The quantum nature of gravity remains experimentally unverified, despite recent proposals to probe it using tabletop experiments such as gravity-mediated entanglement schemes. In parallel, consistent formulations of classical--quantum dynamics have been developed as alternative descriptions of gravity, in which quantum matter interacts with a classical mediator assumed to be fundamentally classical. In this work, we show that classical--quantum dynamics arise generically as an effective description of fully quantum systems under decoherence, providing a bridge between fully quantum and classical--quantum dynamics. We derive the reduced dynamics, which are generically non-Markovian, using an explicit hidden model in which the mediator is coupled to unobserved environmental degrees of freedom. We identify a concrete criterion for when a classical--quantum interpretation is valid: the semi-Wigner operator associated with the mediator sector must remain positive semidefinite, which can be expressed as a positivity condition on nonlocal kernels governing the evolution. In the short-memory limit, the reduced evolution reproduces Markovian classical--quantum dynamics of Oppenheim and collaborators. Our results imply that a classical mediator can arise effectively from decohered quantum dynamics, so that experimental agreement with classical-quantum models does not uniquely determine whether the mediator is fundamentally classical.
The quantum nature of gravity remains experimentally unverified, despite recent proposals to probe it using tabletop experiments such as gravity-mediated entanglement schemes. In parallel, consistent formulations of classical--quantum dynamics have been developed as alternative descriptions of gravity, in which quantum matter interacts with a classical mediator assumed to be fundamentally classical. In this work, we show that classical--quantum dynamics arise generically as an effective description of fully quantum systems under decoherence, providing a bridge between fully quantum and classical--quantum dynamics. We derive the reduced dynamics, which are generically non-Markovian, using an explicit hidden model in which the mediator is coupled to unobserved environmental degrees of freedom. We identify a concrete criterion for when a classical--quantum interpretation is valid: the semi-Wigner operator associated with the mediator sector must remain positive semidefinite, which can be expressed as a positivity condition on nonlocal kernels governing the evolution. In the short-memory limit, the reduced evolution reproduces Markovian classical--quantum dynamics of Oppenheim and collaborators. Our results imply that a classical mediator can arise effectively from decohered quantum dynamics, so that experimental agreement with classical-quantum models does not uniquely determine whether the mediator is fundamentally classical.
We present a unified and minimal theoretical framework addressing the quantum-classical transition, the measurement problem, and the emergence of the arrow of time, grounded in gravitational physics. Building on the insight that quantum superpositions of mass distributions necessarily imply superpositions of gravitational potentials, we show that such states are unavoidably associated with superpositions of proper times due to gravitational time dilation. By extending the system Hilbert space to include a temporal degree of freedom, we demonstrate that matter-time entanglement leads naturally to effective decoherence when the time degree of freedom is traced out. The resulting reduced dynamics takes the form of a Lindblad master equation derived from an underlying unitary evolution. This framework reproduces the correct microscopic and macroscopic limits, clarifies the relation to Diósi-Penrose-type models, and provides a coherent account of classicality and temporal irreversibility without postulating observers, environmental decoherence as fundamental, or objective wavefunction collapse.
We propose a conservative theoretical framework addressing the emergence of classical time and the uniqueness of the past within quantum mechanics, without modifying its fundamental postulates or introducing objective collapse mechanisms. The central observation is that quantum superpositions of mass–energy distributions necessarily imply superpositions of gravitational potentials, and therefore superpositions of proper times. By treating time as an effective quantum degree of freedom and extending the system Hilbert space accordingly, we show that matter–time entanglement leads to an intrinsic dispersion of proper time. Gravitational fluctuations and irreversible memory formation dynamically suppress this dispersion, giving rise to an effective closure of temporal histories. The resulting dynamics admits stable fixed points corresponding to classical time and a unique past, while remaining fully compatible with global unitarity. We analyze the stability of this closure, its relation to decoherence and quantum Darwinism, and its implications for cosmology, including the interpretation of the Big Bang as a temporal closure process rather than a fundamental beginning of time. The framework clarifies the role of gravity in the quantum–classical transition and delineates the physical conditions under which a unique history can emerge.
Abstract We propose a covariant multiverse framework in which quantum phenomena arise effectively for observers restricted to a single semiclassical universe. Each universe is endowed with a classical Lorentzian metric and a scalar--tensor sector that realizes a spacetime- and age-dependent gravitational coupling G(x,τ), where τ is a cosmic-age scalar defined with respect to a preferred cosmological congruence. Inter-universe interactions are modeled by proximity-weighted couplings that permit coherent amplitude exchange between nearby universes and by stochastic sector events that induce transitions and decoherence. For an observer in a given universe, tracing over the remaining sectors yields a reduced, open-system description that is linear and completely positive in the weak-coupling/Markovian regime, providing an explicit route to Lindblad-type generators. Metric superpositions are excluded within each universe by construction; apparent ``superpositions of geometries'' are instead identified with coherence over universe labels, with a definite classical geometry in every sector. We relate this construction to Many-Interacting-Worlds models, semiclassical and stochastic gravity, and standard open-quantum-systems techniques, and we outline phenomenological constraints from bounds on G˙/G and proposed gravitationally mediated entanglement tests. We outline concrete next steps, including toy-model demonstrations of interference and tunneling, parameterization of scalar–tensor functions consistent with Solar-System bounds, and quantitative predictions for gravitationally mediated entanglement experiments. We supplement the conceptual framework with quantitative toy-model demonstrations (qutrit and oscillator), including plots of coherence C01(t), purity P(t), and interferometric visibility V(t), and we illustrate departures from Markovian LGKS dynamics via time-dependent rates in a non-Markovian variant.
