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

Physicists refer to the loss of these distinct quantum effects as decoherence. Exactly why decoherence occurs—and whether gravity plays a role—remains one of the major open questions in fundamental physics.

Now, new experiments supported by the Foundational Questions Institute (FQxI) have ruled out one prominent explanation involving gravity. The results were published in 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 subatomic particles seems to disappear in the macroscopic world we experience every day,” says Catalina Cruceanu, an FQxI member, research director at the Frascati National Institute of the Italian National Institute of Nuclear Physics (INFN-LNF), and spokesperson for the VIP Collaboration.

Testing gravity’s role in quantum decoherence

The experiment was conducted at the INFN Gran Sasso National Laboratory (INFN-LNGS), the world’s largest underground laboratory specializing in fundamental physics.

The researchers focused on a model proposing that gravity itself could break quantum superposition.

According to Einstein’s theory of general relativity, massive objects curve the fabric of space-time. In the 1960s, Hungarian theoretical physicist Frigyes Károlyházy suggested that small, unavoidable fluctuations in space and time could occur.

In his model, those fluctuations would gradually destroy quantum superposition. This process could help explain why large objects do not remain in the strange combinations of states allowed by quantum mechanics, such as the life-and-death scenario imagined in Schrödinger’s cat thought experiment.

Károlyházy’s ideas have recently been revived, refined and reformulated by Angelo Bassi and colleagues at FQxI.

Searching for traces of weak radiation

The predicted fluctuations in space-time cannot be detected directly. But if they exist, they should produce measurable side effects.

According to the model, the fluctuations would cause charged particles to move randomly and accelerate. Those particles should then emit extremely weak electromagnetic radiation.

Such faint signals are difficult to detect because they can be buried under radiation from other sources, including cosmic rays.

That is why Gran Sasso is useful. The laboratory sits beneath 1.4 kilometers of radiation-attenuating rock, blocking much of the background interference that complicates the search.

“The natural shielding provided by the rock creates one of the quietest environments on Earth to detect very unusual physical phenomena,” Cruceanu says.

The researchers used a detector built around a high-purity germanium crystal about the size of a coffee mug. The crystal was protected by additional layers of copper and lead.

They collected data for 62 days and removed the expected background radiation. The remaining data was then compared with the radiation pattern predicted by the Károlyházy model.

The result was simple: no signal appeared.

What the null result means for quantum gravity

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

Instead, it rules out one important version of the idea, giving physicists clearer boundaries for future theories linking gravity and quantum mechanics.

“This lack of signal is itself an important scientific achievement,” Cruceanu says. “By ruling out one of the oldest and most natural gravity-induced decoherence models, this study narrows the search for a theory that explains the interaction between gravity and quantum mechanics and brings us one step closer to understanding one of the deepest mysteries of fundamental physics.”

From quantum theory to testable physics

Károlyházy’s model is based on the idea that nature may impose fundamental limits on how accurately we can determine the location of objects or measure distances.

Since the model was proposed, similar ideas have appeared in several modern attempts to integrate gravity and quantum mechanics, including string theory and loop quantum gravity.

“Every quantum gravity approach ends by predicting the existence of a minimum length associated with the uncertainties in the measurements of space-time,” says Christian Pisicchia, a quantum physicist at Italy’s Enrico Fermi Research Center/INFN/VIP and principal investigator of the new study.

Quantum gravity is often considered a subject far beyond the reach of current experiments. However, this study adds to evidence that at least some predictions involving gravity and quantum mechanics can already be tested.

“Precision experiments have now reached a level of sensitivity that allows them to test ideas that until recently belonged almost exclusively to the realm of theoretical speculation,” Cruceanu says. “As sensitivity improves, the boundaries between theory and measurement continue to move, opening new possibilities for discovering the fundamental principles governing the universe.”

This research was supported by the Foundational Questions Institute (FQxI) through its Awareness in the Physical World program.

“The type of research that FQxI encourages connects teams across generations, across boundaries, and across disciplines,” Cruceanu says. “It really acts as an incubator for new ideas.”