In 2017, a striking thought experiment was thrown into the physics community. It proposed preparing two particles with a visible mass on the order of micrograms and checking whether quantum entanglement could arise between them mediated solely by gravity. If such entanglement were observed, it would constitute decisive evidence that gravity itself possesses quantum properties. Verifying "quantum gravity"—the unification of Einstein's general relativity with quantum mechanics—had long been thought to require an enormous energy at the Planck scale, roughly $1.2 \times 10^{19} \text{ GeV}$, a scale unreachable even by giant particle accelerators. Compared to that, this tabletop experimental approach offered humanity great hope.
A Century-Long Gap in Physics and the Rise of Tabletop Experiments
Two Theoretical Frameworks That Never Meet
Unifying general relativity and quantum mechanics is the greatest challenge physics has faced over the past century. The Standard Model of quantum mechanics elegantly unified three of the four fundamental forces—electromagnetism, the strong nuclear force, and the weak nuclear force—as quantum field theories. Gravity alone remains isolated from this framework. Within the structure of general relativity, gravity is not described as an exchange of particles flying through space, as the other three forces are; instead, it is described as a distortion in the very geometric structure of space itself. Quantum mechanics calculates the behavior of actors performing on a stage, whereas general relativity calculates the effects of the stage itself bending and stretching—the two theories rest on fundamentally different worldviews.
These two theories normally have no overlapping domain. When calculating the motion of stars in the universe, one need not worry about atomic fluctuations, and when calculating the behavior of an electron, Earth's gravity is negligibly small. However, matters change at the singularity at the center of a black hole, or at the moment of the universe's birth—the Big Bang. Because enormous mass is compressed into an extremely tiny region, microscopic and macroscopic laws must be applied simultaneously. When the smooth canvas of spacetime is directly mixed mathematically with the violently fluctuating behavior of quantum mechanics, the calculation results diverge to infinity and break down.
The Limits of Accelerators and the Hope of Tabletop Experiments
To avoid this, it became necessary to construct a "quantum gravity theory" that quantizes gravity itself. Many mathematical frameworks have been proposed, including string theory and loop quantum gravity, yet none has obtained decisive confirmation. For a long time, experimental verification of quantum gravity was considered practically impossible. The Planck-scale energy region where quantum effects of gravity would clearly manifest lies roughly fifteen orders of magnitude beyond what even the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) can reach.
A ray of light pierced this hopeless situation in 2017. Physicists including Sougato Bose, Chiara Marletto, and Vlatko Vedral proposed a scenario in which the effects of quantum gravity could be indirectly verified through tabletop experiments using masses on the order of micrograms, without relying on giant accelerators.
Is "Gravitationally Induced Entanglement" the Only Proof?
Searching for Correlations by Superposing Mass
At the core of the proposed experiment lies the observation of gravitationally induced quantum entanglement. In the quantum world, a particle can exist in a "superposition" of simultaneously being in a "state on the right" and a "state on the left."
The experimental procedure is as follows. First, two minute masses (for example, tiny diamond crystals) are each placed into a spatial superposition state and positioned at extremely close range. All external electromagnetic forces and collisions with gas molecules are completely shielded, narrowing the interaction between the two masses down to gravity alone. If, over time, the states of the two particles become correlated with each other—such that, if one is on the right, the other is drawn to settle into a specific corresponding position—how should we interpret this special correlation?
A Phenomenon That Classical Gravity Cannot Explain
According to modern quantum information theory, if the mediating channel (in this case, gravity) is purely classical, it cannot generate new quantum entanglement between two spatially separated systems. This principle is mathematically proven within the framework known as LOCC (Local Operations and Classical Communication). No matter how complex the instructions exchanged through classical means, it is impossible to create quantum-specific entanglement between separated systems. Interaction mediated by a classical gravitational field falls squarely within this LOCC framework.
If two pieces of matter become entangled via gravity, then the gravitational field itself, which mediates that interaction, must not be a classical entity but must instead be in a quantum superposition state. This has been the robust logical premise underlying the field.
For roughly a decade since then, observing this phenomenon has been regarded as the decisive evidence demonstrating the quantum nature of gravity. Experimental physicists around the world have competed to develop precision apparatus that eliminates interference such as thermal noise to the utmost limit, in order to verify this theoretical prediction. If entanglement were observed, it was expected to become a historic milestone demonstrating that Einstein's spacetime is quantum.
Spacetime as an "Optical Illusion" Brought About by a Change in Perspective
Rubin's Vase That Reverses Interpretation
If gravitationally induced entanglement were observed, quantum gravity would be proven. In the midst of this enthusiasm, an international research team led by Associate Professor Hu Joshua of Kyushu University's Institute for Advanced Study and Dr. Magdalena Zych of Stockholm University revisited the premise lurking beneath this established view. They constructed a new theoretical framework called "the relativity of superpositions of spacetime" and published it in May 2024 in the academic journal npj Quantum Information.
What the research team mathematically proved is the fact that a scenario in which the gravitational field exists in a quantum superposition can be fully translated into an entirely different physical phenomenon.
