Two remarkably successful theories explain nearly everything we observe in the universe, from tiny atoms and molecules to planets, stars, and galaxies. Quantum mechanics describes the behavior of matter at very small scales, while Einstein’s theory of gravity explains how stars and black holes move and how the Universe expands.
Despite their enormous success, the two theories still do not fit neatly together. Physicists have spent decades searching for a theory of “quantum gravity” that could combine them into one consistent description of nature.
The Challenge of Quantum Gravity
A central expectation is that gravity itself should ultimately follow the rules of quantum mechanics. That possibility quickly becomes difficult to visualize.
Quantum mechanics allows an object to be delocalized across multiple locations at the same time, an effect that has been repeatedly demonstrated with atoms and even small pieces of metal. Einstein’s theory, meanwhile, treats gravity as a property of space and time itself — it can bend, flatten, and support waves that travel through it, as gravitational wave detectors have confirmed.
Because of this, many physicists have assumed that the spacetime surrounding a quantum object could also occupy several “states” simultaneously.
But what would that actually look like in an experiment?
Researchers from Kyushu University, the University of Waterloo, and Stockholm University may now have part of the answer. Their findings were published in npj Quantum Information.
When Quantum Gravity Looks Classical
The researchers developed a theoretical framework showing that many situations described as a “quantum superposition of gravity” can also be interpreted in a very different way.
In these cases, the quantum particles can remain in superpositions while moving through ordinary gravity and spacetime. Under that description, no genuinely quantum behavior of gravity is required.
“Many researchers have proposed experiments that could potentially reveal the quantum nature of gravity,” explains Associate Professor Joshua Foo of Kyushu University’s Institute for Advanced Study and lead author of the study. “What we found is that some of these scenarios can be viewed from two equally valid perspectives. One interpretation describes gravity as being in a quantum superposition, while the other describes quantum particles moving in an ordinary gravitational field.”
The team calls this idea the “Relativity of Spacetime Superpositions.”
One way to picture it is to think of two maps showing the same landscape using different projections. The maps may look different, yet both can describe the same underlying terrain. In a similar way, the researchers found that some situations that appear to involve quantum gravity can instead be expressed using classical gravity and spacetime, as long as the motion of each particle is represented with the appropriate quantum state.
A Key Ambiguity in Quantum Gravity Experiments
The findings do not show that gravity is classical, and they do not rule out quantum gravity.
Instead, they expose an important ambiguity in how experiments designed to probe gravity’s quantum properties may be interpreted. An observation that appears to reveal quantum gravity might, in some cases, also be explained without requiring gravity itself to behave quantum mechanically.
“Our work does not tell us that such experiments rule out quantum gravity,” says Magdalena Zych of Stockholm University and a co-author on the paper. “Rather, it helps us identify which experimental signatures would genuinely require a quantum description of gravity and which ones could arise from more familiar physics. That distinction is crucial for designing future experiments.”
Searching for a True Quantum Gravity Signature
Although the work deals with some of the most fundamental questions in physics, research into basic laws of nature has historically produced major practical benefits.
GPS navigation, lasers, and modern electronics all emerged from advances rooted in theoretical quantum physics and Einstein’s theory of gravity.
The more immediate impact of the new framework is that it gives physicists a clearer roadmap for future experiments. By showing which observations can truly distinguish a classical description of gravity from a quantum one, the research helps narrow the search for convincing evidence of quantum gravity.
“Understanding how gravity and quantum mechanics fit together is one of the greatest challenges in physics,” concludes Foo. “Before we can test gravity’s quantum nature, we first need to know what evidence would prove that we’ve found it. Our work helps clarify that question.”