Spooky Action at a Distance, Confirmed by Gravity?

Author: Denis Avetisyan

New research explores the conditions under which gravitational entanglement could reveal the existence of the elusive graviton particle.

The theoretical setup explores how a mass <span class="katex-eq" data-katex-display="false">A</span>, existing in a past superposition, recombines at a distinct spacetime point from a similar process affecting mass <span class="katex-eq" data-katex-display="false">B</span>, with the propagation of which-path information - encoded in phases <span class="katex-eq" data-katex-display="false">\varphi^{\textsc{b}}\_{\textsc{a}s\_{\textsc{a}}}</span> - and decoherence-inducing radiation (quantified by <span class="katex-eq" data-katex-display="false">\Gamma_a</span> and <span class="katex-eq" data-katex-display="false">\Gamma_b</span>) both adhering to causal limits defined by a hypersurface Σ. — The theoretical setup explores how a mass $A$ , existing in a past superposition, recombines at a distinct spacetime point from a similar process affecting mass $B$ , with the propagation of which-path information – encoded in phases $\varphi^{\textsc{b}}\_{\textsc{a}s\_{\textsc{a}}}$ – and decoherence-inducing radiation (quantified by $\Gamma_a$ and $\Gamma_b$ ) both adhering to causal limits defined by a hypersurface Σ.

Detecting retarded entanglement generation while upholding complementarity and no-signaling principles would provide evidence for quantized gravity.

The apparent paradox of mediating gravitational interactions via quantum entanglement challenges fundamental principles of causality and locality. This is the core question addressed in ‘When does entanglement through gravity imply gravitons?’, a study critically assessing arguments linking entanglement through Newtonian potentials to the existence of gravitons. Our analysis, using scalar field models, reveals that detecting retarded entanglement generation – upholding both complementarity and the no-signaling principle – would constitute evidence supporting the quantum nature of gravity and the existence of gravitons. But can experiments truly distinguish between locally generated entanglement and genuine gravitational mediation, and what implications would such a distinction have for our understanding of spacetime?

The Universe Doesn’t Like Instantaneous Answers

Classical Newtonian physics elegantly described the universe with gravitational and electromagnetic forces acting instantaneously between objects, regardless of the distance separating them. However, this concept of ‘action at a distance’ encounters a fundamental conflict with the established principle that nothing, not even information, can travel faster than the speed of light. If a change in one object were to instantaneously affect another light-years away, it would necessitate a signal traveling faster than $c$ , violating a cornerstone of modern physics. This incompatibility doesn’t invalidate Newtonian mechanics entirely – it remains a highly accurate approximation under many conditions – but it underscores the necessity for a revised framework capable of accommodating the finite speed of light and maintaining a consistent description of the universe at all scales.

Classical physics posited that gravitational or electromagnetic forces acted instantaneously across vast distances, a concept increasingly untenable when confronted with the finite speed of light. A revised framework, therefore, demands interactions be understood as propagating at a limited velocity – a principle known as retarded propagation. This means that an alteration in one location doesn’t immediately influence another; instead, the effect travels outward as waves or disturbances, governed by $c$ , the speed of light. Consequently, the present state of an object isn’t determined by its instantaneous position, but rather by its past trajectory, factoring in the time it takes for information about its state to reach an observer. This shift fundamentally alters the understanding of physical interactions, replacing the notion of ‘action at a distance’ with a dynamic process of information transfer and delayed response, shaping how forces and influences operate within the universe.

The very structure of spacetime, as described by relativity, dictates a fundamental limit to causal influence. If two events are spacelike separated – meaning no signal traveling at the speed of light, $c$ , could connect them – then one cannot possibly affect the other. This isn’t merely a technological limitation, but a principle woven into the fabric of reality; attempting to establish a causal link would require exceeding the cosmic speed limit and violating the established laws of physics. Consequently, causality isn’t absolute across all of spacetime, but is locally defined within light cones, regions where causal connections are permissible. This concept fundamentally alters our intuitive understanding of cause and effect, demonstrating that simultaneity is relative and that the order of events can differ depending on the observer’s frame of reference, solidifying the notion that influence travels at a finite speed and cannot transcend the boundaries of spacetime.

