The fundamental divide between the microscopic realm of subatomic particles and the macroscopic world of human experience remains one of the most enduring enigmas in modern science. At the heart of this mystery lies the transition where the strange, fluid laws of quantum mechanics give way to the rigid, predictable physics described by Isaac Newton and Albert Einstein. While quantum mechanics allows for particles to exist in a "superposition" of multiple states simultaneously—the conceptual foundation of Erwin Schrödinger’s famous thought experiment involving a cat that is both dead and alive—the objects we interact with daily never exhibit such duality. This transition, known as decoherence, is the process by which quantum systems lose their "quantumness" and settle into a single, definite state. For decades, theorists have speculated that gravity might be the hidden hand forcing this transition. However, a landmark experiment conducted at the INFN Gran Sasso National Laboratory and published in the New Journal of Physics in June 2026 has significantly narrowed the field of possibility, ruling out a major gravity-based explanation for why the quantum world disappears.
The Mystery of Quantum Decoherence and the Classical World
In the quantum world, an electron does not exist in a single point in space until it is measured; rather, it exists as a wave of probability. This phenomenon, superposition, is the bedrock of quantum computing and particle physics. Yet, as systems grow larger—moving from single atoms to molecules, and eventually to the trillions of atoms that make up a coffee cup or a human being—these superpositions vanish. This disappearance is called decoherence.
Physicists have long sought to identify the mechanism responsible for decoherence. Some argue that it is simply the result of a system interacting with its environment, such as hitting air molecules or stray photons. However, a more radical school of thought suggests that decoherence is an intrinsic property of nature, perhaps tied to the fundamental structure of spacetime itself. If gravity is involved, it would mean that the very fabric of the universe acts as a constant "observer," preventing large-scale objects from maintaining quantum states.
Catalina Curceanu, a member of the Foundational Questions Institute (FQxI) and the director of research for the VIP Collaboration at the National Laboratory of Frascati, notes that understanding this boundary is vital for the future of physics. "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," Curceanu explains. Her team’s recent work at the Gran Sasso National Laboratory (INFN-LNGS) represents a decisive step in testing whether gravity is the culprit.
The Károlyházy Model: A Gravity-Based Solution
The experiment specifically targeted a model proposed in the 1960s by the Hungarian physicist Frigyes Károlyházy. At the time, Károlyházy suggested that the inherent incompatibility between general relativity and quantum mechanics could be resolved if spacetime itself was not smooth, but instead subject to tiny, unavoidable fluctuations.
According to Einstein’s general theory of relativity, massive objects curve the fabric of spacetime. Károlyházy hypothesized that at extremely small scales, this fabric "shimmers" or fluctuates. These fluctuations would be so minute that they would have no effect on a single electron, allowing it to remain in superposition. However, for a larger object with more mass, these spacetime jitters would accumulate, eventually becoming strong enough to collapse the quantum state. This would explain why a cat—composed of a vast number of atoms—cannot remain in a superposition of being alive and dead; the gravitational fluctuations of spacetime would "nudge" it into one state or the other almost instantaneously.
This model remained a theoretical curiosity for decades until it was recently revived and refined by Angelo Bassi of FQxI and his colleagues. They reformulated the Károlyházy model into a version that could finally be tested using modern, high-precision instrumentation.
Searching for the Ghost of Radiation Under a Mountain
The challenge with testing the Károlyházy model is that the spacetime fluctuations themselves are far too small to be detected directly with current technology. However, the laws of physics dictate that if these fluctuations exist and interact with matter, they must have a measurable side effect.
Specifically, if spacetime is constantly fluctuating, it should cause electrically charged particles (like protons and electrons) to move and accelerate in a random, "jittery" fashion. According to classical electromagnetism, an accelerating charged particle must emit radiation. Therefore, if the Károlyházy model were correct, all matter should be emitting a very faint, constant "hiss" of electromagnetic radiation—a signature of gravity-induced decoherence.
