Gravity has been proposed as a reason quantum superpositions become difficult to observe in everyday objects. A new experiment challenges one longstanding model of this process.
A detector buried deep beneath an Italian mountain has challenged a decades-old theory about why quantum behavior disappears from the world we experience. After 62 days of measurements, researchers found no trace of the faint radiation predicted by a model in which gravity disrupts quantum states. The result rules out this particular explanation while leaving open the possibility that gravity contributes to quantum decoherence.
Decoherence helps explain why particles can display unusual quantum behavior, such as existing in superpositions of different states, while everyday objects appear to follow classical physics. “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, 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.
Could Fluctuations in Spacetime Disrupt Quantum Behavior?
In the 1960s, Hungarian physicist Frigyes Károlyházy proposed that spacetime undergoes constant, extremely small fluctuations. According to his model, these disturbances gradually destroy quantum superpositions, preventing large objects from displaying effects such as those illustrated by Schrödinger’s famous thought experiment involving a cat that is both alive and dead.
The proposal connects quantum mechanics with Einstein’s general theory of relativity, which describes gravity as the curvature of spacetime. Although Károlyházy’s original idea dates back decades, it has continued to attract attention. FQxI researcher Angelo Bassi and his colleagues recently reformulated the model, allowing its predictions to be examined experimentally.
The hypothesized fluctuations cannot be measured directly, but they should have a detectable consequence. They would cause electrically charged particles to undergo tiny random accelerations, producing extremely weak electromagnetic radiation. Detecting that radiation, or establishing its absence, provides a way to test the theory.
Searching for a Faint Signal Deep Underground
The FQxI-funded experiment took place at the INFN Gran Sasso National Laboratory (INFN-LNGS), the world’s largest underground laboratory dedicated to fundamental physics research. Located beneath 1.4 kilometers (0.87 miles) of rock, the facility is shielded from much of the cosmic radiation that could otherwise overwhelm such a weak signal.
“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 high-purity germanium crystal roughly the size of a coffee mug, enclosed within layers of copper and lead shielding. After collecting measurements for 62 days, they accounted for the expected background radiation and searched the remaining data for the signature predicted by Károlyházy’s model. They found no matching signal. The findings were published in the New Journal of Physics.
What the Missing Radiation Means for Gravity
“This absence of a signal is itself a major scientific result,” says Curceanu. The measurements exclude the tested version of Károlyházy’s model, placing new constraints on explanations that attribute quantum decoherence to fluctuations in spacetime.
The findings do not establish that gravity has no role in decoherence. Other gravitational mechanisms remain possible, and the broader relationship between quantum mechanics and general relativity is unresolved. “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,” says Curceanu.
Bringing Quantum Gravity Within Experimental Reach
Károlyházy’s proposal is based on the idea that nature imposes a fundamental limit on how precisely distances and positions can be measured. Related concepts appear in modern efforts to reconcile gravity with quantum mechanics, including string theory and loop quantum gravity. In these approaches, the familiar picture of perfectly measurable spacetime may break down at extremely small scales.
“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 study. The new measurements do not establish whether such a minimum length exists, but they constrain one proposed physical consequence of spacetime uncertainty.
“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.”
Reference: “Experimental exclusion of a generalized Károlyházy gravity-induced decoherence model” by Nicola Bortolotti, Kristian Piscicchia, Alessio Porcelli, Matthias Laubenstein, Simone Manti, Antonino Marcianò, Federico Nola and Catalina Curceanu, 19 June 2026, New Journal of Physics.
DOI: 10.1088/1367-2630/ae774c
The research was supported by the Foundational Questions Institute, FQxI, through the Consciousness in the Physical World program.
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