This document presents a critical peer review of the manuscript “Probing a gravitational cat state: Experimental possibilities” (arXiv:1603.04430v1).The analysis identifies fundamental conceptual and theoretical flaws in the definition, interpretation, and proposed probing of so-called gravitational cat states. Starting from standard non-relativistic quantum mechanics coupled to Newtonian gravity, it is shown explicitly that a quantum superposition of a massive source does not generate a quantum superposition of the gravitational field. The gravitational interaction is fully determined by the classical mass density derived from the wave function modulus squared, and all observable quantities reduce to expectation values and classical correlation functions. The review demonstrates that the promotion of the Newtonian force and gravitational potential to operator status is unjustified, that the notion of a gravitational field cat state is a misuse of quantum language, and that the analogy with quantum optical models such as the Jaynes–Cummings Hamiltonian is physically incorrect. Explicit appendices provide a direct derivation from the source wave function and show the absence of gravitational field superposition at both the dynamical and observable levels. It is concluded that the manuscript does not define a consistent quantum state of the gravitational field, does not identify experimentally discriminating observables between classical and quantized gravity, and relies on phenomenological assumptions without variational or ontological closure. The results should therefore be interpreted within semiclassical gravity, and claims of gravitational field superposition are
This document presents a critical peer review of the manuscript “Probing a gravitational cat state: Experimental possibilities” (arXiv:1603.04430v1).The analysis identifies fundamental conceptual and theoretical flaws in the definition, interpretation, and proposed probing of so-called gravitational cat states. Starting from standard non-relativistic quantum mechanics coupled to Newtonian gravity, it is shown explicitly that a quantum superposition of a massive source does not generate a quantum superposition of the gravitational field. The gravitational interaction is fully determined by the classical mass density derived from the wave function modulus squared, and all observable quantities reduce to expectation values and classical correlation functions. The review demonstrates that the promotion of the Newtonian force and gravitational potential to operator status is unjustified, that the notion of a gravitational field cat state is a misuse of quantum language, and that the analogy with quantum optical models such as the Jaynes–Cummings Hamiltonian is physically incorrect. Explicit appendices provide a direct derivation from the source wave function and show the absence of gravitational field superposition at both the dynamical and observable levels. It is concluded that the manuscript does not define a consistent quantum state of the gravitational field, does not identify experimentally discriminating observables between classical and quantized gravity, and relies on phenomenological assumptions without variational or ontological closure. The results should therefore be interpreted within semiclassical gravity, and claims of gravitational field superposition are
Semiclassical gravity, in which a classical spacetime is sourced by the quantum expectation value of the stress-energy tensor, is a standard framework for describing the gravitational interaction of quantum matter. In the nonrelativistic limit this approach leads to the Schrödinger-Newton equation, which is often assumed to be consistent at least in the weak-field regime. In this work, we reexamine this assumption for spatial quantum superpositions of massive particles. We show that, when the quantum state is properly normalized, no modification of the Newtonian gravitational potential arises at leading order. However, at first post-Newtonian order the semiclassical coupling generically produces state-dependent contributions involving the mass density and the mass current of the superposition. These terms have a parametric scaling which is different from that of the corresponding relativistic corrections and which does not have Planck mass suppression. Our results therefore impose a strong post-Newtonian consistency constraint on deterministic semiclassical gravity, indicating that sourcing the metric solely by expectation values is insufficient to recover a consistent relativistic weak-field expansion.
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.
Abstract We extend our earlier work [1] on probing a gravitational cat state (gravcat) to the quantum superposition of two gravcats in an exemplary model and in Bose–Einstein condensates (BEC). In addition to its basic theoretical values in gravitational quantum physics and macroscopic quantum phenomena, this investigation can provide some theoretical support to experimental proposals for measuring gravity-induced entanglement and the quantum nature of perturbative gravity. In the first part we consider cat states generated by double-well potentials. A pair of gravcats, each approximated as a two-level system, is characterized by gravity - induced Rabi oscillations , and by gravity - induced entanglement of its energy eigenstates. In the second part we turn to a (non-relativistic) quantum field theory description and derive a gravitational Gross – Pitaevsky equation for gravcats formed in BEC. Using a mathematical analogy to quantum rotors, we explore the properties of the two-gravcat system for BECs, its physical consequences and observational possibilities. Finally we discuss our results in comparison to predictions of alternative quantum theories, and we explain their implications.
