Gravity may be able to push instead of pull \u2014 at least in a exquisitely and very carefully engineered quantum experiment \u2014 according to a new theoretical proposal that also outlines a simpler path to testing whether gravity itself is fundamentally quantum.<\/p>\n
Before anyone starts investing in anti-gravity machines or an anti-gravity machine startup, the proposal does not suggest that gravity itself becomes repulsive; rather, quantum interference \u2014 which we\u2019ll touch on soon \u2014 between two possible gravitational pulls can produce an average momentum shift in the opposite direction.<\/p>\n
In the paper posted on the pre-print server arXiv<\/a>, physicists Pablo L. Saldanha of the Federal University of Minas Gerais and Chiara Marletto and Vlatko Vedral of the University of Oxford describe a tabletop experiment in which a single mass placed in a quantum superposition could cause a second particle to experience an effective gravitational repulsion. Such a result would have no classical explanation and would serve as evidence that gravity possesses quantum properties, according to the researchers.<\/p>\n The proposal builds on a growing research program aimed at probing gravity at small scales using quantum technologies rather than high-energy particle colliders or cosmological observations. For decades, physicists have struggled to reconcile quantum theory \u2014 which governs atoms and subatomic particles \u2014 with general relativity, which describes gravity as the curvature of spacetime. While the other fundamental forces have been successfully incorporated into quantum theory, gravity remains the outlier.<\/p>\n The new work refines earlier ideas that sought to detect \u201cgravitationally induced entanglement,\u201d a phenomenon in which two tiny masses placed in quantum superpositions of position become entangled solely through their mutual gravitational attraction. Entanglement is a distinctly quantum effect in which particles become linked so that measuring one instantaneously influences the other. Because classical forces cannot create entanglement, observing it would strongly suggest that gravity itself must carry quantum degrees of freedom.<\/p>\n Previous proposals required placing two relatively massive objects into spatial superposition \u2014 a technically daunting task. The new scheme reduces that burden by requiring only one mass to be placed in a superposition. The second object, known as the probe, can remain in an ordinary quantum state without being split between two locations.<\/p>\n Quantum Interference<\/p>\n The central idea relies on quantum interference, a well-established phenomenon in which different possible outcomes combine in ways that can enhance or cancel each other. In the proposed experiment, a \u201csource\u201d mass is placed in a superposition of two positions inside a Mach-Zehnder interferometer, a device commonly used in optics and atomic physics to split and recombine wave-like particles. While in superposition, the source exerts a gravitational pull on a nearby probe particle.<\/p>\n If gravity obeys the quantum superposition principle, the probe should experience a superposition of two gravitational attractions \u2014 one from each possible location of the source mass. Under certain conditions, and with a carefully chosen postselection of the source\u2019s final quantum state, the resulting interference could cause the probe to receive a momentum kick opposite to the direction of the gravitational pull.<\/p>\n In classical physics, gravity is always attractive between two positive masses. There is no known mechanism by which gravity could produce a repulsive force between ordinary matter. In the proposed quantum scenario, however, the average momentum transfer to the probe particle could be negative \u2014 meaning it moves as if pushed away. The effect does not imply that gravity becomes fundamentally repulsive. Instead, it reflects interference between two quantum alternatives of the gravitational field.<\/p>\n The researchers show mathematically that this anomalous momentum transfer arises from entanglement between the source and probe particles mediated by gravity. In their formal treatment, they analyze the interaction using both the standard Schr\u00f6dinger picture and the Heisenberg picture of quantum mechanics. In both descriptions, the outcome depends on what is known as a weak value \u2014 a quantity that can exceed the range of ordinary measurement results when a quantum system is preselected in one state and postselected in another nearly orthogonal state.<\/p>\n Weak values have been studied for decades and have been demonstrated in optical experiments. In this proposal, they allow the small gravitational effect to be amplified, potentially by orders of magnitude. The amplification, however, comes at a cost: the probability of successfully postselecting the desired state becomes correspondingly small.<\/p>\n The paper includes preliminary feasibility estimates. Earlier gravitational entanglement proposals envisioned nanodiamonds with embedded nitrogen-vacancy centers, weighing around 10^-14 kilograms \u2014 or, one hundred-trillionth of a kilogram, placed into superposition across distances of tens to hundreds of micrometers. Using similar parameters, the new scheme finds that observing a measurable effect may require either increasing the mass of the source particle or adjusting other experimental variables.<\/p>\n One scenario described in the study suggests that with a source mass of roughly 10^-14 kilograms and a probe mass of 10\u207b\u00b2\u2070 kilograms \u2014 or, one hundred-quintillionth of a kilograms, combined with micrometer-scale separations and interaction times of fractions of a second, the anomalous momentum shift could reach about 0.2 percent of the probe\u2019s intrinsic momentum uncertainty. According to the team\u2019s estimates, a shift on the order of 0.1 percent should be measurable using established techniques for detecting momentum distributions in ultracold atoms or Bose-Einstein condensates.<\/p>\n Limitations And Future Work<\/p>\n The proposal also highlights practical challenges and room for future work. For example, because the gravitational force between such small objects is extraordinarily weak, competing forces \u2014 such as electromagnetic interactions or forces arising from quantum fluctuations between closely spaced objects (called Casimir\u2013Polder forces) \u2014 could mask the signal. Distinguishing a genuine gravitational effect from these backgrounds would require careful experimental design and shielding.<\/p>\n Another limitation is the reliance on weak-value amplification. While it enhances the observable signal, it reduces the number of successful measurement runs, since most attempts would not meet the required post-selection criteria. This tradeoff is familiar from other weak measurement experiments but adds complexity when dealing with already tiny gravitational effects.<\/p>\n![\"Responsive](\"https:\/\/www.newsbeep.com\/uk\/wp-content\/uploads\/2025\/11\/1764239594_934_Website-Banner-Quantum-2.gif\")
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