A new theoretical study suggests that some proposed signs of quantum gravity may not prove that gravity itself is quantum after all.
For decades, physicists have tried to unite quantum mechanics, which describes atoms and particles, with Einsteinโs theory of gravity, which explains planets, stars, black holes, and the structure of spacetime. The challenge is that quantum mechanics allows objects to exist in multiple possible states at once, while general relativity treats gravity as the curvature of spacetime.
This raises a major question: if a quantum object can exist in more than one location at the same time, does the gravity around it also exist in more than one state?
Many scientists have assumed that proving such a situation would reveal the quantum nature of gravity. But researchers from Kyushu University, the University of Waterloo, and Stockholm University now suggest that some of these apparent signs may also be explained without requiring gravity itself to behave quantum mechanically.
Their work was published in npj Quantum Information.
The researchers developed a framework showing that some situations described as a โquantum superposition of gravityโ can also be interpreted differently. Instead of gravity being quantum, the same result may be explained by quantum particles moving through ordinary classical spacetime.
In other words, the matter may behave quantum mechanically, while gravity remains classical in the description. Both views can sometimes produce the same observable outcome.
The team calls this idea the Relativity of Spacetime Superpositions. It suggests that the same physical situation can be represented in more than one mathematically valid way. From one perspective, spacetime appears to be in a quantum superposition. From another, spacetime remains ordinary while particles carry the quantum behavior.
This does not prove that gravity is classical, and it does not rule out quantum gravity. Instead, it shows that interpreting future experiments may be more difficult than expected.
The finding is important because many proposed experiments aim to detect whether gravity has quantum properties. The new study suggests that scientists must be careful: an experimental result that looks like quantum gravity may still have a classical explanation.
To truly prove that gravity is quantum, future experiments must identify effects that cannot be reproduced by ordinary spacetime combined with quantum matter.
The researchers say their framework can help physicists design better experiments by clarifying which signatures would genuinely require gravity to be quantum and which could arise from familiar physics.
The study adds a new layer to one of the biggest questions in modern physics: how gravity and quantum mechanics fit together. Before scientists can confirm the quantum nature of gravity, they must first know what kind of evidence would count as real proof.
Journal Reference:
Foo, J., Suryaatmadja, C., Mann, R. B., & Zych, M. (2026). Relativity and decoherence of spacetime superpositions. npj Quantum Information. https://doi.org/10.1038/s41534-026-01234-x