Matter-wave interferometry has confirmed the superposition principle for objects as large as fullerenes and, most recently, nanoparticles of thousands of atoms. Because every massive object sources a gravitational field, a long-standing question follows: does the gravitational field of a delocalized quantum system track the realized quantum state, or the probability-weighted expectation value of its mass distribution (semiclassical gravity)? We first revisit a distant-observer thought experiment in which gravitational which-path information appears to threaten complementarity, and summarize its modern resolution, which turns the apparent paradox into an argument for the quantization of the gravitational field. We then propose a variant of the 1981 Page–Geilker experiment that generalizes a protocol of Kent: the position of a macroscopic source mass is controlled by a quantum random trigger whose firing probability p is continuously tunable, a precision gravimeter monitors the local field, and an otherwise identical run with a classical pseudo-random trigger serves as a control. For no-collapse semiclassical gravity we derive, for a repeated protocol with reset, the field offset in intervals without a release and the suppression of the gravimeter step at quantum-triggered releases; both depend on p, whereas every event-sourced theory predicts a p-independent, event-following field, and the control run cancels systematics that scale with the release rate. Order-of-magnitude estimates indicate that a superconducting gravimeter and a 100–1000 kg source can bound the fraction of “counterfactual” mass that gravitates at the 10−3 level as a function of p. We state which hypotheses the experiment can falsify, why decoherence restricts its scope to no-collapse semiclassical theories, and how the design could be migrated to mesoscopic masses, where the question remains open. Version 2 (September 2026) of a manuscript first written in June 2026. 10 pages, 1 table, 44 references. Also submitted to arXiv (quant-ph, cross-list gr-qc).
In this thesis, we consider the implications of solving the quantum measurement problem for the Newtonian description of semiclassical gravity. First we review the formalism of the Newtonian description of semiclassical gravity based on standard quantum mechanics - the Schroedinger-Newton theory - and two well-established predictions that come out of it, namely, gravitational 'cat states' and gravitationally-induced wavepacket collapse. Then we review three quantum theories with 'primitive ontologies' that are well-known known to solve the measurement problem - Schroedinger's many worlds theory, the GRW collapse theory with matter density ontology, and Nelson's stochas- tic mechanics. We extend the formalisms of these three quantum theories to Newtonian models of semiclassical gravity and evaluate their implications for gravitational cat states and gravitational wavepacket collapse. We find that (1) Newtonian semiclassical gravity based on Schroedinger's many worlds theory is mathematically equivalent to the Schroedinger-Newton theory and makes the same predictions; (2) Newtonian semiclassical gravity based on the GRW theory differs from Schroedinger-Newton only in the use of a stochastic collapse law, but this law allows it to suppress gravitational cat states so as not to be in contradiction with experiment, while allowing for gravita- tional wavepacket collapse to happen as well; (3) Newtonian semiclassical gravity based on Nelson's stochastic mechanics differs significantly from Schroedinger-Newton, and does not predict gravita- tional cat states nor gravitational wavepacket collapse. Considering that gravitational cat states are experimentally ruled out, but gravitational wavepacket collapse is testable in the near future, this implies that only the latter two are viable theories of Newtonian semiclassical gravity and that they can be experimentally tested against each other in future molecular interferometry experiments that are anticipated to be capable of testing the gravitational wavepacket collapse prediction.
We propose that gravitational-wave distinguishability provides a universal, information-theoretic mechanism for decoherence in quantum superpositions involving massive systems. When alternative quantum histories imprint distinguishable gravitational radiation states into spacetime, which-branch information exists in principle even in the absence of measurement. This suppresses interference purely through unitary entanglement with the gravitational field. Unlike collapse models, this framework relies only on standard quantum mechanics and the information-carrying capacity of gravitational radiation. We argue heuristically that the Planck mass emerges as an information-theoretic crossover scale beyond which gravitationally induced decoherence becomes unavoidable. To clarify the conceptual content, we introduce a Schrödinger’s-cat–type thought experiment in which one branch of a quantum superposition produces colliding black holes and gravitational waves, while the other does not. In this limit case, spacetime itself functions as an irreversible which-branch environment. The analysis positions gravity not as a modifier of quantum mechanics, but as a universal information channel contributing to the quantum–classical transition.