What kills Schrödinger’s cat? Gravity may not be the answer

A deep-underground experiment found no sign that gravity erases quantum weirdness in the way a famous decades-old theory predicted.

At some point between the tiny world of particles and atoms and the much larger world we experience every day, quantum behavior appears to fade away.

Quantum mechanics allows particles to exist in combinations of possible states, a phenomenon known as superposition. That strange feature inspired Schrödinger's famous thought experiment involving a cat that is both alive and dead until it is observed. Yet objects in the everyday world do not behave that way.

Physicists call the loss of these distinctly quantum effects decoherence. Exactly why decoherence happens, and whether gravity plays a role, remains one of the major open questions in fundamental physics.

Now, a new experiment supported by FQxI has eliminated one prominent explanation involving gravity. The findings were published 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.

Testing Gravity's Role in Quantum Decoherence

The experiment was carried out at the INFN Gran Sasso National Laboratory (INFN-LNGS), the world's largest underground laboratory devoted to fundamental physics.

Researchers focused on a model proposing that gravity itself could help destroy quantum superpositions.

According to Einstein's general theory of relativity, massive objects curve the fabric of spacetime. In the 1960s, Hungarian theoretical physicist Frigyes Károlyházy suggested that spacetime may also undergo tiny, unavoidable fluctuations.

In his model, those fluctuations would gradually disrupt quantum superpositions. That process could help explain why large objects do not remain in the strange combinations of states allowed by quantum mechanics, such as the dead-and-alive situation imagined in Schrödinger's cat thought experiment.

Károlyházy's idea has continued to attract attention and was recently revived, refined and reformulated by FQxI's Angelo Bassi and colleagues.

Searching for a Faint Radiation Signature

The predicted spacetime fluctuations cannot be detected directly. But if they exist, they should have a measurable side effect.

They would cause electrically charged particles to move and accelerate randomly. In turn, those particles should emit extremely faint electromagnetic radiation.

Detecting such a weak signal is difficult because it could easily be buried beneath radiation from other sources, including cosmic rays.

That is why Gran Sasso is so useful. The laboratory sits beneath 1.4 kilometers of radiation-dampening rock, which blocks much of the background interference that would otherwise complicate 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 built around a coffee-mug-sized piece of high-purity germanium crystal, protected by additional layers of copper and lead.

They gathered data for 62 days, then removed the amount of background radiation they expected to see. What remained was compared with the radiation pattern predicted by the Károlyházy model.

The result was simple: no signal appeared.

A Major Theory Takes a Hit

The finding does not prove that gravity has nothing to do with quantum decoherence.

Instead, it rules out one important version of that idea and gives physicists tighter boundaries for future theories connecting gravity with quantum mechanics.

"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."

From Quantum Theory to Testable Physics

Károlyházy's model is based on the idea that nature may impose a fundamental limit on how precisely we can determine the position of an object or measure 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.

Quantum gravity is often viewed as a subject that lies far beyond the reach of present-day experiments. But this study adds to growing evidence that at least some predictions involving both gravity and quantum mechanics can already be tested.

"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."

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."