A Coffee-Mug-Sized Germanium Crystal Under 1.4 Kilometers of Italian Rock Listened for 62 Days for the Radiation a 1960s Theory Says Gravity Should Emit While It Kills Schrödinger's Cat. It Heard Nothing, and That Rules the Theory Out.

Somewhere between an atom and a cat, the rules change. Quantum mechanics lets a particle sit in two states at once, but nobody has ever seen a cat that is both alive and dead, and the process by which quantum fuzziness gives way to everyday certainty, called decoherence, is still not fully explained. One of the oldest proposals is that gravity does the job. A new experiment deep under the Apennines has now tested one of the most natural versions of that idea and found no trace of it.

The theory dates to the 1960s, when the Hungarian physicist Frigyes Károlyházy proposed that spacetime itself constantly ripples with tiny fluctuations, and that those ripples gradually erode superpositions, preventing large objects from ever existing in the kind of quantum combination the cat paradox describes. The model has been revived and reformulated in recent years, notably by Angelo Bassi and colleagues, because it rests on an idea that keeps reappearing in modern attempts to unify quantum theory with gravity: that there is a fundamental limit to how precisely a length can be measured. "Every quantum gravity approach ends up predicting the existence of a minimal length connected to the uncertainty in the measurement of spacetime," said Kristian Piscicchia of the Enrico Fermi Research Center and INFN, the experimental lead on the study.

The fluctuations cannot be seen directly, but they leave a fingerprint. If spacetime jitters, charged particles should jitter with it, accelerating randomly and shedding faint trails of electromagnetic radiation. The trouble is that this radiation would be swamped by cosmic rays and ordinary background at the surface. So the VIP collaboration took the measurement to the INFN Gran Sasso National Laboratory, the world's largest underground physics lab, where 1.4 kilometers of rock overhead filter out most of the noise. "The natural shielding provided by the rock creates one of the quietest environments on Earth for detecting extremely rare physical phenomena," said Catalina Curceanu, research director at INFN's Frascati laboratory and the collaboration's spokesperson.

The detector was a piece of high-purity germanium crystal about the size of a coffee mug, wrapped in layers of copper and lead. The team, led by Nicola Bortolotti, collected data for 62 days, subtracted the expected background, and searched for the specific spectral signature the generalized Károlyházy model predicts. It was not there. The result, published in the New Journal of Physics, excludes the model across the parameter range it tested.

"This absence of a signal is itself a major scientific result," Curceanu said. "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." The finding does not eliminate gravity as a player in decoherence altogether; other gravitational collapse models make different predictions, and some remain untested. What it does is remove one specific mechanism from the table and show where the remaining ones have to live.

The larger point, the researchers argue, is methodological. Quantum gravity is usually described as physics that can only be probed at energies no machine will ever reach. This experiment probed one of its consequences with a crystal, some shielding and two months of patience. "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," Curceanu said. "As sensitivity improves, the boundary between theory and measurement continues to move."

Put simply: a popular explanation for why the quantum world looks ordinary at our scale predicted a faint glow from charged particles being shaken by spacetime. A very quiet detector in a very deep tunnel looked for that glow and found none. Gravity may still be involved in collapsing the cat's superposition, but not the way this theory said.