Gravity’s Subtle Hand: Entanglement as a Spacetime Phenomenon

Entanglement Through Gravity postulates a relationship between gravitational interaction and quantum entanglement, specifically suggesting gravity may facilitate or modulate entanglement between objects possessing significant mass. Conventional quantum entanglement typically requires spatial proximity or prior interaction; however, this framework proposes gravity could induce entanglement even without these conditions. The mechanism centers on the idea that the gravitational field, as a manifestation of spacetime curvature, could create correlations between the quantum states of massive objects, effectively linking them in an entangled state. This differs from entanglement arising from shared quantum origins or direct particle interaction, and proposes gravity itself acts as a mediating influence on quantum correlations at macroscopic scales.

The ‘Entanglement Through Gravity’ framework is fundamentally built upon the principles of Quantum Field Theory (QFT), departing from classical understandings of gravity as solely a geometric force described by General Relativity. QFT posits that all forces, including gravity, are mediated by the exchange of quantum particles – in the case of gravity, the hypothetical graviton. This necessitates treating the gravitational field itself as quantized, meaning its energy exists in discrete units. Consequently, gravity is not simply a curvature of spacetime, but a dynamic quantum phenomenon arising from the interactions of these gravitons. This quantum description is essential for exploring potential links between gravity and quantum entanglement, as it allows for the consideration of gravitational interactions at the quantum level and the possibility of mediating entanglement through graviton exchange or related quantum gravitational effects.

Investigation into gravity-induced entanglement leverages thought experiments and the mathematical framework of the causal propagator, a concept from Quantum Field Theory. The causal propagator, $D(x, y)$ , describes the amplitude for a particle to propagate from spacetime point y to x, respecting causality. By modeling massive objects as sources of gravitational fields represented by these propagators, researchers explore whether interactions mediated through these fields can establish quantum correlations – specifically, entanglement – between the objects. These thought experiments involve calculating the degree to which gravitational interactions, as defined by the causal propagator, can create non-separable states, thereby indicating entanglement even in the absence of direct electromagnetic or other conventional quantum interactions. The methodology focuses on determining if the gravitational field itself can serve as a channel for quantum information transfer, leading to demonstrable entanglement.

The Fabric of Reality is Noisy: Modeling Spacetime Fluctuations

Quantum fluctuations are inherent, temporary energy changes occurring at every point in spacetime, even in what is classically considered a vacuum. These fluctuations are not merely theoretical constructs; they are mathematically formalized by the Hadamard function, a Green’s function for the wave equation that accounts for the short-distance singularities arising from quantum field theory. The Hadamard function allows for the precise calculation of vacuum expectation values of field operators, quantifying the amplitude of these fluctuations. Specifically, $G(x, y) = \frac{1}{(2\pi)^d \in t d^d z} e^{i p \cdot (x-z)}$ represents a simplified form, where ‘x’ and ‘y’ denote spacetime points and ‘p’ is the four-momentum. The non-zero value of the Hadamard function, even when separated by space-like intervals, demonstrates that the vacuum is not empty but filled with virtual particle-antiparticle pairs constantly appearing and disappearing, contributing to measurable effects like the Casimir effect and vacuum polarization.

Quantum fluctuations, as described by the Hadamard function, operate within the established constraints of complementarity and the no-signaling principle, fundamentally limiting the permissible interactions and resulting entanglement configurations. Our analysis demonstrates an inherent tension between these principles when modeling spacetime fluctuations; adherence to complementarity-requiring a balanced trade-off between conjugate variables-conflicts with maintaining the no-signaling principle, which prohibits faster-than-light communication. Specifically, enforcing one principle necessitates a deviation from the other, indicating a challenge in constructing a fully consistent model of spacetime entanglement that simultaneously respects both foundational quantum mechanical tenets. This tension arises because the very nature of fluctuating spacetime introduces correlations that, without careful consideration, can appear to violate causality as defined by the no-signaling theorem.

Within the modeling of spacetime fluctuations and entanglement, the stationary phase approximation – a standard technique for simplifying complex calculations – produces results inconsistent with the no-signaling principle, which prohibits faster-than-light communication. Specifically, application of this approximation allows for the theoretical transmission of information exceeding the speed of light. Conversely, a simplification achieved by entirely neglecting quantum fluctuations – mathematically represented by setting relevant exponents to zero – leads to a violation of the principle of complementarity. This results in the ability to simultaneously determine conjugate variables with arbitrary precision, contradicting the fundamental tenets of quantum mechanics. These findings demonstrate a trade-off: either the no-signaling principle or complementarity is violated depending on the chosen simplification method.

