The fundamental boundary between the quantum realm, where particles exist in multiple states simultaneously, and the macroscopic world of everyday experience remains one of the most significant enigmas in modern science. In a landmark study published in the New Journal of Physics in June 2026, an international team of researchers operating within the depths of the Gran Sasso National Laboratory in Italy has delivered a definitive blow to a long-standing hypothesis: the idea that gravity is the primary force responsible for the "collapse" of quantum states. This research, supported by the Foundational Questions Institute (FQxI), utilizes one of the most sensitive environments on Earth to probe the intersection of Einstein’s general relativity and quantum mechanics, narrowing the search for a unified "Theory of Everything."
The Quantum-Classical Divide: A Century of Mystery
Since the early 20th century, physicists have grappled with the apparent contradiction between the laws governing the very small and the very large. According to quantum mechanics, particles such as electrons and photons exist in a state of superposition—a mathematical combination of all possible locations and states. This phenomenon was famously illustrated by Erwin Schrödinger’s 1935 thought experiment involving a cat that is simultaneously alive and dead until an external observation is made.
However, in the macroscopic world, we never observe cats, chairs, or planets in superpositions. Instead, objects appear to have definite positions and states. The transition from the fuzzy, probabilistic quantum world to the deterministic classical world is known as decoherence. While interaction with the environment—such as air molecules or stray photons—explains much of this transition, physicists have long wondered if there is a more fundamental mechanism at play. Specifically, many have theorized that gravity, the force that shapes the cosmos, might be the silent hand that destroys quantum superpositions.
The Károlyházy Hypothesis and the Fluctuations of Spacetime
The experiment conducted at the National Institute for Nuclear Physics (INFN) targeted a specific model of gravity-induced decoherence proposed in the 1960s by the Hungarian physicist Frigyes Károlyházy. Károlyházy suggested that spacetime is not a perfectly smooth fabric, as envisioned by Albert Einstein, but is instead subject to minute, unavoidable fluctuations at the smallest scales.
In this theoretical framework, these "jitters" in the structure of spacetime act as a source of noise. For a tiny particle, the effect is negligible, allowing it to remain in a quantum superposition. However, as an object grows in mass and size, it interacts with more of these spacetime fluctuations. Eventually, the cumulative effect of these jitters becomes so great that the quantum state is disrupted, causing the object to "settle" into a single classical state.
Recently, this model was refined and reformulated by FQxI member Angelo Bassi and his colleagues, providing specific, testable predictions that could be scrutinized using modern experimental technology. The core prediction of the Károlyházy model is that if spacetime fluctuations exist, they must exert a physical influence on matter.
Hunting for Faint Radiation in the "Silent" Mountain
The primary challenge in testing gravity-induced decoherence is that the predicted effects are incredibly subtle. According to the Károlyházy model, spacetime fluctuations would cause electrically charged particles to undergo a form of random, "jittery" motion. In physics, any time a charged particle is accelerated, it emits electromagnetic radiation. Therefore, if gravity truly causes quantum decoherence through spacetime fluctuations, all matter should be emitting an extremely faint, constant "glow" of radiation.
Detecting this signal is akin to trying to hear a pin drop in the middle of a heavy metal concert. On the surface of the Earth, the planet is bombarded by cosmic rays and background radiation that would instantly overwhelm such a weak signature. To circumvent this, the researchers utilized the INFN Gran Sasso National Laboratory (INFN-LNGS). Located 1.4 kilometers beneath the Gran Sasso d’Italia mountain, the facility is shielded by a massive layer of rock that acts as a natural filter, reducing cosmic ray flux by a factor of one million.
"The natural shielding provided by the rock creates one of the quietest environments on Earth for detecting extremely rare physical phenomena," explained Catalina Curceanu, FQxI member and director of research at the National Laboratory of Frascati (INFN-LNF). "It is only in such an environment that we can hope to isolate the faint radiation predicted by gravity-based quantum models."
Experimental Methodology and the 62-Day Vigil
The experimental setup centered on a high-purity germanium (HPGe) crystal, roughly the size of a coffee mug. Germanium is prized in particle physics for its exceptional energy resolution, making it an ideal candidate for detecting the specific energy signatures of faint radiation. To further isolate the crystal from local background radiation—such as that emitted by the laboratory walls or the researchers themselves—it was encased in multiple layers of ultra-pure lead and copper.
