The boundary between the microscopic world of subatomic particles and the macroscopic reality of human experience remains one of the most significant frontiers in modern science. In a landmark study published in the New Journal of Physics in June 2026, researchers at the Italian National Institute for Nuclear Physics (INFN) have announced findings that challenge a long-standing hypothesis regarding why the strange behavior of quantum mechanics does not manifest in everyday objects. Supported by the Foundational Questions Institute (FQxI), the experiment conducted deep beneath the Apennine Mountains has effectively ruled out a prominent model of gravity-induced decoherence, narrowing the search for a unified theory of the universe.
For over a century, physicists have grappled with the apparent contradiction between two pillars of science: General Relativity, which describes gravity and the large-scale structure of the cosmos, and Quantum Mechanics, which governs the behavior of atoms and particles. While quantum mechanics allows for particles to exist in a state of superposition—being in multiple places or states simultaneously—the objects we interact with daily, from coffee mugs to planets, always appear to occupy a single, definite state. This transition from the quantum to the classical is known as decoherence, and identifying its underlying mechanism is essential to understanding the fundamental nature of reality.
The Mystery of the Macroscopic World
The conceptual cornerstone of this mystery is often illustrated by Erwin Schrödinger’s famous 1935 thought experiment involving a cat that is simultaneously alive and dead until observed. While the experiment was originally intended to highlight the perceived absurdity of quantum mechanics when applied to large-scale objects, it remains a literal description of how particles behave. In a laboratory setting, photons and electrons regularly demonstrate superposition. However, as systems grow in size and complexity, these quantum effects seem to "leak" into the environment, causing the system to collapse into a single state.
Physicists have long debated whether this decoherence is merely a result of interaction with the surrounding environment—such as air molecules or stray light—or if there is an intrinsic "collapse" mechanism built into the laws of nature. One of the most compelling theories suggests that gravity itself might be the culprit. If gravity is the force that prevents quantum behavior from scaling up, then the very fabric of spacetime would be responsible for the classical world we perceive.
The Károlyházy Model and Spacetime Fluctuations
The specific theory under investigation in this new study dates back to the 1960s, proposed by the Hungarian physicist Frigyes Károlyházy. He suggested that the structure of spacetime is not smooth, as Einstein’s General Relativity suggests, but is instead subject to tiny, fundamental fluctuations. Károlyházy hypothesized that these fluctuations impose a limit on how precisely any distance or position can be measured.
According to this model, when a massive object is placed in a quantum superposition, the different "versions" of the object would interact with these spacetime fluctuations in slightly different ways. Over a very short period, these interactions would cause the phases of the quantum states to drift apart, leading to the rapid destruction of the superposition. This "intrinsic decoherence" would happen regardless of how well an experiment is shielded from external environmental noise, making gravity the ultimate arbiter of classical reality.
In recent years, this model was refined and reformulated by Angelo Bassi, a professor at the University of Trieste and an FQxI member, along with his colleagues. Their work turned Károlyházy’s theoretical framework into a set of testable predictions, specifically identifying a "faint radiation signature" that should exist if these spacetime fluctuations are real.
Searching for Silence in Gran Sasso
Testing such a theory requires an environment of near-absolute "cosmic silence." Any experiment designed to detect the subtle effects of gravity on quantum states would be overwhelmed by the background noise of the Earth’s surface, including cosmic rays, radio waves, and seismic activity. To overcome this, the VIP (Violation of the Pauli Exclusion Principle) Collaboration utilized the INFN Gran Sasso National Laboratory (LNGS).
Located 1,400 meters beneath the rock of the Gran Sasso mountain in Italy, the laboratory is the largest underground facility of its kind in the world. The massive overhead rock provides a natural shield equivalent to 3,800 meters of water, reducing the flux of cosmic ray muons by a factor of one million compared to the surface.
"The natural shielding provided by the rock creates one of the quietest environments on Earth for detecting extremely rare physical phenomena," explained Catalina Curceanu, the director of research and spokesperson for the VIP Collaboration at the National Laboratory of Frascati.
