Somewhere between the microscopic realm of elementary particles and the macroscopic world of human beings, something strange happens: The rules of quantum physics, which work so exquisitely for tiny atoms, seem to lose their grip as objects grow larger. Pondering where and how this shift from small-scale quantum fuzziness to everyday sharp certainty happens gives rise to thought-experiment oddities like Schrödinger's famous dead-and-alive cat. The process by which quantum phenomena like superposition—the paradoxical affliction of Schrödinger's cat—fade into the classical reality we experience is known as decoherence. Now, a new FQxI-funded experiment has narrowed the field of possible explanations for decoherence, in particular ruling out a prominent theory linking gravity to the process. The results appeared in a paper in the New Journal of Physics in June 2026.
"One of the deepest questions in modern physics is why the strange quantum behavior that governs atoms and elementary particles seems to disappear in the macroscopic world we experience every day," says FQxI member Catalina Curceanu, director of research and spokesperson for the VIP Collaboration at the National Laboratory of Frascati of the National Institute for Nuclear Physics (INFN-LNF) in Italy.
"One of the deepest questions in modern physics is why the strange quantum behavior that governs atoms and elementary particles seems to disappear in the macroscopic world we experience every day," says Catalina Curceanu.
Conducted at the INFN Gran Sasso National Laboratory (INFN-LNGS), the world's largest underground laboratory for fundamental physics research, the experiment tested one model in which decoherence is caused by gravity. Einstein's general theory of relativity states that gravity manifests due to the warping of spacetime's fabric around massive objects. In the 1960s, the Hungarian theoretical physicist Frigyes Károlyházy posited that spacetime is constantly rippling with tiny fluctuations that gradually erode quantum superpositions, preventing macroscopic objects from existing in the kind of quantum combinations imagined in Schrödinger's famous cat paradox. His model continues to intrigue physicists and was recently revived, refined and reformulated by FQxI's Angelo Bassi and colleagues.
Telltale Trails
The fluctuations predicted by Károlyházy can't be observed directly but, if they exist, they should cause charged particles to jiggle and accelerate randomly, giving telltale trails of electromagnetic radiation. This radiation would be so faint that it could easily be lost in electromagnetic background noise from sources like cosmic rays. That makes the Gran Sasso National Laboratory, which is tucked beneath 1.4 kilometers of radiation-dampening rock, an ideal place to conduct the search. "The natural shielding provided by the rock creates one of the quietest environments on Earth for detecting extremely rare physical phenomena," says Curceanu.
The researchers used a detector made up of a coffee-mug-sized piece of high-purity germanium crystal, surrounded by layers of copper and lead shielding. They collected data for a total of 62 days. Then, they subtracted the expected background radiation from their measurements and looked for a signature that matched that predicted by the model.
The result: No signal.
"This absence of a signal is itself a major scientific result," says Catalina Curceanu.
This doesn't entirely rule out the possibility that gravity plays a role in quantum decoherence. But it does provide important information about where to look for a possible gravitational link. "This absence of a signal is itself a major scientific result," says Curceanu. "By ruling out one of the oldest and most natural gravity-induced decoherence models, this work narrows the search for the theory describing the interplay between gravity and quantum mechanics, bringing us one step closer to understanding one of the deepest mysteries in fundamental physics."
Exiting the Realm of Speculation
Károlyházy's model rests on the notion that there is a fundamental limit to the precision with which we can locate objects and measure length. In the years since he proposed the model, this feature has emerged as a common thread in many contemporary theories seeking to unite quantum physics and gravity, including string theory and loop quantum gravity. "Every quantum gravity approach ends up with predicting the existence of a minimal length connected to the uncertainty in the measurement of spacetime," says Kristian Piscicchia, a quantum physicist at the Enrico Fermi Research Center/INFN/VIP, in Italy, and the experimental lead on the new study.
Although many assume that quantum gravity cannot be probed by current technologies, the new research joins a growing body of work demonstrating that some ideas that involve both gravity and quantum theory are testable today. "Precision experiments are now reaching a level of sensitivity where they can test ideas that, until recently, belonged almost exclusively to the realm of theoretical speculation," says Curceanu. "As sensitivity improves, the boundary between theory and measurement continues to move, opening new possibilities for discovering the fundamental principles that govern our universe."
"Precision experiments are now reaching a level of sensitivity where they can test ideas that, until recently, belonged almost exclusively to the realm of theoretical speculation," says Catalina Curceanu.
The research was supported by the Foundational Questions Institute, FQxI, through the Consciousness in the Physical World program . "The type of research that FQxI is encouraging brings teams together across generations, across boundaries, across disciplines," says Curceanu. "It really can act as incubators of new ideas."
