No signal underground: Scientists challenge a decades-old idea about how quantum reality breaks down. Schrödinger’s cat has a new problem

While Schrödinger's famed cat was simply a thought experiment, the actual physics question being asked in the thought experiment remains quite real – namely, why does the quantum realm behave so differently than our familiar classical world?

According to quantum mechanics, particles and other systems at the microscopic level can be described by a combination of potential states, a phenomenon called superposition.

However, we do not observe such effects in macroscopic objects.


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One of the theories proposed that gravity might be what causes quantum phenomena not to manifest in the macroscopic world. In a recently conducted experiment, this hypothesis was subjected to a very thorough test – and proved to be unsupported by any findings.

The scientists conducting research at the Gran Sasso National Laboratory in Italy searched for 62 days for a very weak form of radiation that would be created if quantum superpositions were disrupted by fluctuations in the fabric of space-time.

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No trace of the expected radiation was detected.

This negative result, reported in the New Journal of Physics in June 2026, does not resolve the question of quantum decoherence. It does, however, rule out one of the proposed mechanisms of how it could be caused by gravity.

Deep beneath a mountain, physicists looked for a quantum clue

The experiment took place at INFN’s Gran Sasso National Laboratory, located beneath around 1.4 kilometres of rock in central Italy.

The location matters because experiments searching for extremely weak signals face a basic problem: Earth is constantly being bombarded by radiation. Cosmic rays and naturally occurring radioactive materials can create signals that overwhelm the phenomenon researchers are trying to detect.

The rock above Gran Sasso provides substantial shielding, making the underground facility one of the world's important sites for experiments involving exceptionally rare events.

For this study, scientists used a highly pure germanium crystal roughly the size of a coffee mug. The detector was surrounded by layers of protective materials, including copper and lead, to reduce unwanted background radiation.

The team collected measurements for 62 days.

Their target was not spacetime itself. The proposed fluctuations could not be observed directly. Instead, researchers looked for a secondary effect that the theory predicted should be easier to detect.

That distinction is important. The experiment was effectively asking whether an invisible process could leave behind a tiny, measurable fingerprint.

It did not.

The theory predicted a faint flash of radiation

The idea tested by the researchers goes back to Hungarian physicist Frigyes Károlyházy, who proposed in the 1960s that spacetime might possess unavoidable fluctuations at very small scales.

The proposal starts with a difficult problem in modern physics. General relativity describes gravity as the curvature of spacetime, while quantum mechanics describes matter and energy at microscopic scales. Bringing the two descriptions together remains an unresolved challenge.

Károlyházy suggested that there could be a fundamental limit to how precisely spacetime can be defined. Those fluctuations, in turn, could interfere with quantum superpositions.

In simple terms, a quantum system could begin in a combination of possible states but gradually lose its distinctly quantum character because of these fluctuations.

Modern versions of the idea have been developed by other physicists, including FQxI member Angelo Bassi and collaborators.

The experimental prediction provided a way to test the proposal.

If spacetime fluctuations disturbed charged particles, those particles should undergo tiny random accelerations. Accelerating electric charges emit electromagnetic radiation. The predicted radiation would be incredibly weak, but a sufficiently sensitive detector in a sufficiently quiet environment could potentially reveal it.

That gave researchers something concrete to search for.

Sixty-two days of data produced no predicted signal

After collecting the data, the researchers accounted for the background radiation expected from the experimental environment. They then compared what remained with the radiation pattern predicted by the Károlyházy model.

The expected signal did not appear.

For a theory that predicts a specific experimental signature, that absence matters. It means the tested version of gravity-induced quantum decoherence cannot simply be assumed to operate at the strength required by the model.

But the result needs to be interpreted carefully.

The researchers have not demonstrated that gravity plays no role in quantum mechanics. They have also not explained why quantum superpositions disappear from the macroscopic world.

Instead, the experiment narrows the range of possibilities.

That is particularly relevant to the broader question of quantum decoherence — the process through which a quantum system loses the observable characteristics of superposition when it interacts with its surroundings.

Environmental interactions are already known to cause decoherence. What remains less clear is whether there is also a more fundamental mechanism built into nature itself, perhaps involving gravity or the structure of spacetime.

This experiment removes one proposed mechanism from the list of possibilities, at least within the range tested.

Schrödinger’s cat still has no final answer

Schrodinger himself came up with the well-known idea of the cat experiment in 1935 in order to point out the bizarre implications of the quantum mechanics when applied to objects found in everyday life.

The idea was never supposed to suggest putting an actual cat into a quantum superposition state in order to see what would happen; rather, it was to show an absurdity of the concept if quantum laws hold everywhere, why there are no combined states for macroscopic systems, like they have in microscopic scale?

More than 90 years have passed since then, but the issue is still actual.

It can be demonstrated by the recent Gran Sasso experiment, which proves the approach taken nowadays to the solution of the problem.

Instead of discussing where the border between the two types of physics exists, the scientists can design experiments aimed at testing the mechanisms, which allow for the transition.

The absence of the expected radiation does not mean that physicists have no results.

It allows establishing a stricter limit on the theories explaining connection of gravity, space-time fluctuations and quantum decoherence.

FAQ

1. What is the concept of Schrödinger's cat?

Schrödinger's cat refers to a well-known thought experiment meant to highlight a fundamental conceptual difficulty in quantum mechanics. In this thought experiment, the cat's status is made to hinge on some quantum process, resulting in a situation where the cat exists in a superposition of being both alive and dead prior to observation.

2. What was Gran Sasso experiment searching for?

The scientists looked for very faint emissions of electromagnetic waves that could occur as a result of spacetime fluctuations making charges oscillate randomly, according to the gravity-induced decoherence hypothesis tested.

3. Was it confirmed that there is no relationship between gravity and quantum mechanics?

No. There were no observed signals as predicted by the specific gravity-induced decoherence hypothesis tested. This experiment has put constraints on this particular model but has not excluded all possible relations between gravity and quantum decoherence.

4. Why was it done underground?

Gran Sasso is located under about 1.4 kilometers of rock that helps shield the experiments from cosmic radiation and other interferences.