Some Signs of Quantum Gravity May Be an Illusion, Physicists Find

Physicists have identified an important source of confusion in experiments designed to test whether gravity itself follows the rules of quantum mechanics, one of the deepest open questions in physics.

Quantum mechanics describes how matter behaves at very small scales — allowing an object to be delocalized across multiple locations at once, an effect that has been demonstrated with atoms and even small pieces of metal. Einstein's theory of gravity, meanwhile, treats gravity as a property of space and time itself, which can bend and support waves, as gravitational-wave detectors have confirmed. The two theories do not fit neatly together, and physicists have spent decades searching for a theory of "quantum gravity" to combine them.

Many researchers have proposed experiments that could reveal whether gravity itself behaves quantum mechanically, based on the idea that the spacetime around a quantum object could occupy multiple "states" at once. A team from Kyushu University, the University of Waterloo, and Stockholm University, writing in the journal npj Quantum Information, found that many of these proposed scenarios can also be explained without gravity behaving quantum mechanically at all.

The researchers developed a theoretical framework showing that a quantum particle can remain in a superposition — existing in more than one state at once — while moving through ordinary, classical gravity and spacetime, producing the same predictions some scientists expected only from genuinely quantum gravity. They call the idea the "Relativity of Spacetime Superpositions," comparing it to two different map projections that describe the same landscape.

"Our work does not tell us that such experiments rule out quantum gravity," said co-author Magdalena Zych of Stockholm University. "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."

Lead author Joshua Foo, an associate professor at Kyushu University's Institute for Advanced Study, said the finding gives physicists a clearer roadmap for designing future experiments. "Before we can test gravity's quantum nature, we first need to know what evidence would prove that we've found it," he said.