The experimental setup centered on a high-purity germanium (HPG) crystal, roughly the size of a coffee mug. Germanium is a semiconductor that is exceptionally sensitive to low-energy radiation. The theory predicted that if Károlyházy’s spacetime fluctuations existed, they would cause electrically charged particles within the crystal—specifically protons and electrons—to undergo a jittery, random motion. According to the laws of electromagnetism, any accelerating charged particle must emit radiation. In this case, the radiation would be an extremely faint X-ray or gamma-ray signal.
Chronology of the 62-Day Search
The experiment followed a rigorous timeline of data acquisition and analysis to ensure the highest possible accuracy:
- Preparation and Shielding: The germanium detector was encased in additional layers of ultra-pure copper and ancient Roman lead (which is naturally low in radioactivity) to block any trace radiation from the laboratory walls.
- Data Collection: The researchers monitored the detector for a continuous period of 62 days. During this time, every energy pulse within the crystal was recorded.
- Background Characterization: Parallel to the experiment, the team conducted an exhaustive audit of "known" background noise, including the decay of trace isotopes within the detector materials themselves.
- Signal Comparison: Using the mathematical refinements provided by Bassi’s team, the researchers calculated the exact spectrum of radiation that the Károlyházy model should produce.
- Statistical Analysis: In June 2026, after months of post-collection processing, the team compared the theoretical prediction against the experimental data.
The results were definitive: no signal matching the Károlyházy prediction was observed. The data perfectly matched the expected background noise, leaving no room for the radiation signature predicted by this specific gravity-induced decoherence model.
Implications for Quantum Gravity
The "null result" of the experiment is a significant milestone in fundamental physics. While it might seem like a disappointment to some that a long-standing theory was not confirmed, in the world of high-stakes physics, ruling out a possibility is as valuable as finding a signal.
"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 failure to detect the Károlyházy signal provides physicists with "tighter boundaries." It suggests that if gravity does cause quantum collapse, it must do so through a mechanism that is much weaker or more complex than previously thought. This has direct implications for other major frameworks in physics, such as String Theory and Loop Quantum Gravity. Most modern attempts to reconcile Einstein with quantum mechanics predict the existence of a "minimal length" or a "pixelation" of spacetime. The Gran Sasso findings suggest that certain versions of these predictions are now empirically untenable.
Kristian Piscicchia, the experimental lead on the study from the Enrico Fermi Research Center, emphasized that the field is entering a new era. "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," he said.
The Broader Scientific Reaction
The physics community has reacted with cautious optimism toward the findings. While the Károlyházy model was a "natural" candidate for explaining decoherence because it relied on the inherent properties of spacetime, its exclusion allows researchers to focus on other competing theories, such as the Diósi-Penrose model. Proposed by Lajos Diósi and Nobel laureate Roger Penrose, this alternative model also links gravity to quantum collapse but through a different mathematical pathway that does not necessarily produce the same radiation signature as the Károlyházy model.
Furthermore, the study demonstrates the growing power of "tabletop" or small-scale precision physics. Traditionally, progress in fundamental physics was associated with massive particle accelerators like the Large Hadron Collider (LHC). However, the VIP Collaboration’s work shows that deep-underground, low-energy experiments can provide equally profound insights into the nature of the universe at a fraction of the cost.
Future Directions and the Role of FQxI
The research was made possible through the support of the Foundational Questions Institute (FQxI) as part of its "Consciousness in the Physical World" program. FQxI focuses on high-risk, high-reward research that explores the very foundations of physics and cosmology.
As technology improves, the VIP Collaboration intends to increase the sensitivity of their detectors even further. Future iterations of the experiment may involve using different materials or cooling the detectors to temperatures near absolute zero to further reduce thermal noise. The goal is to push the boundaries of detection until either a signal is found or all current models of gravity-induced collapse are ruled out.
"As sensitivity improves, the boundary between theory and measurement continues to move," Curceanu concluded. "The type of research that FQxI is encouraging brings teams together across generations and disciplines. It acts as an incubator for new ideas that eventually lead us to the fundamental principles governing our universe."
The June 2026 findings mark a closing chapter for one specific 60-year-old theory, but they simultaneously open a new chapter in the quest to understand why the world of the very small looks so different from the world of the very large. For now, Schrödinger’s cat remains in a state of theoretical limbo, but the walls are closing in on the mysteries of the quantum-classical divide.