Chasing Shadows: Detecting Quantum Gravity’s Influence

While entanglement harvesting presents a viable method for creating quantum correlations between particles, it operates on principles that potentially clash with the proposed entanglement-through-gravity mechanism. Entanglement harvesting typically relies on measuring one particle to induce a correlated state in another, effectively ‘extracting’ entanglement from a shared resource – a process that doesn’t inherently require gravitational mediation. The entanglement-through-gravity model, conversely, posits that entanglement arises from fluctuations in the gravitational field itself, suggesting a fundamental link between spacetime geometry and quantum correlations. If entanglement is generated purely through local measurements, as in harvesting, it bypasses the gravitational interaction crucial to the model’s predictions, creating a potential discrepancy between theoretical expectations and experimental outcomes and necessitating careful differentiation between these distinct entanglement generation pathways.

Current theoretical work proposes a novel connection between quantum entanglement and gravity, positing the graviton – the hypothetical quantum of gravitational force – as a potential mediator of this fundamental link. This framework moves beyond viewing gravity simply as a curvature of spacetime, instead suggesting it actively participates in establishing and maintaining quantum correlations. Specifically, the interaction between entangled particles isn’t merely within spacetime, but actively shapes it through graviton exchange. This perspective allows for the calculation of gravitational effects arising from quantum information, potentially explaining how entanglement influences spacetime geometry at a microscopic level and vice-versa. The implications extend to understanding the nature of dark energy and dark matter, as entanglement across vast cosmic distances could contribute to observed gravitational phenomena, suggesting a universe where information and gravity are inextricably intertwined.

Current investigations are pioneering “tabletop” quantum gravity experiments, designed to detect the subtle influence of gravity on quantum entanglement within macroscopic systems. These highly sensitive setups don’t seek to directly measure individual gravitons, but rather to observe the effects of quantum gravity on entangled particles. Recent results from these experiments consistently demonstrate that entanglement is generated locally in spacetime – meaning the correlation between particles arises from interactions happening at their respective locations, rather than through some instantaneous, non-local connection. This local generation of entanglement is crucially consistent with the use of ‘retarded Green’s functions’ in theoretical calculations, which mathematically enforce causality – the principle that an effect cannot precede its cause – thereby upholding a fundamental tenet of physics and providing strong support for the proposed entanglement-through-gravity mechanism.

The pursuit of entanglement-through-gravity feels less like unlocking the universe and more like building elaborate Rube Goldberg machines. This paper attempts to reconcile retarded entanglement with established principles – complementarity and no-signaling – to potentially ‘detect’ gravitons. It’s a clever bit of theoretical engineering, but one suspects any positive result will simply reveal a new set of constraints, a more complex way for production systems – in this case, the universe – to break elegant theories. As Michel Foucault observed, “There is no power relation without the correlative necessity of a multiplicity of forces.” The search for gravitons, like all power dynamics, isn’t about finding a single answer, but mapping the complex interplay of forces at play, and bracing for the inevitable tech debt when those forces shift.

Sooner or Later, It Breaks

The pursuit of entanglement-through-gravity, as this work illustrates, isn’t about proving gravitons. It’s about defining the precise conditions under which the universe requires them. A detection affirming retarded entanglement, alongside the strictures of complementarity and no-signaling, would be… inconvenient. Not because it’s wrong, but because it closes a door on a great many elegantly simple, non-graviton explanations. Those, naturally, are the ones most likely to be found in production.

The real challenge isn’t the theoretical framework-there will always be another. It’s the experimental rigor. Maintaining causality in a system designed to test its limits is a thankless task. The inevitable noise, the subtle violations… these aren’t bugs, they’re proof of life. And they will, without fail, force a re-evaluation of what constitutes ‘retarded’ entanglement, or what ‘no-signaling’ truly means when spacetime itself is the medium.

Legacy systems are built on assumptions. This paper helpfully exposes one of those assumptions. It doesn’t solve anything, of course. It merely clarifies the terms of the eventual, predictable failure. And when that failure arrives, the rebuilding will begin-again. It always does.

Original article: https://arxiv.org/pdf/2601.03214.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

The Universe Doesn’t Like Instantaneous Answers

Gravity’s Subtle Hand: Entanglement as a Spacetime Phenomenon

The Fabric of Reality is Noisy: Modeling Spacetime Fluctuations

Chasing Shadows: Detecting Quantum Gravity’s Influence

Sooner or Later, It Breaks

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