experiment also has an interesting connection to Nobel Prize-winning physicist Sir Roger Penrose, who was part of the international research team.

Penrose has proposed that quantum mechanics might eventually break down when sufficiently massive objects remain in quantum superpositions for long enough.

The new experiment does not test that proposal because the atoms involved were far too light and the superposition did not persist for the necessary timescale.

So the result neither confirms nor rules out Penrose’s idea.

However, the researchers believe the technique could eventually be adapted to study much heavier objects. One possibility involves using nanodiamonds, which could provide a substantially larger system for investigating the boundary between quantum mechanics and gravity. Experiments aimed at exploring such possibilities are already being pursued by the research group at Ben-Gurion University.

A tiny atom could open the door to much bigger questions

The experiment ultimately demonstrates something remarkable about modern physics: even an extremely small quantum system can be used to probe questions that concern the fundamental structure of the Universe.

By splitting an atomic wave, allowing one part to fall and holding another part in place, researchers created an experimental test of Einstein’s equivalence principle in a regime where classical and quantum descriptions meet.

The result supports Einstein’s prediction under these conditions — but it also highlights how much remains unknown.

The next challenge is to push these experiments toward heavier objects, longer-lasting quantum superpositions and increasingly sensitive measurements.

If researchers can eventually reach that regime, experiments like this could provide new ways to investigate one of physics’ deepest questions: does gravity ultimately obey the rules of quantum mechanics, or does something more fundamental happen at the boundary between the quantum and gravitational worlds?