To grasp this intuitively, consider the trompe-l'oeil structure known as "Rubin's vase," which can be seen either as two faces looking at each other or as a single large vase. The arrangement of the ink that composes the image does not change at all, yet depending on where the observer places their viewpoint, the perceived object completely switches from "faces" to "vase." What this research demonstrated is that a structurally corresponding phenomenon also occurs in the description of spacetime.
Equivalent Transformation of States Induced by the Coordinate System
A state involving a quantum superposition of spacetime can be re-described, through a certain coordinate transformation, as a state in which a quantum particle moves through a single, fixed classical gravitational field. The research team showed that when two states (superposition amplitudes) transform into each other under a change of coordinate system, the following relation holds:
This equation expresses that the physical system on the left-hand side—"a quantum gravitational field and matter"—is equivalently transformed, through an operation that exchanges the observer's coordinate system, into the description on the right-hand side: "matter in a superposition state within a fixed classical gravitational field."
Even if gravity itself is not in a quantum superposition state, the same physical predictions can be reproduced as long as the "test" particle side is in a quantum superposition of position within an ordinary classical gravitational field. As a result, it turns out that a single observed phenomenon—gravitationally induced entanglement—admits two equivalent interpretations: "gravity is quantum" and "gravity is classical, but the particles are quantum."
The Relativity That Exposes the True Nature of Decoherence
The Observer Moves in Step with the Gravitational Source
The research team's discovery was obtained by extending the "principle of general covariance," which lies at the foundation of the general relativity Einstein built, to quantum superposition states. The principle of general covariance is the fundamental rule that physical laws must retain the same form no matter which coordinate system (viewpoint)—whether that of a stationary observer or an accelerating one—is used to describe them. Within general relativity, the gravitational field is itself the distortion of spacetime, and choosing a coordinate system is equivalent to choosing what kind of mesh one uses to measure that distorted spacetime.
Consider a scenario in which the mass that serves as the gravitational source is in a superposition of "right" and "left" positions in space. If the mass is on the right, spacetime is distorted to the right; if on the left, spacetime is distorted to the left. The gravitational field generated by the mass thus also becomes a superposition of "spacetime distorted to the right" and "spacetime distorted to the left." Until now, this superposition of the gravitational field itself has been regarded as unmistakable evidence of quantum gravity.
Here, a transformation is applied that shifts the observer's standpoint (coordinate system) itself in tandem with the position of the mass. One adopts a coordinate system in which, in the branch of the world where the gravitational source is on the right, the observer also moves to the right, and in the branch where it is on the left, the observer also moves to the left. If the difference between each component of the superposition can be described purely by a coordinate transformation, then the entire system can be re-described upon a single fixed classical spacetime. This can be fully transformed into a view in which spacetime itself is not in superposition at all—only the test particle or the observer's side possesses quantum fluctuations within a fixed spacetime.
Decoherence Is Not an Absolute Phenomenon
In their paper, the research team also applied this framework to the "decoherence problem of the gravitational source." Decoherence refers to the phenomenon in which a quantum superposition state collapses due to interaction with the external environment, transitioning into a classical state. They revealed that the decoherence of the surrounding environment caused by the gravitational source being in a superposition is also not an absolute physical phenomenon. It is merely a phenomenon that arises relatively, and only once an external system (a reference frame serving as a clock or ruler) exists to give physical meaning to the spatial superposition. Depending on where the boundary is drawn between the observed object and the reference frame serving as the standard, the face of the physical phenomenon that appears changes.
In Search of a Coordinate-Independent "True Quantum Gravity"
What Have We Actually Proven?
This discovery does not render previous tabletop experiment proposals meaningless. Rather, it brings to light the interpretive limits of what we would actually have proven with the obtained experimental results.
| Point of Comparison | Conventional Established View (Proposals since 2017) | New Framework Presented by This Research |
|---|---|---|
| Interpretation of gravitational entanglement | Decisive evidence that the gravitational field is quantum | Two equivalent interpretations possible (quantum gravity / classical gravity + quantum particle) |
| Treatment of the background spacetime | Assumed to be in a superposition state | Can be re-described as a single fixed classical background |
| Implications for experimental design | Verification complete once entanglement is observed | Necessary to identify true quantum effects that cannot be eliminated by coordinate transformation |
Many of the previous proposals implicitly placed assumptions dependent on a particular coordinate system or external reference frame. If a phenomenon that appears, from one viewpoint, to be a quantum effect of gravity can equally be explained, from another viewpoint, as a quantum effect of a particle, then it cannot be called independent evidence for a theory of quantum gravity.
Searching for Conditions of Evidence That Transcend Viewpoint
Associate Professor Hu Joshua of Kyushu University points out: "Before verifying the quantum nature of gravity, we first need to grasp what would constitute evidence proving that we have discovered it." This research is precisely an attempt to rigorously redefine the conditions for such evidence.
Understanding how gravity and quantum mechanics are connected is one of the deepest inquiries in modern physics. To prove the quantum nature of gravity, one must identify the essential conditions of nature that are not swayed by the arbitrary human choice of how coordinates are set.
Which observational results would allow us to distinguish between a classical description of gravity and a truly quantum description of gravity? Under what conditions does a quantum effect intrinsic to gravity appear—one that can never be eliminated even after a change of viewpoint through a coordinate transformation? On the tabletop stage where theory and experiment intersect, physicists are now exploring more refined ways of questioning nature.