Testing whether gravity is quantum or classical remains a cornerstone challenge in fundamental physics. We introduce a practical tabletop experiment to probe gravity's quantum nature via the gravitational Aharonov-Bohm (AB) effect in atom interferometry. Our approach hinges on two pivotal insights: First, unlike electromagnetic AB effects where constant charge ensures gauge-invariant phases for closed paths, semiclassical gravity couples to the particle's total energy, rendering the AB phase gauge-dependent and unobservable when energies differ between paths due to environmental noise or controlled perturbations. Second, in quantum gravity—treating source, particle, and field as quantum—the total phase splits into a gauge-dependent particle contribution and a gauge-invariant source backreaction, yielding precisely half the nominal AB phase as observable. This halved phase discriminates quantum from classical gravity without requiring massive superpositions, aligning with existing technology. Remarkably, Overstreet et al.'s (Science 375, 226 (2022)) data already reveal a nonzero measured AB phase, providing the first experimental evidence for gravitational quantization.
The quantum nature of gravity is one of physics' greatest unsolved problems, with existing experimental proposals facing formidable challenges in maintaining spatial superpositions of massive objects against decoherence. We propose a fundamentally different approach that bypasses these difficulties. The central insight is that a quantum gravitational field can imprint a gauge-invariant phase on a particle's wavefunction even along an open trajectory, arising from the quantum backreaction of the source. This phase, which manifests prior to the final recombination stage of an interferometer and cannot be generated by a classical gravitational field, provides an operationally direct witness to the quantumness of gravity. Our experimental design uses a single photon whose accumulated gravitational phase due to the Earth is transferred via absorption to auxiliary atoms near the end of the interferometer sequence, preserving the open-path character essential for the test. We scale the interferometer to yield a detectable phase of order
10−3 rad and show how non-local correlation measurements on the atoms can access this gauge-invariant signature. Leveraging established quantum optical and atomic techniques, this proposal offers a feasible path to distinguish quantum from classical gravity. A positive result would provide the first experimental evidence for the quantum nature of gravitational fields, with profound implications for quantum gravity theories.
We analyse the gravitational Aharonov-Bohm (AB) phase within the semiclassical framework. We first review the traditional non-relativistic approximation, where a particle of rest mass
m couples to the Newtonian potential via
Hint=mΦ, yielding a gauge-invariant phase analogous to the electromagnetic AB effect. We then show that this approximation fails when the particle's total energy varies between interferometer branches. A rigorous derivation from linearised general relativity reveals that in the weak-field, non-relativistic limit, gravity couples to the local energy density, not merely the rest mass. In a laser-driven atom interferometer, the two branches of the superposition differ by the location of an excitation energy
δE: either stored in the laser apparatus at
xL or carried by the atom at
xA(t). Under a gauge transformation
δhμν=−∂μξν−∂νξμ, the gravitational AB phase acquires a residual
δ(Δϕgrav)=−ℏc2δE[ξ0(xL,tf)−ξ0(xA(tf),tf)] for a purely temporal generator, which is generically nonzero. Since experiments such as Overstreet et al. (2022) measure a stable, reproducible gravitational AB phase, we are compelled to choose between two mutually exclusive possibilities. If gauge invariance (diffeomorphism invariance) is a fundamental principle, then the semiclassical framework is incomplete and a quantum theory of gravity is necessary. If, on the other hand, gauge invariance fails, then semiclassical gravity can be a complete theory. In this case, the gravitational potentials must be understood as real physical fields, and the gauge-dependent phase is a genuine observable, consistent with experiment. These two resolutions are mutually exclusive, and their implications for the nature of gravity and gauge symmetry are profound.