The team, led by Kristian Piscicchia of the Enrico Fermi Research Center and INFN, monitored the detector for a period of 62 days. During this window, they meticulously recorded every photon detection within the crystal. Following the data collection phase, the researchers performed a rigorous statistical analysis to subtract known sources of background interference, such as trace radioactive isotopes within the shielding materials.
The remaining data was then compared against the mathematical predictions of the Károlyházy-Bassi model. The results, published in mid-2026, were unequivocal: no signal was detected. The lack of spontaneous radiation suggests that the spacetime fluctuations envisioned by Károlyházy either do not exist or are far weaker than previously theorized.
A Major Theory Takes a Hit: Analyzing the Implications
The null result of the Gran Sasso experiment does not mean that gravity has no role in the quantum-to-classical transition, but it does systematically eliminate one of the most prominent and "natural" explanations for it. In science, a negative result is often as valuable as a discovery, as it provides tighter boundaries for future theoretical development.
"This absence of a signal is itself a major scientific result," Curceanu noted. "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."
The experiment’s findings have significant implications for the broader field of quantum gravity. Most modern attempts to reconcile general relativity with quantum mechanics, such as String Theory and Loop Quantum Gravity, predict the existence of a "minimal length"—a scale below which the concepts of distance and time cease to exist. These theories often imply some level of spacetime uncertainty or fluctuation. The Gran Sasso results suggest that any such fluctuations must be extremely subtle and do not interact with matter in the way the Károlyházy model predicted.
Chronology of Quantum Foundations Research
To understand the weight of this experiment, it is helpful to view it within the timeline of quantum foundations:
- 1920s: Development of the Copenhagen interpretation, which posits that observation causes the collapse of the wave function.
- 1935: Schrödinger proposes his cat thought experiment to highlight the absurdity of macroscopic superposition.
- 1966: Frigyes Károlyházy proposes that spacetime fluctuations are the physical mechanism behind the quantum-to-classical transition.
- 1980s-90s: Roger Penrose and Lajos Diósi develop independent models suggesting that the gravitational energy difference between superposed states causes collapse.
- 2000s: Environmental decoherence becomes the standard explanation for most classical behavior, but the "measurement problem" remains.
- 2020-2025: Advances in underground detector technology and ultra-pure materials allow for the first high-precision tests of gravity-induced decoherence.
- June 2026: The VIP Collaboration at Gran Sasso publishes results ruling out the Károlyházy model, significantly narrowing the theoretical landscape.
Reactions from the Scientific Community
The physics community has reacted with a mixture of caution and excitement. While the Károlyházy model was a favorite among many theorists due to its elegance, its dismissal forces researchers to look toward more complex explanations.
Kristian Piscicchia, the experimental lead, emphasized that this study proves that quantum gravity is no longer a purely theoretical field. "Every quantum gravity approach ends up with predicting the existence of a minimal length connected to the uncertainty in the measurement of spacetime," Piscicchia said. "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."
Other experts suggest that the focus may now shift toward the Penrose-Diósi model or other "collapse models" that do not rely on the same radiation signature. The search for a "minimal length" in the universe continues, but the parameters for where that length can hide are shrinking.
The Role of FQxI and Future Outlook
The research was facilitated by the Foundational Questions Institute (FQxI) through its "Consciousness in the Physical World" program. This initiative aims to bridge the gap between abstract physics and the fundamental nature of reality. By funding high-risk, high-reward experiments that traditional agencies might overlook, FQxI has enabled a deeper investigation into the foundations of the universe.
"The type of research that FQxI is encouraging brings teams together across generations, across boundaries, across disciplines," Curceanu said. "It really can act as an incubator of new ideas."
As detector sensitivity continues to improve, the VIP Collaboration and other teams at Gran Sasso plan to refine their search. Future experiments may target even lower energy thresholds or utilize different materials to test alternative theories of the quantum-gravity interface. For now, the boundary between the quantum and the classical remains a mystery, but the 2026 Gran Sasso results have successfully closed one of the most significant chapters in that investigation. The universe, it seems, is even more subtle than our most sophisticated models once imagined.
