Beneath Gran Sasso, a Detector Finds No Sign That Gravity Kills Quantum Superpositions

Physicists have placed a detector far beneath Italy’s Gran Sasso mountain, and it failed to find anything. The experiment was searching for faint radiation that could reveal whether gravity ruins quantum superpositions, and it did not detect it. The team reported no sign of the signal they expected after 62 days of measurements, and the result has now been published in the New Journal of Physics.

The Foundational Questions Institute (FQxi) funded the experiment, which put to the test one of the longest-standing proposals about why quantum mechanics fails at everyday scales. The proposal traces back to Hungarian physicist Frigyes Károlyházy, who suggested in the 1960s that gravity itself could be the reason quantum superpositions collapse.

Decoherence and the Cat

A cat inside a box in Schrödinger’s thought experiment is simultaneously alive and dead until an observer opens the box and looks inside. This illustrates the weird state of superposition that quantum mechanics says allows particles to exist in multiple states at once. In everyday reality, by contrast, objects behave normally: a cat is either alive or dead, not both.

Decoherence describes the move from the odd world of quantum physics to the ordinary. It has puzzled scientists for decades what causes it, and one possible answer suggests gravity plays a part. Károlyházy’s proposal holds that gravity brings about small, unpreventable changes in spacetime, and these changes steadily break down superpositions.

The Detector in the Rock

Beneath 1.4 kilometers of rock, where the layers above shield out much of the background radiation that would otherwise swamp a signal this faint, stands the Gran Sasso National Laboratory. Inside it, the detector at the heart of the experiment is built around a coffee-mug-sized piece of high-purity germanium crystal, encased within layers of copper and lead.

For 62 days they collected their data, then stripped away the expected background radiation. What was left behind was compared against the signal that the Károlyházy model predicted. The outcome was straightforward: no signal appeared.

The fact that the instrument detected nothing is a significant scientific finding in its own right. By eliminating one of the oldest and most natural gravity-induced decoherence models, the research tightens the hunt for the theory that explains how gravity and quantum mechanics interact.

The Quietest Environment on Earth

The leader of the research team and the spokesperson for the VIP Collaboration at the National Laboratory of Frascati of the National Institute for Nuclear Physics (INFN-LNF) in Italy, Curceanu, described the setting directly. She said that “The natural shielding provided by the rock creates one of the quietest environments on Earth for detecting extremely rare physical phenomena,” they were testing for a particle that would be nearly invisible among countless neutrinos, and that the experiment was designed to catch any sign of it.

The whole point of the setup is that quiet. The experiment is hunting for a signal so delicate that it could vanish entirely beneath noise from other sources, including cosmic rays. The rock above the laboratory does most of the hard work, cutting down the noise that would otherwise bury the signal.

The Károlyházy Model

Angelo Bassi and his team at Károlyházy ‘s idea has continued to attract attention and was recently revived, refined and reformulated by FQxi’ have developed a model that rests on the assumption that there is a natural limit to the precision with which we can measure the position of an object or gauge a distance.

Since the model was proposed, similar ideas have appeared in several modern attempts to unite gravity and quantum mechanics, 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.

The model predicts that gravity produces small, unavoidable disturbances in spacetime. As time passes, those disturbances would slowly interrupt quantum superpositions. That mechanism could offer an explanation for why sizable objects do not stay in the bizarre combinations of states permitted by quantum mechanics.

Detecting the Faint Signal

Though the spacetime fluctuations cannot be detected directly, they would leave an observable trace if they existed.

• Electrically charged particles would move and accelerate randomly
• Those particles would give off extremely faint electromagnetic radiation

Picking out that faint signal is hard because it might get lost under radiation from other sources, including cosmic rays.

That is why Gran Sasso serves its purpose so well. The laboratory is buried under 1.4 kilometers of rock, and that rock shields it from radiation, blocking much of the background interference that would otherwise make the search difficult.

A Narrow Path Ahead

This outcome removes one potential explanation rather than settling the debate between gravity and quantum decoherence. The fact that no signal was detected is itself a significant scientific finding. By eliminating one of the oldest and most natural gravity-induced decoherence models, the research tightens the hunt for the theory that explains how gravity and quantum mechanics interact.

This discovery does not establish that gravity has no influence on quantum decoherence. It eliminates one significant explanation for that connection and narrows the limits on what future theories about gravity’s relationship to quantum mechanics can include.

Curceanu called the outcome a major scientific finding. By dismissing one of the most natural and oldest gravity-induced decoherence models, the work narrows down the hunt for the theory that maps the link between gravity and quantum mechanics, bringing us one step closer to grasping one of the deepest mysteries in fundamental physics.

The findings rule out one prominent explanation tied to gravity. They appeared in the New Journal of Physics in June 2026.

The Experiment at a Glance

The study took place at the INFN Gran Sasso National Laboratory (INFN-LNGS), the globe’s largest underground facility dedicated to fundamental physics. The apparatus sits within heavy shielding, and the data collection spanned a significant length of time so that the investigators could separate their signal from the constant background radiation that would otherwise mask it.

The experiment’s method was simple. According to the Károlyházy model, gravity causes fluctuations in spacetime that should produce a weak electromagnetic signal from particles accelerating in response. By removing the expected background radiation, the researchers could compare what was left against the predicted signal. Nothing matched.

This outcome advances the field rather than shutting it down. It demonstrates that one version of the gravity-decoherence hypothesis fails, and it gives physicists a clearer path to testing other ideas in the years ahead.

Source material: “What kills Schrödinger’s cat? Gravity may not be the answer,” ScienceDaily.

experiment · gravity · physics · quantum mechanics · superposition