The nature of gravity has long been a subject of intense debate and investigation in the field of physics. As one of the four fundamental forces of nature, alongside electromagnetism and the strong and weak nuclear forces, gravity has remained a particularly elusive force to understand, especially when compared to the others. While the electromagnetic, strong, and weak nuclear forces have been successfully incorporated into quantum mechanics, gravity has resisted this integration, leading to a key question: is gravity fundamentally classical, or is it quantum in nature?
This question has puzzled physicists for decades, and various theoretical frameworks have attempted to explain gravity in quantum terms, yet the challenge of creating a complete quantum theory of gravity persists. In a recent study published in Physical Review Letters, a team of researchers proposes a novel experimental approach that might finally offer insight into this enduring mystery. Rather than pursuing the daunting task of constructing a fully-fledged quantum theory of gravity or attempting to detect the hypothesized gravitons—the theoretical particles responsible for mediating gravity—the researchers have developed a method designed to identify measurable differences that could indicate whether gravity is quantum or classical in nature.
To better understand this groundbreaking approach, we spoke to the researchers behind the study, who explained the motivation and insights driving their work.
Rethinking the Question of Gravity
The challenge of determining the quantum or classical nature of gravity stems from the difficulty of directly observing or measuring gravity’s behavior in quantum mechanical terms. Traditionally, gravity has been treated as a classical force, as described by Einstein’s general theory of relativity. However, quantum theory predicts that gravity, like the other fundamental forces, might be quantized, suggesting that its fundamental nature could be very different from the classical framework we currently use to describe it.
Serhii Kryhin, a third-year graduate student at Harvard University and co-author of the study, explained the problem: “Several proposals have appeared in the past years that, in principle, allow us to determine gravity’s nature experimentally, but their experimental requirements are extraordinarily challenging. So our motivation was to come up with a more feasible experiment that would have the power to at least falsify that gravity is classical.”
This rethinking of the question involves narrowing down the problem in a way that allows for more practical experimental testing. The key question they aimed to address is: “What measurable differences would tell us whether gravity needs to be quantized?”
A Simple yet Insightful Concept: Quantum vs. Classical Fluctuations
The core insight that underpins the researchers’ approach is deceptively simple. As Vivishek Sudhir, Associate Professor at MIT and co-author of the study, explained: “If gravity is quantum, as a long-range force, it should be able to induce quantum entanglement of distant matter. However, if gravity is fundamentally classical, no entanglement can be produced.”
The team focused on the idea that, if gravity is indeed classical, it must exhibit irreducible stochastic fluctuations. These fluctuations are a crucial aspect of classical gravity, arising from the deterministic nature of the force, which conflicts with the probabilistic nature of quantum mechanics. The researchers recognized that these fluctuations would leave behind a unique signature in the cross-correlation spectrum of gravitational interactions, potentially allowing them to distinguish between quantum and classical gravity.
“Quantum fluctuations always arise as quantum fluctuations of dynamic degrees of freedom of general relativity,” Kryhin noted. “From a practical perspective, the main difference between quantum and classical gravity fluctuations comes in the magnitude. Being relativistic effects, quantum fluctuations are notoriously weak and thus incredibly challenging to measure. On the other hand, classical fluctuations, if they exist and have to remain consistent with everything else we know, appear to be much larger.”
The ability to measure these fluctuations and their effects on the system could provide the breakthrough that physicists have been searching for.
The Mathematical Framework
Building on these concepts, the researchers devised a theoretical framework that could describe how classical gravity and quantum matter interact in a self-consistent way. This framework operates in the Newtonian limit of gravity, where classical gravity is assumed to coexist with quantum matter.
The team constructed a quantum-classical master equation to describe how quantum matter and classical gravity evolve together. They derived a Hamiltonian to describe the interaction of quantum masses with Newtonian gravity through two complementary approaches: Dirac’s theory of constrained systems and the Newtonian limit of gravity. The result was a mathematical model that allowed them to predict how gravitational interactions would play out when classical gravity and quantum matter interact.
Next, they formulated a modified version of Newton’s law of gravitation that accounted for stochastic gravitational effects and, using this, calculated the distinctive correlation patterns between two quantum oscillators that interact gravitationally. These calculations led them to derive a closed Lindblad equation, which serves as a Markovian master equation for quantum matter interacting with classical gravity. The equation contained a term proportional to a parameter ε, which distinguishes between classical gravity (when ε ≠ 0) and quantum gravity (when ε = 0).
Measurable Signatures of Classical Gravity
One of the study’s most significant outcomes is the identification of a measurable experimental signature that could indicate whether gravity is classical or quantum in nature. Contrary to previous claims, the researchers showed that a consistent theory of classical gravity interacting with quantum matter is indeed possible.
Their key prediction is that classical gravity would induce specific fluctuations that are distinct from quantum gravitational fluctuations. The crucial experimental signature lies in the behavior of two quantum harmonic oscillators interacting gravitationally. When these oscillators are coupled and interact under the influence of gravity, their cross-correlation spectrum will display a characteristic phase shift of π or 180 degrees at a particular detuning from resonance, but only if gravity is classical.
The researchers proposed an experimental setup to test this prediction, essentially creating a quantum version of the historic Cavendish experiment—one of the first experiments designed to measure the gravitational constant. In this quantum variant, two highly coherent quantum mechanical oscillators would be gravitationally coupled. By precisely measuring the cross-correlation of their motions, the researchers could observe the phase shift that indicates whether gravity is classical or quantum.
This experimental approach is notably more feasible than previous proposals. While other tests have called for the creation of massive objects in quantum superposition states—an incredibly difficult and technologically demanding task—this new experiment focuses on the interactions between quantum oscillators, which are within the capabilities of current or near-future experimental technology.
Prof. Sudhir emphasized the novelty of this approach: “Semiclassical models of gravity usually explicitly neglect the backaction of quantum fluctuations of matter onto the classical gravity dynamics. In contrast, our theory allows self-consistent dynamics of classical gravity field and quantum matter.”
Implications of a Classical Gravity Discovery
If the experimental results ultimately reveal that gravity is classical in nature, it would have profound implications for our understanding of the universe and our theories of physics. Kryhin noted that, “At present, it is taken as a self-evident fact that gravity has to be quantum, although nobody precisely knows what that means!” The possibility that gravity could be fundamentally classical would challenge many of the assumptions underpinning the search for a quantum theory of gravity, potentially leading to a dramatic reevaluation of the nature of spacetime and the universe itself.
The study acknowledges that the path to experimentally verifying whether gravity is classical or quantum is still fraught with challenges. There are many hurdles to overcome, ranging from developing the necessary formalism and theoretical models to building the experimental apparatus and measurement techniques. As Kryhin pointed out, “From an experimental standpoint, we need two gravitating masses, noise isolation, and measurement techniques, all of which need to come together to realize the sensitivity needed for a decisive experiment.”
More information: Serhii Kryhin et al, Distinguishable Consequence of Classical Gravity on Quantum Matter, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.061501. On arXiv: DOI: 10.48550/arxiv.2309.09105