For more than 100 years, modern physics has been built around two powerful theories. Quantum mechanics explains how extremely small objects such as atoms and particles behave, while Albert Einstein’s theory of gravity explains how gravity affects matter, motion and the structure of the universe.
Both theories have been extraordinarily successful. Yet there is still a major unanswered question: How do quantum mechanics and gravity work together?
Now, an international team of scientists has taken an important step toward answering that question. Researchers from Ben-Gurion University of the Negev, the University of Ulm and the University of Oxford, along with collaborators from other institutions, have observed a long-predicted effect of gravity on a falling quantum object.
The study, published in Science Advances on September 2, involved Nobel Prize-winning physicist Professor Sir Roger Penrose. The experiment shows that a fundamental idea behind Einstein's description of gravity—the equivalence principle—continues to agree with the behavior of matter in the quantum world.
Importantly, the result does not mean scientists have finally unified quantum mechanics and gravity. Instead, it provides a new experimental connection between the two areas of physics.
A 100-Year-Old Principle Enters the Quantum World
At the center of the experiment is Einstein's equivalence principle.
In simple terms, the principle says that gravity can locally appear to disappear for an observer who is freely falling. Imagine being inside an elevator that is falling freely. For a short period, everything inside would appear weightless because the elevator and everything inside it are falling together.
This idea is a cornerstone of Einstein's theory of gravity and has been tested extremely accurately using ordinary objects.
But there was an important unanswered question.
What happens when the falling object is not an ordinary object, but a quantum object?
Quantum objects can behave in ways that seem very different from our everyday experience. An atom, for example, can exist in a quantum superposition, meaning its quantum wave can effectively follow more than one path at the same time.
Testing Einstein's principle under these conditions is extremely challenging.
The new experiment was designed to do exactly that.
The Quantum Galileo Interferometer
The researchers developed a special experimental system called the Quantum Galileo Interferometer.
The name refers to Galileo's famous studies of falling objects, but the new apparatus investigates falling objects at the quantum level.
The experiment used clouds of rubidium atoms cooled to temperatures just above absolute zero. At such extremely low temperatures, the atoms can be controlled with remarkable precision and their quantum behavior becomes easier to observe.
The atoms were positioned close to a specially designed atom chip, which contained tiny electrical wires capable of producing carefully controlled magnetic fields.
The researchers first used microwave pulses to place the atoms into a quantum superposition.
This effectively created two parts of the same atomic quantum wave that could follow different paths.
One part was kept stationary relative to the laboratory, while the other was allowed to fall under gravity.
That difference was crucial.
One Quantum Wave, Two Different Experiences
The scientists used magnetic fields to control the two parts of the atomic wave.
One part experienced a carefully adjusted upward magnetic force that balanced the downward pull of Earth's gravity. As a result, this portion of the wave remained essentially stationary relative to the laboratory.
The second portion was given an upward push using a precisely controlled magnetic pulse. It was then placed into a state that was almost unaffected by the magnetic field.
Gravity could therefore act on it freely.
The second part followed a ballistic trajectory, similar to the path of a ball thrown upward into the air.
However, there was a remarkable difference: this was not simply two ordinary atoms following two paths. The researchers were controlling different components of a single quantum wave.
After the falling portion had traveled along its trajectory, another carefully controlled magnetic pulse brought the two parts back together.
When quantum waves meet, they can interfere with one another.
This interference contains information about how the two paths differed.
Measuring an Extremely Tiny Quantum Effect
The key measurement was the quantum phase difference between the two parts of the atomic wave.
While one part was held in place and the other experienced free fall, the two portions accumulated slightly different quantum phases.
When they were reunited, this difference produced a measurable interference pattern.
The measured phase matched the value predicted when Einstein's equivalence principle is applied to a quantum wave.
In other words, the experiment showed that the behavior of the falling quantum object was consistent with a fundamental principle of Einstein's theory.
The result is significant because previous experiments have already used quantum systems to measure gravity. However, the researchers describe this experiment as the first direct measurement of the predicted quantum phase associated with a freely falling object.
Does This Finally Unite Quantum Mechanics and Gravity?
Not yet.
It is important not to overstate what the experiment has demonstrated.
The study does not provide a complete theory combining quantum mechanics with Einstein's theory of gravity.
It also does not prove that gravity itself is quantum.
Instead, it shows that within the experimental conditions tested, Einstein's equivalence principle remains compatible with quantum mechanics.
This is important because physicists still do not have a complete framework explaining how gravity should behave at the quantum level.
Quantum mechanics works extremely well for microscopic systems, while Einstein's general theory of relativity provides an exceptionally successful description of gravity on larger scales.
The difficulty comes when both descriptions are expected to operate simultaneously.
Experiments such as this one help scientists understand where the two theories agree—and eventually where they might begin to differ.
Roger Penrose's Bigger Question
The experiment is also connected to an important idea proposed by Professor Sir Roger Penrose.
Penrose has argued that quantum mechanics might eventually break down for sufficiently massive objects placed in quantum superpositions for long enough periods.
The current experiment does not test that idea directly.
The rubidium atoms used here are far too small, and the experimental timescales are not sufficient to reach the regime required to investigate such a possible breakdown of quantum mechanics.
However, the researchers see the new technique as a potential step toward much more ambitious experiments.
Their future goal includes testing heavier objects, potentially including nanodiamonds, in quantum superpositions.
If scientists can create and maintain quantum superpositions involving increasingly massive objects, they may eventually be able to investigate whether the laws of quantum mechanics remain valid indefinitely—or whether gravity introduces a fundamental limit.
Why This Experiment Matters
The importance of the study goes beyond simply observing atoms falling under Earth's gravity.
For decades, physicists have been trying to understand whether the principles governing the quantum world can coexist with Einstein's description of gravity.
The new experiment provides evidence that, at least in the regime accessible to current technology, they can.
It demonstrates that a quantum object can be placed in a carefully controlled superposition, allowed to experience free fall, and then measured in a way that reveals the gravitational effect predicted by Einstein's principle.
Professor Ron Folman of Ben-Gurion University described the work as combining a technically difficult experiment with a much broader theoretical question: how gravity and quantum theory could ultimately fit into one understanding of the universe.
University of Oxford physicist Professor Vlatko Vedral similarly emphasized that the experiment pushes quantum mechanics toward one of its most intriguing frontiers—gravity—while showing that its predictions continue to hold.
A Small Atom, A Huge Physics Question
The experiment may involve tiny rubidium atoms, but the question behind it is enormous.
Can the laws of quantum mechanics and gravity ultimately be part of the same fundamental theory?
Scientists still do not know.
What this experiment provides is not the final answer, but another important piece of evidence.
It shows that when a quantum object falls freely under gravity, its quantum behavior follows the prediction associated with Einstein's equivalence principle.
The researchers now hope to push these experiments further, using heavier objects and longer-lasting quantum superpositions.
If future experiments reach a regime where quantum mechanics and gravity begin to disagree, scientists could be looking at clues pointing toward a completely new understanding of nature.
For now, however, the message is clear: even in the strange quantum world, Einstein's century-old principle continues to hold up.
Reference:
- Or Dobkowski et al.
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