Gravity linked to stable interaction chains, not universal emission.

Research demonstrates the consistency of semiclassical gravity by proposing that systems emit gravity only when linked to stable determination chains, specific interaction sequences modelled via decoherence. This challenges universal gravity, retaining the equivalence principle, and offers testable predictions via gravcats and Bose-Marletto-Vedral experiments, potentially deriving the cosmological constant and explaining non-gravitating vacuum energy.

The longstanding incompatibility between quantum mechanics and general relativity continues to challenge theoretical physicists, particularly when attempting to formulate a consistent theory of gravity that incorporates quantum effects, known as semiclassical gravity. A new approach, detailed in the paper ‘Towards a Consistent Semiclassical Theory of Gravity’, proposes a resolution by suggesting gravity is not universally emitted by all systems, but only when those systems engage with specific interaction pathways termed ‘stable determination chains’. Francisco Pipa, from The University of Queensland, and colleagues, argue that disconnecting a system from these chains effectively removes its ability to emit a gravitational field, upholding a modified version of the equivalence principle while addressing inconsistencies within the semiclassical framework. The research further suggests potential avenues for experimental verification, referencing existing investigations into gravitational fields emitted by isolated systems and interactions between systems.

Quantum gravity research persistently seeks to reconcile the seemingly incompatible frameworks of quantum mechanics and general relativity, representing a central challenge in modern physics. Current approaches typically attempt to quantise gravity itself, treating it as a force mediated by hypothetical particles, or alternatively, model gravity as a classical stochastic field influencing quantum events; however, these strategies encounter significant theoretical difficulties, notably infinities arising in calculations and inconsistencies with established physical principles. This work proposes a refined understanding of semiclassical gravity, investigating the conditions under which this approximation remains valid.

Semiclassical gravity suffers from conceptual and mathematical problems, including the prediction of unphysical vacuum energies and inconsistencies with the equivalence principle, which posits the indistinguishability of gravitational and inertial mass. Researchers have sought resolutions, and this work investigates a novel approach by proposing that gravitational effects are not universally present, but emerge only when systems interact with specific, stable chains of interactions. These interactions, modelled using concepts from quantum decoherence, establish a ‘determination chain’ linking a system to the external world; the absence of such a chain implies the system does not emit a gravitational field.

The theoretical framework builds upon a recent reformulation of quantum theory, Environmental Determinacy-based Quantum Theory (EnDQT), which challenges conventional interpretations of quantum measurement by proposing that quantum states are determined by their environment rather than undergoing probabilistic collapse. By adapting EnDQT to incorporate gravity, the authors aim to provide a consistent account of gravitational interactions without resorting to quantization or stochastic fields, resolving inconsistencies within existing frameworks and proposing novel mechanisms for understanding the universe. This potentially offers solutions to long-standing problems in cosmology and vacuum energy calculations.

Specifically, the framework suggests a natural derivation of the cosmological constant, a value currently requiring fine-tuning in standard models, and provides a physical explanation for why the quantum vacuum, despite possessing energy, does not gravitate, resolving a significant discrepancy between theory and observation. The authors emphasize the empirical testability of their theory, highlighting potential experiments that could verify the predicted absence of gravitational emission from isolated systems, including investigations into highly isolated objects, such as those explored in ‘gravcats’ research, and investigations into gravitational interactions between systems, similar to those conducted in the Bose-Marletto-Vedral (BMV) experiment.

Research into reconciling quantum mechanics and general relativity increasingly prioritises methodological innovation, particularly in the design of experiments to detect quantum gravitational effects. A central tenet of current investigation involves a nuanced understanding of decoherence, the process by which quantum systems lose coherence and transition towards classical behaviour, and researchers actively model decoherence as a crucial component in defining ‘stable determination chains’ (SDCs), specific interaction pathways that dictate when a system is considered to emit a gravitational field. This represents a departure from the traditional view of gravity as a universally acting force, instead proposing that gravitational emission is contingent upon a system’s connection to these SDCs.

The development of these SDC models draws heavily on quantum information theory, utilising concepts like entanglement and measurement to characterise the interactions that constitute a stable chain, necessitating the adaptation of techniques from diverse fields, including precision measurement, interferometry, and the analysis of complex quantum states. Experiments designed to test these models, such as ‘gravcats’ and those inspired by the Bose-Marletto-Vedral (BMV) proposal, require exceptionally sensitive apparatus capable of detecting minute gravitational effects. Furthermore, the theoretical framework necessitates a re-evaluation of the equivalence principle, suggesting that gravity isn’t universally acting, but rather conditional on the presence of SDCs.

The emphasis on foundational issues in quantum mechanics, including the interpretation of quantum theory and the emergence of classical behaviour, is also noteworthy, with researchers drawing on concepts from quantum information theory and philosophy of science to address these fundamental questions. This interdisciplinary approach is crucial for developing a coherent understanding of quantum gravity and for designing experiments that can probe the underlying principles governing the universe.

The research centres on reconciling quantum mechanics and general relativity, and a prominent theme is the investigation of decoherence and its potential role in establishing a connection between the quantum and classical realms, and its implications for understanding gravity. A significant portion of the research actively pursues experimental verification of quantum gravity effects, manifesting in proposals for novel experiments, such as those detailed by Marletto and Vedral, who suggest methods for detecting gravity-induced entanglement.

The works reveal a sustained engagement with foundational questions in both quantum mechanics and general relativity, with authors delving into the philosophical underpinnings of these theories, examining concepts such as naturalness and emergence. The research proposes modifications to established theories, highlighting interest in unimodular gravity and alternative black hole solutions that avoid singularities. The authors argue for a consistent semiclassical gravity by positing that systems only emit a gravitational field when interacting with stable determination chains, modelled via decoherence. This denies the universality of gravity while upholding a version of the equivalence principle, and offers a potential explanation for the value of the cosmological constant and the non-gravitating vacuum.

This research demonstrates the consistency of semiclassical gravity through the proposition that gravitational field emission requires interaction with stable determination chains (SDCs), modelled via decoherence processes. The core argument centres on the idea that isolated systems, disconnected from SDCs, do not contribute to the gravitational field, offering a resolution to several longstanding problems in theoretical physics. By linking gravity to the presence of SDCs, the research provides a potential derivation of the cosmological constant from first principles, circumventing the need for ad-hoc assumptions. Furthermore, it offers an explanation for the observed non-gravitation of the vacuum.

Experimental verification of this theory is proposed through two primary avenues: investigations into the gravitational field emitted by truly isolated systems, potentially revealing a deviation from predictions based on universal gravity, and detailed analysis of gravitational interactions between systems, mirroring the approach of the Bose-Marletto-Vedral (BMV) experiment. The emphasis on decoherence and SDCs positions this work within a broader trend towards understanding the quantum-to-classical transition and its implications for gravity, suggesting that gravity may not be a fundamental force in the traditional sense, but rather an emergent phenomenon arising from the interaction of quantum systems with their environment. Future research will focus on refining the mathematical formalism of SDCs and developing more precise experimental protocols for testing the theory’s predictions, with particular attention given to exploring the implications of this framework for cosmology and black hole physics.