Abstract:
Why can atoms and elementary particles exhibit strange quantum behaviors, while the macroscopic objects we interact with every day follow familiar classical physical laws? Physicists have long been trying to solve this mystery. Recently, a new study conducted in an underground laboratory in Italy rigorously tested a theory proposed in the 1960s. The researchers measured continuously for 62 days but did not find the weak radiation signal predicted by the theory, thus ruling out this specific model as an explanation of the mechanism of the disappearance of quantum phenomena.
This research was completed by relevant researchers from the Italian National Institute of Nuclear Physics, and the research results were published in the "New Journal of Physics" on June 19, 2026. The paper is titled "Experimental exclusion of a generalized Karoi-Hazy gravity-induced decoherence model." The research results do not mean that there is no connection between gravity and quantum mechanics, but it eliminates a theoretical path that has attracted attention from the physics community in order to understand why the macroscopic world does not exhibit obvious quantum effects.
The research involves one of the most famous thought experiments in quantum physics - Schrödinger's cat.
In 1935, Austrian physicist Erwin Schrödinger proposed a thought experiment: placing a cat in a closed box with radioactive atoms, poison gas devices, etc. If the atoms decay, the device releases poisonous gas and the cat dies; if the atoms do not decay, the cat survives. According to the description of the entire system by quantum mechanics, the system may be in a quantum superposition of different states before measurement occurs, resulting in the famous paradox of a cat being both alive and dead.
Schrödinger proposed this thought experiment not to claim that cats in reality can really live and die at the same time in the everyday sense, but to reveal the deep problems faced when quantum theory is applied to the macroscopic world.
In the microscopic world, particles can be in a superposition of multiple possible states and exhibit quantum characteristics such as interference. However, objects in everyday life do not exhibit the same obvious phenomenon. Physicists refer to the process in which quantum interference features become increasingly difficult to observe after a quantum system interacts with its surrounding environment as "decoherence."
Environmental interactions can explain why many macroscopic objects appear to follow the laws of classical physics, but what is the relationship between quantum mechanics and gravity, and whether gravity also actively destroys quantum superposition, are still open questions in basic physics.
In the 1960s, the Hungarian theoretical physicist Frijesh Karojhazy proposed a theory that attempted to explain this problem. He believes that there may be extremely small fluctuations in space and time that cannot be completely avoided, and these fluctuations may gradually destroy the superposition state of the quantum system, making it difficult for large objects to continue to exhibit quantum behavior.
This line of thought attempts to establish a connection between quantum mechanics and Einstein's general theory of relativity. General relativity describes gravity as the result of the curvature of space-time, and Karoljhazy's theory goes further to imagine that there may be tiny uncertainties in space-time itself, which could affect the evolution of quantum systems.
Decades after this theory was proposed, relevant researchers have reformulated and improved it to make its predictions subject to more rigorous experimental testing. The researchers hope to experimentally determine whether the hypothetical fluctuations in space-time actually affect matter in measurable ways.
The problem is that the space-time fluctuations envisioned by the theory are extremely weak and cannot be measured directly and routinely. However, if these fluctuations do exist and can randomly accelerate charged particles, then the particles should produce extremely weak electromagnetic radiation.
In other words, researchers don't have to directly observe space-time itself, but simply look for the radiation signals predicted by theory, giving them a chance to test the model. If the signal does exist, it will provide support for the theory; if the experimental sensitivity is high enough but the expected signal is never found, researchers can limit or even exclude the corresponding theoretical parameters and models.
To complete this experiment, the research team chose the Gran Sasso National Laboratory in Italy. The laboratory is located about 1.4 kilometers underground in the Gran Sasso Mountains and is one of the largest underground laboratories for basic physics research in the world.
Underground rock formations can shield a large amount of cosmic rays and other background radiation, reducing external interference. This natural shielding is important when looking for extremely weak signals, since radiation from the environment can be far stronger than the target signal the researchers hope to capture.
In the experiment, the researchers used a high-purity germanium crystal as the core component of the detector, which is roughly the size of a coffee cup. A multi-layer copper and lead shielding structure is also installed on the outside of the detector to further reduce the impact of background radiation.
The research team collected data for 62 consecutive days, then analyzed and subtracted the expected background radiation from the measurement results, and then checked whether the remaining data contained the specific radiation signal predicted by the Karoljhazy model.
Ultimately, the researchers found no signals that matched the model's predictions.
This result means that the experimentally tested Karoyhazy gravity-induced decoherence model cannot be supported by the observational results. The researchers thus ruled out this particular version of the theory, setting new constraints for future studies trying to explain the relationship between quantum decoherence and gravity.
Catarina Curceanu, a member of the research team and head of research at the Frascati National Laboratory of the Italian National Institute of Nuclear Physics, pointed out that the absence of the expected signal is itself an important scientific result. By excluding one long-established and representative model of gravity-induced decoherence, the researchers could further narrow their search for the relevant physical mechanism.
However, this study does not prove that gravity can never affect quantum systems, nor does it solve the larger problem of how to unify quantum mechanics and general relativity.
Different theories may link gravity to quantum decoherence through other mechanisms, and whether these mechanisms are true still requires further experimental verification. What this experiment was able to rule out were specific models that were tested, not all ideas about how gravity affects quantum behavior.
The significance of the research is also that it shows the process of basic physics theories gradually moving from mathematical deduction to precise experimental testing. In the past, some theories involving the structure of space-time and quantum gravity relied mainly on mathematical analysis and conceptual research, making it difficult to design experiments that could directly test their predictions. As detector sensitivity continues to improve, some theories have begun to yield concrete predictions that can be tested against experimental data.
Karoyhazy's thoughts also involve another important question: whether humans can measure distances and positions in space and time with infinite accuracy.
In the classical physics picture, one can generally envision measuring the position of objects with increasing accuracy. However, in theoretical research trying to unify quantum mechanics and gravity, whether space-time still has continuous properties that can be measured with infinite precision on extremely small scales has always attracted much attention. Some lines of quantum gravity research, including string theory and loop quantum gravity, involve re-understanding the structure of space-time at extremely small scales, but different theories handle this issue differently.
Quantum physicist Christian Pischia, who participated in the experiment, pointed out that many ideas in quantum gravity research involve the concept of minimum length scales related to the uncertainty of space-time measurements. However, this experiment does not prove whether the minimum length actually exists, but rather limits a specific model that attempts to link space-time uncertainty to quantum decoherence.
Researchers believe that precision experiments are gradually becoming capable of testing some physical hypotheses that have long remained in the theoretical speculation stage. As experimental technology further develops, scientists are expected to be able to judge in a more rigorous way which theories can describe the real world and which theories need to be revised or abandoned.
From Schrödinger's cat to space-time fluctuations to quantum gravity, this research does not directly reveal all the secrets of why the macroscopic world exhibits the laws of classical physics, but it eliminates a theory that was once thought to possibly explain this phenomenon. It serves as a reminder that the value of scientific theories lies not only in their ability to propose compelling explanations but also in their ability to make clear, testable predictions that can be tested by experimental results.
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