Detecting this signal required an environment of absolute silence. On the surface of the Earth, the planet is bombarded by cosmic rays and background radiation that would easily drown out such a faint signal. To escape this noise, the researchers moved their experiment to the INFN Gran Sasso National Laboratory in Italy. Located 1.4 kilometers beneath the rock of the Apennine Mountains, Gran Sasso is the world’s largest underground laboratory for fundamental physics. The overhead rock acts as a massive shield, reducing the flux of cosmic rays 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," says Curceanu. Inside this subterranean fortress, the team used a detector centered around a high-purity germanium crystal about the size of a coffee mug. This crystal was further shielded by layers of ultra-pure copper and lead to block any radiation from the surrounding laboratory environment.
Data Collection and the Significant "Null Result"
The experimental lead, Kristian Piscicchia, a quantum physicist at the Enrico Fermi Research Center and INFN/VIP, oversaw a rigorous data collection period lasting 62 days. The team monitored the germanium crystal for any signs of the specific radiation pattern predicted by the reformulated Károlyházy model.
After two months of observation, the researchers performed a complex statistical analysis to subtract the known background radiation—radiation from the internal components of the detector and the remaining trace amounts of cosmic rays. Once the noise was removed, the researchers looked for the "residue" that would signify spacetime fluctuations.
The result was definitive: no such signal appeared. The radiation levels detected were consistent with standard background noise, and no additional "hiss" from gravity-induced movement was found.
While a "no signal" result might sound like a failure to a layperson, in the world of fundamental physics, it is a major breakthrough. By failing to find the predicted radiation, the researchers have effectively ruled out the Károlyházy model in its current form. It proves that if gravity does play a role in quantum decoherence, it does not do so in the manner Károlyházy and his successors envisioned.
Implications for Quantum Gravity and String Theory
The dismissal of the Károlyházy model has broad implications for the search for a "Theory of Everything." Currently, the two pillars of physics—General Relativity (which describes the very large) and Quantum Mechanics (which describes the very small)—are mathematically incompatible. Most attempts to unite them, such as String Theory and Loop Quantum Gravity, predict the existence of a "minimal length" or a fundamental "graininess" to spacetime.
"Every quantum gravity approach ends up with predicting the existence of a minimal length connected to the uncertainty in the measurement of spacetime," explains Piscicchia. The Károlyházy model was one of the most natural ways to link this graininess to the observed classical world. By ruling it out, physicists must now look toward more complex or subtle theories.
The experiment also demonstrates that the field of "Quantum Gravity," often thought to be purely theoretical and untestable for centuries to come, is entering an experimental phase. We may not be able to see the "atoms of space" yet, but we are beginning to see where they are not.
Timeline of the Search for Quantum Foundations
To understand the weight of this discovery, one must look at the timeline of how we arrived at this point:
- 1926: Erwin Schrödinger formulates the wave equation, and the debate over the "measurement problem" begins.
- 1935: Schrödinger proposes his cat thought experiment to highlight the absurdity of macroscopic superposition.
- 1960s: Frigyes Károlyházy proposes that spacetime fluctuations cause quantum collapse, creating a bridge between gravity and the classical world.
- 1980s-90s: Theoretical work on "Objective Collapse" models (like the Ghirardi-Rimini-Weber model) gains traction.
- 2010s: The VIP Collaboration begins using ultra-pure germanium detectors at Gran Sasso to test the limits of the Pauli Exclusion Principle and quantum collapse models.
- 2020s: FQxI supports the reformulation of the Károlyházy model, providing specific, testable predictions for radiation emission.
- June 2026: Publication of the Gran Sasso results in the New Journal of Physics, officially ruling out the primary Károlyházy model.
The Role of FQxI and Future Prospects
The research was supported by the Foundational Questions Institute (FQxI) through its Consciousness in the Physical World program. This program focuses on the deep links between the physical laws of the universe and the nature of reality. Curceanu credits this support for allowing the team to tackle high-risk, high-reward questions that mainstream funding might overlook.
"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."
Looking ahead, the VIP Collaboration and other teams at Gran Sasso are already planning more sensitive versions of the experiment. While the Károlyházy model has been constrained, other "Objective Collapse" theories remain. Future experiments will use even larger detectors and longer observation times to search for even fainter signals.
"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," Curceanu concludes. "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 absence of a signal at Gran Sasso has not closed the book on the relationship between gravity and the quantum world. Instead, it has turned the page, forcing physicists to refine their search for the mechanism that keeps our world feeling solid, singular, and classical. The mystery of Schrödinger’s cat remains, but the list of possible explanations is getting shorter.