A consistent theory including gravitational effects in quantum mechanics has not been found. Although many different approaches have been proposed for theory of quantum gravity, several authors have suggested that gravity may be fundamentally classical. As there is no experimental evidence for a fundamentally quantum vs. fundamentally classical theory of gravity, it is hoped that investigating the consequences of semiclassical gravity, one may propose an experiment to distinguish between the quantum or classical nature of gravity. As opto- and electromechanical devices allow quantum limited measurement and control over massive, mechanical degrees of freedom, they are ideal systems to test violations of standard quantum mechanics arising from gravitational effects. After a brief introduction to the background theory and literature review, we begin by introducing a semiclassical model of gravity based on quantum measurement and control. We show under which conditions the theory behaves like a collapse model, and how a Newtonian potential is recovered in the multi particle case.We then go on to analyse our semiclassical model in two scenarios: firstly in a cosmological setting, and secondly by considering quantum clocks on the surface of the earth. In the cosmological description, we show that a violation of energy conservation, resulting from our model, can be reinterpreted as a dark energy fluid in the spacetime, suggesting that perhaps local violations of conservations of energy may be resolved in a cosmological framework. Secondly, we derive a minimum, gravitational dephasing rate for quantum clocks under the semiclassical model, which unsurprisingly, is is well below the resolution of current state of the art clocks. The next chapters diverge from analysing the gravitational model, and instead focus on novel methods of manipulating and measuring micromechanical devices, that may be used to experimentally probe the results of the semi-classical theory. Firstly we show that a sequence of coherently controlled pulses in an optomechanical system can be used to directly probe the momentum of an oscillator - opening up the possibility for direct detection of momentum diffusion, which is a consequence of the semiclassical gravity model. Secondly, we introduce an electro-mechanical device, and sequence of control operations, that may be understood as an interferometer for the wave function of a mechanical oscillator coupled to a qubit. We show how the resulting interference pattern can distinguish between unitary evolution, thermalisation/decoherence, and a purely classical description of the mechanical oscillator. We also show how the qubit-oscillator interaction can be used generate arbitrary wave functions of the mechanical oscillator, using only classical qubit driving.
We study the back-reaction of quantum systems onto classical ones. Taking the starting point that semi-classical physics should be described at all times by a point in classical phase space and a quantum state in Hilbert space, we consider an unravelling approach, describing the system in terms of a classical-quantum trajectory. We derive the general form of the dynamics under the assumptions that the classical trajectories are continuous and the evolution is autonomous, and the requirement that the dynamics is linear and completely positive in the combined classical-quantum state. This requirement is necessary in order to consistently describe probabilities, and forces the dynamics to be stochastic when the back-reaction is non-zero. The resulting equations of motion are natural generalisations of the standard semi-classical equations of motion, but since the resulting dynamics is linear in the combined classical-quantum state, it does not lead to the pathologies which usually follow from evolution laws based on expectation values. In particular, the evolution laws we present account for correlations between the classical and quantum system, which resolves issues associated with other semi-classical approaches. In addition, despite a breakdown of predictability in the classical degrees of freedom, the quantum state evolves deterministically conditioned on the classical trajectory, provided a trade-off between decoherence and diffusion is saturated. As a result, the quantum state remains pure when conditioned on the classical trajectory. To illustrate these points, we numerically simulate a number of semi-classical toy models, including one of vacuum fluctuations as a source driving the expansion of the universe. Finally, we discuss the application of these results to semi-classical gravity, and the black-hole information problem.
A quantum master equation describing the stochastic dynamics of a quantum massive system interacting with a quantum gravitational field is useful for the investigation of quantum gravitational and quantum informational issues such as the quantum nature of gravity, gravity-induced entanglement and gravitational decoherence. Studies of the decoherence of quantum systems by an electromagnetic field shows that a lower temperature environment is more conducive to successful quantum information processing experiments. Likewise, the quantum nature of (perturbative) gravity is far better revealed at lower temperatures than high, minimizing the corruptive effects of thermal noise. In this work, generalizing earlier results of the Markovian ABH master equation [1,2] which is valid only for high temperatures, we derive a non-Markovian quantum master equation for the reduced density matrix, and the associated Fokker-Planck equation for the Wigner distribution function, for the stochastic dynamics of two masses following quantum trajectories, interacting with a graviton field, including the effects of graviton noise, valid for all temperatures. We follow the influence functional approach exemplified in the derivation of the non-Markovian Hu-Paz-Zhang master equation [62,64] for quantum Brownian motion. We find that in the low temperature limit, the off-diagonal elements of the reduced density matrix decrease in time logarithmically for the zero temperature part and quadratically in time for the temperature-dependent part, which is distinctly different from the Markovian case. We end with a summary of our findings and a discussion on how this problem studied here is related to the quantum stochastic equation derived in [77] for gravitational self force studies, and to quantum optomechanics where experimental observation of gravitational decoherence and entanglement may be implemented.