This study explores quantum effects in weak gravitational fields with the aim of identifying feasible pathways towards tabletop tests of quantum gravity. Using numerical simulations of matter-wave interference for nanoparticles with masses between and kg, we investigate how environmental and fundamental decoherence mechanisms shape observable signatures. The results reveal a mass-dependent reduction in interference visibility, dropping from near unity at kg to below 0.2 at kg. Coherence times were found to exceed one second for particles lighter than 10^(-16) kg under cryogenic ultra-high-vacuum conditions, but decreased to sub-millisecond scales for kg particles at room temperature, confirming thermal radiation as the dominant source of decoherence. In parallel, collapse models such as CSL predict additional suppression of visibility for interrogation times of 0.1 s, particularly for masses above kg, enabling discrimination between environmental and intrinsic decoherence mechanisms. These findings underscore the necessity of maintaining ultra-high vacuum and cryogenic environments to detect gravitationally induced quantum phases, thereby providing a practical framework for near-future interferometry experiments. While the present work is limited to phenomenological models and simulated data, it establishes a roadmap for extending investigations to heavier mass regimes, incorporating realistic noise sources, and testing alternative collapse scenarios.
The gravitational Aharonov-Bohm (GAB) effect serves as the gravitational counterpart to the electromagnetic Aharonov-Bohm effect. Despite the concept's inception in 1967, a rigorous experimental test of the GAB effect remains elusive. While recent endeavors with ground-based apparatus show promise in uncovering hints of the GAB effect, we propose a precise yet practical experiment to deterministically measure the GAB phase shift using cold atom interferometry in a microgravity environment, such as an orbital space station or drop tower. We conduct a thorough analysis of the experiment's principles and derive key parameters. Our calculations suggest that it is feasible to precisely measure a significant GAB phase shift within known engineering constraints. Additionally, our design of the cold atom interferometer possesses a unique sensitivity to potential but insensitivity to force, which can be further refined to effectively mitigate adverse effects associated with nonzero inertial forces acting on the cold atoms. These endeavors will ultimately pave the way for experimental exploration into the quantum nature of gravity.
FINDING: Quantum gravity may be testable via tabletop matter-wave/optomechanical interferometry, probing the quantum superposition of gravitational fields at mesoscopic scales. | MATH: The core observable is the gravitational entanglement phase — for two masses
m1,m2 separated by distance
d, the interaction Hamiltonian
HG=−Gm1m2/d generates a phase
ϕ=Gm1m2t/ℏd in a superposition of paths. The experiment requires
ϕ∼π/2 for maximal entanglement detection, implying
m1m2t/d∼ℏ/G — a scale set by the Planck mass squared over
ℏ. No new constants beyond
G,ℏ,c appear; the test is purely quantum-informational (entanglement witness) rather than energy-based. | CONNECTION: The ratio of gravitational to electromagnetic coupling at the Planck scale is
αG/αEM∼(mP/me)2∼(1.22×1019/0.511×106)2∼5.7×1044 — not a golden ratio, but the experimental thre Author: Andrew Stewart Caldin, Independent Researcher, UK. Part of the E8 Intelligence Research series. Platform: e8intelligence.com
The Aharonov-Bohm (AB) effect underscores the fundamental role of electromagnetic and gravitational potentials in quantum mechanics, even in regions where classical fields vanish.In this article, we summarize and expand on our recent work on the AB effect.We explore the gravitational AB effect as a generalization of gravitational redshift, considering a quantum system in orbit around a massive body.In this scenario, time-dependent gravitational potentials give rise to sidebands in the energy spectrum, which could be detected using precision atomic clocks aboard satellites.In parallel, we extend recent investigations of the electric-scalar AB effect.By using a balanced interferometric excitation scheme in a microwave cavity, on resonance, we achieve 25 dB suppression of the electromagnetic fieldwhile enhancing the time-varying scalar electric potential.This offers a promising platform to directly probe the electricscalar AB effect under conditions of a null electric field and a finite scalar potential.Also, it enables the realization of a resonant system exhibiting extreme dispersion.These complementary approaches provide new experimental pathways to test fundamental quantum effects in electromagnetic and gravitational potentials.